Chapter 9 - Modes and Modulation Sources

Chapter 9 - Modes and Modulation Sources
Chapter 9
Modes and Systems
The various modes, modulation types
and protocols we use partly reflect the different types of information we might wish
to transmit, such as data, voice, image or
even multimedia communications. Other
considerations are the behavior of the
radio link including fading, delay, Doppler frequency shift and distortion. We
are also limited by regulatory restrictions
such as bandwidth and following certain
conventions or protocols. Some of the
bands used by amateurs are wide and well
behaved, such as VHF links over short
paths. Others may be narrow, unstable
and hostile to our signals, such as a long
HF path through the auroral zones. Such
conditions dictate which mode will be
most successful.
ISSUES COMMON TO ALL
TRANSMISSION MODES
Bandwidth is the amount of frequency
spectrum that a signal occupies. There are
narrow-band modes, such as CW and
PSK31, and wideband modes, such as TV
and spread spectrum. Not all modes are permitted on all amateur bands. Wideband
modes can be used only where the total
width of the amateur allocation is sufficient
to contain the wide signal. In addition, voluntary agreements and regulatory restrictions keep some wideband modes out of
certain bands or subbands so that one
station’s signal does not preclude operation
by a large number of others using the narrower modes.
All users of the radio spectrum must comply with FCC bandwidth rules. The occupied bandwidth is determined not only by
the mode being used, but by proper opera-
tion of that mode. Many of the permitted
modes can become too wide when improperly adjusted. Perhaps the greatest source of
conflict between ham operators is “splatter”
or “key clicks” caused by overmodulated or
otherwise improperly operated equipment,
regardless of the mode being used. An amateur signal must be no wider than is necessary for good communication, and as clean
as the “state of the art” will allow. Section
97.3 (a)(8) of the FCC rules defines occupied bandwidth as the point where spurious
energy drops to 26 dB below the mean
power of the transmitted signal.
Sensitivity refers to the relative ability of
a mode to decode weak signals. Some
modes are favored by DXers in that they
have a greater ability to “get through” when
the signals are very weak. For local communications, sensitivity may not be the major
concern. Fidelity is not a major issue for
most amateurs, although they rightly take
pride in the clarity of their transmissions and
some amateurs take audio quality quite seriously. Intelligibility is related to fidelity in a
complex way and, sometimes, voice signals
are modified in such a way as to make them
more understandable, perhaps under difficult conditions, even though not as natural
as they might otherwise be.
Quality is the corresponding term for
images, and accuracy describes the degree
to which a text mode reproduces the original
message. Robustness or reliability refers to
the ability of a mode to maintain continuous
communication under difficult conditions.
For example, a very robust signal is desired
when controlling a model airplane. DXers
are not overly concerned with reliability in
that continuous contact is not needed. How-
ever, they do want a signal that gets through
when needed to work a rare station.
Efficiency is the ability of a mode to get
the signal through with minimum energy
expended. Within the regulatory power
limit, energy cost is not a major concern for
most home stations. Thus, efficiency is a
concern mainly to those on battery power—
using handheld or portable stations. Emergency operators also need to consider using
efficient modes. For radio services that use
high power, such as shortwave broadcasters, efficiency is very important. QRP is a
popular activity, where operators take pride
in making contact with a very small amount
of transmitter power (maximum miles per
milliwatt!).
Stability is the ability to maintain the frequency of the transmission very precisely.
Some modes require precise frequency
control. Most modern equipment is very
stable, but some vintage or homemade gear
may be limited in frequency stability.
Higher frequency work can put tight limits
on frequency stability. Channel stability
refers to both frequency, amplitude and
phase variations of the transmission medium itself. The inherent instability of a
radio channel may permit some modes but
preclude others.
Noise immunity is the ability of a radio
system to reject noise of various types that
could otherwise destroy the meaning or impair the quality of the message. This is allimportant in HF mobile operations and for
those living in densely populated areas.
Man-made electrical noise is an increasingly serious threat to ham operations and
requires both regulatory and technical solutions.
Modes and Modulation Sources
9.1
Emission Classifications
Emissions are designated according to their classification and their necessary bandwidth. A minimum of three symbols is
used to describe the basic characteristics of radio waves. Emissions are classified and symbolized according to the
following characteristics:
(6) Two or more channels containing analog
I. First symbol—Type of modulation of the main carrier
information ............................................................... 8
II. Second symbol—Nature of signal(s) modulating the main
(7) Composite system with one or more channel containcarrier
ing quantized or digital information, together with one
III. Third symbol—Type of information to be transmitted
or more channels containing analog information ..... 9
Note: A fourth and fifth symbol are provided for in the ITU
(8) Cases not otherwise covered ................................. X
Radio Regulations. Use of the fourth and fifth symbol is
optional.
Third symbol—type of information to be transmitted2
IV. Details of signal(s)
(1) No information transmitted .................................... N
V. Nature of multiplexing
(2) Telegraphy, for aural reception .............................. A
(3) Telegraphy, for automatic reception ...................... B
First symbol—type of modulation of the main carrier
(4) Facsimile ................................................................. C
(1) Emission of an unmodulated carrier ..................... N
(5) Data transmission, telemetry, telecommand ........ D
(2) Emission in which the main carrier is amplitude(6) Telephony (including sound broadcasting) ........... E
modulated (including cases where subcarriers are
(7) Television (video) .................................................... F
angle-modulated):
(8) Combination of the above ...................................... W
- Double sideband .................................................. A
(9) Cases not otherwise covered ................................. X
- Single sideband, full carrier ................................ H
- Single sideband, reduced or variable level
Where the fourth or fifth symbol is used it shall be used as
carrier ................................................................. R
indicated below. Where the fourth or the fifth symbol is not
used this should be indicated by a dash where each symbol
- Single sideband, suppressed carrier .................. J
would otherwise appear.
- Independent sidebands ....................................... B
- Vestigial sideband ............................................... C
Fourth symbol—Details of signal(s)
(3) Emission in which the main carrier is angle(1) Two-condition code with elements of differing
modulated:
numbers and/or durations ...................................... A
- Frequency modulation ........................................ F
(2) Two-condition code with elements of the same
- Phase modulation ............................................... G
number and duration without error-correction ...... B
Note: Whenever frequency modulation (F) is
(3) Two-condition code with elements of the same
indicated, phase modulation (G) is also acceptable.
number and duration with error-correction .......... C
(4) Emission in which the main carrier is amplitude
(4) Four-condition code in which each condition
and angle-modulated either simultaneously or in a
represents a signal element (of one or
pre-established sequence ..................................... D
more bits) ............................................................... D
(5) Emission of pulses1
(5) Multi-condition code in which each condition
- Sequence of unmodulated pulses ....................... P
represents a signal element (of one or
- A sequence of pulses:
more bits) ................................................................ E
- Modulated in amplitude ........................................ K
(6) Multi-condition code in which each condition or
- Modulated in width/duration ................................ L
combination of conditions represents a
- Modulated in position/phase ............................... M
character ................................................................. F
- In which the carrier is angle-modulated during
(7) Sound of broadcasting quality (monophonic) ....... G
the period of the pulse ......................................... Q
(8) Sound of broadcasting quality (stereophonic
- Which is a combination of the foregoing or in
or quadraphonic) ................................................... H
produced by other means .................................... V
(9) Sound of commercial quality (excluding
(6) Cases not covered above, in which an emission
categories given in (10) and (11) below ............... J
consists of the main carrier modulated, either
(10) Sound of commercial quality with frequency
simultaneously or in a pre-established sequence
inversion or band-splitting .................................... K
in a combination of two or more of the following
(11) Sound of commercial quality with separate
modes: amplitude, angle, pulse ............................ W
frequency-modulated signals to control the
(7) Cases not otherwise covered ................................. X
level of demodulated signal ................................. L
(12) Monochrome ........................................................ M
Second symbol—nature of signal(s) modulating the main
(13) Color ..................................................................... N
carrier
(14) Combination of the above ................................... W
(1) No modulating signal .............................................. 0
(15) Cases not otherwise covered ............................... X
(2) A single channel containing quantized or digital
information without the use of a modulating
Fifth symbol—Nature of multiplexing
subcarrier, excluding time-division multiplex ......... 1
(1) None ........................................................................ N
(3) A single channel containing quantized or digital
(2) Code-division multiplex3 ...................................................... C
information with the use of a modulating subcarrier,
(3) Frequency-division multiplex .................................. F
excluding time-division multiplex ............................ 2
(4) Time-division multiplex ........................................... T
(4) A single channel containing analog information .... 3
(5) Combination of frequency-division and time(5) Two or more channels containing quantized or
division multiplex .................................................... W
digital information .................................................... 7
(6) Other types of multiplexing ..................................... X
1 Emissions
where the main carrier is directly modulated by a signal which has been coded into quantized form (eg, pulse code modulation) should be designated under (2) or (3).
2 In this context the word “information” does not include information of a constant unvarying nature such as is provided by standard
frequency emissions, continuous wave and pulse radars, etc.
3 This includes bandwidth expansion techniques.
9.2
Chapter 9
Emission, Modulation and Transmission Characteristics
Emission designators are generally expressed as characters representing the
necessary bandwidth and emission classification symbols. Necessary bandwidth is expressed as a maximum of five numerals and
one letter. The letter occupies the position of
the decimal point and represents the unit of
bandwidth, as follows: H = hertz, K = kilohertz, M = megahertz and G = gigahertz.
For example, a bandwidth of 2.8 kHz is
expressed as 2K8 or 2K80 and a bandwidth
of 150 Hz is noted as 150H.
Emission classification symbols are (1)
type of modulation of the main carrier, (2)
nature of the signal(s) modulating the main
carrier and (3) the information to be transmitted. They may be supplemented by (4)
details of signal(s) and (5) nature of multiplexing, but the FCC does not require these.
These designators are found in Appendix 1
of the ITU Radio Regulations, ITU-R Recommendation SM.1138 and in the FCC rules
§2.201.
TRANSMISSION IMPAIRMENTS
In addition to attenuation of a signal from
a transmitting station to a receiving station,
the signal is subject to a variety of impairments. These include flat fading, frequencyselective fading, wave polarization rotation
fading, Doppler shift, interference from
other signals, atmospheric noise, galactic
noise and manmade noise. Receiver thermal
noise is not usually an issue at HF because
external noise often dominates but can be a
limiting factor at VHF and above.
Effect of the HF Path on Pulses
Digital signals, including Morse signals
copied by ear, are transmissions in which
the wave abruptly changes state. That means
that something about them is varied in order
to carry the digital information. Understanding what happens to pulses of all types when
they travel via the ionosphere is important
for knowing how to design a workable HF
digital system. VHF and UHF signals generally use more benign paths but some of the
same principles important in HF will also
apply to them.
Since Morse CW using OOK (on-off keying) is the oldest as well as the simplest digital mode, it is useful to analyze the
propagation effects on a simple Morse character, namely the letter “E.” This consists of
a single pulse, which can be distinguished
from the letter “T” by its length, provided
the keying speed is known beforehand. Using the ARRL-recommended values for
shaping to prevent excessive key clicks, a
single Morse “E” will appear as shown on
the left in Fig 9.1 as it leaves the transmitter.
A ham living nearby would observe the
Fig 9.1—Morse “E” as transmitted (left)
and received (right).
pulse essentially unchanged and could comment usefully on the shape of the keying.
However, a receiver far enough removed
so that sky wave was the dominant mode
would see a much different picture. If both
ground wave and sky wave were present, as
could occur on 80 meters at a distance of
20 miles, the received dit might appear as
shown on the right in Fig 9.1. Several things,
however, distort the pulse. One is multipath,
meaning that the signal arrives by more than
one route. One route might be the ground
wave, another the single-hop sky wave, and
others, very likely considerably weaker,
multiple-hop sky waves.
In addition, if the ionosphere is moving
up or down, Doppler shifts will change the
received frequency slightly. If operation is
near the MUF, some energy may be greatly
delayed by reflections from varying heights,
further smearing the pulse. Noise of various
types will be added to the pulse. Finally,
fading effects may be noticed even on such
a short time frame as a single Morse letter.
The successful decoding of a Morse signal consists of simply deciding whether a
pulse was sent or not, and its length. Humans have been given a brain and sense
organs that, when properly trained, can cope
with all these problems to a high degree. Our
brain’s ability to filter out extraneous noise
and signals is remarkable. The dynamic
range and sensitivity of both vision and hearing are close to the theoretical limits. Human
AGC (automatic gain control) operates to
provide a continually variable “decision
threshold.” If all else fails during on-air contacts, repeats can be requested.
A satisfactory non-human Morse decoder
would have to replicate all of the above human functions. As it turns out, Morse, using
OOK, is one of the hardest digital systems to
copy error free, although by using fixed
speed, and with reasonably good propagation, it is possible. Those who have worked
with software or hardware schemes for decoding Morse will point out that narrow filtering will clean up the above pulse
considerably. This is really a form of averaging or integration, which smoothes out the
abrupt changes in amplitude. A phaselocked loop detector provides averaging and
threshold detection in one circuit. Even with
these improvements, OOK signals are difficult to decode reliably.
The second oldest method of transmitting
pulses is with frequency shift keying (FSK)
where, instead of turning a signal on and off,
it is made to jump between different frequencies to correspond to the “key-up” and “keydown” conditions. RTTY (radio teletype)
does not require the use of FSK and some
have used OOK for RTTY. However, because of long use, most of us associate FSK
transmissions with RTTY, the mechanical
or computer decoding of text (plus crude
images) using the Baudot code. Digital text
modes have used AM (of which OOK is an
example, but not the only one), FSK or PSK
(phase shift keying).
As with FM analog transmissions, FSK
and PSK tend to discriminate against amplitude noise, which is common on HF. Thus,
amplitude changes resulting from fading
and/or static can be reduced. However, as
discussed in this Handbook’s AC/RF
Sources chapter, AM noise can change into
phase noise, so this is not a complete solution. Modes based on phase modulation,
such as the PSK modes, are not noise free.
You need only observe the little phase “compass” included in some PSK31 software to
be aware that even when transmitting an idle
tone, the received signal’s phase will jump
around.
Doppler shifts introduce noise into phaseand frequency-shift systems. If we are
counting on seeing a certain frequency shift
as a signal, you don’t want the path adding
any shifts that will show up as noise. Thus,
Doppler shift places a lower limit on the
amount of frequency or phase shift needed
to distinguish between the various data bits.
On HF, ionospheric Doppler, which can be
up to 5 Hz, places a lower limit on the carrier
spacing of multi carrier modes. Lengthening of pulses of up to several milliseconds is
common on HF paths. The general solution
for channel-produced lengthening of pulses
is simply to use a slower rate of transmission. Thus, the pulse has time to settle before
being interpreted.
The fastest CW operators slow down
when unusual multipath propagation conditions smear the characters together. A machine can operate at speeds where even
normal multipath can smear the characters,
not just in extreme cases. This factor puts an
upper limit on the symbol rate. If a bit rate is
to be increased beyond this point, each pulse
must carry more than one bit. This calls for
the use of complex modulation schemes
involving multiple states per pulse. These
can be represented by a number of different
frequencies in an FSK mode, many phases
Modes and Modulation Sources
9.3
VHF applications. Error-correction schemes
will always be useful for the more difficult
channels, since on HF there is no such thing
as a 100% reliable channel. On all frequencies, hams have a habit of “pushing the limits” so that marginal paths are often used.
The design of digital communications systems will always be an exercise in trade-offs.
EFFECT OF THE HF PATH ON
ANALOG SIGNALS
Analog signals undergo the same impairments as digital signals along an ionospheric
transmission path. However, the signals are
normally decoded by the combination of the
human ear and brain, which overcome problems, often without much notice. Frequencyselective fading of double-sideband AM
signals manifests itself as distortion or
mushiness at the receiver audio that may be
ignored by the human operator. Even if the
signal becomes temporarily unreadable, often the operator can fill in the blanks because the information is familiar or
expected. Single-sideband AM (SSB) usually suffers less mushiness in the receiver
due to frequency selective fading because
the signal occupies a narrower bandwidth,
in which low and high-frequency audio tend
to fade together. The human operator can
usually cope with such temporary fadeouts
or can request a retransmission for any part
missed.
Fig 9.2—An example of multilevel phase modulation.
in a PSK mode, many levels in an AM mode
or a combination of several or even all of the
above. An example of multi-level phase
modulation is illustrated in Fig 9.2. One type
of a complex system would be QAM (quadrature amplitude modulation), which requires
special equipment to observe.
With complex modulation forms, noiseinduced changes in both amplitude and
phase will cause some pulses to fall into
oblivion, where you simply cannot say what
was sent. Thus, a higher signal to noise ratio
is needed for complex types of modulation
such as 64QAM, whereas binary modes may
work closer to the noise.
For high bit rates on HF, many carriers
may be used, each one carrying multiple
amplitude and phase levels. There are, however, regulatory and practical limits. Thus,
while it is feasible to transmit digital speech
in a 3-kHz bandwidth, full-motion, highresolution full-motion TV signals would be
very difficult to transmit on HF, even if an
entire amateur band were used.
9.4
Chapter 9
Schemes that use more than one frequency, such as FSK and multicarrier systems, must cope with selective fading, where
not all frequencies fade at the same time.
Users of RTTY FSK systems have long observed that either the mark or space frequency may momentarily fade away leaving
only the other. A good decoder will work
with only one of the two tones present.
All present and future digital modes must
address these problems, and the development of new text modes has seen a steady
progression from the original and hallowed
CW and RTTY modes. As digital voice,
image, text and control modes develop, they
will all cope with the above channel limitations in various ways and with varying degrees of success. Some will be very resistant
to QRM; others will be efficient in use of
spectrum or in throughput—the amount of
data that can be sent in a given time.
Modes that are optimum for a QRM-free
VHF channel may work poorly on HF. Optimum HF modes will be too slow for some
MULTIPLE ACCESS AND
MULTIPLEXING TECHNOLOGIES
To appreciate some of the more complex
communications systems, you need an understanding of the different methods of sharing a carrier or accessing the frequency
spectrum. Multiple access refers to more
than one originating source having use of
the media. Multiplexing means combining
of two or more information streams into one
carrier or transmission path.
Frequency Division Multiple Access
(FDMA)
FDMA is probably the oldest and most
familiar method of accessing the frequency
spectrum, since individual signals are on
different frequency channels. It is also the
least efficient, since each frequency occupies a slot that is reserved for one user at a
time.
Frequency Division Multiplexing
(FDM)
FDM uses more than one subcarrier, imposed on a carrier, to convey different information. It traditionally was used for
multiplexed telephone systems but is rarely
used in the Amateur Radio Service.
Fig 9.3—Spectrum of an individual OFDM
carrier.
Time Division Multiple Access
(TDMA)
TDMA is simply time-sharing a frequency. In a general sense, this occurs naturally as stations in a QSO take turns
transmitting. TDMA is also used in digital
systems that reverse the direction of a circuit
automatically to send information and
acknowledgements.
Time Division Multiplex (TDM)
TDM is transmission of two or more signals over a common channel by interleaving
so that the signals occur in different time
slots. Some cellular telephone systems, such
as Global System for Mobile Communications (GSM) use TDM. In the Amateur Radio Service it is used mostly for telemetry,
such as from amateur satellites and remote
repeaters.
Orthogonal Frequency Division
Multiplexing (OFDM)
The term orthogonal is derived from the
Fig 9.4—Overall OFDM spectrum.
fact that multiple carriers are closely spaced
in frequency, but positioned such that they
do not interfere with one another. The center
frequency of one carrier’s signal falls within
the nulls of the signals on either side of it.
Figs 9.3 and 9.4, illustrate how the carriers
are interleaved to prevent intercarrier interference.
Because each carrier is modulated at a
relatively low rate, OFDM links suffer less
intersymbol interference (ISI) on HF ionospheric paths than single-carrier modulation
at a higher rate. See Smith, Doug, KF6DX,
“Distortion and Noise in OFDM Systems,”
QEX Mar/Apr 2005.
Code Division Multiple Access
(CDMA)
CDMA is a form of spread spectrum and
is generated by modulating a carrier with a
spreading code sequence known to both the
sender and receiver. Unlike FDMA and
TDMA, there is no fixed limit on the number
of users but the number is not infinite.
Major Modulation Systems
The broadest category of modulation is
how the main carrier is modulated. The
major types are amplitude modulation, angle
modulation and pulse modulation.
AMPLITUDE MODULATION
Amplitude modulation (AM) covers a
class of modulation systems in which the
amplitude of the main carrier is the characteristic that is varied. AM is sometimes simplistically described as varying the
amplitude of the carrier from zero power to
a peak power level. In fact, the carrier itself
stays at the same amplitude when modulated
by an analog (such as voice) baseband signal. The modulation itself produces sidebands, which are bands of frequencies on
both sides of the carrier frequency. AM is
basically a process of heterodyning or non-
linear mixing. As in any mixer, when a carrier and baseband modulation are combined,
there are three products in the frequency
range of interest: (1) the carrier, (2) the lower
sideband (LSB), and (3) the upper sideband
(USB). Thus, if a carrier of 10 MHz were
modulated by a 1-kHz sine wave, the outputs would be as shown in Fig 9.5.
The bandwidth of the modulated signal in
this example would be 2 kHz, the difference
between the lowest and highest frequencies.
In AM, the difference between the carrier
and farthest-away component of the sideband is determined by the highest frequency
component contained in the basebandmodulating signal.
ANGLE MODULATION
Two particular forms of angle modula-
Fig. 9.5—A 10-MHz carrier AM-modulated
by a 1-kHz sine wave.
Modes and Modulation Sources
9.5
tion are frequency modulation (FM) and
phase modulation (PM). Frequency and
phase modulation are not independent, since
the frequency cannot be varied without also
varying the phase, and vice versa.
The communications effectiveness of FM
and PM depends almost entirely on the receiving methods. If the receiver can respond
to frequency and phase changes but is insensitive to amplitude changes, it will discriminate against most forms of noise,
particularly impulse noise, such as that from
ignition systems.
Frequency Modulation
Fig 9.6 is a representation of frequency
modulation. When a modulating signal is
applied, the carrier frequency is increased
during one half cycle of the modulating signal and decreased during the half cycle of
the opposite polarity. This is indicated in the
drawing by the fact that the RF cycles occupy less time (higher frequency) when the
modulating signal is negative.
The change in the carrier frequency (frequency deviation) is proportional to the instantaneous amplitude of the modulating
signal. Thus, the deviation is small when the
instantaneous amplitude of the modulating
signal is small and is greatest when the
modulating signal reaches its peak, either
positive or negative. The drawing shows that
the amplitude of the RF signal does not
Fig 9.7—Amplitude of the FM carrier and sidebands with modulation index. This is a
graphical representation of mathematical functions developed by F. W. Bessel. Note that
the carrier completely disappears at modulation indexes of 2.405 and 5.52.
change during modulation. This is an oversimplification and is true only in the overall
sense, as the amplitude of both the carrier
and sidebands do vary with frequency
modulation. FM is capable of conveying dc
levels, as it can maintain a specific frequency.
Phase Modulation
In phase modulation, the characteristic
varied is the carrier phase from a reference
value. In PM systems, the demodulator responds only to instantaneous changes in frequency. PM cannot convey dc levels unless
special phase-reference techniques are used.
The amount of frequency change, or deviation, is directly proportion to how rapidly
the phase is changing and the total amount
of the phase change.
Fig 9.6—Graphical representation of
frequency modulation. In the unmodulated
carrier (A) each RF cycle occupies the
same amount of time. When the
modulating signal (B) is applied, the radio
frequency is increased and decreased
according to the amplitude and polarity of
the modulating signal (C).
9.6
Chapter 9
Bessel Functions
Bessel functions are employed—using
the carrier null method—to set deviation.
Some version of the chart shown in Fig 9.7
has appeared in the ARRL Handbook for
50 years. This chart is unlike previous ones
in that the values are plotted here in dB,
which is more familiar to anyone who uses
an S meter or a spectrum analyzer to observe
the various FM sidebands. This version also
plots all values as positive because receiv-
ers, including spectrum analyzers, do not
distinguish between positive and negative
phase values. Thus, this plot will give values
directly in dB below the unmodulated carrier of each component of a frequencymodulated wave, based on the modulation
index.
Since the carrier and each sideband of a
frequency-modulated signal change amplitude according to fixed rules as the deviation and modulating frequency change, we
can use those rules to set deviation, provided
we have a way to observe the FM spectrum.
Based on a set of mathematical functions
named after F.W. Bessel, who developed
them, we know that a modulation index of
2.405 will produce what is called the “first
carrier null.” Thus, if we wish to set our
deviation to 5 kHz, we can use an audio tone
of 5000/2.405 or 2079 Hz. While observing
the spectrum, we can then increase the deviation from zero until the carrier is in a null.
This guarantees that the deviation is now
5000 Hz. If we use a frequency counter to
set the audio tone accurately, the exactness
of the deviation setting should be very high.
Similarly, for setting the deviation to 3 kHz,
we could use the audio frequency of
1247 kHz and adjust for the first carrier null.
If a spectrum analyzer is not available, an
all-mode receiver using a narrow CW filter
could be used to detect the carrier null, using
the S meter and carefully tuning to the carrier. Additional carrier nulls occur with
modulation indices of 5.52 and 8.654. These
would be useful for wideband FM or when
using low audio frequencies for setting the
deviation with narrow-band FM.
Other methods of setting deviation include observing the bandwidth on a spectrum analyzer using a very low frequency
audio tone and using a deviation meter—an
FM receiver whose audio output is metered
on a scale calibrated directly in kHz of deviation. An FM service monitor may include
both a deviation meter and a spectrum dis-
play. The carrier-null method is the most
accurate of the three methods and can even
be used to calibrate a deviation meter.
You can also use the plot in Fig 9.7 to
predict the bandwidth of any given audio
frequency and deviation combination. Consider this example. We wish to keep our
bandwidth narrow enough to pass through a
15-kHz receive filter and we are transmitting a tone of 3 kHz. Since the third set of
sidebands will be 18 kHz apart, we would
do well to keep them, and all higher sidebands, below –40 dB. A quick look at the
chart shows that this means the modulation
index must be no more than about 0.7, mean-
ing the deviation should be 0.7 × 3 = 2.1 kHz.
If we are willing to allow the third set of
sidebands to be only 34 dB down, we can
use a modulation index of 1, meaning the
deviation will be 3 kHz—close to the recommended value for 9600 bits/s digital signals on FM. The above calculations strictly
apply only when the highest audio frequency (3 kHz) is present. If there is little
chance of 100% modulation at the highest
audio frequency as, for example, with a
normal voice signal, higher deviation could
be used. When new digital modems and
modes are used on FM, the above procedures should be part of the design.
use. Of all modes, CW is the most versatile
in terms of signaling speed. It is used at
speeds—measured in words per minute
(WPM)—of less than one, and up to several
hundred. Depending on ability of the operator, direct human copy works well between
about 5 and 60 WPM, but for very slow or
very fast speeds, the signals may be recorded
and the speed adjusted to allow human decoding. Very slow speeds and extremely
narrow filters make possible communication using signals below the noise, while
very fast speeds are useful for meteor scatter
communication where bandwidth is large,
but the reflection path lasts only a second or
less.
The bandwidth occupied by a CW signal
depends on the keying rate (See the Mixers,
Modulators and Demodulators chapter of
this Handbook), with higher speeds requiring a wider filter to pass the sidebands. In
addition, occupied bandwidth depends on
the rise and fall time and the shape of the
keyed RF envelope. That shape should be
somewhat rounded (no abrupt transitions)
in order to prevent “key clicks”—harmonics of the keying pulse. These can extend
over several kHz and cause unnecessary
interference. The ideal RF envelope of a
code element would rise and fall in the shape
of a sine wave. See Figs 9.8 and 9.9. ARRL
has long recommended a 5-ms rise time for
CW, up to 60 WPM, which keeps the signal
within a 150-Hz bandwidth. Use of a nar-
Operating Modes
This chapter examines various operating
modes used in the Amateur Radio Service,
including text modes, data, telemetry and
telecommand, voice, image, spread spectrum and multimedia. While modes once fit
into neat categories, there is now a blurring
of the definitions. For example, data transmissions could include images.
TELEGRAPHY MODES
These are basically text modes; that is,
transmission of letters, figures and punctuation, in a format suitable for printing at the
receiving station. Morse telegraphy and
radioteletype (Baudot and ASCII) are described, but you should be aware that the
term “telegraphy” includes facsimile transmission as well.
Morse Telegraphy (CW)
Text messages sent by on-off keying
(OOK, also known as amplitude-shift keying, or ASK) is the original mode for both
amateur and commercial radio. It is alive
and well today and is not expected to fade
away. For many amateurs, it is the principal,
or even the only, mode they use and many
take great and justifiable pride in their proficiency with it. The complete international
Morse (including the new @ character
· --· - ·) code itself is defined in ITU-R Recommendation M.1677, International Morse
code.
CW continues in use, however, not just
for reasons of nostalgia. When used by an
experienced operator, it can rival most any
mode for “getting the message through”
under marginal conditions and is absolutely
unrivaled in terms of the simplicity of the
equipment needed. Methods of generating
the code characters, and even of decoding
them, have used the latest technology, but
the straight key and “copy by ear” are still in
Fig 9.8—Optimum CW keying waveforms. The on-off transitions of the RF envelope
should be smooth, ideally following a sine-wave curve. See text.
Modes and Modulation Sources
9.7
need of a shared language. In skilled hands,
CW can achieve a QSO or traffic rate approaching that of phone operation while
using a fraction of the bandwidth.
Fig 9.9—Keying speed vs rise and fall times vs bandwidth for fading and non-fading
communications circuits. For example, to optimize transmitter timing for 25 WPM on a
non-fading circuit, draw a vertical line from the WPM axis to the K = 3 line. From there
draw a horizontal line to the rise/fall time axis (approximately 15 ms). Draw a vertical
line from where the horizontal line crosses the bandwidth line and see that the
bandwidth will be about 60 Hz.
Baudot Radioteletype (RTTY)
One of the first data communications
codes to receive widespread use had five
bits (also called “levels”) to present the alphabet, numerals, symbols and machine
functions. In the US, we use International
Telegraph Alphabet No. 2 (ITA2), commonly called Baudot, as specified in FCC
§97.309(a)(1). The code is defined in the
ITA2 Codes table on the CD-ROM included
with this book. In the United Kingdom, the
almost-identical code is called Murray code.
There are many variations in five-bit coded
character sets, principally to accommodate
foreign-language alphabets.
Five-bit codes can directly encode only
25 = 32 different symbols. This is insufficient to encode 26 letters, 10 numerals and
punctuation. This problem can be solved by
using one or more of the codes to select from
multiple code-translation tables. ITA2 uses
a LTRS code to select a table of upper-case
letters and a FIGS code to select a table of
numbers, punctuation and special symbols.
Certain symbols, such as carriage return,
occur in both tables. Unassigned ITA2 FIGS
codes may be used for the remote control of
receiving printers and other functions.
FCC rules provide that ITA2 transmissions must be sent using start-stop pulses, as
illustrated in Fig 9.10. The bits in the figure
are arranged as they would appear on an
oscilloscope.
Speeds and Signaling Rates
The signaling speeds for RTTY are those
used by the old TTYs, primarily 60 WPM or
45.45 bauds. The baud (Bd) is a unit of signaling speed equal to one pulse (event) per
second. The signaling rate, in bauds, is the
reciprocal of the shortest pulse length.
Fig 9.10—A typical Baudot timing sequence for the letter “D.”
rower filter than this on receive end is uncommon for ear-copied CW; therefore, narrower bandwidth is unnecessary and would
make the signal sound “mushy.” Very fast
pulses, such as would be used for HighSpeed CW (HSCW) meteor scatter work, are
computer generated and can occupy a normal SSB filter bandwidth.
Morse code is one of the most efficient
modes in terms of information sent per baud.
The commonly accepted ratio for bauds to
WPM is WPM = 1.2 × B. Thus, a keying
9.8
Chapter 9
speed of 25 dots per second or 50 bauds is
equal to 60 WPM. The efficiency of Morse
text messages is based on the use of the
shortest code combinations to represent the
most commonly used letters and symbols.
Efficiency is further achieved by extensive
use of abbreviations and “Q signals.” By
making use of these multiple levels of universally recognized coding schemes, CW
can get essential information across quickly.
CW abbreviations are universal so that
simple contacts can be made without the
Transmitter Keying
When TTYs and TUs (terminal units)
roamed the airwaves, frequency-shift keying (FSK) was the order of the day. DC signals from the TU controlled some form of
reactance (usually a capacitor or varactor)
in a transmitter oscillator stage that shifted
the transmitter frequency. Such direct FSK
is still an option with some new radios.
AFSK
Multimode communications processors
(MCPs), however, generally connect to the
radio AF input and output, often through the
speaker and microphone connectors, and
sometimes through auxiliary connectors.
They simply feed AF tones to the microphone input of an SSB transmitter or trans-
ceiver. This is called AFSK for “audio frequency-shift keying.”
When using AFSK, make certain that
audio distortion, carrier and unwanted sidebands do not cause interference. Particularly
when using the low tones discussed later,
the harmonic distortion of the tones should
be kept to a few percent. Most modern AFSK
generators are of the continuous-phase
(CPFSK) type. Also remember that equipment is operating at a 100% duty cycle for
the duration of a transmission. For safe operation, it is often necessary to reduce the
transmitter power output (25 to 50% of normal) from the level that is safe for CW operation.
What are High and Low Tones?
US amateurs customarily use the same
modems (2125 Hz mark, 2295 Hz space)
for both VHF AFSK and HF via an SSB
transmitter. Because of past problems (when
850-Hz shift was used), some amateurs use
“low tones” (1275 Hz mark, 1445 Hz
space). Both high and low tones can be used
interchangeably on the HF bands because
only the amount of shift is important. The
frequency difference is unnoticed on the air
because each operator tunes for best results.
On VHF AFSK, however, the high and low
tone pairs are not compatible.
Transmit Frequency
It is normal to use the lower sideband
mode for RTTY on SSB radio equipment.
In order to tune to an exact RTTY frequency, remember that most SSB radio
equipment displays the frequency of its
(suppressed) carrier, not the frequency of
the mark signal. Review your MCP’s
manual to determine the tones used and calculate an appropriate display frequency. For
example, to operate on 14,083 kHz with a
2125-Hz AFSK mark frequency, the SSB
radio display (suppressed-carrier) frequency should be 14,083 kHz + 2.125 kHz
= 14,085.125 kHz.
Receiving Baudot
TUs (Terminal Units) have been replaced by multi-mode communications
processors (MCPs), which accept AF signals from a radio and translate them into
common ASCII text or graphics file formats (see Fig 9.11). Because the basic interface is via ASCII, MCPs are compatible
with virtually any PC running a simple terminal program. Many MCPs handle CW,
RTTY, ASCII, packet, fax, SSTV and new
digital modes as they come into amateur
use. To an increasing extent, personal computer sound cards with appropriate software are a viable and low-cost alternative
to MCPs. However, sound cards have their
limitations and dedicated hardware can
Fig 9.11—A typical multimode communications processor (MCP) station. MCPs can do
numerous data modes as well as SSTV and fax.
more efficiently perform some operations.
AFSK Demodulators
An AFSK demodulator takes the shifting
tones from the audio output of a receiver
and produces TTY keying pulses. FM is a
common AFSK demodulation method. The
signal is first bandpass filtered to remove
out-of-band interference and noise. It is then
limited to remove amplitude variations. The
signal is demodulated in a discriminator or a
PLL. The detector output is low pass filtered
to remove noise at frequencies above the
keying rate. The result is fed to a circuit that
determines whether it is a mark or a space.
AM (limiterless) detectors, when properly
designed, permit continuous copy even
when the mark or space frequency fades out
completely. At 170-Hz shift, however, the
mark and space frequencies tend to fade at
the same time. For this reason, FM and AM
demodulators are comparable at 170-Hz
shift.
Diversity Reception
Although not restricted to RTTY, diversity reception can be achieved by using two
antennas, two receivers and a dual demodulator. Some amateurs are using it with good
results. One of the antennas would be the
normal station antenna for that band. The
second antenna could be either another
antenna of the same polarization located at
least 3/8-wavelength away, or an antenna of
the opposite polarization located near the
first antenna. A problem is to get both receivers on the same frequency without carefully tuning each one manually. Two
demodulators are needed for this type of diversity. Also, some type of diversity combiner, selector or processor is needed. Many
commercial or military RTTY demodulators
are equipped for diversity reception.
The payoff for using diversity is a worthwhile improvement in copy. Depending on
fading conditions, adding diversity may be
equivalent to raising transmitter power
severalfold.
BAUDOT RTTY BIBLIOGRAPHY
Ford, Steve, WB8IMY, ARRL’s HF Digital Handbook, Third Ed., ARRL, 2004.
Henry, “Getting Started in Digital Communications,” Part 3 (RTTY), QST, May
1992.
Hobbs, Yeomanson and Gee, Teleprinter
Handbook, Radio Society of Great Britain.
Nagle, “Diversity Reception: an Answer
to High Frequency Signal Fading,” Ham
Radio, Nov 1979, pp 48-55.
ASCII (IA5) RADIOTELETYPE
The American National Standard Code for
Information Interchange (ASCII) is a coded
character set used for information-processing systems, communications systems and
related equipment. Current FCC regulations
provide that amateur use of ASCII shall conform to ASCII as defined in ANSI Standard
X3.4-1977. Its international counterparts are
ISO 646-1983 and International Alphabet
No. 5 (IA5) as specified in ITU-T Recommendation V.3.
ASCII uses 7 bits to represent letters,
figures, symbols and control characters.
Unlike ITA2 (Baudot), ASCII has both
upper- and lower-case letters. A table of
ASCII characters is presented as “ASCII
Character Set” on the CD-ROM that accompanies this book.
Parity
While not strictly a part of the ASCII standard, an eighth bit may be added for parity
(P) checking. FCC rules permit optional use
of the parity bit. The applicable US and inModes and Modulation Sources
9.9
Glossary of Digital Communications Terminology
ACK—Acknowledgment, the control signal sent to indicate the
correct receipt of a transmission block.
Address—A character or group of characters that identifies a
source or destination.
AFSK—Audio frequency-shift keying.
ALE—Automatic link establishment.
AMRAD—Amateur Radio Research and Development
Corporation, a nonprofit organization dedicated to experimentation.
AMSAT—Radio Amateur Satellite Corporation.
AMTOR—Amateur teleprinting over radio, an amateur
radioteletype transmission technique employing error
correction as specified in several ITU-R Recommendations
M.476-2 through M.476-4 and M.625.
ANSI—American National Standards Institute.
Answer—The station intended to receive a call. In modem
usage, the called station or modem tones associated
therewith.
APCO—Association of Public Safety Communications Officials.
ARQ—Automatic Repeat reQuest, an error-sending station,
after transmitting a data block, awaits a reply (ACK or NAK)
to determine whether to repeat the last block or proceed to
the next.
ASCII—American National Standard Code for Information
Interchange, a code consisting of seven information bits.
AX.25—Amateur packet-radio link-layer protocol. Copies of
protocol specification are available from ARRL HQ.
Backwave—An unwanted signal emitted between the pulses
of an on/off-keyed signal.
Balanced—A relationship in which two stations communicate
with one another as equals; that is, neither is a primary
(master) or secondary (slave).
Baud—A unit of signaling speed equal to the number of
discrete conditions or events per second. (If the duration of
a pulse is 20 ms, the signaling rate is 50 bauds or the
reciprocal of 0.02, abbreviated Bd).
Baudot code—A coded character set in which five bits
represent one character. Used in the US to refer to ITA2.
Bell 103—A 300-baud full-duplex modem using 200-Hz-shift
FSK of tones centered at 1170 and 2125 Hz.
Bell 202—A 1200-baud modem standard with 1200-Hz mark,
2200-Hz space, used for VHF FM packet radio.
BER—Bit error rate.
BERT—Bit-error-rate test.
Bit stuffing—Insertion and deletion of 0s in a frame to
preclude accidental occurrences of flags other than at the
beginning and end of frames.
Bit—Binary digit, a single symbol, in binary terms either a one
or zero.
Bit/s—Bits per second.
BLER—Block error rate.
BLERT—Block-error-rate test.
Break-in—The ability to hear between elements or words of a
keyed signal.
Byte—A group of bits, usually eight.
Carrier detect (CD)—Formally, received line signal detector, a
physical-level interface signal that indicates that the receiver
section of the modem is receiving tones from the distant
modem.
CDMA—Code division multiple access.
Chirp—Incidental frequency modulation of a carrier as a result
of oscillator instability during keying.
CLOVER—Trade name of digital communications system
developed by Hal Communications.
COFDM—Coded Orthogonal Frequency Division Multiplex,
OFDM plus coding to provide error correction and noise
immunity.
Collision—A condition that occurs when two or more transmissions occur at the same time and cause interference to
the intended receivers.
Constellation—A set of points in the complex plane which
represent the various combinations of phase and amplitude
in a QAM or other complex modulation scheme.
9.10
Chapter 9
Contention—A condition on a communications channel that
occurs when two or more stations try to transmit at the
same time.
Control field—An 8-bit pattern in an HDLC frame containing
commands or responses, and sequence numbers.
CRC—Cyclic redundancy check, a mathematical operation.
The result of the CRC is sent with a transmission block.
The receiving station uses the received CRC to check
transmitted data integrity.
CSMA—Carrier sense multiple access, a channel access
arbitration scheme in which packet-radio stations listen on
a channel for the presence of a carrier before transmitting
a frame.
CTS—clear to send, a physical-level interface circuit generated by the DCE that, when on, indicates the DCE is ready
to receive transmitted data (abbreviated CTS).
DARPA—Defense Advanced Research Projects Agency.
DBPSK—Differential binary phase-shift keying.
DQPSK—Differential quadrature phase-shift keying.
DCE—Data circuit-terminating equipment, the equipment (for
example, a modem) that provides communication between
the DTE and the line radio equipment.
Domino—A conversational HF digital mode similar in some
respects to MFSK16.
DRM—Digital Radio Mondiale. A consortium of broadcasters,
manufacturers, research and governmental organizations
which developed a system for digital sound broadcasting in
bands between 100 kHz and 30 MHz.
EIA—Electronic Industries Alliance.
EIA-232—An EIA standard physical-level interface between
DTE (terminal) and DCE (modem), using 25-pin connectors. Formerly RS-232, a popular serial line standard,
equivalent of ITU-T V.24 and V.28.
Envelope-delay distortion—In a complex waveform,
unequal propagation delay for different frequency components.
Equalization—Correction for amplitude-frequency and/or
phase-frequency distortion.
Eye pattern—An oscilloscope display in the shape of one or
more eyes for observing the shape of a serial digital stream
and any impairments.
Facsimile (fax)—A form of telegraphy for the transmission of
fixed images, with or without half-tones, with a view to their
reproduction in a permanent form.
FCS—Frame check sequence. (See CRC.)
FDM—Frequency division multiplexing
FDMA—Frequency division multiple access
FEC—Forward error correction, an error-control technique in
which the transmitted data is sufficiently redundant to
permit the receiving station to correct some errors.
FSK—Frequency-shift keying.
GNU—A project to develop a free UNIX style operating
system.
G-TOR—A digital communications system developed by
Kantronics.
HDLC—High-level data link control procedures as specified in
ISO 3309.
Hellschreiber—A facsimile system for transmitting text.
Host—As used in packet radio, a computer with applications
programs accessible by remote stations.
IA5—International Alphabet No. 5, a 7-bit coded character
set, ITU-T version of ASCII.
IBOC—In Band On Channel. A method of using the same
channel on the AM or FM broadcast bands to transmit
simultaneous digital and analog modulation.
Information field—Any sequence of bits containing the
intelligence to be conveyed.
ISI—Intersymbol interference; slurring of one symbol into the
next as a result of multipath propagation.
ISO—International Organization for Standardization.
ITA2—International Telegraph Alphabet No. 2, a ITU-T 5-bit
coded character set commonly called the Baudot or Murray
code.
ITU—International Telecommunication Union, a specialized
agency of the United Nations. (See www.itu.int.)
ITU-R—Radiocommunication Sector of the ITU, formerly CCIR.
ITU-T—Telecommunication Standardization Sector of the
ITU, formerly CCITT.
Jitter—Unwanted variations in amplitude or phase in a digital
signal.
Key clicks—Unwanted transients beyond the necessary
bandwidth of a keyed radio signal.
LAP—Link access procedure, ITU-T Recommendation X.25
unbalanced-mode communications.
LAPB—Link access procedure, balanced, ITU-T Recommendation X.25 balanced-mode communications.
Layer—In communications protocols, one of the strata or
levels in a reference model.
Level 1—Physical layer of the OSI reference model.
Level 2—Link layer of the OSI reference model.
Level 3—Network layer of the OSI reference model.
Level 4—Transport layer of the OSI reference model.
Level 5—Session layer of the OSI reference model.
Level 6—Presentation layer of the OSI reference model.
Level 7—Application layer of the OSI reference model.
Linux—A free Unix-type operating system originated by
Linus Torvalds, et al. Developed under the GNU General
Public License.
Loopback—A test performed by connecting the output of a
modulator to the input of a demodulator.
LSB—Least-significant bit.
MFSK16—A multi-frequency shift communications system
Modem—Modulator-demodulator, a device that connects
between a data terminal and communication line (or radio).
Also called data set.
MSB—Most-significant bit.
MSK—Frequency-shift keying where the shift in Hz is equal
to half the signaling rate in bits per second.
MT-63—A keyboard-to-keyboard mode similar to PSK31 and
RTTY.
NAK—Negative acknowledge (opposite of ACK).
Node—A point within a network, usually where two or more
links come together, performing switching, routine and
concentrating functions.
NRZI—Nonreturn to zero. A binary baseband code in which
output transitions result from data 0s but not from 1s.
Formal designation is NRZ-S (nonreturn-to-zero-space).
Null modem—A device to interconnect two devices both
wired as DCEs or DTEs; in EIA-232 interfacing, back-toback DB25 connectors with pin-for-pin connections except
that Received Data (pin 3) on one connector is wired to
Transmitted Data (pin 3) on the other.
Octet—A group of eight bits.
OFDM—Orthogonal Frequency Division Multiplex. A method
of using spaced subcarriers that are phased in such a way
as to reduce the interference between them.
Originate—The station initiating a call. In modem usage, the
calling station or modem tones associated therewith.
OSI-RM—Open Systems Interconnection Reference Model
specified in ISO 7498 and ITU-T Recommendation X.200.
Packet radio—A digital communications technique involving
radio transmission of short bursts (frames) of data containing addressing, control and error-checking information in
each transmission.
PACTOR®—Trade name of digital communications protocols
offered by Special Communications Systems GmbH & Co
KG (SCS).
Parity check—Addition of non-information bits to data,
making the number of ones in a group of bits always either
even or odd.
PID—Protocol identifier. Used in AX.25 to specify the
network-layer protocol used.
Primary—The master station in a master-slave relationship;
the master maintains control and is able to perform actions
that the slave cannot. (Compare secondary.)
Project 25—Digital voice system developed for APCO, also
known as P25.
Protocol—A formal set of rules and procedures for the
exchange of information within a network.
PSK—Phase-shift keying.
PSK31—A narrow-band digital communications system
developed by Peter Martinez, G3PLX.
Q15X25—A DSP-intensive mode intended as an error-free
mode more reliable on HF than packet.
QAM—Quadrature Amplitude Modulation. A method of
simultaneous phase and amplitude modulation. The
number that precedes it, eg, 64QAM, indicates the number
of discrete stages in each pulse.
QPSK—Quadrature phase-shift keying.
RAM—Random access memory.
Router—A network packet switch. In packet radio, a networklevel relay station capable of routing packets.
RTS—Request to send, physical-level signal used to control
the direction of data transmission of the local DCE.
RTTY—Radioteletype.
RxD—Received data, physical-level signals generated by the
DCE are sent to the DTE on this circuit.
SCAMP—Sound Card Automated Message Protocol, an
inexpensive alternative to hardware for passing e-mail
traffic on narrow-bandwidth channels.
Secondary—The slave in a master-slave relationship.
Compare primary.
Source—In packet radio, the station transmitting the frame
over a direct radio link or via a repeater.
SSID—Secondary station identifier. In AX.25 link-layer
protocol, a multipurpose octet to identify several packetradio stations operating under the same call sign.
TAPR—Tucson Amateur Packet Radio Corporation, a nonprofit
organization involved in packet-radio development.
TDM—Time division multiplexing
TDMA—Time division multiple access
Telecommand—The use of telecommunication for the
transmission of signals to initiate, modify or terminate
functions of equipment at a distance.
Telemetry—The use of telecommunication for automatically
indicating or recording measurements at a distance from
the measuring instrument.
Telephony—A form of telecommunication primarily intended
for the exchange of information in the form of speech.
Telegraphy—A form of telecommunication in which the
transmitted information is intended to be recorded on
arrival as a graphic document; the transmitted information
may sometimes be presented in an alternative form or may
be stored for subsequent use.
Teleport—A radio station that acts as a relay between
terrestrial radio stations and a communications satellite.
Television—A form of telecommunication for the transmission of transient images of fixed or moving objects.
Throb—A multi-frequency shift mode like MFSK16.
TNC—Terminal node controller, a device that assembles and
disassembles packets (frames); sometimes called a PAD.
TR switch—Transmit-receive switch to allow automatic
selection between receive and transmitter for one antenna.
TTY—Teletypewriter.
Turnaround time—The time required to reverse the direction
of a half-duplex circuit, required by propagation, modem
reversal and transmit-receive switching time of transceiver.
TxD—Transmitted data, physical-level data signals transferred on a circuit from the DTE to the DCE.
UI—Unnumbered information frame.
V.24—An ITU-T Recommendation defining physical-level
interface circuits between a DTE (terminal) and DCE
(modem), equivalent to EIA-232.
V.28—An ITU-T Recommendation defining electrical characteristics for V.24 interface.
Virtual circuit—A mode of packet networking in which a
logical connection that emulates a point-to-point circuit is
established (compare Datagram).
Window—In packet radio at the link layer, the range of frame
numbers within the control field used to set the maximum
number of frames that the sender may transmit before it
receives an acknowledgment from the receiver.
X.25—An ITU-T packet-switching protocol Recommendation.
Modes and Modulation Sources
9.11
ternational standards (ANSI X3.16-1976;
ITU-T Recommendation V.4) recommend
an even parity sense for asynchronous and
odd parity sense for synchronous data communications. The standards, however, generally are not observed by hams. By
sacrificing parity, the eighth bit can be used
to extend the ASCII 128-character code to
256 characters.
ASCII Serial Transmission
Serial transmission standards for ASCII
(ANSI X3.15 and X3.16; ITU-T Recommendation V.4 and X.4) specify that the bit
sequence shall be least-significant bit (LSB)
first to most-significant bit (MSB); that is, b0
through b6 (plus the parity bit, P, if used).
Serial transmission may be either synchronous or asynchronous. In synchronous
transmissions, only the information bits (and
optional parity bit) are sent, as shown in
Fig 9.12A.
Asynchronous serial transmission adds a
start pulse and a stop pulse to each character. The start pulse length equals that of an
information pulse. The stop pulse may be
one or two bits long. There is some variation, but one stop bit is the convention.
ASCII Data Rates
Data-communication signaling rates depend largely on the medium and the state of
the art when the equipment was selected.
The most-used rates tend to progress in
2:1 steps from 300 to 9600 bits/s and in
8 kbits/s increments from 16 kbits/s upward.
The “baud” (Bd) is a unit of signaling
speed equal to one discrete condition or
event per second. In single-channel transmission, such as the FCC prescribes for
Baudot transmissions, the signaling rate in
bauds equals the data rate in bits per second.
However, the FCC does not limit ASCII to
single-channel transmission. Some digital
modulation systems have more than two
(mark and space) states. In dibit (pronounced “die-bit”) modulation, two ASCII
bits are sampled at a time. The four possible
states for a dibit are 00, 01, 10 and 11. In
four-phase modulation, each state is assigned an individual phase of 0º, 90º, 180º
and 270º respectively. For dibit phase modulation, the signaling speed in bauds is half
the information-transfer rate in bits/s. Since
the FCC specifies the digital sending speed
in bauds, amateurs may transmit ASCII at
higher information rates by using digital
modulation systems that encode more bits
per signaling element.
Amateur ASCII RTTY Operations
On April 17, 1980, the FCC first permitted ASCII in the Amateur Radio Service.
Amateurs have been slow to abandon
Baudot in favor of asynchronous serial
9.12
Chapter 9
ASCII. Rather than transmitting start-stop
ASCII, this code has become embedded in
more sophisticated data transmission
modes, which are described later in this
chapter.
ASCII BIBLIOGRAPHY
ANSI X3.4-1977, “Code for Information
Interchange,” American National Standards Institute.
ANSI X3.15-1976, “Bit sequencing of the
American National Standard Code for
Information Interchange in Serial-byBit Data Transmission.”
ANSI X3.16-1976, “Character Structure
and Character Parity Sense for Serialby-Bit Data Communication Information Interchange.”
ANSI X3.25-1976, “Character Structure
and Character Parity Sense for Parallelby-Bit Communication in American National Standard Code for Information
Interchange.”
Bemer, “Inside ASCII,” Interface Age,
May, June and July 1978.
ITU-T Recommendation V.3, “International Alphabet No. 5.”
ITU-T Recommendation V.4, “General
Structure of Signals of International Alphabet No. 5 Code for Data Transmission
over the Public Telephone Network.”
Mackenzie, Coded Character Sets, History
and Development, Addison-Wesley Publishing Co, 1980.
AMTOR
AMTOR is derived from ITU-R Recommendation M.476, and is known
“narrowband direct printing” (NBDP) and
commercially as “SITOR.” It has been
largely overtaken by newer protocols.
AMTOR uses two forms of time diversity
in either Mode A (ARQ, Automatic Repeat
reQuest) or Mode B (FEC, Forward Error
Correction). In Mode A, a repeat is sent only
when requested by the receiving station. In
Mode B, each character is sent twice. In
Mode A or Mode B, the second type of time
diversity is supplied by the redundancy of
the code itself.
Mode B (FEC)
When transmitting to no particular station
(for example calling CQ, in a net operation
or during bulletin transmissions) there is no
(one) receiving station to request repeats.
Mode B uses a simple forward-error-control
(FEC) technique: It sends each character
twice. Burst errors are virtually eliminated
by delaying the repetition for a period
thought to exceed the duration of most noise
bursts. In AMTOR, groups of five characters are sent (DX) and then repeated (RX).
At 70 ms per character, there is 280 ms between the first and second transmissions of
a character.
The Information Sending Station (ISS)
transmitter must be capable of 100% dutycycle operation for Mode B. Thus, it may be
necessary to reduce power level to 25% to
50% of full rating.
Mode A (ARQ)
This synchronous system transmits
blocks of three characters from the Information Sending Station (ISS) to the Information Receiving Station (IRS). After each
block, the IRS either acknowledges correct
receipt (based on the 4/3 mark/space ratio)
or requests a repeat. The station that initiates
the ARQ protocol is known as the Master
Station (MS). The MS first sends the selective call of the called station in blocks of
three characters, listening between blocks.
Four-letter AMTOR calls are normally derived from the first character and the last
three letters of the station call sign. For example, W1AW’s AMTOR call would be
WWAW. The Slave Station (SS) recognizes
its selective call and answers that it is ready.
The MS now becomes the ISS and will send
traffic as soon as the IRS says it is ready.
On the air, AMTOR Mode A signals have
a characteristic “chirp-chirp” sound. Because of the 210/240-ms on/off timing,
Mode A can be used with some transmitters
at full power levels.
AMTOR BIBLIOGRAPHY
Ford, Steve, WB8IMY, Your RTTY/AMTOR
Companion (Newington, CT: ARRL,
1993.)
Henry, Bill, “Getting Started in Digital Communications-AMTOR,” QST, Jun 1992.
ITU-R Recommendations M.476 and 625,
“Direct-Printing Telegraph Equipment
Fig 9.12—Typical serial synchronous and asynchronous timing for the ASCII character “S.”
in the Maritime Mobile Service.”
Martinez, Peter, “AMTOR, An Improved
RTTY System Using a Microprocessor,”
Radio Communication, RSGB, Aug 1979.
Newland, Paul, “A User’s Guide to
AMTOR Operation,” QST, Oct 1985.
PSK31
Peter Martinez, G3PLX, who was instrumental in bringing us AMTOR, also developed PSK31 for real time keyboard-tokeyboard QSOs. This section was adapted
from an article in RadCom, Jan 1999. The
name derives from the modulation type
(phase-shift keying) and the data rate, which
is actually 31.25 bauds. PSK31 is a robust
mode for HF communications that features
the 128 ASCII (Internet) characters and the
full 256 ANSI character set. This mode
works well for two-way QSOs and for nets.
Time will tell if PSK31 will replace Baudot
RTTY on the amateur HF bands.
Morse code uses a single carrier frequency
keyed on and off as dits and dahs to form
characters. RTTY code shifts between two
frequencies, one for mark (1) the other for
space (0). Sequences of marks and spaces
comprise the various characters.
Martinez devised a new variable-length
code for PSK31 that combines the best of
Morse and RTTY. He calls it Varicode because a varying number of bits are used for
each character (see Fig 9.13). Much like
Morse code, the more commonly used letters in PSK31 have shorter codes.
As with RTTY, there is a need to signal
the gaps between characters. The Varicode
does this by using “00” to represent a gap.
The Varicode is structured so that two zeros
never appear together in any of the combinations of 1s and 0s that make up the characters. In on-the-air tests, Martinez has
verified that the unique “00” sequence
works significantly better than RTTY’s stop
code for keeping the receiver synchronized.
With Varicode, a typing speed of about
50 words per minute requires a 32 bit/s
transmission rate. Martinez chose 31.25 bit/s
because it can be easily derived from
the 8-kHz sample rate used in many DSP
systems.
The shifting carrier phase generates sidebands 31.25 Hz from the carrier. These are
used to synchronize the receiver with the
transmitter. The bandwidth of a PSK31 signal is shown in Fig 9.14.
PSK31 Error Correction
Martinez added error correction to
PSK31 by using QPSK (quaternary phase
shift keying) and a convolutional encoder
to generate one of four different phase
shifts that correspond to patterns of five
successive data bits. At the receiving end,
a Viterbi decoder is used to correct errors.
Fig 9.13—Codes for the word “ten” in ASCII, Baudot, Morse and Varicode.
Fig 9.14—The spectrum of a PSK31 signal.
There are 32 possible sequences for five
bits. The Viterbi decoder tracks these possibilities while discarding the least likely
and retaining the most likely sequences.
Retained sequences are given a score that
is based on the running total. The most
accurate sequence is reported, and thus
errors are corrected.
Operating PSK31 in the QPSK mode
should result in 100% copy under most conditions, but at a price. Tuning is twice as
critical as it is with BPSK. An accuracy of
less than 4 Hz is required for the Viterbi
decoder to function properly.
Getting Started
In addition to a transceiver and antenna,
you only need a computer with a Windows
operating system and a 16-bit sound card to
receive and transmit PSK31. Additional information and software is available for free
download over the Web. Use a search engine to find PSK31 information and links to
downloads.
An interesting wrinkle is to generate text,
transmit it via PSK31 or some other RTTY or
data mode, receive it and use a speech synthesizer to read the message. An example of
this technique was described by W3NRG in
the October 2004 issue of CQ Magazine
(p 48). Synthesized speech takes some getting used to, as everybody sounds pretty
much alike, and the personality of the
speaker does not come through.
PSK31 BIBLIOGRAPHY
Ford, Steve, WB8IMY, ARRL’s HF Digital Handbook, Third Ed., ARRL, 2004.
DATA MODES
The difference between text and data
modes is not abrupt but a blur. Data could be
Modes and Modulation Sources
9.13
used to mean text, numbers, telecommand,
telemetry and in some cases images. The
third letter of the emission symbol “D” is
used in common for data, telecommand and
telemetry.
Packet Radio
Data communications is telecommunications between computers. Packet switching
is a form of data communications that transfers data by subdividing it into “packets,”
and packet radio is packet switching using
the medium of radio. This description was
written by Steve Ford, WB8IMY.
Packet radio has its roots in the Hawaiian
Islands, where the University of Hawaii began using the mode in 1970 to transfer data
to its remote sites dispersed throughout the
islands. Amateur packet radio began in
Canada after the Canadian Department of
Communications permitted amateurs to use
the mode in 1978. (The FCC permitted amateur packet radio in the US in 1980.)
In the first half of the 1980s, packet radio
was the habitat of a small group of experimenters who did not mind communicating
with a limited number of potential fellow
packet communicators. In the second half of
the decade, packet radio “took off” as the
experimenters built a network that increased
the potential number of packet stations that
could intercommunicate and thus attracted
tens of thousands of communicators who
wanted to take advantage of this potential.
Packet radio provides error-free data
transfer. The receiving station receives information exactly as the transmitting station
sends it, so you do not waste time deciphering communication errors caused by interference or changes in propagation.
Packet uses time efficiently, since packet
bulletin-board systems (PBBSs) permit
packet operators to store information for
later retrieval by other amateurs. And it uses
the radio spectrum efficiently, since one
radio channel may be used for multiple communications simultaneously, or one radio
channel may be used to interconnect a number of packet stations to form a “cluster” that
provides for the distribution of information
to all of the clustered stations. The popular
DX PacketClusters are typical examples (see
Fig 9.15).
Each local channel may be connected to
other local channels to form a network that
affords interstate and international data communications. This network can be used by
interlinked packet bulletin-board systems to
transfer information, messages and thirdparty traffic via HF, VHF, UHF and satellite
links. Primary node-to-node links are also
active on the Internet.
It uses other stations efficiently, since any
packet-radio station can use one or more
other packet-radio stations to relay data to
9.14
Chapter 9
its intended destination. It uses current station transmitting and receiving equipment
efficiently, since the same equipment used
for voice communications may be used for
packet communications. The outlay for the
additional equipment necessary to make
your voice station a packet-radio station
may be as little as $100. It also allows you to
use that same equipment as an alternative to
costly landline data communications links
for transferring data between computers.
The TNC
The terminal node controller—or TNC—
is at the heart of every packet station. A TNC
is actually a computer unto itself. It contains
the AX.25 packet protocol firmware, along
with other enhancements depending on the
manufacturer. The TNC communicates with
you through your computer or data terminal. It also allows you to communicate with
other hams by feeding packet data to your
transceiver.
The TNC accepts data from a computer
or data terminal and assembles it into packets (see Fig 9.16). In addition, it translates
the digital packet data into audio tones that
can be fed to a transceiver. The TNC also
functions as a receiving device, translating
the audio tones into digital data a com-
Fig 9.15—DX PacketClusters are networks comprised of individual nodes and stations
with an interest in DXing and contesting. In this example, N1BKE is connected to the
KC8PE node. If he finds a DX station on the air, he’ll post a notice—otherwise known as
a spot—which the KC8PE node distributes to all its local stations. In addition, KC8PE
passes the information along to the W1RM node. W1RM distributes the information and
then passes it to the KR1S node, which does the same. Eventually, WS1O—who is
connected to the KR1S node—sees the spot on his screen. Depending on the size of the
network, WS1O will receive the information within minutes after it was posted by
N1BKE.
Fig 9.16—The functional block diagram of a typical TNC.
puter or terminal can understand. The part
of the TNC that performs this tone-translating function is known as a modem (see
Fig 9.17).
If you’re saying to yourself, “These TNCs
sound a lot like telephone modems,” you’re
pretty close to the truth! The first TNCs were
based on telephone modem designs. If
you’re familiar with so-called smart modems, you’d find that TNCs are very similar.
You have plenty of TNCs to choose from.
The amount of money you’ll spend depends
directly on what you want to accomplish.
Most TNCs are designed to operate at 300
and 1200 bit/s, or 1200 bit/s exclusively
(see Fig 9.18). There are also TNCs dedicated to 1200 and 9600 bit/s operation, or
9600 bit/s exclusively. Many of these TNCs
include convenient features such as personal
packet mailboxes, where friends can leave
messages when you’re not at home. Some
TNCs also include the ability to easily disconnect the existing modem and substitute
another. This feature is very important if you
wish to experiment at different data rates.
For example, a 1200 bit/s TNC with a modem disconnect header can be converted to
a 9600 bit/s TNC by disconnecting the
1200 bit/s modem and adding a 9600 bit/s
modem.
If you’re willing to spend more money,
you can buy a complete multimode communications processor, or MCP. These devices
not only offer packet, they also provide the
capability to operate RTTY, CW, AMTOR,
PACTOR, FAX and other modes. In other
words, an MCP gives you just about every
digital mode in one box.
TNC Emulation and Internal TNCs
TNC-emulation systems exist for IBM
PCs and compatibles. One is known as
BayCom, which uses the PC to emulate the
functions of a TNC/terminal while a small
external modem handles the interfacing.
BayCom packages are available in kit form
for roughly half the price of a basic TNC.
PC owners also have the option of buying
full-featured TNCs that mount inside their
computers. TNC cards are available on the
market. They are complete TNCs that plug
into card slots inside the computer cabinet.
No TNC-to-computer cables are necessary.
Connectors are provided for cables that attach to your transceiver. In many cases, specialized software is also provided for
efficient operation.
Transceiver Requirements
Packet activity on the HF bands typically
takes place at 300 bit/s using common SSB
transceivers. The transmit audio is fed from
the TNC to the microphone jack or auxiliary
audio input. Receive audio is obtained from
the radio’s external speaker jack or auxil-
Fig 9.17—A block diagram of a typical modem.
Fig 9.18—Four popular 1200 bit/s packet TNCs: (clockwise, from bottom left) the MFJ1270C, AEA PK-88, Kantronics KPC-3 and the DRSI DPK-2.
iary audio output. Tuning is critical for
proper reception; a visual tuning indicator—
available on some TNCs and all MCPs—is
recommended.
These simple connections also work for
1200 bit/s packet, which is common on the
VHF bands (2 m in particular). Almost any
FM transceiver can be made to work with
1200 bit/s packet by connecting the transmit audio to the microphone jack and taking
the receive audio from the external speaker
(or earphone) jack.
At data rates beyond 1200 bit/s, transceiver requirements become more rigid. At
9600 bit/s (the most popular data rate above
1200 bit/s), the transmit audio must be injected at the modulator stage of the FM transceiver. Receive audio must be tapped at the
discriminator. Most 9600 bit/s operators use
modified Amateur Radio transceivers or
commercial radios.
In the mid 1990s amateur transceiver
manufacturers began incorporating data
ports on some FM voice rigs. The new
“data-ready” radios are not without problems, however. Their IF filter and discriminator characteristics leave little room for
error. If you’re off frequency by a small
amount, you may not be able to pass data.
In addition, the ceramic discriminator coils
used in some transceivers have poor group
delay, making it impossible to tune them
for wider bandwidths. With this in mind,
some amateurs prefer to make the leap to
9600 bit/s and beyond using dedicated
amateur data radios.
Regardless of the transceiver used, setting the proper deviation level is extremely
critical. At 9600 bit/s, for example, optimum
performance occurs when the maximum
deviation is maintained at 3 kHz. Deviation
adjustments involve monitoring the transmitted signal with a deviation meter or service monitor. The output level of the TNC is
adjusted until the proper deviation is
achieved.
PACKET NETWORKING
Digipeaters
A digipeater is a packet-radio station capable of recognizing and selectively repeating packet frames. An equivalent term
used in industry is bridge. Virtually any
TNC can be used as a single-port digipeater,
because the digipeater function is included
in the AX.25 Level 2 protocol firmware.
Although the use of digipeaters is waning
today as network nodes take their place,
the digipeater function is handy when you
Modes and Modulation Sources
9.15
need a relay and no node is available, or for
on-the-air testing.
TCP/IP
If you’re an active packeteer, sooner or
later someone will bring up the subject of
TCP/IP—Transmission Control Protocol/
Internet Protocol. Despite its name, TCP/IP
is more than two protocols; it’s actually a set
of several protocols. Together they provide
a high level of flexible, “intelligent” packet
networking. TCP/IP enthusiasts see a future
when the entire nation, and perhaps the
world, will be linked by high-speed TCP/IP
systems using terrestrial microwave and satellites.
TCP/IP has a unique solution for busy
networks. Rather than transmitting packets
at randomly determined intervals, TCP/IP
stations automatically adapt to network delays as they occur. As network throughput
slows down, active TCP/IP stations sense the
change and lengthen their transmission delays accordingly. As the network speeds up,
the TCP/IP stations shorten their delays to
match the pace. This kind of intelligent network sharing virtually guarantees that all
packets will reach their destinations with the
greatest efficiency the network can provide.
With TCP/IP’s adaptive networking
scheme, you can chat using the telnet protocol with a ham in a distant city and rest assured that you’re not overburdening the
system. Your packets simply join the constantly moving “freeway” of data. They
might slow down in heavy traffic, but they
will reach their destination eventually. (This
adaptive system is used for all TCP/IP packets, no matter what they contain.)
TCP/IP excels when it comes to transferring files from one station to another. By
using the TCP/IP file transfer protocol (ftp),
you can connect to another station and transfer computer files—including software. As
you can probably guess, transferring large
files can take time. With TCP/IP, however,
you can still send and receive mail (using
the SMTP protocol) or talk to another ham
while the transfer is taking place.
When you attempt to contact another station using TCP/IP, all network routing is
performed automatically according to the
TCP/IP address of the station you’re trying
to reach. In fact, TCP/IP networks are transparent to the average user.
To operate TCP/IP, all you need is a computer (it must be a computer, not a terminal),
a 2-m FM transceiver and a TNC with KISS
capability. As you might guess, the heart of
your TCP/IP setup is software. The TCP/IP
software set was written by Phil Karn,
KA9Q, and is called NOSNET or just NOS
for short.
There are dozens of NOS derivatives
available today. All are based on the origi9.16
Chapter 9
Fig 9.19—PACTOR
data packet format.
nal NOSNET. The programs are available
primarily for IBM-PC compatibles and
Macintoshes. You can obtain NOS software from on-line sources such as the
CompuServe HAMNET forum libraries,
Internet ftp sites, Amateur Radio-oriented
BBSs and elsewhere. NOS takes care of all
TCP/IP functions, using your “KISSable”
TNC to communicate with the outside
world. The only other item you need is your
own IP address. Individual IP Address Coordinators assign addresses to new TCP/IP
users.
PACKET BIBLIOGRAPHY
ARRL/TAPR, proc. Digital Communications Conferences, ARRL, annually
1983-present.
Ball, Bob, WB8WGA, “An Inexpensive
Terminal Node Controller for Packet
Radio,” QEX, Mar/Apr 2005.
Fox, Terry, WB4JFI, AX.25 Packet-Radio
Link-Layer Protocol, ARRL, 1984
(maintained by Tucson Amateur Packet
Radio–TAPR)
Horzepa, Stan, WA1LOU, Your Gateway
to Packet Radio, ARRL, 1989.
Roznoy, Rich, K1OF, Packet: Speed, More
Speed and Applications, ARRL, 1997.
PACTOR
PACTOR (PT), now often referred to as
PACTOR-I, is an HF radio transmission system developed by German amateurs HansPeter Helfert, DL6MAA, and Ulrich Strate,
DF4KV. It was designed to overcome the
shortcomings of AMTOR and packet radio.
It performs well under both weak-signal and
high-noise conditions. PACTOR-I has been
overtaken by PACTOR-II and PACTOR-III
but remains in use.
TRANSMISSION FORMATS
Information Blocks
All packets have the basic structure
shown in Fig 9.19, and their timing is as
shown in Table 9.1:
• Header: Contains a fixed bit pattern to
simplify repeat requests, synchronization
and monitoring. The header is also important for the Memory ARQ function. In
each packet carrying new information, the
Table 9.1
PACTOR Timing
Object
Packet
CS receive time
Control signals
Propagation delay
Cycle
Length (seconds)
0.96 (200 bd:
192 bits; 100 bd:
96 bits)
0.29
0.12 (12 bits at
10 ms each)
0.17
1.25
Table 9.2
PACTOR Status Word
Bit
0
1
2
3
4
5
6
7
Meaning
Packet count (LSB)
Packet count (MSB)
Data format (LSB)
Data format (MSB)
Not defined
Not defined
Break-in request
QRT request
Data Format Bits
Format
ASCII 8 bit
Huffman code
Not defined
Not defined
bit 3
0
0
1
1
bit 2
0
1
0
1
Bits 0 and 1 are used as a packet count;
successive packets with the same value are
identified by the receiver as repeat packets.
A modulus-4 count helps with unrecognized
control signals, which are unlikely in practice.
bit pattern is inverted.
• Data: Any binary information. The format is specified in the status word. Current choices are 8-bit ASCII or 7-bit ASCII
(with Huffman encoding). Characters are
not broken across packets. ASCII RS (hex
1E) is used as an IDLE character in both
formats.
• Status word: See Table 9.2
• CRC: The CRC is calculated according to
the CCITT standard, for the data, status
and CRC.
Acknowledgment Signals
The PACTOR acknowledgment signals
are shown in Table 9.3. Each of the signals
is 12 bits long. The characters differ in pairs
in 8 bits (Hamming offset) so that the chance
of confusion is reduced. If the CS is not
correctly received, the TX reacts by repeating the last packet. The request status can be
uniquely recognized by the 2-bit packet
number so that wasteful transmissions of
pure RQ blocks are unnecessary.
Timing
The receiver pause between two blocks is
0.29 s. After deducting the CS lengths,
0.17 s remain for switching and propagation delays so that there is adequate reserve
for DX operation.
CONTACT FLOW
Listening
In the listen mode, the receiver scans any
received packets for a CRC match. This
method uses a lot of computer processing
resources, but it’s flexible.
CQ
A station seeking contacts transmits CQ
packets in an FEC mode, without pauses for
acknowledgment between packets. The
transmit time length, number of repetitions
and speed are the transmit operator’s choice.
(This mode is also suitable for bulletins and
other group traffic.) Once a listening station
has copied the call, the listener assumes the
TX station role and initiates a contact. Thus,
the station sending CQ initially takes the RX
station role. The contact begins as shown in
Table 9.4
Speed Changes
With good conditions, PACTOR’s normal signaling rate is 200 bauds, but the system automatically changes from 200 to
100 bauds and back, as conditions demand.
In addition, Huffman coding can further
increase the throughput by a factor of 1.7.
There is no loss of synchronization speed
changes; only one packet is repeated.
When the RX receives a bad 200-baud
packet, it can acknowledge with CS4. TX
immediately assembles the previous packet
in 100-baud format and sends it. Thus, one
packet is repeated in a change from 200 to
100 bauds.
The RX can acknowledge a good 100baud packet with CS4. TX immediately
switches to 200 bauds and sends the next
packet. There is no packet repeat in an upward speed change.
Change of Direction
The RX station can become the TX station by sending a special change-over
packet in response to a valid packet. RX
sends CS3 as the first section of the
changeover packet. This immediately
changes the TX station to RX mode to read
the data in that packet and responds with
CS1 and CS3 (acknowledge) or CS2 (reject).
End of Contact
PACTOR provides a sure end-of-contact
procedure. TX initiates the end of contact by
sending a special packet with the QRT bit set
in the status word and the call of the RX
station in byte-reverse order at 100 bauds.
The RX station responds with a final CS.
PACTOR-II
This is a significant improvement over
PACTOR-I, yet it is fully compatible with
the older mode. Also invented in Germany,
PACTOR uses 16PSK to transfer up to
800 bit/s at a 100-baud rate. This keeps the
bandwidth less than 500 Hz.
PACTOR-II uses digital signal processing (DSP) with Nyquist waveforms,
Huffman and Markov compression, and
powerful Viterbi decoding to increase transfer rate and sensitivity into the noise level.
The effective transfer rate of text is over
1200 bit/s. Features of PACTOR II include:
• Frequency agility—It can automatically
adjust or lock two signals together over a
±100-Hz window.
• Powerful data reconstruction based upon
computer power—With over 2 MB of
available memory.
• Cross correlation—Applies analog
Memory ARQ to acknowledgment
frames and headers.
• Soft decision making—Uses artificial intelligence (AI), as well as digital information received to determine frame validity.
• Extended data block length—When transferring large files under good conditions,
the data length is doubled to increase the
transfer rate.
• Automatic recognition of PACTOR-I,
PACTOR-II and so on, with automatic
mode switching.
• Intermodulation products are canceled by
the coding system.
• Two long-path modes extend frame tim-
Table 9.3
PACTOR Control Signals
Code
CS1
CS2
CS3
Chars (hex)
4D5
AB2
34B
CS4
D2C
Function
Normal acknowledge
Normal acknowledge
Break-in (forms header
of first packet from
RX to TX)
Speed change request
All control signals are sent only from RX to
TX.
ing for long-path terrestrial and satellite
propagation paths.
This is a fast, robust mode, possibly the
most powerful in the ham bands. It has excellent coding gain as well. It can also communicate with all earlier PACTOR-I
systems. PACTOR-II stations acknowledge each received transmission block.
PACTOR-II employs computer logic as
well as received data to reassemble defective data blocks into good frames. This reduces the number of transmissions and
increases the throughput of the data.
PACTOR-III
PACTOR-III is a software upgrade for
existing PACTOR-II modems that provides
a data transmission mode for improved
speed and robustness. PACTOR-III is not a
new modem or hardware device. Most current PACTOR-II modems are upgradeable
to use PACTOR-III via a software update,
since PACTOR-II firmware accommodates
the new PACTOR-III software. Both the
transmitting and receiving stations must support PACTOR-III for end-to-end communications using this mode.
PACTOR-III’s maximum uncompressed
speed is 2722 bit/s. Using online compression, up to 5.2 kbit/s is achievable. This requires an audio passband from 400 Hz to
2600 Hz (for PACTOR-III speed level 6).
On an average channel, PACTOR-III is
more than three times faster than PACTORII. On good channels, the effective throughput ratio between PACTOR-III and
PACTOR-II can exceed five. PACTOR-III
is also slightly more robust than PACTORII at their lower SNR edges.
The ITU emission designator for
PACTOR-III is 2K20J2D. Because
PACTOR-III builds on PACTOR-II,
most specifications like frame length
and frame structure are adopted from
PACTOR-II. The only significant difference is PACTOR III’s multi-tone waveform
that uses up to 18 carriers while PACTORII uses only two carriers. PACTOR-III’s
carriers are located in a 120-Hz grid and
modulated with 100 symbols per second
DBPSK or DQPSK. Channel coding is also
adopted from PACTOR-II’s Punctured
Convolutional Coding.
PACTOR-III Link Establishment
The calling modem uses the PACTOR-I
FSK connect frame for compatibility. When
the called modem answers, the modems
negotiate to the highest level of which both
modems are capable. If one modem is only
capable of PACTOR-II, then the 500 Hz
PACTOR-II mode is used for the session.
With the MYLevel (MYL) command a user
may limit a modem’s highest mode. For
example, a user may set MYL to “1” and
Modes and Modulation Sources
9.17
Table 9.4
PACTOR Initial Contact
Master Initiating Contact
Size (bytes)
Content
Speed (bauds)
1
/Header
100
8
/SLAVECAL
100
6
/SLAVECAL/
200
Slave Response
The receiving station detects a call, determines mark/space polarity, decodes 100-bd and
200-bd call signs. It uses the two call signs to determine if it is being called and the quality
of the communication path. The possible responses are:
First call sign does not match slave’s
(Master not calling this slave)
none
Only first call sign matches slave’s
(Master calling this slave, poor communications)
CS1
First and second call signs both match the slaves
(good circuit, request speed change to 200 bd)
CS4
Table 9.5
CLOVER-II Modulation Modes
As presently implemented, CLOVER-II supports a total of 7 different modulation formats: 5
using PSM and 2 using a combination of PSM and ASM (Amplitude Shift Modulation).
Name
Description
In-Block Data Rate
16P4A
16PSM
8P2A
8PSM
QPSM
BPSM
2DPSM
16 PSM, 4-ASM
16 PSM
8 PSM, 2-ASM
8 PSM
4 PSM
Binary PSM
2-Channel Diversity BPSM
750 bps
500 bps
500 bps
375 bps
250 bps
125 bps
62.5 bps
only a PACTOR-I connection will be made,
set to “2” and PACTOR-I and II connections
are available, set to “3” and PACTOR-I
through III connections are enabled. The
default MYL is set to “2” with the current
firmware and with PACTOR-III firmware it
will be set to “3”. If a user is only allowed to
occupy a 500 Hz channel, MYL can be set
to “2” and the modem will stay in its
PACTOR-II mode. The PACTOR-III Protocol Specification is available on the Web at
www.scs-ptc.com/pactor.html.
PACTOR Bibliography
ARRL Web, Technical Descriptions,
www.arrl.org/FandES/field/regulations/techchar/.
Ford, Steve, WB8IMY, ARRL’s HF Digital Handbook, Third Ed., ARRL. 2004.
G-TOR
This brief description has been adapted
from “A Hybrid ARQ Protocol for Narrow
Bandwidth HF Data Communication” by
Glenn Prescott, WBØSKX, Phil Anderson,
WØXI, Mike Huslig, KBØNYK, and Karl
Medcalf, WK5M (May 1994 QEX).
G-TOR is short for Golay-TOR, an innovation of Kantronics, Inc. It was inspired by
9.18
Chapter 9
HF Automatic Link Establishment (ALE)
concepts and is structured to be compatible
with ALE.
The purpose of the G-TOR protocol is to
provide an improved digital radio communication capability for the HF bands. The
key features of G-TOR are:
Standard FSK tone pairs (mark and space)
• Link-quality-based signaling rate: 300,
200 or 100 bauds
• 2.4-second transmission cycle
• Low overhead within data frames
• Huffman data compression—two types,
on demand
• Embedded run-length data compression
• Golay forward-error-correction coding
• Full-frame data interleaving
• CRC error detection with hybrid ARQ
• Error-tolerant “Fuzzy” acknowledgments.
The G-TOR Protocol
Since one of the objectives of this protocol is ease of implementation in existing
TNCs, the modulation format consists of
standard tone pairs (FSK), operating at 300,
200 or 100 bauds, depending upon channel
conditions. (G-TOR initiates contacts and
sends ACKs only at 100 bauds.) The G-TOR
waveform consists of two phase-continuous
tones (BFSK), spaced 200 Hz apart (mark =
1600 Hz, space = 1800 Hz); however, the
system can still operate at the familiar 170Hz shift (mark = 2125 Hz, space = 2295 Hz),
or with any other convenient tone pairs. The
optimum spacing for 300-baud transmission
is 300 Hz, but you trade some performance
for a narrower bandwidth.
Each transmission consists of a synchronous ARQ 1.92-s frame and a 0.48-s interval for propagation and ACK transmissions
(2.4 s cycle). All advanced protocol features
are implemented in the signal-processing
software.
Data Compression
Data compression is used to remove redundancy from source data. Therefore,
fewer bits are needed to convey any given
message. This increases data throughput and
decreases transmission time—valuable features for HF. G-TOR uses run-length coding
and two types of Huffman coding during
normal text transmissions. Run-length coding is used when more than two repetitions
of an 8-bit character are sent. It provides an
especially large savings in total transmission time when repeated characters are being transferred.
The Huffman code works best when the
statistics of the data are known. G-TOR applies Huffman A coding with the upper- and
lower-case character set, and Huffman B
coding with upper-case-only text. Either
type of Huffman code reduces the average
number of bits sent per character. In some
situations, however, there is no benefit from
Huffman coding. The encoding process is
then disabled. This decision is made on a
frame-by-frame basis by the informationsending station.
Golay Coding
The real power of G-TOR resides in the
properties of the (24,12) extended Golay
error-correcting code, which permits correction of up to three random errors in three
received bytes. The (24,12) extended
Golay code is a half-rate error-correcting
code: Each 12 data bits are translated into
an additional 12 parity bits (24 bits total).
Further, the code can be implemented to
produce separate input-data and parity-bit
frames.
The extended Golay code is used for
G-TOR because the encoder and decoder
are simple to implement in software. Also,
Golay code has mathematical properties that
make it an ideal choice for short-cycle synchronous communication.
G-TOR Bibliography
ARRL Web, Technical Descriptions,
www.arrl.org/FandES/field/regula-
tions/techchar/.
Ford, Steve, WB8IMY, ARRL’s HF Digital Handbook, Third Ed., ARRL. 2004.
CLOVER-II
The desire to send data via HF radio at
high data rates and the problems encountered when using AX.25 packet radio on
HF radio led Ray Petit, W7GHM, to develop a unique modulation waveform and
data transfer protocol that is now called
“CLOVER-II.” Bill Henry, K9GWT, supplied this description of the Clover-II system. CLOVER modulation is characterized
by the following key parameters:
• Very low base symbol rate: 31.25 symbols/second (all modes).
• Time-sequence of amplitude-shaped
pulses in a very narrow frequency spectrum. Occupied bandwidth = 500 Hz at
50 dB below peak output level.
• Differential modulation between pulses.
• Multilevel modulation.
The low base symbol rate is very resistant
to multipath distortion because the time between modulation transitions is much longer
than even the worst-case time-smearing
caused by summing of multipath signals. By
using a time-sequence of tone pulses, DolphChebychev “windowing” of the modulating
signal and differential modulation, the total
occupied bandwidth of a CLOVER-II signal
is held to 500 Hz.
The CLOVER Waveform
Multilevel tone, phase and amplitude
modulation give CLOVER a large selection
of data modes that may be used (see Table
9.5). The adaptive ARQ mode of CLOVER
senses current ionospheric conditions and
automatically adjusts the modulation mode
to produce maximum data throughput.
When using the “Fast” bias setting, ARQ
throughput automatically varies from
11.6 byte/s to 70 byte/s.
The CLOVER-II waveform uses four
tone pulses that are spaced in frequency by
125 Hz. The time and frequency domain
characteristics of CLOVER modulation are
shown in Figs 9.20, 9.21 and 9.22. The
time-domain shape of each tone pulse is
intentionally shaped to produce a very
compact frequency spectrum. The four
tone pulses are spaced in time and then
combined to produce the composite output
shown. Unlike other modulation schemes,
the CLOVER modulation spectra is the
same for all modulation modes.
Modulation
Data is modulated on a CLOVER-II signal by varying the phase and/or amplitude
of the tone pulses. Further, all data modulation is differential on the same tone pulse—
Fig 9.20—Amplitude vs time plots for CLOVER-II’s four-tone waveform.
PSK of a continuous carrier is avoided. This
is true for all CLOVER-II modulation formats. The term “phase-shift modulation”
(PSM) is used when describing CLOVER
modes to emphasize this distinction.
Fig 9.21—A frequency-domain plot of a
CLOVER-II waveform.
data is represented by the phase (or amplitude) difference from one pulse to the next.
For example, when binary phase modulation is used, a data change from “0” to “1”
may be represented by a change in the phase
of tone pulse 1 by 180º between the first and
second occurrence of that pulse. Further, the
phase state is changed only while the pulse
amplitude is zero. Therefore, the wide frequency spectra normally associated with
Coder Efficiency Choices
CLOVER-II has four “coder efficiency”
options: 60%, 75%, 90% and 100% (“efficiency” being the approximate ratio of real
data bytes to total bytes sent). “60% efficiency” corrects the most errors but has the
lowest net data throughput. “100% efficiency” turns the encoder off and has the
highest throughput but fixes no errors. There
is therefore a tradeoff between raw data
throughput versus the number of errors that
can be corrected without resorting to retransmission of the entire data block.
Note that while the “In Block Data Rate”
numbers listed in the table go as high as
750 bit/s, overhead reduces the net throughput or overall efficiency of a CLOVER transmission. The FEC coder efficiency setting
and protocol requirements of FEC and ARQ
modes add overhead and reduce the net efficiency. Table 9.6 and Table 9.7 detail the
relationships between block size, coder efficiency, data bytes per block and correctable
byte errors per block.
CLOVER FEC
All modes of CLOVER-II use ReedSolomon forward error correction (FEC) data
encoding, which allows the receiving station to correct errors without requiring a
Modes and Modulation Sources
9.19
Table 9.6
Data Bytes Transmitted Per Block
Block Reed-Solomon Encoder Efficiency
Size
60%
75%
90%
100%
17
8
10
12
14
51
28
36
42
48
85
48
60
74
82
255
150
188
226
252
Table 9.7
Correctable Byte Errors Per Block
Block Reed-Solomon Encoder Efficiency
Size
60%
75%
90%
100%
17
1
1
0
0
51
9
5
2
0
85
16
10
3
0
255
50
31
12
0
Fig 9.22—Spectra plots of AMTOR, HF packet-radio and CLOVER-II signals.
repeat transmission. This is a very powerful
error-correction technique that is not available in some other common HF data modes.
CLOVER ARQ
Reed-Solomon data coding is the primary means by which errors are corrected
in CLOVER “FEC” mode. In ARQ mode,
CLOVER-II employs a three-step strategy
to combat errors. First, channel parameters
are measured and the modulation format is
adjusted to minimize errors and maximize
data throughput. This is called the “Adaptive ARQ Mode” of CLOVER-II. Second,
Reed-Solomon encoding is used to correct
a limited number of byte errors per transmitted block. Finally, only those data
blocks in which errors exceed the capacity
of the Reed-Solomon decoder are repeated
(selective block repeat).
With seven different modulation formats,
four data-block lengths (17, 51, 85 or
255 bytes) and four Reed-Solomon coder
efficiencies (60%, 75%, 90% and 100%),
there are 112 (7 × 4 × 4) different waveform
modes that could be used to send data via
CLOVER. Once all of the determining factors are considered, however, there are eight
different waveform combinations that are
actually used for FEC and/or ARQ modes.
CLOVER-2000
CLOVER-2000 is a faster version of
CLOVER (about four times faster) that uses
eight tone pulses, each of which is 250 Hz
wide, spaced at 250-Hz centers, contained
within the 2-kHz bandwidth between 500
and 2500 Hz. The eight tone pulses are sequential, with only one tone being present
at any instant and each tone lasting 2 ms.
Each frame consists of eight tone pulses
lasting a total of 16 ms, so the base modu9.20
Chapter 9
lation rate of a CLOVER-2000 signal is
always 62.5 symbols per second (regardless of the type of modulation being used).
CLOVER-2000’s maximum raw data rate
is 3000 bit/s.
Allowing for overhead, CLOVER-2000
can deliver error-corrected data over a
standard HF SSB radio channel at up to
1994 bit/s, or 249 characters (8-bit bytes)
per second. These are the uncompressed
data rates; the maximum throughput is typically doubled for plain text if compression is
used. The effective data throughput rate of
CLOVER-2000 can be even higher when
binary file transfer mode is used with data
compression.
The binary file transfer protocol used by
HAL Communications operates with a terminal program explained in the HAL E2004
engineering document listed under references. Data compression algorithms tend to
be context sensitive—compression that
works well for one mode (eg, text), may not
work well for other data forms (graphics,
etc). The HAL terminal program uses the
PK-WARE compression algorithm, which
has proved to be a good general-purpose
compressor for most computer files and programs. Other algorithms may be more efficient for some data formats, particularly for
compression of graphic image files and digitized voice data. The HAL Communications
CLOVER-2000 modems can be operated
with other data compression algorithms in
the users’ computers.
CLOVER-2000 is similar to the previous
version of CLOVER, including the transmission protocols and Reed-Solomon error
detection and correction algorithm. The
original descriptions of the CLOVER Control Block (CCB) and Error Correction Block
(ECB) still apply for CLOVER-2000, except
for the higher data rates inherent to CLOVER-2000. Just like CLOVER, all data sent
via CLOVER-2000 is encoded as 8-bit data
bytes and the error-correction coding and
modulation formatting processes are transparent to the data stream—every bit of
source data is delivered to the receiving terminal without modification.
Control characters and special “escape
sequences” are not required or used by
CLOVER-2000. Compressed or encrypted
data may therefore be sent without the need
to insert (and filter) additional control characters and without concern for data integrity. Five different types of modulation may
be used in the ARQ mode—BPSM (Binary
Phase Shift Modulation), QPSM (Quadrature
PSM), 8PSM (8-level PSM), 8P2A (8PSM +
2-level Amplitude-Shift Modulation), and
16P4A (16 PSM plus 4 ASM).
The same five types of modulation used
in ARQ mode are also available in Broadcast (FEC) mode, with the addition of 2Channel Diversity BPSM (2DPSM). Each
CCB is sent using 2DPSM modulation, 17byte block size and 60% bias. The maximum ARQ data throughput varies from
336 bit/s for BPSM to 1992 bit/s for 16P4A
modulation. BPSM is most useful for weak
and badly distorted data signals, while the
highest format (16P4A) needs extremely
good channels, with high SNRs and almost
no multipath.
Most ARQ protocols designed for use
with HF radio systems can send data in only
one direction at a time. CLOVER-2000
does not need an “OVER” command; data
may flow in either direction at any time.
The CLOVER ARQ time frame automatically adjusts to match the data volume sent
in either or both directions. When first
linked, both sides of the ARQ link exchange information using six bytes of the
CCB. When one station has a large volume
of data buffered and ready to send, ARQ
mode automatically shifts to an expanded
time frame during which one or more
255 byte data blocks are sent.
If the second station also has a large volume of data buffered and ready to send, its
half of the ARQ frame is also expanded.
Either or both stations will shift back to CCB
level when all buffered data has been sent.
This feature provides the benefit of full-duplex data transfer but requires use of only
simplex frequencies and half-duplex radio
equipment. This two-way feature of CLOVER can also provide a back-channel orderwire capability. Communications may be
maintained in this “chat” mode at 55 words
per minute, which is more than adequate for
real-time keyboard-to-keyboard communications.
CLOVER Bibliography
ARRL Web, Technical Descriptions,
www.arrl.org/FandES/field/regulations/techchar/.
Ford, Steve, WB8IMY, ARRL’s HF Digital Handbook, Third Ed., ARRL. 2004.
SCAMP AND RDFT
SCAMP (Sound Card Amateur Message
Protocol) is intended as a low-cost alternative to commercial modems (TNCs). A paper describing SCAMP was presented by
Rick Muething, KN6KB, at the 2004 ARRL/
TAPR Digital Communications Conference.
It is a new digital sound card protocol suitable for both HF and VHF for transmission
of text messages with binary attachments. It
is compatible with Winlink 2000 and is designed for manually initiated message forwarding. SCAMP is not a keyboard (chat)
mode.
SCAMP incorporates the work by Barry
Sanderson, KB9VAK, on Redundant Digital File Transfer (RDFT) and adds an ARQ
wrapper around RDFT to ensure error-free
transmission. There are four redundancy
levels: 10%, 20%, 40% and 70%, the latter
being the most robust and requires the most
transmission time. Audio data is sent at a
standard rate of 11.025 kHz, with 16-bit
samples using a PC sound card. SCAMP
occupies a bandwidth of about 2 kHz and a
net throughput of 2 to 4 kbytes/minute, depending on conditions. It employs an automated “channel-busy” detector for
reduction of QRM and to protect against
QRM from “hidden transmitters.” For more
details, please see the SCAMP Bibliography,
below.
On-the-air peer-to-peer testing began in
November 2004 and the first transcontinental transmission was made in December
2004 between N6KZB in Temecula, CA,
and W3QA in West Chester, PA. Beta testing began of SCAMP with WinLink 2000 in
March 2005.
SCAMP and RDFT Bibliography
ARRL Letter, The Vol 23, No 48, “SCAMP
On-Air Testing Commences,” Dec 10,
2005.
Muething, “SCAMP (Sound Card Amateur
Message Protocol),” proc., 2004 ARRL/
TAPR Digital Communications Conference.
Muething, “SCAMP Protocol Specification,” winlink.org/Presentations/
SCAMPspec.pdf.
AUTOMATIC LINK ESTABLISHMENT
The US military services have found it
difficult to maintain a sufficient number of
qualified radio operators to operate MF/HF
radios. So the Defense Department contracted with MITRE Corporation for the development of a method of operating MF/HF
radios without skilled operators. MITRE
studied what skilled operators do and developed Automatic Link Establishment (ALE)
to operate radios and make contact with
another station without human intervention
and under computer control. ALE automatically finds the best frequency among a prearranged list using techniques such as
selective calling, handshaking, link quality
analysis, polling, sounding, etc.
ALE is used by the Military Affiliate Ra-
dio Service (MARS). It has also been
adopted by some radio amateurs.
ALE Waveform
The ALE waveform is designed to be
compatible with the audio passband of a
standard SSB radio. It has a robust waveform for reliability during poor path conditions. It consists of 8-ary frequency-shift
keying (FSK) modulation with eight orthogonal tones, a single tone for a symbol.
These tones represent 3 bits of data, with
least significant bit to the right, as follows:
Frequency
Data
750 Hz
000
1000 Hz
001
1250 Hz
011
1500 Hz
010
1750 Hz
110
2000 Hz
111
2250 Hz
101
2500 Hz
100
The tones are transmitted at a rate of
125 tones per second, 8 ms per tone. The
resultant transmitted bit rate is 375 bit/s.
The basic ALE word consists of 24 bits of
information. Details can be found in Federal Standard 1045, Detailed Requirements, www.its.bldrdoc.gov/fs-1054a/
45-detr.htm.
ALE Bibliography
Adair, Robert, KAØCKS, et al, “A Federal
Standard for HF Radio Automatic Link
Establishment,” QEX January 1990.
Adair, Robert, KAØCKS, et al, “The
Growing Family of Federal Standards
for HF Radio Automatic Link Establishment (ALE)—Part I,” QEX July 1993;
Part II, QEX August 1993; Part III, QEX,
September 1993; Part IV, QEX October
1993; Part V, QEX November 1993;
Part VI QEX December 1993.
Brain, Charles, G4GUO, PC-ALE Project,
www.chbrain.dircon.co.uk/peale.html.
Menold, Ronald, AD4TB, “ALE—The
Coming of Automatic Link Establishment,” QST February 1995, p. 68
(Technical Correspondence).
National Communications System, “Telecommunications: HF Radio Automatic
Link Establishment,” Federal Standard
1045A, October 1993.
Internetworking
Although it has been a goal of some radio
amateurs to develop a digital communications network independent of the Internet, interconnection with the Internet
provides a good bridge between isolated
amateur radio nets. Several methods of
transferring data, e-mail or linking repeaters
have been developed.
WINLINK 2000
WinLink 2000 is a Windows application
that permits messages to be transferred automatically between the Internet and remote
amateur stations, which may be on recreational vehicles or at sea. The Internet is used
as a backbone to allow WinLink mailbox
operation (MBO) stations to share their data-
bases. Its original author was Victor Poor,
W5SSM. See: winlink.org/.
IRLP
Created by David Camerpon, WE7LTD,
the Internet Radio Linking Project (IRLP)
uses Voice over Internet Protocol (VoIP) to
form a voice communications network
Modes and Modulation Sources
9.21
of servers and nodes between amateur
repeaters and/or simplex stations. See:
www.irlp.net/.
EchoLink
EchoLink was developed by Jonathan
Taylor, K1RFD, to link a personal computer
to communicate by VoIP with several thousand repeaters having EchoLink capabilities.
Or, it can be used to permit amateur stations
within range of your station to connect with
the Internet. See: www.echolink.org/.
eQSO
eQSO, created by Paul Davies, MØZPD,
was designed to operate like a worldwide
amateur radio net. See: www.eqso.net/.
Internetworking Bibliography
Brone, Jeff, WB2JNA, “EchoLink for
Beginners,” QST, January 2005.
Ford, Steve, WB8IMY, ARRL’s HF Digital
Handbook, Third Ed., ARRL. 2004.
Ford, Steve, WB8IMY, “VoIP and Amateur Radio,” QST, February 2003.
Horzepa, Stan, WA1LOU, “WinLink
2000: A Worldwide HF BBS,” QST,
March 2000.
Linden, Louis, KI5TO, “Winlink 2000 in
the Jungle,” QST, November 2004.
TELEMETRY, TRACKING AND
TELECOMMAND
According to FCC Part 97 rules, telemetry
is a one-way transmission of measurements
at a distance from the measuring instrument,
whereas telecommand is a one-way transmission to initiate, modify or terminate functions of a device at a distance. Actually, the
two go hand in hand, since it is important to
have telemetry first, then modify the remote
device, then look once again in the telemetry to see if the desired action took place.
Telemetry, tracking and telecommand
(often seen as TT&C) are attracting increasing attention because Amateur Radio rules
permit higher power transmitters than allowed under Part 15 of the FCC rules. TT&C
is distinct from traditional forms of Amateur
Radio (telegraphy, voice and image intended to be heard or seen by human operators), since it receives information from an
object and commands the object to take an
action. Although pulse modulation systems
are common, TT&C also uses familiar communications modes, such as television (in
this case used as a form of telemetry), packet
radio (such as ASCII used for telemetry coding, commands or uploading programs).
This section provides only a sampling of
telemetry, tracking and telecommand systems involving Amateur Radio. APRS (Automatic Position Reporting System) is a
marriage of an application of the Global
Positioning System and Amateur Radio to
relay position and tracking information.
Telemetry and telecommand are also used
to manage remote terrestrial stations, as well
as amateur satellites. Radio Control (R/C) of
remote objects has long been a part of Amateur Radio because of the versatility offered
by Part 97 rules to licensed operators. R/C is
not limited to model cars, boats and airplanes
but is vital for the growing field of robotics.
APRS
Bob Bruninga, WB4APR, developed Automatic Position Reporting System (APRS)
as a result of trying to use packet radio for
real-time communications for public service
events. Packet radio is not well suited for those
real-time events, where information has a
very short lifetime. APRS avoids the complexity and limitations of trying to maintain a
connected network. It uses UI (unconnected)
frames to permit any number of stations to
participate and exchange data, just like voice
users would on a voice net. Stations that have
information to contribute simply transmit it,
and all stations monitor and collect all data on
frequency. APRS also recognizes that one of
the greatest real-time needs at any special
event or emergency is the knowledge of
where all stations and other key assets are
located. APRS accomplishes the real-time
display of operational traffic via a split screen
and map displays.
Since the object of APRS is the rapid dissemination of real-time information using
packet UI frames, a fundamental precept is
that old information is less important than
new information. All beacons, position reports, messages and display graphics are
redundantly transmitted, but at longer and
longer repetition rates. Each new beacon is
transmitted immediately, then again 20 seconds later. After every transmission, the
period is doubled. After ten minutes only six
packets have been transmitted. After an hour
this results in only three more beacons; and
only three more for the rest of the day! Using
this redundant UI broadcast protocol, APRS
is actually much more efficient than if a fully
connected link had to be maintained between all stations.
The standard configuration for packet
radio hardware (radio-to-TNC-to-computer)
also applies to APRS until you add a GPS
(Global Positioning System) receiver to the
mix. You don’t need a GPS receiver for a
stationary APRS installation (nor do you
need a computer for a mobile or tracker
APRS installation). In these cases, an extra
port or special cable is not necessary. It is
necessary, however, when you desire both a
computer and a GPS receiver in the same
installation.
One way of accomplishing this is by using a TNC or computer that has an extra
serial port for a GPS receiver connection.
Alternatively, you can use a hardware
single port switch (HSP) cable to connect a
TNC and GPS receiver to the same serial
port of your computer. The HSP cable is
available from a number of sources including TNC manufacturers Kantronics, MFJ
and PacComm.
Whichever GPS connection you use,
make sure that you configure the APRS software so it is aware that a GPS receiver is part
of the hardware configuration and how the
GPS receiver connection is accomplished.
APRS also supports an optional weather
station interface. The wind speed, direction,
temperature and rainfall are inserted into the
station’s periodic position report. The station shows up on all APRS maps as a large
blue dot, with a white line showing the wind
speed and direction. Several automatic
APRS weather reporting stations, supported
with additional manual reporting stations,
can form a real-time reporting network in
support of SKYWARN activities. For additional information see the book, APRS
Tracks, Maps and Mobiles by Stan Horzepa,
WA1LOU, published by ARRL.
Radio Control (R/C)
Amateur Radio gave birth to the radio
control (R/C) hobby as we know it today.
FCC §97.215 rules specifically permit “remote control of model craft” as a licensed
amateur station activity. Station identification is not required for R/C, and the transmitter power is limited to 1 W. FCC §97.215
states:
9.22
Chapter 9
“Telemetry transmitted by an
amateur station on or within
50 km of the Earth’s surface is
not considered to be codes or
ciphers intended to obscure the
meaning of communications.”
This section was contributed by H. Warren Plohr, W8IAH. The simplest electronic
control systems are currently used in lowcost toy R/C models. These toys often use
simple on/off switching control that can be
transmitted by on/off RF carrier or tone
modulation. More expensive toys and R/C
hobby models use more sophisticated con-
Fig 9.23—Photo of three R/C model electric cars.
trol techniques. Several simultaneous proportional and switching controls are available, using either analog or digital coding
on a single RF carrier.
R/C hobby sales records show that control of model cars is the most popular segment of the hobby. Battery powered cars
like that shown in Fig 9.23 are the most
popular. Other popular types include models powered by small internal combustion
gas engines.
R/C model aircraft are next in the line of
popularity and include a wide range of styles
and sizes. Fixed-wing models like those
shown in Fig 9.24 are the most popular.
They can be unpowered (gliders) or powered by either electric or gas engines. The
basic challenge for a new model pilot is to
operate the model in flight without crashing.
Once this is achieved, the challenge extends
to operating detailed scaled models in realistic flight, performing precision aerobatics,
racing other models or engaging in modelto-model combat.
The challenge for the R/C glider pilot is to
keep the model aloft in rising air currents.
The most popular rotary-wing aircraft models are helicopters. The sophistication of
model helicopters and their control systems
can only be appreciated when one sees a
skilled pilot perform a schedule of precision
flight maneuvers. The most exotic maneuver is sustained inverted flight, a maneuver
not attainable by a full-scale helicopter.
R/C boats are another facet of the hobby.
R/C water craft models can imitate full-scale
ships and boats, from electric motor powered scale warships that engage in scale
battles, to gas powered racing hydroplanes,
model racing yachts and even submarines.
Most R/C operation is no longer on
Amateur Radio frequencies. The FCC currently authorizes 91 R/C frequencies between 27 MHz and 76 MHz. Some
frequencies are for all models, some are
only for aircraft and others for surface
(cars, boats) models only. Some frequencies are used primarily for toys and others
for hobbyist models. Amateur Radio R/C
(A)
Fig 9.24—Photo of two R/C aircraft
models.
operators use the 6-m band almost exclusively. Spot frequencies in the upper part
of the band are used in geographical areas
where R/C operation is compatible with
6-m repeater operation and TV Channel-2
signals that can interfere with control.
Eight spot frequencies, 53.1 to 53.8 MHz,
spaced 100 kHz apart, are used. There is
also a newer 200 kHz R/C band from 50.8
to 51.0 MHz providing ten channels
spaced 20 kHz apart. The close channel
spacing in this band requires more selective receivers than do the 53-MHz channels. The Academy of Model Aeronautics
(www.modelaircraft.org) Membership
Manual provides a detailed list of all R/C
frequencies in current use as well as other
useful information. The ARRL Repeater
Directory lists current Amateur Radio R/C
frequencies.
Fig 9.25 shows a typical commercial R/C
system, consisting of a hand-held aircraft
transmitter (A), a multiple-control receiver,
four control servos and a battery (B). This
particular equipment is available for any of
the ten R/C frequencies in the 50.851.0 MHz band. Other commercially available control devices include relays
(solid-state and mechanical) and electric
motor speed controllers.
Some transmitters are tailored to specific
kinds of models. A helicopter, for example,
requires simultaneous control of both collective pitch and engine throttle. A model
helicopter pilot commands this response
with a linear motion of a single transmitter
control stick. The linear control stick signal
(B)
Fig 9.25—A, photo of Futaba’s Conquest
R/C aircraft transmitter. B shows the
matching airborne system.
Fig 9.26—Photo of Airtronics Infinity 660
R/C aircraft transmitter.
Modes and Modulation Sources
9.23
is conditioned within the transmitter to provide the encoder with a desired combination
of nonlinear signals. These signals then
command the two servos that control the
vertical motion of the helicopter.
Transmitter control-signal conditioning is
provided by either analog or digital circuitry. The signal conditioning circuitry is
often designed to suit a specific type of
model, and it is user adjustable to meet an
individual model’s control need. (Low-cost
transmitters use analog circuitry.) They are
available for helicopters, sailplanes and pattern (aerobatic) aircraft.
More expensive transmitters use digital
microprocessor circuitry for signal conditioning. Fig 9.26 shows a transmitter that
uses a programmable microprocessor. It is
available on any 6-m Amateur Radio R/C
frequency with switch-selectable PPM or
PCM coding. It can be programmed to suit
the needs of a helicopter, sailplane or pattern aircraft. Nonvolatile memory retains up
to four user-programmed model configurations.
Many R/C operators use the Amateur
Radio channels to avoid crowding on the
non-ham channels. Others do so because
they can operate home-built or modified
R/C transmitters without obtaining FCC type
acceptance. Still others use commercial R/C
hardware for remote control purposes
around the shack.
R/C RF MODULATION
The coded PPM or PCM information for
R/C can modulate an RF carrier via either
amplitude- or frequency-modulation techniques. Commercial R/C systems use both
AM and FM modulation for PPM, but use
FM exclusively for PCM.
The AM technique used by R/C is 100%
“down modulation.” This technique
switches the RF carrier off for the duration
of the PPM pulse, usually 250 to 350 μs. A
typical transmitter design consists of a thirdovertone transistor oscillator, a buffer amplifier and a power amplifier of about 1/2 W
output. AM is achieved by keying the 9.6-V
supply to the buffer and final amplifier.
The FM technique used by R/C is frequency shift keying (FSK). The modulation
is applied to the crystal-oscillator stage,
shifting the frequency about 2.5 or 3.0 kHz.
The direction of frequency shift, up or down
with a PPM pulse or PCM code, can be in
either direction, as long as the receiver detector is matched to the transmitter. R/C
manufacturers do not standardize, so FM
receivers from different manufacturers may
not be compatible.
SIGNALING TECHNIQUES
Radio control (R/C) of models has used
many different control techniques in the
9.24
Chapter 9
Fig 9.27—Diagram of a pulse-feedback servo.
past. Experimental techniques have included both frequency- and time-division
multiplexing, using both electronic and
mechanical devices. Most current systems
use time-division multiplexing of pulsewidth information. This signaling technique, used by hobbyist R/C systems, sends
pulse-width information to a remotely located pulse-feedback servomechanism. Servos were initially developed for R/C in the
1950s and are still used today in all but lowcost R/C toys.
Fig 9.27 is a block diagram of a pulsefeedback servo. The leading edge of the
input pulse triggers a linear one-shot
multivibrator. The width of the one-shot
output pulse is compared to the input pulse.
Any pulse width difference is an error signal
that is amplified to drive the motor. The
motor drives a feedback potentiometer that
controls the one-shot timing. When this
feedback loop reduces the error signal to a
few microseconds, the drive motor stops.
The servo position is a linear function of the
input pulse width. The motor-drive electronics are usually timed for pulse repetition rates
of 50 Hz or greater and a pulse width range
of 1 to 2 ms. A significantly slower repetition rate reduces the servomechanism slew
rate but not the position accuracy.
In addition to motor driven servos, the
concept of pulse-width comparison can be
used to operate solid-state or mechanical
relay switches. The same concept is used in
solid-state proportional electric motor speed
controllers. These speed controllers are used
to operate the motors powering model cars,
boats and aircraft. Currently available model
speed controllers can handle tens of amperes
of direct current at voltages up to 40 V dc
using MOSFET semiconductor switches.
Requirements
The signaling technique required by R/C
is the transmission of 1- to 2-ms-wide pulses
with an accuracy of ±1 μs at repetition rates
Fig 9.28—Diagram of a four-channel PPM
RF envelope.
of about 50 Hz. A single positive-going dc
pulse of 3 to 5 V amplitude can be hard
wired to operate a single control servomechanism. If such a pulse is used to modulate an RF carrier, however, distortion of the
pulse width in the modulation/demodulation
process is often unacceptable. Consequently, the pulse-width information is usually coded for RF transmission. In addition,
most R/C systems require pulse-width information for more than one control. Time-division multiplexing of each control provides
this multichannel capability. Two coding
techniques are used to transfer the pulsewidth information for multiple control channels: pulse-position modulation (PPM) and
pulse-code modulation (PCM).
Pulse-Position Modulation
PPM is analog in nature. The timing between transmitted pulses is an analog of the
encoded pulse width. A train of pulses encodes multiple channels of pulse-width information as the relative position or timing
between pulses. Therefore the name, pulseposition modulation. The transmitted pulse
is about 300 μs in width and uses slow rise
and fall times to minimize the transmitter
RF bandwidth. The shape of the received
waveform is unimportant because the desired information is in the timing between
pulses. Fig 9.28 diagrams a frame of five
pulses that transmits four control channels
of pulse-width information. The frame of
modulation pulses is clocked at 50 Hz for a
frame duration of 20 ms. Four multiplexed
pulse widths are encoded as the times between five 300-μs pulses. The long period
between the first and the last pulse is used
by the decoder for control-channel synchronization.
PPM is often incorrectly called digital
control because it can use digital logic circuits to encode and decode the control
pulses. A block diagram of a typical encoder
is shown in Fig 9.29. The 50-Hz clock frame
generator produces the first 300-μs modulation pulse and simultaneously triggers the
first one-shot in a chain of multivibrators.
The trailing edge of each one-shot generates
a 300-μs modulation pulse while simultaneously triggering the succeeding multivibrator one-shot. In a four-channel system
the fifth modulation pulse, which indicates
control of the fourth channel, is followed by
a modulation pause that is dependent on the
frame rate. The train of 300-μs pulses are
used to modulate the RF carrier.
Received pulse decoding can also use
digital logic semiconductors. Fig 9.30
shows a simple four-control-channel decoder circuit using a 74C95 CMOS logic IC.
The IC is a 4-bit shift register operated in the
right-shift mode. Five data pulses spaced 1
to 2 ms apart, followed by a synchronization pause, contain the encoded pulse-width
information in one frame. During the sync
pause, the RC circuit discharges and sends a
logic-one signal to the 74C95 serial input
terminal. Subsequent negative going data
pulses remove the logic-one signal from the
serial input and sequentially clock the logic
one through the four D-flip-flops. The output of each flip-flop is a positive going pulse,
with a width corresponding to the time between the clocking pulses. The output of
each flip-flop is a demultiplexed signal that
is used to control the corresponding servo.
Pulse Code Modulation
PCM uses true digital code to transfer
R/C signals. The pulse width data of each
control channel is converted to a binary
word. The digital word information of each
control channel is coded and multiplexed to
permit transmission of multiple channels of
control on a single RF carrier. On the receiving end, the process is reversed to yield the
servo control signals.
There is no standard for how the digital
word is coded for transmission. Therefore
PCM R/C transmitters and receivers from
different makers are not interchangeable.
Some older PCM systems provide only 256
discrete positions for 90° of servo motion,
thereby limiting servo resolution. Newer
systems use more digital bits for each word
and provide smooth servo motion with 512
and 1024 discrete positions. All PCM and
Fig 9.29—Diagram of a PPM encoder.
Fig 9.30—Diagram of a 74C95 PPM decoder.
PPM systems use the same servo inputsignal and supply voltages. Therefore the
servos of different manufacture are interchangeable once compatible wiring connectors have been installed.
AMATEUR SATELLITE TT&C
TT&C plays a vital part of the launching
and management of amateur satellites. Satellites have onboard intelligence and are
increasing able to make their own decisions
but Article 25 of the international Radio
Regulations requires the following:
“Administrations authorizing
space stations in the amateursatellite service shall ensure
that sufficient earth command
stations are established before
launch to ensure that any
harmful interference caused by
emissions from a station in the
amateur-satellite service can
be terminated immediately.”
The U.S. implementation of the Radio
Regulations in Part 15 of the FCC rules has
these provisions:
Ҥ97.211 Space telecommand
station.
(a) Any amateur station desig-
nated by the licensee of a space
station is eligible to transmit as
a telecommand station for that
space station, subject to the
privileges of the class of operator license held by the control
operator.
(b) A telecommand station may
transmit special codes intended to obscure the meaning
of telecommand messages to
the station in space operation.
…..
(d) A telecommand station may
transmit one-way communications.”
Telemetry from amateur satellites, such
as from “engineering beacons” is available
to all amateurs. Computer programs are
available from AMSAT for decoding the
telemetry to monitor the health of the spacecraft and other measurements. However,
telecommand of amateur satellites is closely
held in order to maintain effective control.
Amateur Satellite TT&C References
AMSAT, www.amsat.org.
Davidoff, Martin, K2UBC, The Radio
Amateur’s Satellite Handbook, ARRL,
Rev first ed, 2003.
Modes and Modulation Sources
9.25
Voice Modes
AMPLITUDE MODULATION (AM)
This material was written by John O.
Stanley, K4ERO. The first AM broadcast of
speech and music occurred nearly a century
ago, when on Christmas eve of 1906,
Fessenden, using a modulated high frequency alternator, surprised ship operators
with a program of music, Bible readings and
poetry. The development of a continuous
wave transmitter, one that produced a constant sine wave output, rather than the rough
spark signal, made AM practical. Thus, CW,
as this pure wave was called, not only greatly
enhanced Morse communications, but allowed voice transmissions as well. By
changing the strength or amplitude of this
smooth continuous wave, a voice could be
superimposed on the radio frequency carrier.
The decade of the 1920s saw not only the
rapid development of the broadcast industry, but also enabled many hams to try the
new voice mode. Indeed, in those early
years, there was sometimes little difference
between a ham who used voice and a broadcaster. The situation was a mess, and QRM
was king! By 1929 it was permissible to use
AM voice in limited portions of our amateur
spectrum and, on some bands, only the most
qualified licensees had the privilege.
Users of AM had to learn that an RF wave
could have only a certain amount of audio
imposed upon it before overmodulation
occurred. Trying to go above 100% modulation produced severe distortion and splatter. AM remained the dominant voice mode
for ham operations well into the second half
of the 20th century, when it was gradually
eclipsed by SSB (SSB is actually a form of
AM) and FM. We can still hear AM on the
ham bands today, mostly coming from stations using vintage gear. AMers usually
choose operating times when the bands are
less crowded, and often take pride in a clean
and clear signal.
The great advantage of AM, and one reason for its long history, is the ease with which
a full carrier AM signal can be received. This
was all important in broadcasting where, for
every transmitter, there were thousands or
even millions of receivers. With modern
integrated circuits, complex detectors now
cost very little. Therefore, the biggest reason for keeping AM broadcasting, at present,
is to avoid obsolescing the billions of existing receivers. These will gradually have to
be replaced when digital broadcasts begin
in the AM and shortwave bands.
There are many ways to produce an AM
signal, but all of them involve multiplying
the amplitude of the information to be transmitted by the amplitude of the radio wave
9.26
Chapter 9
(A)
(B)
Fig 9.31—Electronic displays of AM signals in the frequency and time domains. A
shows an unmodulated carrier or single-tone SSB signal. B shows a full-carrier AM
signal modulated 20% with a sine wave.
that will carry it. When multiplication of two
signals takes place, as opposed to their
simple addition, mixing is involved. The
result is multiple signals, including the sum
and difference of the AF and RF frequencies. These two “products” will appear as
sidebands alongside what was the original
RF frequency. Mixing, modulation, detection, demodulation, and heterodyning all
refer to this multiplication process and can
all be analyzed by the same mathematical
treatment. See the Mixers, Modulators and
Demodulators chapter of this Handbook for
a more detailed discussion of this process.
If an RF signal is modulated by a single
audio tone, and observed on an oscilloscope, it will appear as shown on the right in
Fig 9.31B. Observing the same signal on a
spectrum analyzer will show that the composite signal observed on the scope is composed of three discrete parts as shown on the
left in Fig 9.31B. The center peak, which is
identical with the original unmodulated
wave shown in Fig 9.31A, is usually called
the carrier, although this terminology is
deceiving and imprecise. It is the composite
RF signal, as seen on the oscilloscope, which
actually carries the audio in the form of
variations in its amplitude, so we might well
have referred to the center frequency as a
“reference” or some other such term.
As a reference signal, the carrier contains
important, though not indispensable, infor-
mation. For a signal with both sidebands
present, it provides a very important frequency and phase reference that allows
simple and undistorted detection, using
nothing more than a diode. The carrier also
provides an amplitude reference, which is
used by AM receivers to set the gain of the
receiver, using AGC or automatic gain control. The carrier also contains most of the
power of the transmitted signal, while most
of the important information is in the sidebands. See the Mixers, Modulators and
Demodulators chapter in this Handbook,
which gives details of power distribution in
an AM signal.
SINGLE SIDEBAND (SSB)
Telephone engineers developed a system
of using only one of the two sidebands,
which, being mirror images of each other,
contain the same information. SSB systems
attracted the attention of hams soon after
WWII and gradually became the voice mode
of choice for the HF bands. SSB is considered a form of AM, in that it is identical to an
AM signal with one sideband, and with all or
part of the carrier removed. The complexity
of generating a SSB signal, plus the difficulty of tuning the generally unstable receivers common in the 1950s, slowed the
changeover to the new mode, but its adoption was inevitable. SSB became popular
because of its greater power efficiency,
SSB ON 20 AND 75 METERS
—THE 9 TO 5 CONNECTION
Fig 9.32—How an occupied radio
frequency spectrum shifts with application
of an audio (baseband) signal. The dotted
line represents the RF carrier point, or in
the case of the 3-kHz audio signal, the
reference frequency, 0 kHz.
which allowed each watt of RF to go further.
The fact that it occupied less bandwidth was
a plus also and very welcome on the most
crowded bands. See the sidebar SSB on 20
and 75 Meters in this chapter.
While systems used for telephone relays
used pilot carriers so that the signal could be
reproduced without distortion, hams chose
to eliminate the carrier entirely. This required generating a reference frequency at
the receiver, which, if accurate to within
20 Hz, allowed intelligible speech to be recovered. Since amateur regulations have
long prohibited transmission of music, the
distortion produced by loss of the exact
phase and frequency reference was not serious. The loss of the amplitude reference was
overcome with the development of the
“hang” AGC, which works on the average
value of the received sideband, which is
constantly changing. While not as fast or
accurate as the carrier-based AGC available
in AM, this has proven satisfactory, if proper
attention is given to its design (See the Receivers and Transmitters chapter of this
Handbook.)
Thus, SSB, while giving up some fidelity
and while increasing complexity, has
proven superior to full-carrier AM for
speech communication because of its power
and bandwidth efficiency. And under certain circumstances, such as selective fading,
it can actually have less distortion than DSB
AM. On HF, it is possible for the carrier to
fade in an DSB AM signal, leaving less than
is needed for envelope detection. Medium
wave AM broadcasts often have this problem at night. It can be overcome with “exalted carrier detection.” Synchronous
detection is a refinement of this method. (See
the Mixers, Modulators and Demodulators
chapter of this Handbook.) SSB, in effect,
uses exalted carrier detection all the time.
An SSB signal is best visualized as an
audio or baseband signal that has simply
SSB experiments began on
75 meters because it was the lowest
frequency phone band in widespread
use. Due to perpetual crowding and its
DX potential, 20 meters also seemed
to call for use of SSB. Some early rigs
included only these two bands. The
popular homebrew W2EWL rig was
built on the chassis of a war surplus
ARC-5 transmitter using its 5 MHz
VFO, and generated the sideband
signal on 9 MHz using the phasing
method. Nine plus five is 14 MHz, and
nine minus five is 4 MHz, yielding 75
or 20 meter coverage by choosing
which of the two mix products we
would filter out and amplify. Thus, two
bands were covered with the same
VFO/IF combination. Other rigs used a
tunable IF from 5.0 to 5.5 MHz. This
was subtracted from a 9-MHz crystal
to obtain 4.0 to 3.5 MHz, and added to
9 MHz to cover 14.0 to 14.5 MHz. This
process reversed the sidebands, and
eventually led to the convention of
using LSB on the lower bands and
USB on the higher bands. This also
explains why on some vintage rigs the
75-meter band dial reads backwards!—K4ERO
been shifted upwards into the radio frequency spectrum, as shown in Fig 9.32. The
relative frequencies, phases and amplitudes
of all the components will be the same as the
original frequency components except for
having had a fixed reference frequency
added to them. Surprisingly, this process,
called heterodyning, is not done by directly
adding the signals together, but by multiplying them and subsequently filtering or phasing out the carrier and one of the sidebands.
The Mixers, Modulators and Demodulators chapter of this Handbook explains this
interesting process in detail.
The relative frequencies within the band
of information being transmitted may appear inverted; that is, lower frequencies in
the original audio signal are higher in the RF
signal. When this happens, we call the signal lower sideband or LSB. LSB is produced
when the final frequency is the result of
subtraction rather than addition. If a tone of
1 kHz is heterodyned to 14201 kHz by mixing with a 14200 kHz carrier, the result will
be upper sideband, since 14200 + 1 gives us
that result. When the same tone appears at
3979 kHz by mixing it with a 3980 kHz
carrier, we know that an LSB signal was
produced since 3980 – 1 gives us the 3979
result. Whenever the audio tone needs a
minus sign to find the result, we are on LSB.
Fig 9.33—A method of changing
sidebands with virtually no change in the
frequency spectrum occupied. Note that
the carrier point position has changed
concurrent with the change from LSB to
USB.
In most mixing schemes there will be three
frequencies involved (carrier, VFO, and
band select crystal) but the principle still
holds.
The frequency of an SSB transmission is
designated as that of the carrier, which is the
frequency (or the sum of several frequencies) used to shift the baseband information
into the RF spectrum. In a good SSB signal,
little or no energy actually appears on the
frequency we say we are using. It is strictly
a reference. For this reason, some radio services have chosen to designate SSB channels by the center of the occupied bandwidth
rather than the carrier frequency. Ham practice is to designate the carrier frequency and
whether the upper or lower sideband is in
use. An interesting exception is the new fivechannel, 60-m amateur band (a secondary
allocation) where the FCC specified a 2.8kHz bandwidth on five center frequencies:
5332, 5348, 5368, 5373 and 5405 kHz.
Only USB voice (2K8J3E emission) is permitted.
Most hams will find it more natural to
remember USB at corresponding carrier frequencies of 5330.5, 5346.5, 5366.5, 5371.5
and 5403.5 kHz. Since the USB or the LSB
is considered “normal” for each of our
bands, it is assumed that the sideband in use
is understood. We need to remember when
switching sidebands that we will be occupying a different portion of the spectrum than
before the switch, and we may inadvertently
cause QRM, unless we check for a clear frequency. If you wish to change from LSB to
USB without changing the spectrum occupied, you must retune your dial down about
3 kHz, as a careful study of Fig 9.33 should
make clear. This principle applies to digital
as well as voice modes, but usually not to
CW, where modern rigs make the above
adjustment for us. This means that the frequency readout with a CW signal will be the
actual frequency occupied, but with analog
Modes and Modulation Sources
9.27
voice and digital modes this will probably
not be the case.
Another need for understanding where
sideband signals actually fall is in operating
close to the edge of a band or subband. For
example, on 20 meters where USB is used,
you must not operate above approximately
14.347 MHz, since the transmission will be
outside the band if you operates much
higher. Operation with a suppressed carrier
exactly on 4.0 MHz could be done on LSB
if the signal is very clean, but is not recommended. Most modern rigs prevent out of
allocated band transmissions but do not preclude the above cases of improper operation.
Today there are many new modes for text,
speech and image transmission, and more
will be developed in the future. Often these
are transmitted using SSB. Knowing exactly
where the signal will appear on the band
depends on understanding how LSB and
USB signals are produced. These modes use
either a separate circuit or more recently a
computer sound card to produce audio frequency tones that represent the information
in coded form. This is then fed into the audio
input of an SSB transmitter. They are then
heterodyned to the desired amateur band for
transmission. In a transceiver, the incoming
signals are similarly heterodyned back to the
audio range for processing in the computer
sound card or other circuitry. Some computerbased digital modes allow reading the actual
signal frequency off the screen, provided the
transceiver dial is properly set.
Voice signals and some text and image
modes require linear amplification. This
means that the amplifiers in the transmitter
must faithfully represent the amplitude as
well as the frequency of the baseband signal. If they fail to do so, intermodulation
distortion (IMD) products appear and the
signal becomes much wider than it should
be, producing interference (QRM) on
nearby frequencies. CW and FM do not require a linear amplifier, but you can use one
for these modes also, at a small price in efficiency. Some VHF “brick” amplifiers have
a choice of either the more efficient class C
amplification or the more linear class B
amplification. The linear or SSB mode must
be chosen if SSB voice and some digital
modes are being used. Whenever linear
amplification is needed, flat-topping must
be prevented. This results from overdriving
the amplifier so that it goes above the design
power limit and becomes non-linear.
SSB transmitters and most linear amplifiers use automatic level control (ALC) to
prevent overdrive and flat topping. However, there are limits to ALC and flat topping
can still occur if the amplifier is grossly over
driven. The surest way to create ill will on
any band is to cause spatter by over driving
9.28
Chapter 9
Fig 9.34—Block diagrams of filter-method SSB generators. They differ in the manner that
the upper and lower sideband are selected.
your amplifier, regardless of the mode.
Amplifiers suitable for both linear and nonlinear signals are discussed in the RF Power
Amplifiers chapter of this Handbook. The
effects of non-linear amplification are also
further treated in the Mixers, Modulators
and Demodulators chapter.
How an SSB Signal is Produced
When the proper receiver bandwidth is
used, an SSB signal will show an effective
gain of up to 9 dB over an AM signal of the
same peak power. Because the redundant
information is eliminated, the required bandwidth is half that of a comparable AM (DSB)
emission. Unlike DSB, the phase of the local
carrier generated in the receiver is unimportant.
SSB Generation: The Filter Method
If the DSB signal from the balanced
modulator is applied to a narrow bandpass
filter, one of the sidebands can be greatly
attenuated. Because a filter cannot have infinitely steep skirts, the response of the filter
must begin to roll off within about 300 Hz of
the phantom carrier to obtain adequate suppression of the unwanted sideband. This
effect limits the ability to transmit bass frequencies, but those frequencies have little
value in voice communications. The filter
rolloff can be used to obtain an additional
20 dB of carrier suppression. The bandwidth
of an SSB filter is selected for the specific
application. For voice communications,
typical values are 1.8 to 3.0 kHz.
Fig 9.34 illustrates two variations of the
filter method of SSB generation. In A, the
heterodyne oscillator is represented as a
simple VFO, but may be a premixing system
or synthesizer. The scheme at B is perhaps
less expensive than that of A, but the heterodyne oscillator frequency must be shifted
when changing sidebands in order to maintain dial calibration.
SSB Generation: The Phasing Method
Fig 9.35 shows another method to obtain
an SSB signal. The audio and carrier signals
are each split into equal components with a
90° phase difference (called quadrature)
and applied to balanced modulators. When
the DSB outputs of the modulators are combined, one sideband is reinforced and the
other is canceled. The figure shows sideband selection by means of transposing the
audio leads, but the same result can be
achieved by switching the carrier leads. The
phase shift and amplitude balance of the two
channels must be very accurate if the unwanted sideband is to be adequately attenuated. Table 9.8 shows the required phase
accuracy of one channel (AF or RF) for various levels of opposite sideband suppression.
Fig 9.35—Block diagram of a phasing SSB generator.
Table 9.8
Unwanted Sideband Suppression as
a Function of Phase Error
Phase Error
(deg.)
0.125
0.25
0.5
1.0
2.0
3.0
4.0
5.0
10.0
15.0
20.0
30.0
45.0
Suppression
(dB)
59.25
53.24
47.16
41.11
35.01
31.42
28.85
26.85
20.50
16.69
13.93
9.98
6.0
The numbers given assume perfect amplitude balance and phase accuracy in the other
channel.
The shows that a phase accuracy of 1° is
required to achieve unwanted sideband suppression of greater than 40 dB. It is difficult
to achieve this level of accuracy over the
entire speech band. The phase-accuracy tolerance can be loosened to 2° if the peak
deviations can be made to occur within that
spectral gap. The major advantage of the
phasing system is that the SSB signal can be
generated at the operating frequency without the need of heterodyning. Phasing can
be used to good advantage even in fixedfrequency systems. A loose-tolerance (4°)
phasing exciter followed by a simple twopole crystal filter can generate a high-quality signal at low cost.
Audio Phasing Networks
Since the phasing method requires that all
baseband signals be presented to the bal-
anced modulators in both a normal (in phase)
and quadrature (90° phase shifted) signal,
we must provide, in the case of an audio
signal, a network that can produce a constant 90° phase shift over a wide frequency
range. Fortunately, the absolute phase shift
is not as important as the relative phase between the two channels. Various circuits
have been devised that will provide this relative shift. Robert Dome, W2WAM, pioneered a simple network using precision
components that achieved this and his network was used in early SSB work. The
polyphase network, which appeared in this
Handbook for several editions, required
more—but less precise—components.
Methods using active filter techniques are
also available.
With DSP (Digital Signal Processing),
producing a 90° phase shift over a wide frequency range is easily accomplished using
the Hilbert transformer. This will likely give
new life to the phasing method of SSB generation since many new radios already have
DSP capability present for other reasons. See
the Receivers and Transmitters chapter of
this Handbook for an example of an SSB
receiver using DSP with the phasing
method. See also the Digital Signal Processing chapter.
Producing 90° phase-shifted signals at RF
frequencies has also used several approaches. For VHF and up, a quarter-wave
section of coax is possible. Generating an
RF signal at four times the desired frequency
and dividing down with flip-flops generates
quadrature signals accurate over a wide
range of frequencies. Phase lock loops provide yet another approach.
The phasing method is useful not only for
generating an SSB signal, but for any mixing or frequency-conversion task. In-Phase
and Quadrature (I&Q) modulators, demodu-
lators and mixers are in common use in
modern communication technology. These
allow elimination of image frequencies without filters, or greatly relax the specification
of filters that are used. Digital modulation
can be generated in an I&Q format that can
be directly heterodyned into the RF spectrum using I&Q modulators. The Digital
Signal Processing chapter of this Handbook
discusses many of these concepts.
Fig 9.36 – Graphical representation of
frequency modulation. In the unmodulated
carrier (A) each RF cycle occupies the
same amount of time. When the
modulating signal (B) is applied, the radio
frequency is increased and decreased
according to the amplitude and polarity of
the modulating signal (C).
Modes and Modulation Sources
9.29
FREQUENCY MODULATION (FM)
Unlike AM, which changes the amplitude
of a radio wave in accordance with the
strength of the modulation signal, FM
changes the frequency of the wave so that
the instantaneous value of frequency represents a voltage level in the modulating signal as is shown in Fig 9.36. This means that
the demodulator must extract the information by generating an output whose amplitude is determined by the frequency of the
received wave. Thus, FM transmission involves amplitude to frequency conversion
and vice-versa. Producing these conversions was not as easy as it was in the case of
AM, and thus FM was not employed as early
as was AM.
As you can see in Fig 9.37, the circuits
required for FM were especially difficult in
the case of the receiver. See also the AMand Angle-Demodulation subsections of the
Mixers, Modulators and Demodulators
chapter in this Handbook. In addition, mathematical analysis seemed to show that FM
would require a very large bandwidth (theoretically infinite), and this discouraged early
experimenters.
Edwin Armstrong was a ham before the
days of call signs. While a young man, he
invented the regenerative, super-regenerative and superheterodyne receivers. He went
on to challenge the prevailing wisdom and
developed a practical FM system. His “Yankee Network” provided high fidelity broadcasts throughout the northeastern United
States in the late 1930s, using frequencies
below our 6-meter band. After WWII, FM
was moved to 88-108 MHz and became FM
broadcasting as we now know it. Dependable day and night reception was a result of
the frequency chosen, not the mode, but
wideband FM, which had dictated the use of
a VHF frequency where bandwidth was
available, provided the wide audio response,
high signal to noise ratio, and freedom from
static that AM could never have provided,
even at VHF. The advantages of FM were
proven even when bandwidths were less
than infinity. The math had not been wrong,
but had just been taken a bit too literally.
Hams experimented with narrowband FM
(NBFM) on the HF bands during the 1950s,
but nothing much came of it. The explosion
in the use of FM in the amateur bands came
after surplus commercial FM equipment,
using frequencies near 150 MHz, became
available in the 1960s and 1970s. Two
meters was the first to use this equipment
and is still the workhorse of the VHF FM
bands. Hams, like the commercial and public service users before them, discovered that
FM has certain advantages—less noise, ease
of operation, no fussy tuning and suitability
for use through repeaters.
A mathematical analysis of FM is com9.30
Chapter 9
plex, and well beyond the scope of this chapter. Readers who are interested in more details can consult the Mixers, Modulators
and Demodulators chapter of this Handbook. Unlike AM, where the occupied bandwidth is simple to calculate (twice the
highest modulating frequency), FM bandwidth depends on both the modulating frequency and the deviation, which is equal to
the peak frequency excursion above and
below the central carrier frequency. As the
math predicts, there are sidebands that extend to infinity but, fortunately, these drop
off in amplitude rather quickly. As
Armstrong surmised, ignoring sidebands
that contain only a tiny portion of the total
energy does not impair the quality of the
received signal.
As a rule of thumb, adequate bandwidth
for an FM voice system using narrowband
modulation (5 kHz or so) is Bn = 2 (M+D)
where Bn is the necessary bandwidth in
hertz, M is the maximum modulation frequency in hertz, and D is the peak deviation
in hertz. For narrowband FM with voice, the
bandwidth equals 2 × (3000+5000) =
16 kHz. This defines the filter through which
the signal can be received without noticeable distortion.
Examples of FM spectra using various
modulation indices are found in the Mixers, Modulators and Demodulators chapter of this Handbook. Note that as more and
more sidebands appear, the amplitude of
each is reduced. This is because all of the
sidebands, plus the carrier, must add together (vectorially) to produce a total wave
of constant amplitude. This is characteristic of an FM signal. This constant amplitude signal has the advantage of being easy
to amplify without the need for a linear
amplifier. Many VHF and UHF brick-type
amplifiers have separate settings for FM
and SSB. The FM setting is more efficient
since, by giving up the requirement for linearity, we can bias the transistors for greater
efficiency. Thus, an FM amplifier is easier
to build than one suitable for AM or SSB.
However, this constant amplitude characteristic of FM comes at a price. The full
power is being transmitted, even between
words or when one is holding down the push
to talk, but not actually speaking. For normal speech, the power advantage FM gains
by amplifier efficiency is lost compared to
SSB, where power is only transmitted when
the voice requires it. One should not, however, conclude that the unmodulated FM
signal serves no purpose. Its presence “quiets” the channel, opens the squelch of the
receiver(s), and turns on any repeater(s) that
might be in the circuit. There may also be
various control tones (squelch, etc.) present,
even though these may be inaudible because
they are in a frequency range that the human
ear does not easily perceive.
Using FM and PM with Digital Modes
Frequency-shift keying (FSK) is a means
of producing frequency modulation that has
discrete states; that is, the instantaneous frequency takes on definite values representing digital information. FSK is a form of FM
and some of the same principles apply. FSK
was covered earlier in the section on RTTY
and other digital modes.
Phase modulation (PM) is very similar to
FM in that it is not possible to change the
frequency of a signal without impacting its
phase, and vice versa. Instantaneous frequency can be considered to be the rate of
change of phase of a signal. Some FM
modulators have used this relationship to
produce FM by phase modulation along with
audio frequency shaping to convert the PM
signal into the equivalent of an FM signal.
This issue is discussed further in the Mixers,
Modulators and Demodulators chapter of
Fig 9.37—At A, block diagram of an AM receiver. At B, an FM receiver. Dark borders
outline the sections that are different in the FM set.
this Handbook.
Phase shift keying (PSK) is a form of
phase modulation suitable for digital transmissions. It is discussed further in the following pages of this chapter. Both FSK and
PSK produce sidebands in accordance with
the same principles discussed above. However, in order to control bandwidth, digital
signals using PSK may depart from the requirement that an FM signal have a constant
amplitude. Such signals are really a combination of FM and AM, and linear amplification must be used.
DIGITAL VOICE MODES
There is a risk in saying anything about an
area that is developing rapidly both inside
and outside Amateur Radio. Amateurs are
watching digital voice developments in
other radio services but not all are suitable
models for Amateur Radio applications.
On MF and HF, transmission of digital
voice is difficult owing to multipath propagation, QRM and noise. Several digital voice
systems have been developed and more are
expected. The most prominent contender is
Digital Radio Mondiale (DRM). DRM is a
non-profit consortium of broadcasters,
manufacturers, educational and governmental organizations devoted to developing
a single standard for digital sound broadcasting in long, medium and short wave
bands. As of 2005, thousands of software
radios were being used to hear regular DRM
broadcasts from a growing number of countries. The software radio consists of a modified HF receiver, a sound card and computer
software downloaded from DRM. See
www.drmx.org/ for details. As of mid 2005,
there were few if any hardware DRM receivers available even at relatively high
prices.
The DRM standard has several modes,
some aimed at high fidelity music and others suitable for voice. The broadcaster can
select the most appropriate mode, and the
receiver will switch automatically to that
mode. The various DRM modes occupy 4.5,
5, 9, 10 or 20 kHz according to the spectrum
available and the quality desired. See
Fig 9.38. DRM produces excellent quality
but is more subject to the effects of interference and propagation than DSB AM.
Another digital sound system used in the
broadcasting service is called IBOC—InBand On-Channel. The basic idea is to send
a digital signal underneath an existing AM
or FM program without one interfering with
the other. Although used in the United
States, IBOC hasn’t caught on for international broadcasting. An article on IBOC at
New York station WOR was presented in the
March 2003 issue of QST (p 28).
The International Telecommunication
Union approved a standard known as ITUR Recommendation BS.1514, System for
digital sound broadcasting in the broadcasting bands below 30 MHz. It describes
DRM and IBOC, and compares the systems.
Amateur Radio Digital Voice
For HF Amateur Radio, digital voice has
the potential to provide better quality than
SSB. It could have other yet-to-be-exploited possibilities, such as adapting to
conditions from a “robotic” sounding
speech under marginal propagation to
“arm-chair copy” when conditions are
good. It is possible to imbed some ancillary
information in the digital stream so the receiver will be able to display call signs,
graphics and other information of interest
to the stations in QSO.
In 2000, the ARRL Board of Directors
Fig 9.38—A DRM HF
digital broadcast
signal. Per-division
resolution is 5 kHz
horizontal and 10 dB
vertical.
created a Digital Voice Working Group to
investigate and promote digital voice in the
Amateur Radio Service. The pioneering
work done by Charles Brain, G4GUO, and
Andy Talbot, G4JNT, was published in the
May-June 2000 issue of QEX. Their system
was based on use of the AMBE 2020 encoder-decoder. It uses Orthogonal Frequency Division Multiplexing (OFDM) with
36 carriers in a band of 300-2500 Hz. At
least one commercial version of AMBE
2020/G4GUO system is available in the
amateur market. See Hallas, Joel, W1ZR,
“AOR ARD9800 Digital Voice Modem,”
QST, February 2004.
The January-February 2003 issue of QEX
(p 49) described a special Amateur Radio
adaptation of the DRM system to fit inside a
3-kHz bandwidth. This system has taken on
the name HamDream and some information
can be found on the Web at www.qslnet.de/
member/hb9tlk/.
These OFDM standards use many carriers spaced about 50 Hz apart, each using
16QAM (Quadrature Amplitude Modulation with 16 discrete states in each symbol)
or some similar modulation scheme. To
mitigate the effects of multipath propagation, the symbol rate must be limited to a few
hundred bauds. Thus, the high bit rate
needed for voice requires both multiple carriers and complex modulation. There are
tradeoffs between complexity, weak signal
sensitivity, reliability under difficult conditions, speech quality and latency. The most
obvious way to generate and demodulate
such a signal is to use a computer and a
sound card.
While the same digital voice encoderdecoders could be used at MF/HF as well as
VHF and above, it may be desirable to optimize the system for best performance in each
frequency range. At MF/HF, the emphasis is
naturally on reliability in the presence of
fading and interference, while at VHF and
UHF, it is possible to design for quality of
speech reproduction and possibly multimedia (voice/data/image). See High Speed
Multimedia Radio later in this chapter.
There is much room for innovation and
experimentation in this field. A great deal of
work will go into developing whatever digital voice mode we will be using 10 years
from now. Those interested in being a part
of this exciting technology should begin by
mastering the material in the Electrical Signals and Components and DSP chapters of
this Handbook, and keeping up with QST
and QEX material on digital speech. Also
check the following Web sites: www.arrl.
org/tis/info/digivoice.html; www.dougsmith.net; www.temple.edu/k3tu/digital_
voice.htm;www.tapr.org/tapr/dv/ ;
www.rac.ca/opsinfo/infodig.htm
#Digital%20Speech and www.DRM.org.
Modes and Modulation Sources
9.31
Image Modes
FACSIMILE
This section, by Dennis Bodson,
W4PWF, Steven Karty, N5SK, and Ralph
Taggart, WB8DQT, covers the several facsimile systems most commonly used in
Amateur Radio today. For further information on the area of facsimile, its history and
the development of related standards asso-
ciated with this mode, refer to FAX: Facsimile Technology and Systems.1 The subject of Weather fax, while of interest to many
amateurs, is not a primary activity of the
Amateur Radio Service. Information on this
subject is contained in the Weather Satellite
Handbook2 and the ARRL Image Communications Handbook.3
Facsimile Overview
Facsimile (fax) is a method for transmit-
ting very high resolution still pictures using
voice-bandwidth radio circuits. The narrow
bandwidth of the fax signal, equivalent to
SSTV (Slow Scan TV), provides the potential for worldwide communications on the
HF bands. Fax is the oldest of the imagetransmitting technologies and has been for
years the primary method of transmitting
newspaper photos and weather charts. Fax
is also used to transmit high-resolution cloud
images from both polar-orbit and geostation-
Fig 9.39—Amateur Image Communications encompass a wide range of activities, a few of which are illustrated here. Narrowband
Television (NBTV) experimenters explore the history and technology of the earliest days of television by restoring or recreating
mechanical TV gear while exploring the possibilities of narrowband, full motion TV, primarily using computer technology. Amateur
Television (ATV) operators use standard broadcast television, typically in color, to communicate on UHF and microwave frequencies.
The scope of their operating activities ranges from point-to-point communication (simplex or via local ATV repeaters), roving or
portable operation for a variety of reasons, including emergency and public service communications, and the application of ATV to
remote sensing via aircraft, high-altitude balloons, and remote-control vehicles of all sorts. Slow-scan Television (SSTV) involves the
transmission of medium and high-resolution images, usually in full-color, using standard Amateur voice equipment (typically SSB or
FM). Most modern SSTV activity is computer-based, offering international DX on HF frequencies and local, regional, or space
communications (satellite, MIR, and now the International Space Station) on VHF and UHF. Facsimile (Fax) encompasses the
transmission and reception of very high-resolution still images (typically using computers) over a period of several to many minutes.
One of the most popular areas of Amateur experimentation and operation has involved the reception of imagery from polar-orbit and
geostationary weather satellite. While this Handbook will provide a brief introduction to some of these activities, all of them and more
are covered in much greater detail in the ARRL Image Communications Handbook.
9.32
Chapter 9
ary satellites. Many of these images are retransmitted using fax on the HF bands.
The resolution of typical fax images
greatly exceeds what can be obtained using
SSTV or even conventional television (typical images will be made up of 800 to 1600
scanning lines). This high resolution is
achieved by slowing down the rate at which
the lines are transmitted, resulting in image
transmission times of 4 to 10 minutes.
Modern personal computers have virtually eliminated bulky mechanical fax recorders from most amateur installations.
Now the incoming image can be stored in
computer memory and viewed on a standard TV monitor or a high-resolution computer graphics display. The use of a color
display system makes it entirely practical to
transmit color fax images when band conditions permit.
The same computer-based system that
handles fax images is often capable of SSTV
operation as well, blurring what was once a
clear distinction between the two modes.
The advent of the personal computer has
provided amateurs with a wide range of
options within a single imaging installation.
SSTV images of low or moderate resolution
can be transmitted when crowded band conditions favor short-frame transmission times.
When band conditions are stable and interference levels are low, the ability to transmit
very high resolution fax images is just a few
keystrokes away!
Hardware and Software
The computer allows reception and transmission of various fax modes, where parameters such as line-per-minute rates and
indices of cooperation can be altered by simply pressing a key or by pointing and clicking a mouse. Many fax programs are available as either commercial software or
shareware. Usually, the shareware packages
(and often trial versions of the commercial
packages) are available by downloading
from the Internet.
A good starting point is the ARRL software repositories. To get to them, set your
browser to the ARRL Web and go to the FTP
(files) link in the site index. You can use any
commercial search site to look for “fax” AND
“software.” Examples of several fax programs are as follows:
• JVFAX is a very popular fax program. It is
DOS-based program with a large number
of options for installation. It can receive
and transmit several fax formats, blackand-white and color. Your computer’s
serial port, connected to a very simple
interface, provides the connection to your
transceiver.
• The FAX 480 software program can also be
used with fax as well as SSTV. For more
information on this program and others
including website addresses, see the July
1998 QST article “FAX 480 and SSTV
Interfaces and Software,” p 32. A copy for
downloading of the free software program
vester_n.zip for FAX 480 can be found
online at the Oakland University FTP site.
This program also uses a simple interface
almost identical to that for JVFAX.
• Weatherman is a DOS-based program,
using a SoundBlaster (or compatible) card
as the interface. The program is shareware
and provides receive-only capability. A
single, shielded wire from your receiver
audio output to the computer audio input
is the only connection needed.
• WXSat operates under Windows 3.X. While
specifically set up to decode and store
weather-satellite APT pictures, it can also
be used for HF-fax reception.
Both Weatherman and WXSat are
samples of what you can find during a search
on the Internet. Often, programs are offered
and then either withdrawn or improved over
the versions previously distributed—To get
the latest and greatest you have to periodically search and see what comes up. If you
use an online service such as CompuServe
or AOL, they are another source of fax software. Check their ham forums or sections
for listings.
Many commercial multimode controllers
either contain software to receive and transmit fax, or are compatible with PC-hosted
software. Available controller suppliers include MFJ, Timewave, and Kantronics; additional software may be required for the
Kam Plus. Check the advertising pages of
QST for the latest units available.
One well-known fax page on the Internet,
complete with downloadable software, is
posted and maintained by Marius Rensen; it
contains listings of commercial fax transmissions for you to test your software or just
SWL for interest. Before using a program
taken from any Internet source, check other
sources for newer versions. It is not uncommon to have older versions posted on one
place and newer versions in another. It is
always a good idea to virus check software
before and after unzipping.
Image transmission using voice bandwidth is a trade-off between resolution and
time. In the section on slow-scan television,
standards are described that permit 240-line
black-and-white images to be transmitted in
about 36 seconds, while color images of
similar resolution require anywhere from 72
to 188 seconds, depending on the color format. In terms of resolution, 240-line SSTV
images are roughly equivalent to what you
would obtain with a standard broadcast TV
signal recorded on a home VCR. This is
more than adequate for routine video com-
munication, but there are many situations
that demand images with higher resolution.
HAL Communications Corporation has
developed an interesting system that enables
a standard fax machine (Group 3 or G3) to
send commercial fax images over HF radio.
HAL Communications accomplishes this
with just two small ancillary devices, which
connect between a standard fax machine and
an ordinary HF radio transceiver. This
method is frequently referred to as “G3 fax
over radio.” Any G3 fax machine can be
connected to the HAL FAX-4100 controller
with just a standard RJ-11 modular connector. The FAX-4100 controller connects
directly to the HAL CLOVER-2000 (DSP4100) radio data modem, which in turn connects to the HF transceiver. This entire setup
is duplicated at the opposite end of the link.
A “call” is initiated from the fax machine
keypad just as if the fax machine were connected to a phone line. The FAX-4100 controller includes a built-in 9600-baud G3
modem that emulates the telephone system.
The controller at the initiating end answers
the ring from the originating fax machine,
establishes the HF radio link (based on the
“phone number”), and handshakes with the
controller at the other end to start the receiving fax machine. Fax image data then passes
from the fax machine into the controller’s
memory at the originating end. The controller also establishes a data link between the
CLOVER-2000 modems at both ends, then
passes the fax data through them and the
controller at the receiving end, and finally
into the receiving G3 fax machine. HAL has
automated the HF radio operating procedures. To the user, sending a fax over HF
radio is a simple three-step process:
1. Lay the page(s) on the fax machine.
2. Enter the ID number of the other station.
3. Push GO on the fax machine.
Housekeeping control functions and indications are also automated, feeding messages back to the fax machine whenever
possible (link failed, other station not available, etc). A full page can be sent in 2 to
6 minutes, depending upon ionospheric
conditions and the image density of the page
being transmitted. The entire link set up and
maintenance procedure is transparent to the
fax operator, who need not know nor care
that an HF radio system is part of the fax
link. It all works just like a standard fax telephone transmission. An additional piece of
equipment is available from HAL to enable
the same fax machine to be shared between
HF radio and conventional telephone lines.
The HAL LI-4100 Line Interface is a “smart
switch” that can be connected between the
fax machine, the FAX-4100 controller, and
up to two telephone lines.
Modes and Modulation Sources
9.33
Courteous SSTV
Operating
• Recommended frequencies:
3.845, 7.171, 14.230, 14.233,
21.340, 28.680, 145.5 MHz.
• 14.230 is the most active.
• Make contact by voice before
sending SSTV.
• Not all systems recognize the VIS
code, so it is good manners to
announce the mode before transmitting.
Fig 9.40—Early SSTV operators developed
a basic 8-second black and white
transmission format. The sync pulses are
often called “blacker than black.” A
complete picture would have 120 lines
(8 seconds at 15 ms per line). Horizontal
sync pulses occur at the beginning of
every line; a 30 ms vertical sync pulse
precedes each frame.
Facsimile References
McConnell, Bodson, and Urban, FAX:
Facsimile Technology and Systems, 3rd
Ed., Artech House, 1999,
Taggart, Ralph, WB8DQT, Weather Satellite Handbook, 5th Ed. (Newington:
ARRL, 1994).
Taggart, Ralph, WB8DQT, “A New Standard for Amateur Radio Facsimile,”
QST, Feb 1993.
Taggart, Ralph, WB8DQT, ARRL Image
Communications Handbook, 1st Ed.
(Newington: ARRL, 2002).
SLOW-SCAN TELEVISION (SSTV)
An ancient Chinese proverb states: “A
picture is worth a thousand words.” It’s still
true today. Sight is our highest bandwidth
sense and the primary source of information
about the world around us. What would you
think about a TV news program without pictures about the stories? Would you enjoy
9.34
Chapter 9
reading the comics if there were no drawings with the text? Do you close your eyes
when talking to someone in person? Many
hams feel the same way about conversing
with Amateur Radio: Sending images is a
wonderful way to enhance communication.
This material was written by John Langner,
WB2OSZ.
For decades only a dedicated few kept
SSTV alive. The small numbers of commercial equipment were very expensive and
home-brewing was much too complicated
for most people. Early attempts at computerbased systems were rather crude and frustrating to use.
The situation has changed dramatically in
recent years. There is now a wide variety of
commercial products and home-brew
projects to fit every budget. SSTV activity is
experiencing rapid growth. There is much
software that uses computer sound cards for
SSTV.
The early SSTV 8-second transmission
standard is illustrated in Fig 9.40. Audio
tones in the 1500 to 2300-Hz range represent black, white and shades of gray. A short
1200-Hz burst separates the scan lines and a
longer 1200-Hz tone signals the beginning
of a new picture.
Color SSTV History
The early experimenters weren’t content
with only black and white (B&W) images
and soon devised a clever way to send color
pictures with B&W equipment. The transmitting station sends the same image three
times, one each with red, green and blue
filters in front of the TV camera lens. The
receiving operator took three long-exposure photographs of the screen, placing
red, green and blue filters in front of the
film camera’s lens at the appropriate times.
This was known as the “frame sequential”
method.
In the 1970s, it became feasible to save
these three images in solid-state memory
and simultaneously display them on an ordinary color TV. The frame-sequential
method had some drawbacks. As the first
frame was received you’d see a red and
black image. During the second frame, green
and yellow would appear. Blue, white, and
other colors wouldn’t show up until the
final frame. Any noise (QRM or QRN) could
ruin the image registration (the overlay of
the frames) and spoil the picture.
The next step forward was the “line sequential” method. Each line is electronically
scanned three times before being transmitted: once each for the red, green, and blue
picture components. Pictures could be seen
in full color as they were received and registration problems were reduced. The
Wraase SC-1 modes are examples of early
line-sequential color transmission. They
have a horizontal sync pulse for each of the
color component scans. The major weakness here is that if the receiving end gets out
of step, it won’t know which scan represents
which color.
Rather than sending color images with the
usual RGB (red, green, blue) components,
Robot Research used luminance and
chrominance signals for their 1200C modes.
The first half or two thirds of each scan line
contains the luminance information, which
is a weighted average of the R, G and B
components. The remainder of each line
contains the chrominance signals with the
color information. Existing B&W equipment could display the B&W-compatible
image on the first part of each scan line and
the rest would go off the edge of the screen.
This compatibility was very beneficial when
most people still had only B&W equipment.
The luminance-chrominance encoding
made more efficient use of the transmission time. A 120-line color image could be
sent in 12 s, rather than the usual 24 s. Our
eyes are more sensitive to details in changes
of brightness than color, so the time could
be used more efficiently by devoting more
time to luminance than chrominance. The
NTSC and PAL broadcast standards also
take advantage of this vision characteristic
and use less bandwidth for the color part of
the signal.
The 1200C introduced another innovation: It encoded the transmission mode in
the vertical sync signal. By using narrow
FSK encoding around the sync frequency,
compatibility was maintained. This new signal just looked like an extra-long vertical
sync to older equipment. The luminancechrominance encoding offers some benefits,
but image quality suffers. It is acceptable for
most natural images but looks bad for sharp,
high-contrast edges, which are more and
more common as images are altered via
computer graphics. As a result, all newer
modes have returned to RGB encoding.
The Martin and Scottie modes are essentially the same except for the timings. They
have a single horizontal sync pulse for each
set of RGB scans. Therefore, the receiving
end can easily get back in step if synchronization is temporarily lost. Although they
have horizontal sync, some implementations ignore them on receive. Instead, they
rely on very accurate time bases at the transmitting and receiving stations to keep in step.
The advantage of this “synchronous” strategy is that missing or corrupted sync pulses
won’t disturb the received image. The disadvantage is that even slight timing inaccuracies produce slanted pictures.
In the late 1980s, yet another incompatible mode was introduced. The AVT mode
is different from all the rest in that it has no
horizontal sync. It relies on very accurate
oscillators at the sending and receiving stations to maintain synchronization. If the
beginning-of-frame sync is missed, it’s all
over. There is no way to determine where a
scan line begins. However, it’s much harder
to miss the 5-s header than the 300-ms VIS
code. Redundant information is encoded 32
times and a more powerful error-detection
scheme is used. It’s only necessary to receive a small part of the AVT header in order to achieve synchronization. After this,
noise can wipe out parts of the image, but
image alignment and colors remain correct.
Table 9.9 lists characteristics of common
modes.
Scan Converters
A scan converter is a device that converts
signals from one TV standard to another. In
this particular case we are interested in converting between SSTV, which can be sent
through audio channels, and fast scan
(broadcast or ATV), so we can use ordinary
camcorders and color televisions to generate and display pictures. From about 1985
to 1992, the Robot 1200C was king.
Fig 9.41 shows a typical SSTV station
built around a scan converter such as the
Robot 1200C or a SUPERSCAN 2001. The
scan converter has circuitry to accept a TV
signal from a camera and store it in memory.
It also generates a display signal for an
ordinary television set. The interface to the
radio is simply audio in, audio out and a
push-to-talk (PTT) line. In the early days,
pictures were stored on audio tape, but now
computers store them in memory. Once a
picture is in a computer, it can be enhanced
with paint programs.
This is the easiest approach. Just plug in
the cables, turn on the power and it works.
Many people still prefer special dedicated
hardware, but most of the recent growth of
SSTV has been from these lower cost PCbased systems using sound cards and software.
SSTV with a Computer
There were many attempts to use early
home computers for SSTV. Those efforts
were hampered by very small computer
memories, poor graphics capabilities and
poor software development tools.
Surprisingly, little was available for the
ubiquitous IBM PC until around 1992, when
several systems appeared in quick succession. By this time, all new computers had a
VGA display, which is required for this application. Most modern SSTV stations look
like Fig 9.42. Some sort of interface is used
to get audio in and out of the computer.
These can be external interfaces connected
to a serial or printer port, an internal computer card specifically designed for SSTV
or even a peripheral audio card.
Table 9.9
SSTV Transmission Characteristics
Mode
Designator
AVT
Martin
HQ
Scan Time
(sec)
24
90
94
188
125
Scan Lines
Notes
24
90
94
188
125
Color
Type
RGB
RGB
RGB
RGB
BW
120
240
200
400
400
D
D
D
D
D
M1
M2
M3
M4
RGB
RGB
RGB
RGB
114
58
57
29
240
240
120
120
B
B
C
C
YC
YC
90
112
240
240
G
G
203
305
406
16+480
16+480
16+480
HQ1
HQ2
Pasokon TV
P3
P5
P7
RGB
RGB
RGB
Robot
8
12
24
36
12
24
36
72
BW
BW
BW
BW
YC
YC
YC
YC
Scottie
S1
S2
S3
S4
DX
Wraase SC-1
Wraase SC-2
8
12
24
36
12
24
36
72
120
120
240
240
120
120
240
240
A,E
E
E
E
RGB
RGB
RGB
RGB
RGB
110
71
55
36
269
240
240
120
120
240
B
B
C
C
B
24
48
96
RGB
RGB
RGB
24
48
96
120
240
240
C
B
B
30
60
120
180
RGB
RGB
RGB
RGB
30
60
120
180
128
256
256
256
J120
RGB
120
240
GVA 125
GVA 125
GVA 250
BW
RGB
RGB
125
125
250
480
240
480
JV Fax
JV Fax Color
RGB
variable
FAX480
Fax 480
Truscan
Colorfax 480
BW
BW
RGB
138
128
384
Pro-Skan
WinPixPro
variable
F
480
480
480
H
I
Notes
RGB—Red, green and blue components sent separately.
YC—Sent as Luminance (Y) and Chrominance (R-Y and B-Y).
BW—Black and white.
A—Similar to original 8-second black & white standard.
B—Top 16 lines are gray scale. 240 usable lines.
C—Top 8 lines are gray scale. 120 usable lines.
D—AVT modes have a 5-second digital header and no horizontal sync.
E—Robot 1200C doesn’t really have B&W mode but it can send red, green or blue memory
separately. Traditionally, just the green component is sent for a rough approximation of
a b&w image.
F—JV Fax Color mode allows the user to set the number of lines sent, the maximum
horizontal resolution is slightly less than 640 pixels. This produces a slow but very high
resolution picture. SVGA graphics are required.
G—Available only on Martin 4.6 chipset in Robot 1200C.
H—Vester version of FAX480 (with VIS instead of start signal and phasing lines).
I—Trucolor version of Vester Truscan.
Modes and Modulation Sources
9.35
Perhaps the single most significant
breakthrough in computer-based SSTV is
the wide range of Windows- and DOSbased programs using the PC’s soundcard
as the main transmit/receive interface.
Many operators nowadays use the popular
freeware program MMSSTV by JE3HHT
(see mmhamsoft.ham-radio.ch/mmsstv/),
with a simple hardware interface to go into
and to come out of the PC’s soundcard.
Information on current computer SSTV
software is available at www.tima.
com/~djones. The subject of computer
SSTV software and interfacing is also discussed at length in the Image Communications Handbook published by ARRL.
A simple “clipper” hardware interface to
the computer’s soundcard can be built with
less than $15 worth of RadioShack parts.
Fig 9.43 shows such an interface circuit
used for receiving and transmitting. Connect the output of T2 to the phone patch
input (sometimes labeled LINE INPUT) of
your transceiver, if it has one. Otherwise,
you’ll have to use the microphone input. R3
is set to the proper level for the audio going
to the transmitter. You must set the audio
signal into the transceiver at a level it can
handle without distortion.
There is no low-pass filtering in the audio
line between the computer output and transmitter audio input. On-the-air checks with
many stations reveal that no additional external filtering is required when using SSB
transmitters equipped with mechanical or
Fig 9.41—Diagram of an older SSTV station based on a scan converter.
Fig 9.42—A modern SSTV station that utilizes the soundcard in a PC.
Fig 9.43—Schematic of the simple SSTV receive and transmit circuit from July 1998 QST. T1 and T2 are RadioShack 273-1380 audiooutput transformers; the 20-μF, 50-V capacitor is a parallel combination of two RadioShack 272-999 10-μF, 50-V non-polarized
capacitors; equivalent parts can be substituted. Unless otherwise specified, resistors are 1/4-W, 5%-tolerance carbon composition or
film units. At J1, numbers in parentheses are for 25-pin serial port connectors; other numbers are for 9-pin connectors.
9.36
Chapter 9
crystal filters. If you intend to use this circuit
with an AM or phasing-type SSB rig (or with
VHF/UHF FM transmitters), add audio filtering to provide the required spectral purity. An elliptical low-pass filter such as
described by Campbell (see references)
should be adequate for most cases.
Circuit component values aren’t critical,
nor is the circuit’s physical construction. A
PC board is available from FAR Circuits, but
perf-board construction employing short
leads works fine.
Digital Slow-Scan Television
DSSTV is a method of transmitting computer image files, such as JPEG or GIF over
Amateur Radio, as described in an article by
Ralph Taggart, WB8DQT, in the Feb 2004
issue of QST. The signal format phase modulates a total of eight subcarriers (ranging
from 590 to 2200 Hz at intervals of 230 Hz.
Each subcarrier has nine possible modulation states. This signal modulation format is
known as redundant digital file transfer
(RDFT) developed by Barry Sanderson,
KB9VAK. RDFT is also used with SCAMP,
described earlier in this chapter.
SSTV Summary
For decades there was a convenient excuse for not trying SSTV: it cost kilobucks to
buy a specialized piece of equipment. But
you can’t use that excuse anymore. There
are free programs that only require trivial
hardware interfaces to receive and transmit
slow-scan pictures. Once you get hooked,
there are plenty of other home-brew projects
and commercial products available at affordable prices. You need not be a computer
wizard to install and use these systems.
SSTV Bibliography
Battles, B. and Ford, S., “Smile—You’re
on Ham Radio!” QST, Oct 1992.
Bodson, D., W4PWF, and Karty, S., N5SK,
“FAX480 and SSTV Interfaces and Software,” QST, Jul 1998.
Campbell, R, “High-Performance, SingleSignal Direct-Conversion Receivers,”
QST, Jan 1993. See also Feedback, QST,
Apr 1993, p 75.
Langner, J. WB2OSZ, “Slow Scan Television—It isn’t expensive anymore,” QST,
Jan 1993,.
SSTV Glossary
ATV—Amateur Television. Sending pictures by Amateur
Radio. You’d expect this abbreviation to apply equally to
fast-scan television (FSTV), slow-scan television (SSTV)
and facsimile (fax), but it’s generally applied only to FSTV.
AVT—Amiga Video Transceiver. 1) Interface and software
for use with an Amiga computer, developed by Ben BlishWilliams, AA7AS, and manufactured by Advanced
Electronic Applications (AEA); 2) a family of transmission
modes first introduced with the AVT product.
Back porch—The blank part of a scan line immediately
following the horizontal sync pulse.
Chrominance—The color component of a video signal.
NTSC and PAL transmit color images as a black-andwhite compatible luminance signal along with a color
subcarrier. The subcarrier phase represents the hue and
the subcarrier’s amplitude is the saturation. Robot color
modes transmit pixel values as luminance (Y) and
chrominance (R-Y [red minus luminance] and B-Y [blue
minus luminance]) rather than RGB (red, green, blue).
Demodulator—For SSTV, a device that extracts image
and sync information from an audio signal.
Field—Collection of top to bottom scan lines. When
interlaced, a field does not contain adjacent scan lines
and there is more than one field per frame.
Frame—One complete scanned image. The Robot 36second color mode has 240 lines per frame. NTSC has
525 lines per frame with about 483 usable after subtracting vertical sync and a few lines at the top containing
various information.
Frame Sequential—A method of color SSTV transmission
that sent complete, sequential frames of red, then green
and blue. Now obsolete.
Front porch—he blank part of a scan line just before the
horizontal sync.
FSTV—Fast-Scan TV. Same as common, full-color,
motion commercial broadcast TV.
Interlace—Scan line ordering other than the usual
sequential top to bottom. For example, NTSC sends a
field with just the even lines in 1/60 second, then a field
with just the odd lines in 1/60 second. This results in a
complete frame 30 times a second. AVT “QRM” mode is
the only SSTV mode that uses interlacing.
Line Sequential—A method of color SSTV transmission
that sends red, green, and blue information for each
sequential scan line. This approach allows full-color
images to be viewed during reception.
Luminance—The brightness component of a video
signal. Usually computed as Y (the luminance signal) =
0.59 G (green) + 0.30 R (red) + 0.11 B (blue).
Martin—A family of amateur SSTV transmission modes
developed by Martin Emmerson, G3OQD, in England.
NTSC—National Television System Committee. Television standard used in North America and Japan.
PAL—Phase alteration line. Television standard used in
Germany and many other parts of Europe.
Pixel—Picture element. The dots that make up images
on a computer’s monitor.
P7 monitor—SSTV display using a CRT having a verylong-persistence phosphor.
RGB—Red, Green, Blue. One of the models used to
represent colors. Due to the characteristics of the
human eye, most colors can be simulated by various
blends of red, green, and blue light.
Robot—(1) Abbreviation for Robot 1200C scan converter; (2) a family of SSTV transmission modes
introduced with the 1200C.
Scan converter—A device that converts one TV
standard to another. For example, the Robot 1200C
converts SSTV to and from FSTV.
Scottie—A family of amateur SSTV transmission modes
developed by Eddie Murphy, GM3SBC, in Scotland.
SECAM—Sequential color and memory. Television
standard used in France and the Commonwealth of
Independent States.
SSTV—Slow Scan Television. Sending still images by
means of audio tones on the MF/HF bands using
transmission times of a few seconds to a few minutes.
Sync—That part of a TV signal that indicates the beginning of a frame (vertical sync) or the beginning of a
scan line (horizontal sync).
VIS—Vertical Interval Signaling. Digital encoding of the
transmission mode in the vertical sync portion of an
SSTV image. This allows the receiver of a picture to
automatically select the proper mode. This was
introduced as part of the Robot modes and is now used
by all SSTV software designers.
Wraase—A family of amateur SSTV transmission modes
first introduced with the Wraase SC-1 scan converter
developed by Volker Wraase, DL2RZ, of Wraase
Electronik, Germany.
Modes and Modulation Sources
9.37
Montalbano, J., KA2PYJ, “The ViewPort
VGA Color SSTV System,” 73, Aug
1992.
Taggart, R., WB8DQT, “Digital SlowScan Television,” QST, Feb 2004, p 4751.
Taggart, R., WB8DQT, Image Communications Handbook, Published by ARRL,
Newington, CT, 2002. ARRL Order No.
8616.
Vester, B., K3BC “Vester SSTV/FAX80/
Fax System Upgrades,” Technical Correspondence, QST, Jun 1994.
Vester, B., K3BC, “SSTV: An Inexpensive System Continues to Grow,” Dec
1994 QST.
Vester, B., K3BC, “K3BC’s SSTV Becomes TRUSCAN,” Technical Correspondence, QST, Jul 1996.
FAST-SCAN TELEVISION
Fast-scan amateur television (FSTV or
just ATV) is a wide-band mode that uses
standard broadcast, or NTSC, television scan
rates. It is called “fast scan” only to differentiate it from slow-scan TV. In fact, no scan
conversions or encoder/decoders are necessary with FSTV. Any standard TV set can
display the amateur video and audio. Standard (1 V P-P into 75 Ω) composite video
Fig 9.44—Students enjoy using ATV to
communicate between science and
computer classes.
Fig 9.45—The ATV view shows the aft end
of the Space Shuttle cargo bay during
mission STS-9.
9.38
Chapter 9
Table 9.10
Line-of-Sight Snow-Free 70-cm ATV Communication Distances
This table relates transmit and receive station antenna gains to communication distances in
miles for 1/10/100 W PEP at 440 MHz. To find the possible snow-free distance under lineof-sight conditions, select the column that corresponds to transmit antenna gain and the
row for the receive antenna gain. Read the distance where the row and column intersect.
Multiply the result by 0.5 for 902 MHz and 0.33 for 1240 MHz.
The table assumes 2 dB of feed-line loss, a 3 dB system noise figure at both ends and
snow-free is greater than 40 dB picture:noise ratio (most home cameras give 40 to 45 dB
picture:noise; this is used as the limiting factor to define snow-free ATV pictures). The P
unit picture rating system goes down about 6 dB per unit. For instance, P4 pictures would
be possible at double the distances in the table.
TX Antenna
RX Antenna
0 dBd
4 dBd
9 dBd
15.8 dBd
0 dBd
4 dBd
9 dBd
15.8 dBd
0.8/2.5/8
1/3.5/11
2/7/22
5/15/47
1/3.5/11
2/6/19
3.5/11/34
7.5/23/75
2/7/22
3.5/11/34
6/19/60
13/42/130
5/15/47
7.5/23/75
13/42/130
29/91/290
from home camcorders, cameras, VCRs or
computers is fed directly into an AM ATV
transmitter. The audio has a separate connector and goes through a 4.5 MHz FM
subcarrier generator that is mixed with the
video. This section was written by Tom
O’Hara, W6ORG.
Amateurs regularly show themselves in
the shack, zoom in on projects, show home
video tapes, computer programs and just
about anything that can be shown live or by
tape (see Figs 9.44 and 9.45). Whatever the
camera “sees” and “hears” is faithfully
transmitted, including color and sound information. Picture quality is about equivalent to that of a VCR, depending on video
signal level and any interfering carriers. All
of the sync and signal-composition information is present in the composite-video
output of modern cameras and camcorders.
Most camcorders have an accessory cable
or jacks that provide separate video and
audio outputs. Audio output may vary from
one camera to the next, but usually it has
been amplified from the built-in microphone
to between 0.1 to 1 V P-P (into a 10-kΩ
load).
ATV transmitters have been carried by
helium balloons to above 100,000 ft, to the
edge of space. The result is fantastic video
transmissions, showing the curvature of the
Earth, that have been received as far as
500 miles from the balloon. Small cameras
have been put into the cockpits of R/C model
airplanes to transmit a pilot’s-eye view.
Many ATV repeaters retransmit Space
Shuttle video and audio from NASA during
missions. This is especially exciting for
schools involved with SAREX. ATV is used
for public service events, such as parades,
races, Civil Air Patrol searches and remote
damage assessment.
Emergency service coordinators have
found that live video from a site gives a better
Table 9.11
Bit encoding for 5.5 Mbps and
11 Mbps CCK transmissions.
Data Rate
Mbps
5.5
11
CCK
encoded bit
2
6
DQPSK
encoded bit
2
2
understanding of a situation than is possible
from voice descriptions alone. Weather-radar video, WEFAX, or other computer generated video has also been carried by ATV
transmitters for RACES groups during significant storms. This use enables better allocation of resources by presenting real-time
information about the storm track. Computer
graphics and video special effects are often
transmitted to dazzle the viewers.
How Far Does ATV Go?
The theoretical snow-free line-of-sight
distance for 10 W, given 15.8-dBd antennas
and 2-dB feed-line loss at both ends, is
91 miles. (See Table 9.11.) However, except for temperature-inversion skip conditions, reflections, or through high hilltop
repeaters, direct line-of-sight ATV contacts
seldom exceed 25 miles. The RF horizon
over flat terrain with a 50-ft tower is
10 miles. For best DX, use low-loss feed line
and a broadband high-gain antenna, up as
high as possible. The antenna system is the
most important part of an ATV system because it affects both receive and transmit
signal strength.
A snow-free, or “P5,” picture rating (see
Fig 9.46) requires at least 200 μV
(–61 dBm) of signal at the input of the ATV
receiver, depending on the system noise
figure and bandwidth. The noise floor increases with bandwidth. Once the receiver
system gain and noise figure reaches this
P5 —Excellent
P4—Good
P3—Fair
P2—Poor
P1—Barely perceptible
Fig 9.46—An ATV quality reporting system.
floor, no additional gain will increase sensitivity. At 3-MHz bandwidth the noise
floor is 0.8 μV (–109 dBm) at standard temperature. If you compare this to an FM voice
receiver with 15 kHz bandwidth; there is a
23 dB difference in the noise floor. However the eye, much like the ear of experienced CW operators, can pick out sync bars
in the noise below the noise floor. Sync
lock and large well contrasted objects or
lettering can be seen between 1 and 2 μV.
Color and subcarrier sound come out of the
noise between 2 and 8 μV depending on
their injection level at the transmitter and
TV-set differences.
Two-meter FM is used to coordinate ATV
contacts. Operators must take turns transmitting on the few available channels and
the 2-m link allows full-duplex audio from
many receiving stations to the ATV transmitting station, who speaks on the sound
subcarrier. This is great for interactive show
and tell. It is also much easier to monitor a
squelched 2-m channel using an omni antenna rather than searching out each station
with a beam. Depending on the third-harmonic relationship to the video on 70 cm,
144.34 MHz and 146.43 MHz (simplex) are
the most popular frequencies. They are often mixed with the subcarrier sound on ATV
repeater outputs.
Getting the Picture
Since the 70-cm band corresponds to
cable TV channels 57 through 61, seeing
your first ATV picture may be as simple as
connecting a good outside 70-cm antenna
(aligned for the customary local polarization) to a cable-ready TV set’s antenna input
jack. Cable channel 57 is 421.25 MHz, and
each channel is progressively 6 MHz higher.
(Note that cable and broadcast UHF channel
frequencies are different.) Check the ARRL
Repeater Directory for a local ATV repeater
output that falls on one of these cable channels. Cable-ready TVs may not be as sensitive as a low-noise downconverter designed
just for ATV, but this technique is well worth
a try.
Most stations use a variable tuned
downconverter specifically designed to convert the whole amateur band down to a VHF
TV channel. Generally the 400 and 900MHz bands are converted to TV channel 3
or 4, whichever is not used in the area. For
1200 MHz converters, channels 7 through
10 are used to get more image rejection. The
downconverter consists of a low-noise
preamp, mixer and tunable or crystal-controlled local oscillator. Any RF at the input
comes out at the lower frequencies. All signal processing is done in the TV set. A complete receiver with video and audio output
would require all the TV sets circuitry, less
the sweep and CRT components. There is no
picture-quality gain by going direct from a
receiver to a video monitor (as compared
with a TV set) because IF and detector bandwidth are still the limiting factors.
A good low-noise amateur downconverter with 15 dB gain ahead of a TV set
will give sensitivity close to the noise floor.
A preamp located in the shack will not significantly increase sensitivity, but rather will
reduce dynamic range and increase the
probability of intermodulation interference.
Sensitivity can be increased by increasing
antenna-system gain:
• Reducing feed-line loss
• Increasing antenna gain
• Or adding an antenna mounted preamp
(which will eliminate the coax loss, plus
any loss through transmit linear amplifier
TR relays).
Remember that each 6 dB increase in
combination of transmitted power, reduced
coax loss, antenna gain or receiver sensitivity can double the line-of-sight distance.
Foliage greatly attenuates the signal at
UHF, so place antennas above the tree tops
for the best results. Beams made for 432MHz weak-signal work or 440-MHz FM
may not have enough SWR bandwidth to
cover all the ATV frequencies for transmitting, but they are okay for reception. A number of manufacturers now make ATV beam
antennas to cover the whole band from 420
to 450 MHz. Use low-loss coax (such as
Belden 9913: 2.5 dB/100 ft at 400 MHz) or
Hardline for runs over 100 ft. All outside
connectors must be weatherproofed with
tape or coax sealer—Any water that gets
inside the coax will greatly increase the
attenuation. Almost all ATV antennas use N
connectors, which are more resistant to
moisture contamination than other types.
Antenna polarization varies from area to
area. Technically, the polarization should be
chosen to give additional isolation (up to
20 dB) from other users near the channel. It
is more common to find that the polarity was
Fig 9.47—A 6-MHz video channel with the
video carrier 1.25 MHz up from the lower
edge. The color subcarrier is at 3.58 MHz
and the sound subcarrier at 4.5 MHz above
the video carrier.
Modes and Modulation Sources
9.39
determined by the first local ATV operators
(which antennas they had in place for other
modes). Generally, those on 432 MHz SSB
and weak-signal DX have horizontally polarized antennas, and those into FM, public
service or repeaters will have vertical antennas. Check with local ATV operators before
permanently locking down the antenna-mast
clamps. Circularly polarized antennas let
you work all modes, including satellites,
with only 3 dB sacrificed when working a
fixed polarity.
ATV Frequencies
Standard broadcast TV channels are
6 MHz wide to accommodate the composite
video, 3.58 MHz color and 4.5 MHz sound
subcarriers. (See Fig 9.47.) Given the NTSC
525 horizontal line and 30 frames per second scan rates, the resulting horizontal resolution bandwidth is 80 lines per MHz.
Therefore, with the typical TV set’s 3-dB
rolloff at 3 MHz (primarily in the IF filter),
up to 240 vertical black lines can be seen.
Color bandwidth in a TV set is less than this,
resulting in up to 100 color lines. Lines of
resolution are often confused with the number of horizontal scan lines per frame. The
video quality should be every bit as good as
on a home video recorder.
The lowest frequency amateur band wide
enough to support a TV channel is 70 cm
(420 - 450 MHz), and it is the most popular.
With transmit power, antenna gains and
coax losses equal, decreasing frequency
increases communication range. The 33-cm
band goes half the distance that 70 cm does,
but this can be made up to some extent with
high-gain antennas, which are physically
smaller at the higher frequency. A Technician class or higher license is required to
transmit ATV on this band, and Novices can
transmit ATV only in the 1270 to 1295 MHz
segment of the 23-cm band. Depending on
local band plan options, there is room for no
more than two simultaneous ATV channels
in the 33- and 70-cm bands without interference. Generally, because only two channels
are available in the 70-cm band, an ATV
repeater input on 439.25 or 434.0 MHz is
shared with simplex stations. 421.25 MHz
is the most popular in-band repeater output
frequency. At least 12 MHz of separation is
necessary for in-band repeaters because of
filter-slope attenuation characteristics and
TV-set adjacent-channel rejection. Some
repeaters have their output on the 33-cm or
23-cm bands (the 923.25 and 1253.25 MHz
output frequencies are most popular). This
frees up a channel on 70 cm for simplex.
Such cross-band repeaters also make it
easier for the transmitting operator to monitor the repeated video with only proper antenna separation needed to prevent receiver
desensitization. 426.25 MHz is used for sim9.40
Chapter 9
Fig 9.48—A spectrum-analyzer photo of a
color ATV signal. Each vertical division
represents 10 dB; horizontal divisions are
1 MHz. Spectrum power density varies
with picture content, but typically 90% of
the sideband power is within the first
1 MHz.
plex, public service and R/C models in areas
with cross-band repeaters, or as a alternative
to the main ATV activities on 434.0 or
439.25 MHz. Before transmitting, check
with local ATV operators, repeater owners
and frequency coordinators listed in the
ARRL Repeater Directory for the coordinated frequencies used in your area.
Since a TV set receives a 6-MHz bandwidth, ATV is more susceptible to interference from many other sources than are
narrower modes. Interference 40 dB below
the desired signal can be seen in video.
Many of our UHF (and above) amateur
bands are shared with radar and other government radio positioning services. These
show up as horizontal bars in the picture.
Interference from amateurs who are unaware of the presence of the ATV signal (or
in the absence of a technically sound and
publicized local band plan) can wipe out the
sound or color or put diagonal lines in the
picture.
DSB and VSB Transmission
While most ATV is double sideband
(DSB) with the widest component being the
sound subcarrier out ±4.5 MHz, over 90%
of the spectrum power is in the first 1 MHz
on both sides of the carrier for DSB or VSB
(vestigial sideband). As can be seen in
Fig 9.48, the video power density is down
more than 30 dB at frequencies greater than
1 MHz from the carrier. DSB and VSB are
both compatible with standard TV receivers, but the lower sound and color
subcarriers are rejected in the TV IF filter as
unnecessary. In the case of VSB, less than
5% of the lower sideband energy is attenuated. The other significant energy frequencies are the sound (set in the ATV transmitter
at 15 dB below the peak sync) and the color
at 3.58 MHz (greater than 22 dB down).
Narrowband modes operating greater
than 1 MHz above or below the video carrier are rarely interfered with or know that
Fig 9.49—A photo of an ATV image of the
Space Shuttle interior with K6KMN’s
repeater ID overlaid. Automatic video
overlay in the picture easily solves the 10minute ID requirement for Space Shuttle
retransmissions and other long
transmissions.
the ATV transmitter is on unless the
narrowband signal is on one of the subcarrier
frequencies or the stations are too near one
another. If the band is full and the lower
sideband color and sound subcarrier frequencies need to be used by a dedicated link
or repeater, a VSB filter in the antenna line
can attenuate them another 20 to 30 dB, or
the opposite antenna polarization can be
used for more efficient packing of the spectrum. Since all amateur linear amplifiers reinsert the lower sideband to within 10 dB of
DSB, a VSB filter in the antenna line is the
only cost-effective way to reduce the unnecessary lower sideband subcarrier energy
if more than 1 W is used. In the more populated areas, 2-m calling or coordination frequencies are often used to work out
operating time shifts, and so on, between all
users sharing or overlapping the same segment of the band.
ATV Identification
ATV identification can be on video or the
sound subcarrier. A large high-contrast callletter sign on the wall behind the operating
table in view of the camera is the easiest way
to fulfill the requirement. Transmitting stations fishing for DX during band openings
often make up call-ID signs using fat black
letters on a white background to show up
best in the snow. Their city and 2-m monitoring frequency are included at the bottom
of the sign to make beam alignment and
contact confirmation easier.
Quite often the transmission time exceeds
10 minutes, especially when transmitting
demonstrations, public-service events,
space-shuttle video, balloon flights or a
video tape. A company by the name of Intuitive Circuits makes a variety of boards that
will overlay text on any video looped
through them. Call letters and other information can be programmed into the board’s
non-volatile memory by on-board push buttons or an EIA-232 line from a computer
(depending on the version and model of the
OSD board). There is even a model that will
accept NMEA-0183 GPRMC data from a
GPS receiver and overlay latitude, longitude, altitude, direction and speed, as well as
call letters, on the applied camera video. This
is ideal for ATV rockets, balloons and R/C
vehicles. The overlaid ID can be selected to
be on, off or flashed on for a few seconds
every 10 minutes to automatically satisfy the
ID requirement of §97.119 (see Fig 9.49).
The PC Electronics VOR-2 board has an
automatic nine-minute timer, and it also has
an end-of-transmission hang timer that
switches to another video source for ID.
Driving Amplifiers with ATV
Wide-band AM video requires some special design considerations for linear amplifiers (as compared to those for FM and SSB
amplifiers). Many high-power amateur amplifiers would oscillate (and possibly self
destruct) from high gain at low frequencies
if they were not protected by feedback networks and power RF chokes. These same
Fig 9.50—An oscilloscope used to observe
a video waveform. The lower trace is the
video signal as it comes out of the sync
stretcher. The upper trace is the signal
from the Mirage D1010-N amplifier.
stability techniques can affect some of the
5-MHz video bandwidth. Sync, color and
sound can be very distorted unless the amplifier has been carefully designed for both
stability and AM video modulation.
Mirage, Teletec and Down East Microwave either make special ATV amplifiers or
offer standard models that were designed
for all modes, including ATV. Basically the
collector and base bias supplies have a range
of capacitors to keep the voltage constant
under modulation, while at the same time
using the minimum-value low-resistance
series inductors or chokes to prevent self
oscillation.
Almost all amateur linear power amplifiers have gain compression from half to their
full rated peak envelope power. To compensate for this, the ATV exciter/modulator has a sync stretcher to maintain the
proper transmitted video to sync ratio (see
Fig 9.50). With both video and sound
subcarrier disconnected, the pedestal control is set for maximum power output. Peak
sync should first be set to 90% of the rated
peak envelope power. (This is necessary to
give some head room for the 4.5 MHz
sound that is mixed and adds with the video
waveform.) The TXA5-70 exciter/modulator has a RF power control to set this. Once
this is done, the blanking pedestal control
can be set to 60% of the peak sync value.
For example, a 100-W amplifier would first
be set for 90 W with the RF power control
and then 54 W with the pedestal control.
Then the sound subcarrier can be turned
back on and the video plugged in and adjusted for best picture. If you could read it
on a peak-reading power meter made for
video, the power would be between 90 and
100 W PEP. On a dc oscilloscope connected to a RF diode detector in the antenna
line, it can be seen that the sync and blank-
ing pedestal power levels remain constant
at their set levels regardless of video gain
setting or average picture contrast. On an
averaging meter like a Bird 43, however, it
is normal to read something less than the
pedestal set-up power.
ATV Repeaters
Basically there are two kinds of ATV repeaters: in band and cross band. 70-cm inband repeaters are more difficult to build
and use, yet they are more popular because
equipment is more available and less expensive. Indeed, cable-ready TV sets tune the
70-cm band with no modifications.
Why are 70-cm repeaters more difficult to
build? The wide bandwidth of ATV makes
for special filter requirements. Response
across the 6-MHz passband must be as flat
as possible with minimum insertion loss, but
also must sharply roll off to reject other users as little as 12 MHz away. Special multipole interdigital or combline VSB filters are
used to meet the requirement. An ATV duplexer can be used to feed one broadband
omnidirectional antenna, but an additional
VSB filter is needed in the transmitter line
for sufficient attenuation of noise and IMD
products.
A cross-band repeater, because of the
great frequency separation between the input and output, requires less sophisticated
filtering to isolate the transmitter and receiver. In addition, a cross-band repeater
makes it easier for users to see their own
video (no duplexer is needed, only sufficient antenna spacing). Repeater linking is
easier too, if the repeater outputs alternate
between the 23- and 33-cm bands.
Fig 9.51 shows a block diagram for a
simple 70-cm in-band repeater. No duplexer
is shown because the antennas and VSB filters provide adequate isolation. The repeater
Fig 9.51—A block diagram of a 70-cm in-band ATV repeater. The antennas are Diamond omnidirectional verticals, which require 20 ft
(minimum) of vertical separation to prevent receiver desensitization. The VSB filters are made by DCI; they have the proper band-pass
characteristics and only 1 dB insertion loss. A low pass filter on the receiver is also necessary because cavity type filters repeat a
pass-band at odd harmonics and the third-harmonic energy from the transmitter may not be attenuated enough. The receiver, 10-W
transmitter and VOR are made by PC Electronics. The Communications Specialists DTD-1 DTMF decoder and ID8 Morse identifier
(optional if a video ID is used) are used to remotely turn the repeater transmitter on or off and to create a CW ID, respectively.
Alternatively, an Intuitive Circuits ATV4-4 ATV repeater controller board can do all the control box functions as well as remotely select
from up to four video sources.
Modes and Modulation Sources
9.41
Fig 9.52—Block diagram of an FMATV receiver.
transmitter power supply should be separate
from the receiver and exciter supply. ATV is
amplitude modulated, therefore the current
varies greatly from maximum at the sync tip
to minimum during white portions of the
picture. Power supplies are not generally
made to hold tight regulation with such great
current changes at rates up to several megahertz. Even the power supply leads become
significant inductors at video frequencies;
they will develop a voltage across them that
can be transferred to other modules on the
same power-supply line.
To prevent unwanted key up from other
signal sources, ATV repeaters use a video
operated relay (VOR). The VOR senses the
horizontal sync at 15,734 Hz in much the
same manner that FM repeaters use CTCSS
tones. Just as in voice repeaters, an ID timer
monitors VOR activity and starts the repeater
video ID generator every nine minutes, or a
few seconds after a user stops transmitting.
Frequency Modulated ATV (FMATV)
While AM is the most popular mode because of greater equipment availability,
lower cost, less occupied bandwidth and use
of a standard TV set, FMATV is gaining
interest among experimenters and also repeater owners for links. FM on the 1200MHz band is the standard in Europe because
there is little room for video in their allocated portion of the 70-cm band. FMATV
occupies 17 to 21 MHz depending on deviation and sound subcarrier frequency. The
US 70-cm band is wide enough but has great
interference potential in all but the less popu9.42
Chapter 9
lated areas. Most available FMATV equipment is made for the 1.2, 2.4 and 10.25-GHz
bands. Fig 9.52 is a block diagram of an
FMATV receiver.
The US standard for FMATV is 4 MHz
deviation with the 5.8-MHz sound subcarrier set to 10 dB below the video level.
1252 or 1255 MHz are suggested frequen-
cies in order to stay away from FM voice
repeaters and other users higher in the band,
while keeping sidebands above the 1240MHz band edge. Using the US standard, with
Carson’s rule for FM occupied bandwidth, it
comes out to just under 20 MHz. So
1250 MHz would be the lowest possible frequency. Almost all modern FMATV equip-
Fig 9.53—Two approaches to ATV receiving. This chart compares AM (A) and FM (F) ATV
as seen on a TV receiver and monitor. Signal levels are into the same downconverter
with sufficient gain to be at the noise floor. The FM receiver bandwidth is 17 MHz, using
the US standard.
ment is synthesized, but if yours is not, use
a frequency counter to monitor the frequency for warm up drift. Check with local
frequency coordinators before transmitting
because the band plan permits other modes
in that segment.
Experimentally, using the US standard,
FMATV gives increasingly better picture-tonoise ratios than AMATV at receiver input
signals greater than 5 μV. Because of the
wider noise bandwidth and FM threshold effect, AM video can be seen in the noise well
before FM. For DX work, it has been shown
that AM signals are recognizable signals in
the snow at four times (12 dB) greater distance than FM signals, with all other factors
equal. Above the FM threshold, however, FM
rapidly overtakes AM. Snow-free pictures
occur above 50 μV, or four times farther
away than with AM signals. The crossover
point is near the signal level where sound and
color begin to appear for both systems.
Fig 9.53 compares AM and FMATV across a
wide range of signal strengths.
There are a variety of methods to receive
FMATV. Older satellite receivers have a 70
or 45-MHz input and require a down converter with 40 to 50 dB gain ahead of them.
Also satellite receivers are made for wider
deviation and need some video gain to give
the standard 1 volt peak-to-peak video output when receiving a signal with standard
4-MHz deviation. Current satellite receivers
directly tune anywhere from 900 to
2150 MHz and they only need a preamp
added at the antenna for use on the 33 and
23-cm ham bands. The additional video gain
can often be had by adjusting an internal pot
or changing the gain with a resistor.
Some of the inexpensive Part 15 licensefree wireless video receivers in the 33 cm
band use 4-MHz deviation FM video, and
most of the 2.4-GHz ones are FM, which can
be used directly. However, they may or may
not have the standard de-emphasis video
network, which then may have to be added.
On 2.4 GHz, some of the Part 15 frequen-
Fig 9.54—N8QPJ mounted an ATV setup
aboard this model Humvee.
cies are outside the band and care should be
taken to use only those inside the 2390 to
2450 MHz ham band if modified. Wavecom
Jr has been the most popular 2.4 GHz license-free video transmitter and receiver
(available from ATV Research). These have
been modified for higher power and other
features, as well as having all four of the
channels in the ham band using interface
boards from PC Electronics.
Gunnplexers on 10.4 GHz make inexpensive point to point ATV links for publicservice applications or between repeaters. A
10-mW Gunnplexer with 17-dB horn can
cover over 2 miles line-of-sight when received on a G8OZP low noise 3-cm LNB
and satellite receiver. An application note
for construction of the 3-cm transmitter
comes with the GVM-1 Gunnplexer video
modulator board from PC Electronics.
For short distance ATV from R/C vehicles, low-power FM ATV modules with
50 to 100-mW output in the 33, 23 or 13-cm
bands are often used. These offer less
desense possibility to the R/C receiver. An
example can be seen on the model Humvee
in Fig 9.54.
DIGITAL AMATEUR TELEVISION
(DATV)
German amateurs have lead the way in
digital ATV. For the past few years, Uwe
Kraus, DJ8DW, and others have had a stand
at HamRadio—the large European Amateur
Radio gathering in Friedrichshafen, Germany. The motivation for DATV is about
the same as for commercial digital television, particularly high quality pictures even
with weak signals and a distinctively smaller
bandwidth than that occupied by analog TV.
A breakthrough occurred in September
1998 when the DATV team transmitted digital pictures over a 62-mile path with a 2 MHz
bandwidth at 434 MHz using MPEG-1 encoding.
See: www.von-info-ch/hb9afo/histoire/
news043.htm and www.von-info.ch/
hb9afo/datv_e.htm.
Further ATV Reading
Amateur Television Quarterly Magazine.
CQ-TV, British ATV Club, a quarterly
publication available through Amateur
Television Quarterly Magazine.
Kramer Klaus, DL4KCK, "AGAF e.V.
DATV-Boards-Instructions for starting
up," Amateur Television Quarterly
Magazine, spring 2005.
Ruh, “ATV Secrets for the Aspiring
ATVer,” Vol 1, 1991 and Vol 2, 1992.
Available through Amateur Television
Quarterly Magazine.
Seiler, Thomas, HB9JNX/AE4WA, et al,
“Digital Amateur TeleVision (D-ATV),
proc. ARRL/TAPR Digital Communications Conference, www.baycom.org/
~tom/ham/dcc2001/datv.pdf
Taggart, “An Introduction to Amateur
Television,” April, May and June 1993
QST.
Taggart, R., WB8DQT, Image Communications Handbook, Published by ARRL,
Newington, CT, 2002. ARRL Order No.
8616. See also www.arrl.org/catalog.
Spread Spectrum
Contributors to this section were André
Kesteloot, N4ICK, John Champa, K8OCL,
and Kris Mraz, N5KM. The ARRL Spread
Spectrum Sourcebook contains a more complete treatment of the subject. The following
information takes the subject from early
experiments by the Amateur Radio Research
and Development Corporation (AMRAD) to
contemporary Amateur Radio use of spread
spectrum technology for high-speed multimedia (HSMM) applications.
Spread spectrum originated in the 1930s,
shrouded in secrecy. In 1942, Hollywood
movie actress Hedy Lamarr and composer
George Antheil were granted a patent for
spread spectrum. Despite the fact that John
Costas, W2CRR, published a paper on nonmilitary applications of spread spectrum
communications in 1959, spread spectrum
was used almost solely for military purposes
until the late 1970s. In 1981, the FCC
granted AMRAD a Special Temporary Authorization to conduct Amateur Radio
spread spectrum experiments. In June 1986,
the FCC authorized all US amateurs to use
spread spectrum above 420 MHz. These
FCC grants were intended to encourage the
development of spread spectrum, which was
an important element in commercial wireless systems that emerged in the 1990s.
WHY SPREAD SPECTRUM
Faced with increasing noise and interference levels on most RF bands, traditional
wisdom still holds that the narrower the RF
bandwidth, the better the chances that “the
signal will get through.” This is not so.
In 1948, Claude Shannon published his
famous paper, “A Mathematical Theory of
Modes and Modulation Sources
9.43
Communication” in the Bell System Technical Journal, followed by “Communications
in the Presence of Noise” in the Proceedings
of the IRE for January 1949. A theorem that
follows Shannon’s, known as the ShannonHartley theorem, states that the channel capacity C of a band-limited Gaussian channel
is:
S·
§
C = Wlog2 ¨1 ¸ bits / s
(Eq 1)
N¹
©
where
W is the bandwidth,
S is the signal power and
N is the noise within the channel bandwidth.
This theorem states that should the channel be perfectly noiseless, the capacity of
the channel is infinite. It should be noted,
however, that making the bandwidth W of
the channel infinitely large does not make
the capacity infinite, because the channel
noise increases proportional to the channel
bandwidth.
Within reason, however, you can trade
power for bandwidth. In addition, the power
density at any point of the occupied bandwidth can be very small, to the point that it
may be well below the noise floor of the
receiver. The US Navy Global Positioning
System (GPS) is an excellent example of the
use of what is called direct-sequence spread
spectrum. The average signal at the GPS
receiver’s antenna terminals is approximately –160 dBW (for the C/A code). Since
most sources of interference are relatively
narrowband, spread-spectrum users will
also benefit, as narrowband interfering signals are rejected automatically during the
despreading process, as will be explained
later in this section.
These benefits are obtained at the cost of
fairly intricate circuitry: The transmitter
must spread its signal over a wide bandwidth
in accordance with a certain prearranged
code, while the receiver must somehow synchronize on this code and recombine the
received energy to produce a usable signal.
To generate the code, use is made of pseudonoise (PN) generators. The PN generators
are selected for their correlation properties.
This means that when two similar PN sequences are compared out of phase their
correlation is nil (that is, the output is 0), but
when they are exactly in phase their correlation produces a huge peak that can be used
for synchronization purposes.
This synchronization process has been
(and still is) the major complicating factor in
Fig 9.55—Power vs frequency for frequencyhopping spread spectrum signals.
Emissions jump around to discrete
frequencies in pseudo-random fashion.
Fig 9.56—Power vs frequency for a directsequence-modulated spread spectrum
signal. The envelope assumes the shape
of a (sin x/x)2 curve. With proper
modulating techniques, the carrier is
suppressed.
any spread spectrum link, for how can one
synchronize on a signal that can be well
below the receiver’s noise floor? Because of
the cost associated with the complicated
synchronization processes, spread spectrum
applications were essentially military-related until the late 1970s. The development
of ICs then allowed for the replacement of
racks and racks of tube equipment by a few
plug-in PC boards, although the complexity
level itself did not improve. Amateur Radio
operators could not afford such levels of
complexity and had to find simpler solutions, at the cost of robustness in the presence of interference.
Spread-Spectrum Transmissions
A transmission can be called “spread
spectrum” if the RF bandwidth used is (1)
much larger than that needed for traditional
modulation schemes and (2) independent of
the modulation content. Although numerous spread spectrum schemes are in existence, amateurs can use any of them as long
as the modulation scheme has been published, for example on the ARRL website.
By far, frequency-hopping (FH) and directsequence spread spectrum (DSSS) are the
most popular forms within the Amateur
Radio community.
To understand FH, let us assume a transmitter is able to transmit on any one of 100
discrete frequencies F1 through F100. We
now force this equipment to transmit for
1 second on each of the frequencies, but in
an apparently random pattern (for example,
F1, F62, F33, F47…) See Fig 9.55. Should
some signal interfere with the receiver site
on three of those discrete frequencies, the
system will still have achieved reliable
transmission 97% of the time. Because of
the built-in redundancy in human speech, as
well as the availability of error-correcting
Fig 9.57—A block diagram of the practical spread spectrum link. The success of this arrangement lies in the use of a synchronized
oscillator (right) to recover the transmitter clock signal at the receiving site.
9.44
Chapter 9
Fig 9.58—(A) The envelope of the unfiltered biphase-modulated spread spectrum signals as viewed on a spectrum analyzer. In this
practical system, band-pass filtering is used to confine the spread spectrum signal to the amateur band. (B) At the receiver end of the
line, the filtered spread spectrum signal is apparent only as a 10-dB hump in the noise floor. (C) The despread signal at the output of
the receiver DBM. The original carrier—and any modulation components that accompany it—has been recovered. The peak carrier is
about 45 dB above the noise floor—more than 30 dB above the hump shown at B. (These spectrograms were made at a sweep rate of
0.1 s/division and an analyzer bandwidth of 30 kHz; the horizontal scale is 1 MHz/division.)
codes in data transmissions, this approach is
particularly attractive for systems that must
operate in heavy interference.
In a DSSS transmitter, an RF carrier and
a pseudo-random pulse train are mixed in a
doubly balanced mixer (DBM). In the
process, the RF carrier disappears and is
replaced by a noise-like wideband transmission, as shown in Fig 9.56. At the receiver, a similar pseudo-random signal is
reintroduced and the spread spectrum signal is correlated, or despread, while
narrowband interference is spread simultaneously by the same process.
The technical complexity mentioned
above is offset by several important advantages for military and space applications:
• Interference rejection. If the interference
is not synchronized with the original
spread spectrum signal, it will not appear
after despreading at the receiver.
• Security. The length and sophistication of
the pseudo-random codes used can be
such as to make unauthorized recovery
difficult, if not impossible.
• Power density. Low power density makes
for easy hiding of the RF signal and a
resulting lower probability of detection.
So far as the Amateur Radio community
is concerned, particular benefit will be derived from the interference rejection just
mentioned, since it offers both robustness
and reliability of transmissions, as well as a
low probability of interference to other users. Additionally, spread spectrum has the
potential to allow better utilization of the RF
spectrum allocated to amateurs. There is a
limit as to how many conventional signals
can be placed in a given band before serious
transmission degradation takes place. Additional spread spectrum signals will not cause
severe interference, but may instead only
raise the background noise level. This becomes particularly important in bands
shared with other users and in our VHF and
UHF bands increasingly targeted by wouldbe commercial users. The utilization of a
channel by many transmitters is essentially
the concept behind CDMA (Code Division
Multiple Access), a system in which several
DSSS transmissions can share the same RF
bandwidth, provided they utilize orthogonal pseudo-random sequences.
Amateur Radio Spread Spectrum
Experimentation sponsored by AMRAD
began in 1981 led to the design and construction of a practical DSSS UHF link. This
project was described in May 1989 QST and
was reprinted in The ARRL Spread Spectrum
Sourcebook. In it, André Kesteloot, N4ICK,
offered a simple solution to the problem of
synchronization. The block diagram is
shown in Fig 9.57, and Fig 9.58 shows the
RF signals at the transmitter output, at the
receiver antenna terminals and the recovered signal after correlation. James Vincent,
G1PVZ, replaced the original FM scheme
with a continuously variable delta modulation system, or CVSD. In 1989 in a paper
titled License-Free Spread Spectrum Packet
Radio, Al Broscius, N3FCT, suggested the
use of Part 15 spread spectrum wireless local area network (WLAN) devices that were
becoming available be put to use in amateur
radio.
In 1997 TAPR started the development of
a 1-W, 128-kbit/s, FHSS radio for the amateur radio 902 MHz band. In late 1999 the
FCC considerably relaxed the Amateur
Radio service rules regarding the use of
spread spectrum. These changes allowed
amateurs to use commercial off-the-shelf
(COTS) Part 15 spread spectrum devices
used under § 97.311 of the FCC rules.
Emergence of Commercial Part 15
Equipment
Just as military surplus radio equipment
fueled Amateur Radio in the 1950s, and
commercial FM radios and repeaters snowballed the popularity of VHF/UHF amateur
repeaters in the 1960s and 1970s, the availability of commercial wireless LAN
(WLAN) equipment is driving the direction
and popularity of Amateur Radio use of
spread spectrum in the 2000s. FCC Part 15
documents the technical rules for commercial spread-spectrum equipment. The Institute of Electrical and Electronics Engineers
(IEEE) has provided the standards under
which manufacturers have developed
equipment for sale commercially. IEEE
802.11 standardized FHSS and DSSS for
the 2.4 GHz band at data rates of 1 and
2 Mbit/s. Next came the release of 802.11b,
which provided the additional data rates of
5.5 and 11 Mbit/s but only for DSSS. FHSS
was not carried forward. This was followed
by 802.11g, which does not use SS but uses
OFDM for data rates of 6, 9, 12, 18, 24, 36,
48 and 54 Mbit/s as well as backward compatibility with 802.11b. As of this writing
the most recent release of the standard is
802.11a. This release addresses the use of
OFDM in certain parts of the 5 GHz band. It
provides the same data rates as 802.11g. The
currently unreleased 802.11n standard
promises data rates in excess of 108 Mbit/s.
Frequency Hopping Spread
Spectrum
FHSS radios, as specified in 802.11, hop
among 75 of 79 possible non-overlapping
frequencies in the 2.4 GHz band. Each hop
occurs approximately every 400 ms with a
hop time of 224 μs. Since these are Part 15
devices, the radios are limited to a maximum peak output power of 1 W and a maxiModes and Modulation Sources
9.45
mum bandwidth of 1 MHz (–20 dB) at any
given hop frequency. The rules allow using
a smaller number of hop frequencies at wider
bandwidths (and lower power: 125 mW) but
most manufacturers have opted not to develop equipment using these options. Consequently, off-the-shelf equipment with this
wider bandwidth capability is not readily
available to the amateur.
The hopping sequences are well defined
by 802.11. There are three sets of 26 such
sequences (known as channels) consisting
of 75 frequencies each. The ordering of the
frequencies is designed as a pseudo-random
sequence hopping at least 6 MHz higher or
lower that the current carrier frequency such
that no two channels are on the same frequency at the same time. Channel assignment can be coordinated among multiple
collocated networks so that there is minimal
interference among radios operating in the
same band.
The FHSS radio can operate at data rates
of 1 and 2 Mbit/s. The binary data stream
modulates the carrier frequency using frequency shift keying. At 1 Mbit/s the carrier
frequency is modulated using 2-Level
Gaussian Frequency Shift Keying (2GFSK)
with a shift of ±100 kHz. The data rate can
be doubled to 2 Mbit/s by using 4GFSK
modulation with shifts of ±75 kHz and
±225 kHz.
chipping stream is used to phase modulate
the carrier via phase shift keying. Differential Binary Phase Shift Keying (DBPSK) is
used to achieve 1 Mbit/s, and Differential
Quadrature Phase Shift Keying (DQPSK)
is used to achieve 2 Mbit/s. Fig 9.58 shows
a typical 1 or 2 Mbit/s DSSS signal having
a major lobe bandwidth of ±11 MHz
(–30 dB). The first minor sidelobe is down
at least 30 dB and the second minor sidelobe is down 50 dB as required by Part 15
rules.
The higher data rates specified in 802.11b
are achieved by using a different pseudorandom code known as a Complimentary
Sequence. Recall the 11-bit Barker code can
encode one data bit. The 8-bit Complimentary Sequence can encode 2 bits of data for
the 5.5-Mbit/s data rate or 6 bits of data for
the 11-Mbit/s data rate. This is known as
Complimentary Code Keying (CCK). Both
of these higher data rates use DQPSK for
carrier modulation. DQPSK can encode two
data bits per transition. Table 9.12 shows
how four bits of the data stream are encoded
to produce a 5.5- Mbit/s data rate and eight
bits are encoded to produce an 11-Mbit/s
data rate. There are 64 different combinations of the 8-bit Complimentary Sequence
that have mathematical properties that allow
easy demodulation and interference rejection. At 5.5 Mbit/s, only four of the combinations are used. At 11 Mbit/s, all 64
combinations are used.
As an example, for an input data rate of
5.5 Mbit/s, four bits of data are sampled at
the rate of 1.375 million samples per second. Two input bits are used to select one of
four eight-bit CCK sequences. These eight
bits are clocked out at a rate of 11 Mbit/s.
The two remaining input bits are used to
select the phase at which the eight bits are
transmitted. Fig 9.59A shows a conceptual
block diagram of a 5.5-Mbit/s CCK transmitter modulator, while Fig 9.59B shows an
11-Mbit/s modulator.
Orthogonal Frequency Division
Modulation
OFDM provides its spreading function by
transmitting the data simultaneously on
multiple carriers. 802.11g and 802.11a
specify 20-MHz wide channels with 52 carriers spaced every 312.5 kHz. Of the 52
carriers, four are non-data pilot carriers that
carry a known bit pattern to simplify demodulation. The remaining 48 carriers are
modulated at 250 thousand transitions per
Direct Sequence Spread Spectrum
DSSS uses a fast digital sequence to accomplish signal spreading. That is, a wellknown pseudo-random digital pattern of
ones and zeros is used to modulate the data
at a very high rate. In the simplest case of
DSSS, defined in 802.11, an 11-bit pattern
known as a Barker sequence (or Barker
code) is used to modulate every bit in the
input data stream. The Barker sequence is
10110111000. Specifically, a “zero” data
bit is modulated with the Barker sequence
resulting in an output sequence of
10110111000. Likewise, a “one” data bit
becomes 01001000111 after modulation
(the inverted Barker code). These output
patterns are known as chipping streams;
each bit of the stream is known as a chip. It
can be seen that a 1 Mbit/s input data stream
becomes an 11 Mbit/s output data stream.
The DSSS radio, like the FHSS radio, can
operate at data rates of 1 and 2 Mbit/s. The
Table 9.12
Bit encoding for 5.5 Mbps and 11
Mbps CCK transmissions
Data Rate,
Mbps
5.5
11
9.46
CCK
encoded bit
2
6
Chapter 9
DQPSK
encoded bits
2
2
Fig 9.59—Conceptual block diagram of a modulator for a CCK Spread Spectrum
transmitter. (A) 5.5 Mbit/s data rate. (B) 11 Mbit/s data rate. See text.
second. Taking all 48 transitions in parallel
is known as a symbol. That is, at any given
instant in time 48 bits of data are being transmitted.
The term orthogonal is derived from the
fact that these carriers are positioned such
that they do not interfere with one another.
The center frequency of one carrier’s signal
falls within the nulls of the signals on either
side of it.
OFDM radios can be used to transmit
data rates of 6, 9, 12, 18, 24, 36, 48 and
54 Mbit/s as specified by both 802.11a and
802.11g. In order to transmit at faster and
faster data rates in the same 20-MHz channel, different modulation techniques are
employed: BPSK, QPSK, 16QAM and
64QAM. In addition, some of the bits transmitted are used for error correction, so the
raw data rates could be reduced by up to half
of what they would be without error correction. For instance, assuming BPSK (one bit
Table 9.13
Summary of the modulation
techniques used by OFDM to achieve
the different data rates.
Data Rate
Mbps
6
9
12
18
24
36
48
54
Modulation
BPSK
BPSK
QPSK
QPSK
16QAM
16QAM
64QAM
64QAM
Coding Rate,
R
1/2
3/4
1/2
3/4
1/2
3/4
2/3
3/4
per carrier) and assuming half the bits are
used for error correction (known as the coding rate, R); the resulting data rate would be
6 Mbit/s.
48 carriers × 1 bit per carrier × 1/2 R =
24 bits (effective)
24 bits × 250 kilo transitions per second =
6 Mbit/s.
Table 9.13 shows a complete list of the
modulation methods and coding rates employed by OFDM. The higher data rates will
require better signal strength to maintain
error free reception due to using few error
correction bits and more complex modulation methods.
Spread Spectrum References
Dixon, Spread Spectrum Systems, second
edition, 1984, Wiley Interscience, New
York.
Dixon, Spread Spectrum Techniques,
1976, IEEE Press, New York.
Kesteloot, Ed., The ARRL Spread Spectrum Sourcebook (Newington, CT:
ARRL, 1990). Includes Hershey, QST
and QEX material listed separately here.
“Poisson, Shannon and the Radio Amateur,”
Proceedings of the IRE, Dec 1959.
Multimedia Systems
In January 2001, the ARRL Board of
Directors voted unanimously that the
ARRL should proceed with the development of High Speed Digital Networks for
the Amateur Service. The ARRL President
appointed a group of individuals knowledgeable in the field from the international Amateur Radio community and
industry. The group would report to the
Technology Task Force (TTF). The TTF
established the High Speed Multimedia
(HSMM) Working Group, with John
Champa, K8OCL, as its chairman.
Champa identified two initial goals for the
working group, so as to immediately begin the development of such high speed
digital amateur radio networks:
1. Encourage the amateur adoption and
modification of commercial off-the-shelf
(COTS) IEEE 802.11 spread spectrum
hardware and software for Part 97 uses.
2. Encourage or develop other high-speed
digital radio networking techniques,
hardware, and applications.
These efforts were rapidly dubbed HSMM
Radio. Although initially dependent on
adaptation of COTS 802.11 gear to Part 97,
it is obvious from these goals that HSMM
radio is not a specific operating mode, but
more of a direction or driving force within
amateur radio.
Furthermore, in HSMM radio, the emphasis has shifted away from primarily keyboard
radio communication, as in conventional
packet radio, to multimedia radio. This includes simultaneous voice, video, data and
text over radio.
In HSMM radio these individual medi-
ums have different names, much like their
Internet counterparts. For example, voice
modes, although technically digital voice,
are most often called streaming audio. However, since it is two-way voice over an IP
network similar to the direction being taken
by contemporary commercial telephony
technology, the same technology use to link
many amateur radio repeaters over the
Internet, the name voice-over-IP (VoIP) may
be more appropriate.
Video modes, although sometimes called
amateur digital video (ADV), are also known
as streaming video. Again, perhaps the commercial term for such two-way video QSOs
may be more appropriate: IPVC (IP
videoconferencing).
Text exchanges via a keyboard are often
used in HSMM radio, but they are similarly
called by their Internet or Packet Radio
name: Chat mode. File transfers using FTP
can also be done, just as on the Internet.
This combination of Internet terminology,
coupled with this dramatic shift in emphasis within amateur radio from traditional
analog point-to-point radio toward networked digital radios, has resulted in many
amateurs nick naming HSMM radio The
Hinternet. Although the name implies some
under-dog status to some, the name seems
to be sticking.
HSMM RADIO APPLICATIONS
HSMM radio has some unique ham radio
networking applications and operational
practices that differentiate the Hinternet
from normal Wi-Fi hotspots at coffee
houses and airports, which you may have
read about in the popular press. HSMM
radio techniques are used, for example, for
system RC (remote control) of amateur radio stations.
In this day of environmentally sensitive
neighborhoods, one of the greatest challenges, particularly in high density residential areas, is constructing ham radio
antennas, particularly high, tower-mounted
HF beam antennas. In addition, such amateur installations represent a significant investment in time and resources. This burden
could be easily shared among a small group
of friendly hams, a radio club or a repeater
group.
Implementing a link to a remote HF station via HSMM radio is easy to do. Most
computers now come with built-in multimedia support. Most amateur radio transceivers are capable of PC control. Adding the
radio networking is relatively simple. Most
HSMM radio links use small 2.4-GHz antennas mounted outdoors or pointed
through a window. These UHF antennas
are relatively small and inconspicuous when
compared to a full-size 3-element HF Yagi
on a tall steel tower.
For example, Darwin Thompson,
K6USW, has performed remote control of a
Kenwood TS-480SAT/HX transceiver,
which can be controlled over a LAN and the
Internet, or in this case the Hinternet. The
Kenwood International website provides
two programs for the TS-480SAT/HX at:
www.kenwood.net/indexKenwood.
cfm?do=SupportFileCategory&File
CatID=3.
The ARHP-10 program is the radio host
Modes and Modulation Sources
9.47
program. It operates the computer attached to
the transceiver. Just follow the instructions
included with the software to make the cables
to interface the radio to your computer. The
ARCP-480 program is the radio control software. ARCP-480 operates the computer at the
other end of the remote control link. By attaching a suitable headset to this remote PC,
the operator now has full control of the transceiver via the HSMM radio link and can use
voice-over-Internet-protocol (VoIP) to transmit and receive audio.
A ham does not have to have an antennaunfriendly homeowners association (HOA)
or a specific deed restriction problem to put
RC via HSMM radio to good use. This system RC concept could be extended to other
types of amateur radio stations. For example,
it could be used to link a ham’s home to a
shared, high-performance amateur radio DX
station, EME station or OSCAR satellite
ground station for a special event, or on a
regular basis.
SHARED HIGH-SPEED INTERNET
ACCESS
Sharing high-speed Internet access
(Cable, DSL, etc) with another ham is a
popular application for HSMM radio. Half
of the US population is restricted to slow
dial-up Internet connections (usually around
20 to 40 kbit/s) over regular analog telephone lines. Getting a high-speed Internet
connection, even a shared one, can dramatically change the surfing experience! Just
remember that if you use an HSMM radio to
share high speed access to the Internet,
which Amateur Radio has content restrictions, for example no commercial for-profit
business e-mails, etc. An example might be
an amateur television station (ATV) transmitting an outdoor scene and inadvertently
picking-up a billboard in the station camera.
Such background sources are merely incidental to your transmission. They are not
the primary purpose of your communications, plus they are not intended for rebroad-
cast to the public.
Just as on the Internet, it is possible to do
such things as playing interactive games,
complete with sound effects and full-motion
animation with HSMM radio. This can be
lots of fun for new and old hams alike, plus
it can attract others in the “Internet Generation” to get interested in amateur radio and
perhaps become new radio club members.
In the commercial world these activities are
called “WLAN Parties.” Such e-games are
also an excellent method for testing the true
speed of your station’s Hinternet link.
HSMM RADIO IN EMERGENCY
COMMUNICATIONS
There are a number of significant reasons
and exciting new examples why HSMM
radio is the way of the future for many Emergency Communications (EmComm) situations. These may or may not be under ARES
or RACES auspices.
1. The amount of digital radio traffic on
SS and HSMM Glossary
Ad Hoc Mode—An operating mode of a client RIC that
allows it to associate directly with any other RIC without
having to go through an Access Point. See Infrastructure
mode.
AP—Access Point
APRS—Automatic Position Reporting System
Association—The service used to establish access point/
station mapping and enable station use of the WLANs
services in infrastructure mode.
Authentication—Process by which the wireless communications system verifies the identity of a user attempting
to use a WLAN prior to the user associating with the AP.
Band-limited Gaussian Channel—A “brickwall” linear filter
that is equal to a constant over some frequency band
and equal to zero elsewhere, and by white Gaussian
noise with a constant power spectrum over the channel
bandwidth.
Barker Code—An 11-bit digital sequence used to modulate
(spread) the input data stream. A one bit is represented
by the sequence 10110111000 and a zero bit is represented by the sequence 01001000111.
CCK—Complimentary Code Keying. A spreading technique
in which the input data stream is modulated with a digital
sequence (the complimentary code) depending on the
value of the data stream. In 802.11b, for example, the
complimentary code consists of 64 eight-bit values. Six
data bits from the input stream are used to select which
of the complimentary codes is used to modulate the data.
See Barker Code.
Correlation—A measure of how closely a signal matches a
delayed version of itself shifted n units in time.
COTS—Commercial Off The Shelf equipment.
DBPSK—Differential Binary Phase Shift Keying. A method
of modulating data onto a carrier by changing the phase
of the carrier relative to its current phase. A binary “1” is
represented by a +90 degree phase shift and a binary “0”
is represented by a 0 degree phase shift.
DHCP—Dynamic Host Configuration Protocol. A protocol
used by a client computer to obtain an IP address for use
on a network.
DSSS—Direct Sequence Spread Spectrum. A spread
spectrum system in which the carrier has been modulated by a high speed spreading code and an information
data stream. The high speed code sequence dominates
the “modulating function” and is the direct cause of the
wide spreading of the transmitted signal. (Title 47,
9.48
Chapter 9
Chapter I, Part 2, subpart A, section 2.1 Terms and
Definitions).
DQPSK—Differential Quadrature Phase Shift Keying. A
method of modulating data onto a carrier by changing the
phase of the carrier similar to DBPSK except that two bits
can be represented by a single phase shift such as
following this scheme:
2-Bit Value
Phase Shift (degrees)
00
0
01
+90
10
–90
11
180
FHSS—Frequency Hopping Spread Spectrum. A spread
spectrum system in which the carrier is modulated with
the coded information in a conventional manner causing a
conventional spreading of the RF energy about the
frequency carrier. The frequency of the carrier is not fixed
but changes at fixed intervals under the direction of a
coded sequence. The wide RF bandwidth needed by such
a system is not required by spreading of the RF energy
about the carrier but rather to accommodate the range of
frequencies to which the carrier frequency can hop. The
test of a frequency hopping system is that the near term
distribution of hops appears random, the long term
distribution appears evenly distributed over the hop set,
and sequential hops are randomly distributed in both
direction and magnitude of change in the hop set. (Title
47, Chapter I, Part 2, subpart A, section 2.1 Terms and
Definitions).
GPS—Global Positioning System
IEEE—Institute of Electrical and Electronic Engineering
IEEE 802.11—An IEEE standard specifying FHSS and DSSS
in the 2.4 GHz band at 1 Mbit/s and 2 Mbit/s data rates.
802.11 is also used as a general term for all spread
spectrum devices operating under Part 15. For example
“The 802.11 network” could be referring to a collection of
RICs and APs using 802.11b and 802.11g based devices.
IEEE 802.11a—An IEEE standard specifying OFDM in the
5.8 GHz band at 6, 12, 16, 24, 36, 48, and 54 Mbit/s data
rates.
IEEE 802.11b—An IEEE standard specifying DSSS in the
2.4 GHz band at 5.5 and 11 Mbit/s data rates in addition
to being backward compatible with DSSS at 1 and 2 Mbit/
s specified in 802.11.
IEEE 802.11g—An IEEE standard specifying OFDM in the
2.4 GHz band 6, 12, 16, 24, 36, 48, and 54 Mbit/s data
2.4 GHz is increasing and operating under low powered, unlicensed Part 15 limitations cannot overcome this noise.
2. EmComm organizations increasingly
need high-speed radio networks that can
simultaneously handle voice, video, data
and text traffic.
3. The cost of a commercially installed
high-speed data network can be more
than emergency organizations and communities can collectively afford.
4. EmComm managers also know that they
need to continuously exercise any emergency communications system and have
trained operators for the system in order
for it to be dependable.
Being able to send live digital video images of what is taking place at a disaster site
to everybody on the HSMM radio network
can be invaluable in estimating the severity
of the situation, planning appropriate responding resources and other reactions. The
Emergency Operations Center (EOC) can
actually see what is happening while it is
happening. Submitting a written report
while simultaneously talking to the EOC
using Voice over IP (VoIP) would provide
additional details.
With HSMM radio, often all that is needed
to accomplish such immediacy in the field is
a laptop computer equipped with a wireless
local area network card (PCMCIA) with an
external antenna jack. In HSMM radio jargon such a card is simply called a RIC (radio
interface card). Connect any digital camera
with a video output port or any webcam, and
a headset to the laptop’s sound card. Then
connect the RIC to a short Yagi antenna
(typically 18 inches of antenna boom
length) and point the antenna back to the
EOC.
HSMM RADIO RELAY
There are a number of ways to extend the
HSMM link. The most obvious means would
rates in addition to being backward compatible with DSSS
at 1, 2, 5.5, and 11 Mbit/s specified in 802.11b.
IEEE 802.11n—An IEEE standard specifying data rates up to
250 Mbit/s and being backward compatible with 802.11a
and 802.11g.
IEEE 802.16—An IEEE standard specifying wireless last-mile
broadband access in the Metropolitan Area Network
(MAN). Also known as WiMAX.
ISM—Industrial, Scientific, and Medical. Specific frequency
bands authorized by Part 18 rules for non-communication
equipment such as microwave ovens, RF lighting, etc. The
ISM spectrum where spread spectrum is allowed is located
at 2.4 – 2.5 GHz and 5.725 – 5.875 GHz band.
Infrastructure Mode—An operating mode of a client RIC
that requires all communications to go through an Access
Point.
NMEA 0183—National Marine Electronics Association
interface standard which defines electrical signal requirements, data transmission protocol and time, and specific
sentence formats for a 4800-baud serial data bus.
OFDM—Orthogonal Frequency Division Multiplexing. A
modulation method in which the communication channel is
divided into multiple subcarriers each being individually
modulated. While not meeting the Part 2 definition of
spread spectrum the FCC has given specific authorization
for OFDM systems.
Orthogonal—A mathematical term derived from the Greek
word orthos, which means straight, right, or true. In terms
of RF, orthogonal applies to the frequencies of the
subcarriers which are selected so that at each one of
these subcarrier frequencies, all the other subcarriers do
not contribute to the overall waveform. In other words, the
subcarrier channel is independent of the other channels.
PCMIA—Personal Computer Manufacturer Interface
Adaptor.
Pigtail—A short piece of coaxial cable with a appropriate
connectors to match the RIC antenna port and an external
antenna system.
QAM—Quadrature Amplitude Modulation. A method of
modulating data onto a carrier by changing both the phase
and amplitude of the carrier. In its simplest form, 2QAM,
the modulation is identical to BPSK. 16QAM represents 4
bits by changing among 16 phase/amplitude states.
64QAM represents six bits by changing among 64 phase/
amplitude states.
be to run higher power and to place the
antennas as high as possible, as is the case
with VHF/UHF FM repeaters. In some
densely populated urban areas of the country this approach with 802.11, at least in the
2.4 GHz band, may cause some interference
with other users. Other means of getting
greater distances using 802.11 on 2.4 GHz
or other amateur bands should be considered. One approach is to use highly directive, high-gain antennas, or what is called
the directive link approach.
Another approach used by some HSMM
radio networks is what is called a low-profile radio network design. They depend on
several low power sources and radio relays
of various types. For example, two HSMM
radio repeaters (known commercially as access points, or APs, about $100 devices) may
be placed back-to-back in what is known as
bridge mode. In this configuration they will
simply act as an automatic radio relay for the
high-speed data. Using a series of such radio
RIC—Radio Interface Card. The radio equivalent of a
Network Interface Card (NIC).
RLAN—Radio Local Area Network. See also WLAN.
RMAN—Radio Metropolitan Area Network
Spread Spectrum—An information bearing communications system in which: (1) Information is conveyed by
modulation of a carrier by some conventional means, (2)
the bandwidth is deliberately widened by means of a
spreading function over that which would be needed to
transmit the information alone. (Title 47, Chapter I, Part
2, subpart A, section 2.1 Terms and Definitions).
SSID—Service Set Identifier. A unique alphanumeric string
used to identify a WLAN, or in the case of HSMM, RLAN,
by using the individual call sign and perhaps the name of
the amateur radio club or repeater group.
UNII—Unlicensed National Information Infrastructure. The
UNII spectrum is located at 5.15 - 5.35 GHz, 5.725 5.825 GHz, and the recently added 5.470-5.725 GHz
band.
USB—Universal Serial Bus.
VPN—Virtual Private Network.
WEP—Wired Equivalent Privacy. An encryption algorithm
used by the authentication process for authenticating
users and for encrypting data payloads over a WLAN.
WEP Key—An alphanumeric character string used to
identify an authenticating station and used as part of the
data encryption algorithm.
Wi-Fi—Wireless Fidelity. Refers to products certified as
compatible by the Wi-Fi Alliance. See www.wi-fi.org.
This term is also applied in a generic sense to mean any
802.11 capability.
WiMAX—Familiar name for the IEEE 802.16 standard.
WISP—Wireless Internet Service Provider
WLAN—Wireless Local Area Network.
Modes and Modulation Sources
9.49
relays on a series of amateur towers between
the end-points of the link, it is possible to
cover greater distances with relatively low
power and yet still move lots of multimedia
data.
BASIC HSMM RADIO STATION
How do you set up an HSMM radio base
station? It is really very easy. HSMM radio
amateurs can go to any electronics outlet or
office supply store and buy commercial offthe shelf (COTS) Wireless LAN gear, either
IEEE 802.11b or IEEE 802.11g. They then
connect external outdoor antennas. That is
all there is to it.
There are some purchasing guidelines to
follow. First, decide what interfaces you are
going to need to connect to your computer.
Equipment is available for all standard
computer interfaces: Ethernet, USB and
PCMCIA. If you use a laptop in your station,
get the PCMCIA card. Make certain it is the
type with an external antenna connection. If
you have a PC, get the Wireless LAN adaptor type that plugs into either the USB port or
the RJ45 Ethernet port. Make certain it is the
type that has a removable rubber duck antenna or external antenna port! The included
directions will explain how to install these
devices.
The core of any HSMM radio station is a
computer-operated HSMM 2.4-GHz radio
transceiver, and it will probably cost about
$60 to $80. Start off teaming up with a
nearby ham radio operator. Do your initial
testing in the same room together. Then as
you increase distances going toward your
separate station locations, you can coordinate using a suitable local FM simplex frequency. Frequently hams will use
146.52 MHz or 446.00 MHz, the National
FM Simplex Calling Frequencies for the 2-m
and 70-cm bands, for voice coordination.
More recently, HSMM radio operators have
tended to use 1.2-GHz FM transceivers and
handheld (HT) radios. The 1.2-GHz amateur band more closely mimics the propagation characteristics of the 2.4-GHz amateur
band. The rule of thumb is that if you cannot
hear the other station on the 1.2-GHz FM
radio, you probably will not be able to link
up the HSMM radios either.
Hams frequently ask why 802.11 transmitter output and receiver sensitivity are
stated typically in dBm. The simple answer
is that this convention simplifies certain calculations. For transmitter output, convert
dBm to power using the formula for dB. The
reference power level is 1 mW. That means
that +10 dBm = 10 mW and +20 dBm =
100 mW.
For receive, it’s a bit more complicated if
you want the more familiar units. First, calculate power level and then covert that to
voltage across 50 Ω. A good RIC receiver is
9.50
Chapter 9
Fig 9.60—Back panel of a typical HSMM-style repeater. This device is known
commercially as a wireless access point (AP). It is essentially a computer wireless
network hub to enable multiple radio stations to share the various resources of the
network. This particular model is a Cisco Model 1200. Note that the left, or secondary
antenna’s rubber duck has been removed from the TNC connector to show that the
connector is of the reverse polarity (RP) type. This is designated as a female TNC/RP
connector. Manufacturers of 802.11 gear typically install a RP-type of some type
connector to prevent FCC Part 15 unlicensed users from employing their equipment in a
non-certified manner. Of course, this is not an issue for licensed Part 97 users, however
as in this case, a male RP-type plug will be required in order to connect the device to an
outside antenna. The provision of a secondary antenna is to provide space diversity,
which helps reduce the negative impact of multipath propagation of the radio signals.
The secondary antenna may be ignored when connecting the primary antenna to a
single outside antenna, especially if it is a highly directive antenna, which would help
reduce multipath effects. (Photo: John Champa, K8OCL).
able to receive down to –96 dBm. That
would be equal to 0.00000000025 mW,
which is 3.54 μV across 50 Ω.
HSMM RADIO REPEATERS
Access Points
What hams would call a repeater, and
computer buffs would call a hub, the WiFi
industry refers to as a wireless access point,
or simply AP. This is a device that allows
several amateur radio stations to share the
radio network and all the devices and circuits connected to it.
An 802.11b AP will sell for about $80 and
an 802.11g AP for about $100. The AP acts
as a central collection point for digital radio
traffic, and can be connected to a single
computer or to another radio or wired network.
The AP is provided with an SSID, which
is the station identification it constantly
broadcasts. For ham purposes, the SSID can
be set as your call sign, thus providing automatic, and constant station identification. To
use an AP in a radio network the wireless
computer users have to exit ad-hoc mode
and enter what is called the infrastructure
mode, in their operating software. Infrastructure mode requires that you specify the
radio network your computer station is intended to connect to, so set your computer
station to recognize the SSID you assigned
to the AP (yours or another ham’s AP) to
which you wish to connect.
Point-to-Point Links
The AP can also be used as one end of a
point-to-point radio network. If you want to
extend a radio network connection from one
location to another, for example in order to
remotely operate an HF station, you could
use an AP at the network end and use it to
communicate to a computer at the remote
station location.
An AP allows for more network features
and improved information security than is
provided by ad-hoc mode. Most APs provide DHCP service, which is another way of
saying they will automatically assign an
Internet (IP) address to the wireless computers connected to the radio network. In addition, they can provide filtering, which allows
only known users to access the network.
MOBILE HSMM OPERATING
When hams use the term mobile HSMM
station what they are normally talking
about is a wireless computer set-up in their
vehicle to operate in a stationary portable
fashion. Nobody is suggesting that you try
to drive a vehicle and look at a computer
screen at the same time! That would be
very dangerous. So unless you have somebody else driving the vehicle, keep your
eyes on the road and not on the computer
screen.
What sort of equipment is needed to
operate an HSMM mobile station?
• Some type of portable computer, such
•
•
•
•
•
as a laptop. Some hams use a PDA, notebook or other small computing device.
The operating system can be Microsoft
Windows, Linux, or Mac OS, although
Microsoft XP offers some new and innovative WLAN functionality.
Some type of radio software hams
would call an automatic monitor, and
computer buffs would call a sniffer utility. The most common type being used
by hams is Marius Milner’s Network
Stumbler for Windows frequently just
called, NetStumbler. All operating systems have monitoring programs that are
available. Linux has Kismet; MAC OS
has MacStumbler. Marius Milner has a
version for the PocketPC, which he calls
MiniStumbler.
A RIC (Radio Interface Card = PCMCIA
WiFi computer adapter card with external antenna port), which is supported
by the monitoring utility you are using.
The most widely supported RIC is the
Orinoco line. The Orinoco line is inexpensive and fairly sensitive.
An external antenna attached to your RIC.
This is often a magnetically mounted
omni-directional vertical antenna on the
vehicle roof, but small directional antennas pointed out a window or mounted on
a small ground tri-pod are also frequently
used.
A pigtail or short strain-relief cable will
be needed to connect from the RIC
antenna port to the N-series, RP/TNC or
other type connector on the external
antenna.
A GPS receiver that provides NMEA
0183 formatted data and computer interface cable. This allows the monitoring utility to record where HSMM
stations are located on a map, just as in
APRS. GPS capability is optional, but
just as with APRS capability, it makes
the monitored information much more
useful for locating HSMM stations.
Warning
While operating your HSMM mobile station, if you monitor an unlicensed Part 15
station (non-ham), some types of WiFi
equipment will automatically associate or
link to such stations, if they are not encrypted, and many are not (that is, WEP is
not enabled). Although Part 15 stations share
the 2.4-GHz band on a non-interfering basis
with hams, they are operating in another
service. In another part of this section we
will provide various steps you can take to
prevent Part 15 stations from automatically
linking with HSMM stations. So in like manner, except in the case of a communications
emergency, we recommend that you do not
use a Part 15 station’s Internet connection
for any ham purpose.
Fig 9.61—View of HSMM equipment
(802.11b) inside an antenna-mounted
NEMA-4 box. Mounting the equipment at
the end of the dish antenna’s pigtail
significantly reduces feed line losses and
greatly enhances the performance of the
station. The box contains both a bridge,
and a 500 mW bidirectional amplifier or
BDA (lower left). Amplifier power is
provided by the power insertion module
seen in the upper left corner of the
enclosure. (Photo: John Champa, K8OCL).
HSMM AREA SURVEYS
Both licensed amateurs and unlicensed
(Part 15) stations use the 2.4-GHz band. To
be a good neighbor, find out what others are
doing in your area before designing your
community HSMM radio network. This is
easy to do using IEEE 802.11 modulation.
Unless it has been disabled, an active repeater (AP) is constantly sending out an
identification beacon known as the SSID. In
HSMM practice this is simply the ham station call sign (and perhaps the local radio
club name) entered into the software configuration supplied with the CD that comes
with the repeater. So every HSMM repeater
is also a continuous beacon.
A local area survey using appropriate
monitoring software, for example the free
NetStumbler software downloaded and running on your PC (www.netstumbler.com/
index.php), is recommended prior to starting up any HSMM operations. Slew your
station’s directional antenna through a 360°
arc, or drive your HSMM mobile station
(described earlier) around your local area.
This HSMM area survey will identify and
automatically log most other 802.11 station
activity in your area. There are many different ways to avoid interference with other
users of the band when planning your
HSMM operating. For example, moving
your operating frequency 2-3 channels
away from the other stations is often suffi-
cient. Why several channels and not just
one? Because the channels have considerable overlap. Why this situation exists is
beyond the scope of this section, but here is
the situation: The channels are only 5 MHz
wide, but the DSSS or OFDM modulation of
802.11 is 22 MHz wide. Commercial users
often recommend moving 5 channels away
from the nearest AP to completely avoid
interference. There are six channels within
the amateur 2.4 GHz band, but there are
problems for hams with two of them. Channel 1 centered on 2412 MHz overlaps with
OSCAR satellite downlink frequencies.
Channel 6 centered on 2437 MHz is by far
the most common out-of-the-box default
channel for the majority of WLAN equipment sold in the US, so that often is not the
best choice. Subsequently, most HSMM
radio groups end up using either channel 3
or channel 4, depending on their local situation. Again, an area survey is recommended before putting anything on the air.
However, because of the wide sidebands
used in these inexpensive broad banded
802.11 modulations, even moving 2-3
channels away from such activity may not
be enough to totally avoid interference, especially if you are running what in HSMM is
considered high power (typically 1800 mW
RF output—more on that subject later). You
may have to take other steps. For example,
you may use a different polarization with
your antenna system. Many HSMM stations
use horizontal polarization because much
802.11 activity in their area is primarily
vertically polarized.
HSMM ANTENNA SYSTEMS
There are a number of factors that determine the best antenna design for a specific
HSMM radio application. Most commonly,
HSMM stations use horizontal instead of
vertical polarization.
Furthermore, most HSMM stations use
highly directional antennas instead of omnidirectional antennas. Directional antennas
provide significantly more gain and thus
better signal-to-noise ratios, which in the
case of 802.11 modulations means higher
rate data throughput. Higher data throughput, in turn, translates into more multimedia
radio capability.
Highly directional antennas also have
many other advantages. Such antennas can
allow two hams to shoot over, or shoot
around, or even shoot between, other wireless stations on the band.
However, the nature of 802.11 modulations coupled with the various configurations of many COTS devices allows hams to
economically experiment with many other
fascinating antenna designs. Such unique
antenna system designs can be used to simply help avoid interference, or to extend the
Modes and Modulation Sources
9.51
the path uses a horizontally polarized loop
Yagi. Both antennas have gain, both antennas are broadband width designs, and both
antennas are horizontally polarized. Nonetheless, the hams may experience higher
BER (bit error rate) because of symbol errors
caused by the different manner in which the
two antennas manipulate the digital radio
signal wave front. Further radio amateur
experimentation with HSMM radio signals
is warranted to determine the full impact on
the radio link of using mixed antenna types.
range of HSMM links, or both.
Space Diversity
Some APs and some RICs have spacediversity capability built-into their design.
However, it is not always operated in the
same fashion, so check the literature or the
website of your particular device’s manufacturer to be certain how the dual antenna
ports are used. For example, many APs
come equipped with two rubber ducky antennas and two antenna ports. One antenna
port may be the primary and the other port
the secondary input to the transceiver.
Which signal input is used may depend on
which antenna is providing the best S/N
ratio at that specific instant. Experimentation using two outside high-gain antennas
spaced 10 or more wavelengths apart (that
is only about one meter on the 2.4-GHz
band) may be very worthwhile in improving data throughput on long links. Such
extended radio paths tend to experience
more multipath signal distortion. This
multipath effect is caused by multiple signal
reflections off various objects in the path of
the linking signal. The use of space diversity techniques may help reduce this effect
and thus improve the data rate throughput
on the link. Again, the higher the date rates
the more multimedia radio techniques that
can be used on that network.
Circular Polarization
The use of circular polarization created
using helical antennas, patch feed-points on
dish antennas or other means, warrants further study by radio amateurs. Remember this
is high-speed digital radio. To avoid symbol
errors, circularly polarized antennas should
be used at both ends of the link. Also, be
certain that the antennas are of the same
handedness, for example, right-hand circular polarization (RHCP). The ability of circular polarization to enhance propagation
of long-path HSMM radio signals should not
be overlooked.
Circularly Polarized Space Diversity
A combination or hybrid antenna design
combining both circularly polarized antennas and space diversity could yield some
extraordinary signal propagation results. For
example, it has been suggested that perhaps
using a RHCP for one antenna and LHCP for
the other antenna, especially using spacing
greater than 10 wavelengths, in such a system could provide a nearly “bullet-proof”
design. Only actual field testing of such
designs under different terrain features
would reveal such potential.
Mixed Antenna Design Problem
In conventional wide-bandwidth analog
radio antennas systems, so long as both
antennas at both ends of a radio link have
9.52
Chapter 9
Fig 9.62—FM voice repeater, amateur
television (ATV), and HSMM antennas
mounted on a hydraulically operated mast.
This portable installation was used to
provide shared high-speed Internet access
and other special communications support
to the many hams attending the 2003
Pacificon Hamvention in San Ramon, CA.
The HSMM station is also used to provide
streaming video or amateur digital video
(ADV) to the Mount Diablo ARC’s analog
FM ATV repeater on the nearby mountain.
(Photo: John Champa, K8OCL)
broad bandwidths and the same polarization, all is fine. While this may be true for
wide bandwidth analog signals, such as
amateur television VSB (vestigial sideband)
signals or FM ATV signals, it may not be
true for broad bandwidth high-speed digital
signals.
First, 802.11 modulations produce very
broadband signals, typically 22 MHz. Secondly, the evidence to date indicates that the
use of a same polarized antenna with one
type of feed point at one end of the link and
the use of a same polarized antenna with a
different type of feed point at the other end
of the link, may introduce a problem with
high-speed digital signals. A common example of this potential mixed-antenna issue
would be if one HSMM station uses a horizontally polarized linear Yagi, while the
other HSMM station at the opposite end of
the link uses a horizontally polarized loop
Yagi.
Here is another typical situation. Let us
say the ham at one end of the radio path uses
a dish antenna with a horizontal dipole feedpoint. The other ham at the opposite end of
RUNNING HIGHER POWER
Hams often ask why operate 802.11
modes under licensed Part 97 regulations
when we may also operate such modes under
unlicensed Part 15 regulations, and without
the content restrictions imposed on the
Amateur Radio service?
A major advantage of operating under
Amateur Radio regulations is the feasibility
of operating with more RF power output
and larger, high-gain directive antennas.
These added capabilities enable hams to
increase the range of their operations. The
enhanced signal-to-noise ratio provided by
running high power will also allow better
data packet throughput. This enhanced
throughput, in turn, enables more multimedia experimentation and communication
capability over such increased distances.
In addition, increasing the effective radiated power (ERP) of an HSMM radio link
provides for more robust signal margins and
consequently a more reliable link. These are
important considerations in providing effective emergency communications services
and accomplishing other important public
service objectives in a band increasingly
occupied by unlicensed stations and other
noise sources.
It should be noted that the existing FCC
amateur radio regulations covering spread
spectrum (SS) at the time this is being written were implemented prior to 802.11 being
available. The provision in the existing regulations calling for automatic power control
(APC) for RF power outputs in excess of
1 W is not considered technologically feasible in the case of 802.11 modulations for
various reasons. As a result the FCC has
communicated to the ARRL that the APC
provision of the existing SS regulations are
therefore not applicable to 802.11 emissions
under Part 97.
However, using higher than normal output power in HSMM radio, in the shared
2.4 GHz band, is also something that should
be done with considerable care, and only
after careful analysis of link path conditions
and the existing 802.11 activity in your area.
Using the minimum power necessary for the
communications is the law and has always
been a good operating practice for hams.
There are also other excellent and far less
expensive alternatives to running higher
power when using 802.11 modes. For examples, amateurs are also allowed to use
higher-gain directional antennas. Such
antennas increase both the transmit and receive effectiveness of the transceiver. Also,
by placing equipment as close to the station
antenna as possible, a common amateur
OSCAR satellite and VHF/UHF DXing
technique, the feed-line loss is significantly
reduced. This makes the HSMM station
transceiver more sensitive to received signals, while also getting more of its transmitter power to the antenna.
Only after an HSMM radio link analysis
(see the link calculations portion at
www.arrl.org/hsmm/ or go to logidac.com/
gfk/80211link/pathAnalysis.html) clearly
indicates that additional RF output power is
required to achieve the desired path distance
should more power output be considered.
At that point in the analysis showing that
higher power is required, what is needed is
called a bi-directional amplifier (BDA). This
is a super fast switching pre-amplifier/amplifier combination that is usually mounted
at the end of the antenna pig-tail near the top
of the tower or mast. A reasonably priced
2.4-GHz 1800-mW watt output BDA is
available from the FAB Corporation (www.
fab-corp.com). It is specifically designed for
amateur HSMM radio experimenters. Be
certain to specify HSMM when placing your
order. Also, to help prevent unauthorized
use by unlicensed Part 15 stations, the FAB
Corp may request a copy of your amateur
license to accompany the order, and they
will only ship the BDA to your licensee
address as recorded in the FCC database.
This additional power output of
1800 mW should be sufficient for nearly
all amateur operations. Even those supporting EmComm, which may require
more robust signal margins than normally
needed by amateurs, seldom will require
more power output than this level. If still
greater range is needed, there are other
less expensive ways to achieve such
ranges as described in the section HSMM
Radio Relays.
When using a BDA and operating at
higher than normal power levels on the
channels 2 through 5 recommended for
Amateur Radio use. These channels are arbitrary channels intended for Part 15 operation and are not required for Amateur Radio
use, but they are hard-wired into the gear so
we are stuck with them. You should also be
aware of the sidebands produced by 802.11
modulation. These sidebands are in addition
to the normal 22 MHz wide spread spectrum
signal. Accordingly, if your HSMM radio
station is next door to an OSCAR ground
station or other licensed user of the band,
you may need to take extra steps in order to
avoid interfering with them.
The use of a tuned output filter may be
necessary to avoid causing QRM. Even
when operating on the recommended channels in the 2-5 range, whenever you use
higher than normal power, some of your now
amplified sidebands may go outside the
amateur band, which stops at 2450 MHz. So
from a practical point of view, whenever the
use of a BDA is required to achieve a specific link objective, it is a good operating
practice to install a tuned filter on the BDA
output. Such filters are not expensive and
they’re readily available from several commercial sources. It should also be noted that
most BDAs currently being marketed, while
suitable for 802.11b modulation, they are
often not suitable for the newer, higher
speed 802.11g modulation.
There is another point to consider. Depending on what other 802.11 operating
may be taking place in your area, it may be
a good practice to only run higher power
when using directional or sectional antennas. Such antennas allow hams to operate
over and around other licensed stations, but
also including unlicensed Part 15 activity in
your area that you don’t want to disrupt (a
local school WLAN, WISP, etc). Again,
before running high power, it is recommended that an area survey be conducted
using a mobile HSMM rig as described earlier to determine what other 802.11 activity
is in your area and what channels are already in use.
INFORMATION SECURITY
An HSMM radio station could be considered a form of software defined radio. Your
computer running the appropriate software
combined with the RIC makes a single unit,
which is now your station HSMM transceiver. However, unlike other radios, your
HSMM radio is now a networked radio device. It could be connected directly to other
computers and to other radio networks and
even to the Internet. So each HSMM radio
(PC + RIC + software) needs to be protected.
There are at least two basic steps that should
be taken with regards to all HSMM radios:
The PC should be provided with an antivirus program. This anti-virus software must
be regularly updated to remain effective.
Such programs may have come with the PC
when it was purchased. If that is not the case,
reasonably priced anti-virus programs are
readily available from a number of sources.
Secondly, it is important to use a firewall
software program on your HSMM radio. The
firewall should be configured to allow all
outgoing traffic, but to restrict all incoming
traffic without specific authorization. Commercial personal computer firewall products
are available from Symantec, ZoneLabs and
McAfee Network Associates.
Check this URL for a list of freeware
firewalls for your personal computer:
www.webattack.com/freeware/security/
fwfirewall.shtml.
Check this URL for a list of shareware
firewalls for your personal computer:
www.webattack.com/Shareware/security/swfirewall.shtml.
Once a group of HSMM stations has setup and configured a repeater (AP) into a
radio local area network (RLAN) then additional steps may need to be taken to restrict
access to the repeater. Only Part 97 stations
should be allowed to associate with the
HSMM repeater. Remember, in the case of
802.11 modulations, the 2.4-GHz band is
shared with Part 15 unlicensed 802.11 stations. How do you keep these unlicensed
stations from automatically associating
(auto-associate) with your licensed ham
radio HSMM network?
Many times the steps taken to avoid interference with other stations also limits
those other stations’ capability to autoassociate with the HSMM repeater and to
improve the overall security of the HSMM
station. For example, you could use a different antenna polarization than the Part 15
station, or you could operate with a directional antenna oriented toward the desired
coverage area rather than using an omnidirectional antenna.
The most effective method to keep unlicensed Part 15 stations off the HSMM repeater is to simply enable the Wired
Equivalent Protection (WEP) already built
into the 802.11 equipment. The WEP encrypts or scrambles the digital code on the
HSMM repeater based on the instruction or
“key” given to the software. Such encryption makes it impossible for unlicensed stations not using the specific code to
accidentally auto-associate) with the HSMM
repeater.
The primary purpose of this WEP implementation in the specific case of HSMM
operating is to restrict access to the ham
network by requiring all stations to authenticate themselves. Ham stations do this by
using the WEP implementation with the
appropriate ham key. Hams are permitted
by FCC regulations to encrypt their transmission in specific instances; however,
ironically at the time of this writing, this is
not one of them. Accordingly, for hams to
use WEP for authentication and not for encryption, the key used to implement the
WEP must be published. The key must be
published in a manner accessible by most
of the amateur radio community. This fulfills the traditional ham radio role as a selfpolicing service. The current published
ham radio WEP key is available at the home
page of the ARRL Technology Task Force
High Speed Multimedia Working Group:
www.arrl.org/hsmm/.
Before implementing WEP on your
HSMM repeater be certain that you have
Modes and Modulation Sources
9.53
checked the website to ensure that you are
using the current published WEP key. The
key may need to be occasionally changed.
HSMM FREQUENCIES
Up to this point all the discussion has been
regarding HSMM radio operations on the
2.4-GHz amateur band. However, 802.11
modulations can be used on any amateur
band above 902 MHz.
On the 902 MHz band, using 802.11
modulations would occupy nearly the entire
band. This may not be a problem in your
area depending on the nature of the other
existing users of the band in your area, either
licensed or unlicensed. FM repeaters may
not have a problem with sharing the frequency with 802.11 operations, since they
would likely just hear an 802.11 modulated
signal as weak background noise, and the
802.11 modulation, especially the OFDM
channels used by 802.11g, would simply
work around the FM interference with little
negative impact. There is some older 802.11
gear (FHSS) available on the surplus market
for amateur experimentation. Alternatively,
some form of frequency transverter may be
used to take 2.4 GHz to the 902-MHz band.
The 1.2-GHz band has some potential for
802.11 experimenting. Some areas have
several FM voice repeaters and even ATV
FM repeaters on the band. But again these
relatively narrow bandwidth signals would
likely hear any 802.11 modulations as simply background noise. Looking at the potential interference from the HSMM
perspective, even in the case of the FM ATV,
it is unlikely the signal would significantly
disrupt the 802.11 modulation unless the
two signals were on exactly the same center
frequency or at least with complete overlap
in bandwidth. Keep in mind that the FM
ATV signal is only several megahertz wide,
but the 802.11 modulation is 22 MHz wide.
For the analog signal to wipe out the spread
spectrum signal, it would need to overpower
or completely swamp the 802.11 RIC
receiver’s front end.
The 3.5-GHz band offers some real possibilities for 802.11 developments. Frequency transverters are available to get to
the band from 2.4 GHz and there is little
other activity on the band at this time. Developments in Europe of 802.16 with
108 Mbit/s data throughput may make 3.5GHz gear available for amateur experimentation in the US. Hams are investigating the
feasibility of using such gear when it becomes available in the US for providing a
RMAN or radio metropolitan area networks. The RMAN would be used to link the
individual HSMM repeaters (AP) or RLANs
together in order to provide county-wide or
regional HSMM coverage, depending on the
ham radio population density.
The 5-GHz band is also being investi9.54
Chapter 9
gated. The COTS 802.11a modulation gear
has OFDM channels that operate in this
Amateur Radio band. The 802.11a modulation could be used in a ham RLAN operating
much as 802.11g is in the 2.4-GHz band. It
is also being considered by some HSMM
groups as a means of providing MAN links.
This band is also being considered by
AMSAT for what is known as a C-N-C transponder. This would be an HSMM transponder onboard probably a Phase-3
high-altitude or a Phase-4 geostationary
OSCAR with uplink and downlink bandpass
both within the 5-GHz amateur band. Some
other form of modulation other than 802.11
would likely have to be used because of timing issues and other factors, but the concept
is at least being seriously discussed.
RMAN link alternatives are also being
tested by hams. One of these is the use of
virtual private networks (VPN) similar to
the method currently used to provide worldwide FM voice repeater links via the Internet.
Mark Williams, AB8LN, of the HSMM
Working Group is leading a team to test the
use of various VPN technologies for linking
HSMM repeaters.
HF is not being ignored either. It is possible that a modulation form that, while it is
neither SS nor HSMM, might be able to produce data rates fast enough to efficiently
handle e-mail type traffic on the HF bands,
while still occupying an appropriate bandwidth. Such modulation would be helpful in
an emergency with providing an outlet for
RMAN e-mail traffic. Neil Sablatzky, K8IT,
is leading a team of ham investigators on the
HF and VHF bands.
Finally there are commercial products
being developed such as the Icom D-STAR
system that could readily be integrated into
a RMAN infrastructure.
HSMM REFERENCES
Use of HSMM over Amateur Radio is a
developing story. You can keep up with
developments by visiting ARRLWeb at
arrl.org/hsmm/.
For more details about using HSMM radio for remote control of stations, see the
article “Remote-Control HF Operation
over the Internet,” by Brad Wyatt, K6WR,
QST, November 2001 p 47-48.
For guidelines on using e-games on-the
air in Amateur Radio, see the HSMM column titled “Is (sic) All Data Acceptable
Data” by Neil Sablatzky, K8IT, in the Fall
2003 issue of CQ VHF.
For more information regarding HSMM
on future OSCAR satellites, see the Proceedings of the AMSAT-NA 21st Space
Symposium, November 2003, Toronto,
Ontario, Canada, especially the paper by
Clark, Tom, W3IWI, “C-C RIDER, A New
Concept for Amateur Satellites,” available
from ARRL.
Burger, Michael W, AH7R, and John J.
Champa, K8OCL, “HSMM in a Briefcase,” CQ VHF, Fall 2003, p 32.
Champa, John, K8OCL, and Ron Olexa,
KA3JIJ, “How To Get Into HSMM,” CQ
VHF, Fall 2003.
Champa, John, K8OCL, and Stephensen,
John, KD6OZH, “28 kbps to 9 Mbps
UHF Modems for Amateur Radio Stations,” QEX, Mar/Apr 2005.
Cooper, G.R., and McGillem, C. D., Modern Communications and Spread Spectrum, New York, McGraw-Hill, 1986.
Duntemann, Jeff, K7JPD, Jeff Duntemann’s
Wi-Fi Guide, 2nd Ed, Paraglyph Press,
2004.
Flickenger, Rob, Building Wireless Community Networks, 2nd Ed, O’Reilly,
2003.
Flickenger, Rob, Wireless Hacks,
O’Reilly, 2003.
Ford, Steve, WB8IMY, “VoIP and Amateur
Radio,” QST, February 2003, p 44-47.
Ford, Steve, WB8IMY, ARRL’s HF Digital
Handbook, American Radio Relay
League, 2001.
Fordham, David, KD9LA, “802.11 Experiments in Virginia’s Shenandoah
Valley,” QST, July 2005.
Gast, Matthew S., 802.11 Wireless Networks, The Definitive Guide, O’Reilly,
2002.
Geier, Jim, Wireless LANs, Implementing
High Performance IEEE 802.11 Networks, 2nd Ed, SAMS, 2002.
Husain, Kamran, and Parker, Timothy, PhD,
et al, Linux Unleashed, SAMS, 1995.
McDermott, T., Wireless Digital Communications: Design and Theory, TAPR,
1996.
Mraz, Kris I, N5KM, “High Speed Multimedia Radio,” QST, April 2003, pp 28-34.
Olexa, Ron, KA3JIJ, “Wi-Fi for Hams Part
1: Part 97 or Part 15,” CQ, June 2003,
pp 32-36.
Olexa, Ron, KA3JIJ, “Wi-Fi for Hams Part
2: Building a Wi-Fi Network,” CQ, July
2003, p 34-38.
Patil, Basavaraj, et. Al,. IP in Wireless Networks, Prentice Hall, 2003.
Potter, Bruce and Fleck, Bob, 802.11
Security, O’Reilly, 2003.
Reinhardt, Jeff, AA6JR, “Digital Hamming: A Need for Standards,” CQ,
January 2003, p 50-51.
Rinaldo, Paul L., W4RI, and Champa,
John J., K8OCL, “On The Amateur Radio Use of IEEE 802.11b Radio Local
Area Networks,” CQ VHF, Spring 2003,
p 40-42.
Rotolo, Don, N2IRZ, “A Cheap and Easy
High-Speed Data Connection,” CQ,
February 2003, p 61-64.
Torrieri, D.J., Principles of Secure Communication Systems, Boston, Artech
House, 1985.
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