Precise Cable and Antenna Measurements

Precise Cable and Antenna Measurements
Keysight Technologies
Techniques for Precise Cable and
Antenna Measurements in the Field
Using FieldFox handheld analyzers
Application Note
This application note introduces the
practical aspects of cable and
antenna testing, interpreting measurement results and instrument
operation including calibration
options such as CalReady and
QuickCal using a Keysight Technologies, Inc. FieldFox handheld
analyzer configured as a cable and
antenna analyzer. Measurement
examples are provided showing
techniques for measuring insertion
loss, return loss, and locating faults
in a transmission system. Carry
precision with you.
Introduction
Cable and antenna measurements are often required to verify and troubleshoot the electrical
performance of RF and microwave transmission systems and antennas. In RF and microwave
communications industries, measurements are often made along the coaxial cable connecting a
transmitter to its antenna and/or between an antenna and its receiver. This process is sometimes
referred to as Line Sweeping and results in the measurement of the signal attenuation and return loss
as a function of frequency. Line sweeping may also be used to estimate the physical location of a fault
or damage along the transmission line using the Distance-to-Fault (DTF) measurement available on
many RF and microwave signal analyzers such as FieldFox. Testing the performance of transmission
lines is not limited to only coaxial cable types but systems using waveguide and twisted pair cables
may also be characterized once the appropriate adapter is installed between the transmission line
and the coaxial interface on the analyzer. In addition, antenna measurements, in the form of signal
reflection, or namely return loss and VSWR, may be used to verify the performance of an antenna at
the installation site. When multiple antennas are required at an installation site, whether the antennas
are associated with the same system or different systems, the antenna-to-antenna isolation may also
be verified using FieldFox.
Figure 1 shows a typical return loss measurement (upper trace) and DTF measurement (lower trace)
for an X-band antenna connected to a 3-meter length of coaxial cable. The antenna was mounted
on a vertical mast at a roof-top installation and measured using FieldFox configured with a preset
calibration called CalReady. The return loss measurement includes two frequency markers placed
across the specified operating band at 8 GHz and 12.4 GHz. The DTF measurement includes a
distance marker placed at the location of the antenna at 3.23 meters.
This application note will introduce the practical aspects of cable and antenna testing, interpreting
measurement results and instrument operation including calibration options such as CalReady and
QuickCal using FieldFox.
Figure 1. Measured return loss (upper trace) and distance-to-fault (lower trace) for an X-band antenna
and coaxial feed cable
03 | Keysight | Techniques for Precise Cable and Antenna Measurements in the Field Using FieldFox Handheld Analyzers - Application Note
Why Is Cable and Antenna Testing Important?
During the installation, operation and
maintenance of a telecommunication system, the connecting cables,
adapters and antennas may become
damaged or show reduced performance over time. It is known that fifty
to sixty percent of cellular base station
problems are caused by faulty cables,
connectors and antennas. Failure in
these components causes poor coverage and unnecessary handovers in the
cellular system. In any communication
system, it becomes important to have
a process and the appropriate instrumentation to verify the performance of
the cables and antennas when issues
are discovered at the system level.
Cabling and antennas may be installed
in a variety of environments including outdoor and indoor installations.
Outdoor installations may include
roof-top, tower mounted and underground cabling. Indoor installations
may include equipment shelters, office
buildings, shipboard, aircraft and
certain automotive applications. At an
outdoor installation of a wireless cellular system, the antenna and portions of
the coaxial cabling are often exposed
to extreme weather conditions including rain, snow, ice, wind and lightning.
Severe environmental conditions can
potentially create physical damage
to the system components including
failed waterproof sealing at interfaces
between cables and connectors, failed
sealing at cable splices, and weather-induced cracks in the insulation
resulting in an ingress of moisture into
the cables and interfaces. Sheltered
installations can expose components
to mishandling, stress, heat, vibration, and oils and other containments
that could leak into the system. Cable
fault can also occur at any interface
between the cables and connectors
where solder joints and cable crimps
become fatigued and break. It is also
possible that cables can be dented,
crushed or improperly routed during or
after installation. Conditions may also
be created where the minimum bend
radius is exceeded beyond the value
specified by the cable manufacturer.
For example, low-loss coaxial cable
may have a specified minimum bend
radius of 1 inch to over 10 inches and
bending the cable below the specification will cause damage to the cable
and substantially degrade its electrical
performance.
Verifying and maintaining the operation of the RF and microwave transmission system, including coaxial cable,
waveguide and twisted pair line, and
antennas, requires fast and accurate
measurements of the insertion loss
and the return loss as a function of
frequency. Also important to field
operations is the capability to quickly
troubleshoot and locate faults and
damage in cables and connectors. A
modern handheld analyzer, such as
FieldFox, configured for cable and
antenna testing (CAT), is an extremely
useful tool for characterizing the entire
transmission system as well as the
performance of individual components
in the system. In the next few sections
of this application note, measurement
examples will be provided showing
techniques for measuring insertion
loss, return loss and locating faults in
a transmission system. The measurements will be made using a FieldFox
N991XA analyzer with frequency range
up to 26.5 GHz.
04 | Keysight | Techniques for Precise Cable and Antenna Measurements in the Field Using FieldFox Handheld Analyzers - Application Note
Cable Insertion Loss Measurements
The insertion loss of transmission line
or coaxial cable is often measured as a
function of the intended operating frequency band. The insertion loss, also
referred to as the cable attenuation, is
the amount of energy dissipated in the
cable. The insertion loss also includes
energy lost due to mismatch reflection
between the source and load. It is
expected that the source (transmitter),
the transmission line (coaxial cable),
and the load (antenna) are all designed
for the same characteristic impedance,
Zo, usually 50 ohms or 75 ohms when
using coaxial cable. It has been shown
1 that coaxial cable having an air dielectric has the lowest insertion loss at
75 ohms and the highest power handling capability at 30 ohms. The Cable
TV (CATV) industry has standardized
on 75-ohm cable in order to maximize
signal transmission over very long
cable distances required by the system. On the other hand, the majority
of the RF and microwave industry has
standardized on 50-ohm impedance as
a compromise between the lowest loss
and the highest power handling. Most
RF and microwave analyzers, such as
FieldFox, are configured with 50-ohm
test port impedances. When measuring 75-ohm cables and components,
50-ohm to 75-ohm adapters, such
as the Keysight Technologies, Inc.
N9910X-846, should be connected to
the analyzer.
Once the transmission line cables
are installed into a system, it is often
difficult and costly to remove them
in order to verify their operation and
troubleshoot cable failures. Also with
very long cable runs, access to both
ends of the cable at the same time is
typically impossible, especially when
attempting to connect the cable to
the test instrumentation. Under these
conditions, techniques that allow
cable insertion loss measurements
to be made from only one end of the
cable are preferred. Figure 2 shows
two configurations for measuring the
insertion loss of a cable using Field-
Fox with CAT mode. Figure 2a shows
the traditional method for measuring
the insertion loss of a cable using a
two-port technique. Figure 2b shows a
novel technique, available on the FieldFox, for measuring the insertion loss of
a cable using measurements obtained
from only one end of the cable. Both of
these techniques will be discussed.
In the traditional two-port setup, the
cable under test is connected to two
separate ports on FieldFox. FieldFox
injects a test signal into the cable from
the RF OUT port conveniently positioned along the top of the analyzer.
As the test signal passes through the
cable, a small portion of the energy is
absorbed by resistive and dielectric
losses in the cable. Discontinuities
from cable connectors, cable splices,
damage and other factors will reflect
a portion of the energy back to the
source resulting in an additional increase in the measured insertion loss.
The remaining signal exiting the cable
is then measured by FieldFox at the RF
IN port.
The ratio between the input and output
signals represents the total insertion
loss of the cable usually expressed in
decibels (dB). An ideal lossless cable
would have 0 dB insertion loss. The
cable manufacturer usually provides a
table of the insertion loss as a function of frequency. For example, an
LMR900 coaxial cable would have
2.2 dB insertion loss for a 100 meter
length of cable measured at 150 MHz.
The same cable would have 7.4 dB loss
at 1.5 GHz. Those familiar with vector
network analyzers (VNA) will recognize
this insertion loss measurement as the
absolute value of the S21 transmission
scattering parameter (S-parameter).
Additional information regarding VNA
options on FieldFox and the basics
of network analysis can be found at
the following references [1] and [2]. It
should be noted that accurate insertion loss measurements require an
initial calibration of FieldFox.
Figure 2. Configurations for measuring the insertion loss of a cable using FieldFox with CAT mode
05 | Keysight | Techniques for Precise Cable and Antenna Measurements in the Field Using FieldFox Handheld Analyzers - Application Note
Cable Insertion Loss Measurements (continued)
The calibration removes the insertion
loss effects of the equipment, adapters and jumper cables as a function of
frequency. The calibration of FieldFox
for making one and two-port measurements will be discussed later in this
application note. Figure 3 shows the
insertion loss measurement of a coaxial cable using the 2-port technique
over the frequency range of 30 kHz to
18 GHz. This figure also shows a measurement of the insertion loss using a
1-port cable loss technique to be
discussed next.
As previously mentioned, when a cable
is installed into a system, it is often
difficult to physically connect FieldFox
to both ends of a very long cable without introducing an equally long jumper
cable into the test setup. Fortunately,
FieldFox has a convenient technique
to measure cable insertion loss from
only one end of the cable under test.
This eliminates the need to carry an
extra-long, high-quality test cable as
part of the equipment requirements.
Figure 2b shows the simple 1-port
configuration for measuring cable
insertion loss by attaching one end
of the cable to FieldFox and leaving
the other end open or terminated in
a short. It is preferred at microwave
frequencies to use the short circuit
to eliminate fringing fields found in
an open-ended cable which could
alter the measured results. In this
configuration, FieldFox injects a test
signal into the cable from the RF OUT
port, same as before. The test signal
passes through the cable, is completely reflected from the open end (or
shorted end), passes through the cable
a second time and lastly measured
by FieldFox using the same port.
FieldFox is configured with an internal
factory-calibrated “reflectometer” 1
for measuring reflected signals at the
RF OUT port. This internal calibration, called CalReady, is available at
instrument turn-on and instrument
preset and will be discussed later in
this application note.
Once the reflected measurement is
complete, FieldFox, using a built-in
model for coaxial cable dispersion
and knowing that the measured signal
contains twice the cable insertion loss
(due to the round trip path), can now
report the cable insertion loss as a
function of frequency. Figure 3 shows a
comparison of the measured insertion loss using the 2-port and 1-port
techniques. In this figure, the blue
line represents the 2-port insertion
loss measurement and the yellow line
represents the 1-port cable loss measurement. As observed in figure 3, the
1-port technique contains additional
amplitude ripple across the measured
frequency range due to mismatch
effects between the open (short) and
the input connector. Using FieldFox,
the amplitude ripple may be reduced
with an additional measurement of
a 50-ohm load placed at the end of
the cable under test. In this case, the
measurement of the 50-ohm load
is saved to memory using the Trace,
Data-> Mem.
The load is removed and the cable is
terminated in an open (or short), same
as before. The measurement of the
open (short) is then subtracted from
the memory using the Data Math, Data-Mem. This additional measurement
step may improve the observed ripple
in the 1-port cable measurement. In
general, the traditional 2-port insertion
loss measurement will be more accurate than a 1-port cable measurement
but having a measurement process
that does not require an instrument
connection to both ends of the cable
is a great benefit when characterizing
installed cabled systems.
Figure 3. Measurement of coaxial cable insertion loss using a novel one-port technique
(yellow) and a traditional two-port technique (blue), both available on FieldFox
06 | Keysight | Techniques for Precise Cable and Antenna Measurements in the Field Using FieldFox Handheld Analyzers - Application Note
Cable Insertion Loss Measurements (continued)
There is another configuration for
measuring cable insertion loss that
does not require a long coaxial cable
in the return path back to FieldFox. In
this configuration, FieldFox is configured as a power meter connected to
a Keysight U2000 Series USB power
sensor. As shown in figure 4, one end
of the cable under test is directly connected to the RF OUT port of FieldFox.
The other end of the installed cable is
connected to the USB power sensor.
FieldFox is configured to generate a
CW signal at the RF OUT port. This
CW signal is the test signal that will
be transmitted along the cable and
measured by the USB power sensor. If
the cable ends are physically separated by a large distance, the USB sensor
can be connected back to FieldFox
through a USB cable extender. USB
cable extenders are commercially
available with ranges of 500 meters or
more. This configuration overcomes
the limitation of running an expensive
coaxial cable between one end of the
installed cable back to the instrument.
The main constraint using the power
meter solution is that swept frequency
measurements are not available and
therefore changing test frequencies
will require manual tuning of FieldFox’s
settings.
When measuring cables with high
insertion loss, the displayed measurement trace may exhibit a relatively high
level of noise as the measured signal
amplitude approaches the noise floor
of the analyzer. One way to improve
the relative signal to noise and improving the associated measurement
accuracy is to set the output power
on FieldFox to High Power mode. High
Power mode is the default setting
for FieldFox and is typically used for
most passive device measurements.
The Low Power mode is typically used
when measuring high-gain or lownoise amplifiers to prevent saturation
and potential damage to the amplifier.
The High Power setting results in a
port power of approximately +5 dBm,
the Low Power setting is approximately
–25 dBm.
Other techniques to reduce the
displayed noise level include increasing the number of trace averages or
decreasing the IF bandwidth setting.
Trace averaging will reduce the effects
of random noise on the measurement
by averaging the results from multiple
sweeps. FieldFox trace averaging can
be set over a range from 1 (no averaging) to 1000.
Knowing that trace averaging requires
the measurement of multiple traces,
it should be expected that there will
be an increase in the total measurement time as multiple sweeps are
required. Reducing the IF bandwidth
setting on FieldFox will also reduce
displayed noise. The IF bandwidth can
be adjusted over the range of 100 Hz
to 300 kHz. It should be noted a lower
IF bandwidth will also increase the
overall measurement time as lower
bandwidths increase the instrument
sweep time.
Figure 4. Configuration for measuring cable loss using FieldFox and a USB power sensor
07 | Keysight | Techniques for Precise Cable and Antenna Measurements in the Field Using FieldFox Handheld Analyzers - Application Note
Figure 5a and 5b. Frequency response measurement of a panel antenna showing (a) return loss (dB) and (b) VSWR
Antenna Return Loss and VSWR Measurements
Antennas are specified by their gain
factor and return loss or voltage
standing wave ratio (VSWR). The
return loss and VSWR are standard
measurements reported for most RF
and microwave components and systems and can easily be measured with
FieldFox in the field or in the lab. Antenna gain measurements are typically
performed in a special test facility,
such as an anechoic chamber. During
antenna gain testing, a transmission
measurement is made from the antenna under test to a second calibrated
antenna. The effects of the calibrated
antenna, and any free-space loss,
are removed from the transmission
measurement resulting in the desired
antenna gain. The measurement of
antenna gain in the field is rather
difficult as environmental reflections
and inaccessibility around the antenna prevent this type of testing. When
an antenna is installed in a system,
it will therefore be required to only
measure the reflection properties of
the antenna in order to determine if an
antenna is faulty or damaged. Return
loss and the associated VSWR will be
used to characterize the performance
of an antenna. These 1–port measurements are typically performed over the
intended frequency band of interest.
A typical antenna with a return loss of
10 dB or higher would represent a
reasonably well-matched antenna.
A 10-dB return loss is equivalent to
having 90% of the incident energy
radiated by the antenna with 10% of
the energy being reflected back to
the transmitter. For some applications
where high efficiency is required, such
as base station antennas, the return
loss may be specified at 15 dB or
higher. A 10-dB return loss is approximately equal to a VSWR of 2:1 which
is also quoted in the specifications.
Appendix A includes a conversion
table relating values of return loss to
VSWR. Fortunately, FieldFox handles
this conversion and can be configured
to display the measured response as
return loss (dB) or VSWR.
Figure 5 shows the measured reflection characteristics of a 2.4 GHz patch
panel antenna. This antenna included a
short 6-inch length of 50-ohm coaxial
cable ending with a Type-N female
connector at the end. A standard
Type-N male-to-male adapter was required to interface the antenna to the
Type-N female connector of FieldFox.
When interfacing any component to
an instrument, it is very important to
use high quality adapters to improve
the accuracy and repeatability of the
measurement. For this measurement
example, the adapter and cable was
included as part of the measured
results. The adapter characteristics
can be removed from the measured
results by using the QuickCal or O,S,L
calibration available on FieldFox.
Calibration options will be discussed
later in this application note. Figure 5a
shows the measured return loss over
the range of 2.2 to 2.7 GHz. Two markers are placed at the points of 10-dB
return loss signifying an appropriate
bandwidth for this antenna. A marker
table lists the amplitude and associated frequency for each active marker
up to a total of six. Figure 5b shows
the equivalent VSWR measurement for
the same antenna. When measuring
the reflection characteristics of an
antenna, it is important to attach the
analyzer as close to the antenna as
possible. If a long length of cable is
inserted between the antenna and the
analyzer or if the cable has become
excessively lossy due to damage, then
the high insertion loss of the cable
will mask the true return loss of the
antenna. For example, if the return loss
of a damaged antenna is actually 6 dB
but a jumper cable with 3 dB insertion
loss is connected between the antenna
and the analyzer, then the measured
return loss of the combination of the
antenna and cable would be 12 dB
(3 dB cable loss + 6 dB antenna RL +3 dB cable
loss) In this case it would be necessary
to attach the analyzer directly to the
antenna in order to properly measure
the true performance of the antenna or
perform a user calibration at the end of
the jumper cable.
When making measurements with
FieldFox attached to an antenna,
it may be possible that undesired
interference from outside the system
may be captured by the antenna and
measured by FieldFox. The interference may appear as a spike or lack
of stability in the measured trace.
FieldFox includes an Interference Rejection setting that may minimize the
effects of any interfering signal. The
Interference Rejection setting is found
under the Measurement Setup menu.
Once enabled, up to six measurement
sweeps may be required to reduce or
eliminate the interference.
08 | Keysight | Techniques for Precise Cable and Antenna Measurements in the Field Using FieldFox Handheld Analyzers - Application Note
Antenna-to-Antenna Isolation
In many wireless applications, different systems will be co-located to
share tower and shelter structures.
For example, figure 6a shows a typical repeater where antennas from
multiple systems are mounted onto
the common structure. It is important
that antennas, either operating within
the same system or between different
systems, are physically spaced far
enough apart to maintain an adequate
level of isolation in order to prevent the
creation of intermodulation distortion in the transmit amplifiers and/or
noise desensitization in the receivers.
Intermodulation distortion and receiver
desensitization may be created when
energy from a high power transmitter
radiated from one antenna couples into
a nearby antenna and enters the amplifiers of the coupled system. Systems
are often designed with duplexing
and other filters to reject signals from
nearby transmitters and other interference but system performance may require an antenna-to-antenna isolation
of 60 dB or more. It is therefore necessary to sweep the antenna-to-antenna
isolation across all frequency bands of
interest. Figure 6b shows a configuration for measuring the antenna-to-antenna isolation using FieldFox. FieldFox
is set to measure 2-port insertion loss
and calibrated using short jumper
cables with a “normalize” calibration
type. The measurement of isolation is
reciprocal so it does not matter which
antenna is attached to the RF OUT
port of FieldFox.
Figure 7 shows an example of the
measured isolation between two
panel antennas mounted to a common
structure. One antenna was designed
to operate in the 900-MHz band and
the other at 2.4 GHz. As shown in this
figure, the antenna-to-antenna isolation is above 44 dB across the 902 to
928 MHz band but is only 35 dB in the
2.4 to 2.485 GHz band. This level of
isolation implies that energy transmitted from the 2.4-GHz system will enter
the 900-MHz receiver front-end at a
level of 35 dB lower than the transmitted signal. The 900-MHz
Figure 6a. Microwave repeater showing
multiple co-located antennas.3
Figure 6b. Configuration for measuring
antenna-to-antenna isolation
Figure 7. Measured antenna-to-antenna isolation between a 900-MHz panel antenna to a
2.4-GHz panel antenna mounted on the same structure
system will need to have adequate
filtering to provide additional rejection
of the 2.4-GHz signal otherwise the
900-MHz receiver will be desensitized
by this high level signal.
09 | Keysight | Techniques for Precise Cable and Antenna Measurements in the Field Using FieldFox Handheld Analyzers - Application Note
Locating Faults Along Transmission Lines
Once it has been determined that
the cable insertion loss is higher
than expected or the return loss and
associated VSWR is out of spec, it
becomes necessary to find the possible locations for the fault(s) along the
transmission system. The location of
discrete and periodic cable faults can
be determined using the distance-tofault (DTF) measurement available on
FieldFox. DTF uses reflection measurements taken from the transmission line
and calculates the individual amplitude
response of any discontinuities as a
function of distance. DTF measurements using FieldFox are similar to
time domain measurements found on
a vector network analyzer 4 but DTF
reports distance to the discontinuity
instead of time delay. Figure 1 shows
a dual display of the return loss as a
function of frequency (upper trace)
and the associated DTF measurement
(lower trace) from a section of coaxial cable connected to an antenna.
When examining the DTF display, high
amplitude level signals are located at
the points where discontinuities exist
along the transmission line and at the
load. The location of these high amplitude signals will provide an important
troubleshooting tool when attempting
to determined faults in the cabled
transmission system.
Figure 8 shows a typical configuration
for measuring the DTF on FieldFox. In
this example, it is assumed that some
type of the load is connected to FieldFox through two sections of coaxial
cables. The load can be the system
antenna, a 50-ohm termination or just
an open ended cable. In this example,
the two cable sections are connected
together with an adapter. Figure 8
also shows a table of important cable
specifications as supplied by the manufacturer. The specification for velocity
factor is very important to enter into
FieldFox in order for the instrument to
correctly display the distance to each
cable discontinuity. Velocity factor is
the signal’s propagation velocity in
the transmission line relative to signal
velocity travelling in a vacuum (speed
of light). When a signal travels in a
coaxial cable, the dielectric between
the inner conductor and outer conductor of the cable will reduce the signal’s
velocity resulting in a velocity factor
less than 1.0. For the cable specified
in figure 8, the signal’s velocity factor
in this cable is 0.66 which implies that
the signal travels at a rate of 66% of
the speed of the light in free space.
The velocity factor is entered into
FieldFox under the DTF Cable Specifications located in the Measurement
Setup menu.
Figure 8. Configuration for measuring the distance-to-fault (dB) in a system having two coaxial cables connected to a load; also shown are the specifications for the cables used in this example
10 | Keysight | Techniques for Precise Cable and Antenna Measurements in the FieldUsing FieldFox Handheld Analyzers - Application Note
Locating Faults Along Transmission Lnes (continued)
Figure 9 shows the DTF measurement
for the simple example shown in figure
8. The cables are initially terminated
in a 50-ohm coaxial load. Markers are
placed at the three peaks in the DTF
response. The first marker, shown on
the far left, reports a distance of 0
meters. This marker represents the
interface between FieldFox and the
first coaxial cable. The second marker
reports a distance of 4 meters. This
marker is located at the adapter between the two cables. It also implies
that the length of the first cable is 4
meters. The third marker is located at
the 50-ohm load and is reported at
13.8 meters. This measurement can
be used to calculate the length of the
second cable which is 9.8 meters (13.8
meters – 4 meters). There is a noticeable drop in the measured amplitude
after the 50-ohm load signifying the
location of the end of the cable. It
should be noted that if the cable’s velocity factor was not correctly entered,
the distance measurements would
not be correct making it difficult to
accurately locate the physical distance to a discontinuity in the cable. It
should also be noted when two cable
types with different velocity factors
are included in the measurement, such
as a short jumper cable connected to
a long Helix cable, the velocity factor
of the longer cable should be entered
into FieldFox. Ideally, the short jumper
cable should be included as part of
QuickCal and therefore its effects will
be calibrated out from the measurement.
Figure 10 shows the measurement for
the same configuration as in figure
8 but the 50-ohm load was removed
and the cable was left open ended. As
a comparison, the yellow trace is the
current measurement having the cable
with the open end and the blue trace,
stored to memory, is the measurement
using the 50-ohm load. As expected,
the start, the adapter and the cable
end are measured at the same location
using the open but there is a notice
Enter “velocity factor”
Select “distance
to fault (dB)”
Figure 9. Measurement of the distance-to-fault (dB) for the configuration shown in figure 8
Figure 10. DTF measurement comparison between cable terminated in open circuit (yellow) and 50-ohm load (blue)
able increase in the amplitude of the
reflected signal from the open.
As FieldFox is measuring the two-way
reflection properties from the cables
under test, the displayed DTF measurements are calculated by taking
one-half of the actual measured
distance. From figure 10, it is also
observed that two additional peaks appear to the right of the open. As these
measured cables are not that long,
these peaks are created by re-reflections from the multiple discontinuities
throughout the cable set. For these
two peaks, marker 4 is measured to be
located at 23.7 meters and marker 5 is
located at 27.6 meters.
These measurement peaks are the result of signals that travel through sections of the cables multiple times and
interact with open circuit and adapter
discontinuities along the way.
11 | Keysight | Techniques for Precise Cable and Antenna Measurements in the FieldUsing FieldFox Handheld Analyzers - Application Note
Locating Faults Along Transmission Lines (continued)
Figure 11 shows a simplified diagram
on the various propagation paths
back to FieldFox for the cable configuration shown in figure 8. Figure
11a shows the main reflected signal
from the open-ended cable resulting
in the signal at 13.8 meters (marker
3 in figure 10). Figure 11b shows the
re-reflections between the adapter
and the open resulting in a peak at
approximately 23.7 meters (marker 4).
The last peak, not shown in figure 11,
represents the re-reflections between
FieldFox and the open resulting in a
peak at approximately 27.6 meters
(marker 5 = (4 m + 9.8 m + 9.8 m + 4 m
+ 4 m + 9.8 m + 9.8 m + 4 m)/2).
Under some instrument settings, it is
possible that a measured peak could
be an artifact or “alias” of the calculated DTF response. An alias is not a true
cable response and may appear when
FieldFox reports an “Alias-free Range =
Off” as shown under the DTF Settings
page. Aliases will not appear when
FieldFox reports an “Alias-free Range
= On”. These undesired peaks are more
likely to occur with FieldFox settings
at short stop distances, less than 10
meters, and/or high resolution, 1001
points. When an alias is suspected, a
simple measurement procedure will
help determine which peaks are true
responses and which are aliases. Place
a marker on the response in question
and note the measured distance.
Change the start or stop distance
setting on FieldFox. The displayed
distance of a true response will not
change. On the other hand, the displayed distance of an alias will appear
to move and therefore this peak should
not be recorded as an actual DTF
response.
It should be noted that for the measurement example shown in figure 10,
the expected return loss from an open
circuit should be 0 dB but is measured at 9.9 dB (marker 3). Unless the
specified cable loss (dB/m) is properly
entered into FieldFox,
Figure 11. Signal propagation paths between FieldFox and an open-ended cable; (a) signal
reflection from the open end, (b) double reflection from the open end and the adapter.
Cable type selection menu
RG8U cable parameters
Figure 12. Cable type selection and cable parameter menus on FieldFox
the measured amplitude will not be
cor-rect. For this measurement, the
cable loss was entered as 0 dB/m
(default setting) so the insertion loss
of this transmission system is masking the true return loss from the open
circuit. If the application requires an
accurate amplitude measurement
of the return loss from a particular
reflection on the DTF display, the cable
loss should be entered either manually
into FieldFox or selected from a list of
stored cable types. Figure 12 shows
the cable selection menu (left menu)
on FieldFox. In this case, the RG8U
cable type is highlighted and selected.
The cable parameters can be viewed
and edited as shown in the table on
the right of figure 12. For this RG8U
type, the velocity factor is 0.848 and
the cable loss (dB/m) is stored as a
function of
frequency. Cable loss values are interpolated between the entered frequency points. In general, DTF measurements of transmission systems are
primarily concerned with locating the
physical position of faults along the
line and therefore, precise amplitude
measurements of the return loss and
associated VSWR may not be required.
Accordingly most applications only
require that the cable’s velocity factor
be entered into the FieldFox cable
specification table.
12 | Keysight | Techniques for Precise Cable and Antenna Measurements in the FieldUsing FieldFox Handheld Analyzers - Application Note
Locating Faults Along Transmission Lines (continued)
If the velocity factor (VF) of a cable is
unknown but the physical distance of
the cable is known, then the DTF measurement may be used to estimate the
velocity factor. In this case, the cable
is attached to FieldFox and the VF is
adjusted until the measured distance
matches the known physical length.
For example, figure 13 shows the measurement of a 3-meter length of Category 6 twisted pair cable terminated in
an open end. The cable was attached
to a 100-ohm balanced-to-unbalanced
transformer to match the 100-ohm
impedance of the twisted pair cable to
the 50-ohm impedance of FieldFox’s
test port connector. The VF setting
was adjusted until the measured
length was 3 meters and for this example, the velocity factor was 0.70 (70%).
This velocity factor may now be used
for DTF measurements of other Category 6 cables from this manufacturer.
There are three additional instrument
settings that control how FieldFox
calculates and displays DTF measurements. The first setting is related
to the Frequency Mode of operation
found under the Measurement Setup
menu on FieldFox. There are two frequency modes available when making
DTF measurements - Lowpass and
Bandpass. The Lowpass frequency
mode will provide the optimum resolution in measured distance but requires
test components that have a very wide
frequency bandwidth such as transmission lines, including coaxial cables
and adapters. The Bandpass frequency
mode is required for components that
have a narrow operating band such
as filters, antennas and waveguide
components. As resolution in distance
is inversely proportional to measurement bandwidth, the Bandpass mode
will not provide the same high resolution in a DTF measurement as the
Lowpass mode. In Lowpass mode, the
measured frequency range is automatically determined after the start and
stop distances are entered for the DTF
display.
Figure 13. Measurement of a Category-6 patch cable with adjustment to FieldFox’s
velocity factor correction in order to match the measured distance to the known physical
distance
In Bandpass mode, the measured start
and stop frequencies are manually
entered based on the approximate
frequency range for the components
under test.
The second DTF instrument setting is
related to the Resolution or number
of measurement points. The Resolution can be selected from 101, 201,
401, 601, 801, and 1001 points. A
larger number of points increases the
measurement sweep time but also
increases the maximum distance that
can be measured on the DTF display.
The range resolution is calculated
from the measured frequency span.
The maximum distance that can be
displayed is calculated from the range
resolution multiplied by the number of
points minus one.
The third DTF instrument setting is
the DTF Window. In general, when
troubleshooting faults in transmission systems, the default setting with
a Maximum Window provides the
highest dynamic range for the DTF
measurements. Under conditions when
attempting to resolve two closely-spaced discontinuities, a Minimum
Window would be preferred. Additional information regarding Frequency
Mode, Resolution and Windowing can
be found in the FieldFox User’s Manual.
13 | Keysight | Techniques for Precise Cable and Antenna Measurements in the FieldUsing FieldFox Handheld Analyzers - Application Note
Calibrating FieldFox’s Test Ports
FieldFox offers a set of user calibrations that can remove the effects
of cables and adapters that may be
placed between the instrument and
the device under test. These types of
user calibrations are performed by the
operator on a regular basis and are
independent of the general instrument
calibration that may be required on a
yearly cycle. The yearly instrument calibration is a traceable process that is
performed by a certified test lab, such
as Keysight, and confirms that the
instrument is performing to its stated
specifications. The yearly instrument
calibration should be traceable to
International System (SI) units through
a national metrology institute such as
NIST, NPL or BIPM. With a fully-traceable instrument calibration, FieldFox
is capable of making highly accurate
measurements at the instrument
test ports. In practice, different test
requirements may result in a variety
of jumper cables and adapters being
connected to FieldFox. Often it is preferred that the effects of these cables
and adapters are not included in the
measurement. In this case, FieldFox
allows a user calibration to be performed, with or without a calibration
kit, in order to extend the calibration
plane beyond the jumper cables and
adapters.
Operators familiar with the operation
of vector network analyzers (VNAs)
will be familiar with these types of user
calibrations. The remaining sections
of this application note will introduce
the different types of user calibrations
available for accurately measuring the
reflection characteristics and insertion
loss of components and systems.
Calibrating for reflection
measurements
There are three types of user calibrations available for accurately measuring the reflection characteristics of
transmission systems and antennas.
User calibration will improve the
results for all previously discussed
measurements of return loss, VSWR,
1-port cable loss and DTF. The calibration types (cal types) for reflection
measurements are CalReady, QuickCal
and 1-port OSL. Figure 14 shows the
configuration diagrams for these three
cal types including any jumper cables,
adapters and calibration standards
that may be required. In general, for all
cal types, jumper cables and adapters should be high quality items and
maintained in very good condition to
improve the accuracy and repeatability
in the measured data.
CalReady
The first and easiest calibration to
implement is CalReady, also known as
Preset Cal. CalReady is immediately
available upon instrument power-on or
instrument preset and requires no additional steps to calibrate. This built-in
calibration is performed at the factory
over the full frequency range of FieldFox and applies directly to the RF OUT
test port of FieldFox. CalReady assumes that the device under test (DUT)
will be connected directly to FieldFox’s
Type-N female test port connector or
the 3.5-mm male connector on 26.5
GHz models. As shown in figure 14a,
CalReady has a calibration plane at the
connector of FieldFox and is available
for all reflection measurements including return loss, VSWR, 1-port cable
loss and DTF. When using CalReady,
any jumper cables and/or adapters
connected between FieldFox
and the DUT will be included in the
measured response.
QuickCal
The second cal type available for
reflection measurements is QuickCal.
QuickCal is useful when jumper cables
and adapters are placed between
FieldFox and the DUT and the user
does not want to include their effects in the measured data. QuickCal
corrects for phase shift and loss of
cables and adapters and electrically
moves the calibration plane to the end
of these components (see figure 14b).
QuickCal requires a single measurement sweep of an open at the end of
the jumper cables and adapters, no
additional calibration standards are
required. Once the calibration sweep
is complete, the DUT may be measured without including the effects
of these jumper cables and adapters.
Jumper cables should be high-quality
phase-stable test cables. It should
be noted that if the DUT has a good
return loss, greater than 20 dB to 25
dB, an optional LOAD standard may be
measured during QuickCal to improve
the accuracy of the DUT measurements. Ideally the LOAD standard
should have the same connector type
as the DUT. The Keysight N9910X-800
(801) tee calibration kit contains a
Type-N male (female) 50-ohm termination.
14 | Keysight | Techniques for Precise Cable and Antenna Measurements in the FieldUsing FieldFox Handheld Analyzers - Application Note
Calibrating FieldFox’s Test Ports (continued)
1-port OSL
The third cal type available for reflection measurements is 1-port OSL.
This calibration requires a calibration
kit where the O is the OPEN standard,
the S is the SHORT and the L is the
LOAD standard. Calibration kits are
available in Type-N (50 ohm and 75
ohm), 7/16, 3.5 mm, Type F, 7 mm, TNC
and several waveguide kits including
X-band, P-band and K-band. When
the 1-port OSL cal type is selected
on FieldFox, the connector type must
be chosen including the connector
gender (male or female). The gender is
defined as that of the cal kit connector. As each calibration kit is different,
proper selection of the connector type
and gender will ensure that FieldFox is
using the correct cal kit definitions. For
the highest measurement accuracy,
the cal kit connector type should be
the same as the DUT connector. In this
way the calibration plane is located
directly at the DUT input (see figure
14c). The 1-port OSL cal type is the
most accurate of the three available
cal types but the calibration process
requires three separate measurement
sweeps, one for each of the three cal
standards. For the highest accuracy
when using 1-port OSL, it is important that the OPEN standard from the
calibration kit is used instead of just
leaving the cable open ended.
Calibrating for insertion
less measurements
There are several types of user calibrations available for accurately measuring the insertion loss of cables and
systems. The cal types are CalReady,
QuickCal, Normalization, Enhanced
Response and Full 2-port.
CalReady
CalReady allows users to make cable
and antenna measurements at the
test port without calibration, if a good
quality phase-stable cable is used (25
to 30 dB return loss and much better
than system return loss that is under
test) to connect FieldFox to the DUT.
This is the easiest method to make a
cable and antenna sweep in the field.
CalReady for insertion loss measurements is immediately available upon
instrument power-on or instrument
preset. CalReady is factory-calibrated
at FieldFox test ports and is very useful
to characterize the insertion loss of
jumper cables. In this case, the jumper
cable will be connected directly to
RF OUT (port 1) and RF IN (port 2) of
FieldFox. When measuring the insertion loss of a DUT, jumper cables and/
or adapters connected between FieldFox and the DUT will be included as
part of the DUT’s measured response.
Figure 14. Calibration types and calibration standards required for accurately measuring
reflection properties using FieldFox
15 | Keysight | Techniques for Precise Cable and Antenna Measurements in the FieldUsing FieldFox Handheld Analyzers - Application Note
Calibrating FieldFox’s Test Ports (continued)
Normalization
A very simple user calibration for
removing the effects of the jumper
cable and the frequency response of
the instrument is the Normalization cal
type. During this calibration process,
a short jumper cable is connected between the RF OUT port and the RF IN
port on FieldFox (see figure 15a). Once
the jumper cable is connected, a single
measurement sweep is performed
during the calibration. All DUT measurements are then normalized, or referenced, to this cable. As the calibration now includes the loss and phase
response of the jumper cable, this
cable should also be included when
making measurements of the DUT as
shown in figure 15a. If the jumper cable
is removed after the calibration, the
measurements of the DUT will be offset by the value of the insertion loss of
the jumper cable resulting in a slightly
lower measured value of insertion loss
for the DUT. It is also possible to use
two jumper cables and/or adapters
during the instrument calibration,
with one connected to each port of
FieldFox, as shown in figure 15b. Figure
15b also shows the DUT measurement configuration with connection
between the two jumper cables. While
the Normalization calibration is one of
the easiest to implement, is the least
accurate of all the cal types available
on FieldFox. It is often advisable to use
the one of the other cal types, such as
CalReady or QuickCal, for improved
measurement accuracy. It should also
be noted that the Normalization cal
type is also available for reflection
measurements where an open or short
is used to terminate the test port
during the calibration.
QuickCal for insertion loss measurements begins with the same calibration procedure as that for reflection
testing but the measurement sweep
of the open-ended jumper cable(s),
and the optional LOADs, occurs at
both RF OUT (port 1) and RF IN (port
2) of FieldFox. QuickCal for insertion
loss testing also requires an additional
measurement sweep of a jumper cable(s) connecting RF OUT (port 1) to RF
IN (port 2). This additional measurement of the jumper cable is similar to
the calibration step shown in figure 15.
After calibration, FieldFox will display
insertion loss measurements without
including the effects of any jumper
cables and adapters used as part of
the calibration.
Enhanced response and full
2-Port calibrations
A Full 2-Port calibration requires
measurement sweeps of the OSL
standards placed at both test ports of
FieldFox. This cal type also requires
measurement sweeps of the jumper
cable(s) connecting RF OUT (port
1) to RF IN (port 2). The Enhanced
Response follows the same calibration
procedure as the Full 2-Port but does
not require measurement sweeps of
the OSL standards placed at port 2 of
FieldFox. Without measuring the OSL
standards at port 2, the Enhanced
Response calibration is slightly less
accurate than the Full 2-Port calibration. The Enhanced Response and Full
2-Port cal types can be used for both
insertion loss testing and reflection
testing. While these cal types result
in the highest level of measurement
accuracy, the user will be required to
carry a high-quality calibration kit into
the field.
The Enhanced Response and Full
2-Port cal types provide the highest
level of measurement accuracy of all
the cal types available on FieldFox.
QuickCal
QuickCal allows users to extend the
reference plane to the end of the
jumper or adapter without using a cal
kit and supports many commonly-used
connector types.
Figure 15. Configuration for the Normalization Cal Type using a jumper cable(s) required
for 2-port insertion loss testing of a DUT
16 | Keysight | Techniques for Precise Cable and Antenna Measurements in the FieldUsing FieldFox Handheld Analyzers - Application Note
Conclusion
References
This application note has introduced
measurement and calibration techniques for cable and antenna testing
(CAT) using FieldFox. Measurement
examples were provided for cable
insertion loss, return loss, VSWR
and Distance-to-Fault (DTF) testing.
Antenna measurements were also reviewed and several examples of return
loss and antenna-to-antenna isolation
were provided. Additional information regarding CAT and line sweeping
can be found at the Keysight website. Keysight also provides local and
on-site training of CAT and FieldFox
applications including the Line Sweep
and Antenna Test Training Course
H7215x-121.
[1] Keysight Application Note, “Network Analyzer Basics,” Literature Number 5965-7917E, August 2004.
[2] www.keysight.com/find/FieldFox
[3] Copyright J M Briscoe and licensed for reuse under the Creative
Commons Licence.
[4] Keysight Application Note 1287-12, "Time Domain Analysis Using a Network Analyzer," Literature Number 5989-5723EN, May 2012.
CalReady allows users to make cable and antenna measurements at test
ports without calibration, when a good quality, phase-stable cable is used
to connect FieldFox to the DUT. QuickCal allows users to extend the reference plane to the end of the jumper or adapter without the need of a cal kit;
it supports many commonly-used connector types.
Appendix A: Return Loss and VSWR Conversion Table
Return loss can be expressed as a voltage standing wave ratio (VSWR) value using the following table:
Return Loss
(dB)
VSWR
Return Loss
(dB)
VSWR
Return Loss
(dB)
VSWR
Return Loss
(dB)
VSWR
Return Loss
(dB)
VSWR
4.0
4.42
14.0
1.50
18.0
1.29
28.0
1.08
38.0
1.03
6.0
3.01
14.2
1.48
18.5
1.27
28.5
1.08
38.5
1.02
8.0
2.32
14.4
1.47
19.0
1.25
29.0
1.07
39.0
1.02
10.0
1.92
14.6
1.46
19.5
1.24
29.5
1.07
39.5
1.02
10.5
1.85
14.8
1.44
20.0
1.22
30.0
1.07
40.0
1.02
11.0
1.78
15.0
1.43
20.5
1.21
30.5
1.06
40.5
1.02
11.2
1.76
15.2
1.42
21.0
1.20
31.0
1.06
41.0
1.02
11.4
1.74
15.4
1.41
21.5
1.18
31.5
1.05
41.5
1.02
11.6
1.71
15.6
1.40
22.0
1.17
32.0
1.05
42.0
1.02
11.8
1.69
15.8
1.39
22.5
1.16
32.5
1.05
42.5
1.02
12.0
1.67
16.0
1.38
23.0
1.15
33.0
1.05
43.0
1.01
12.2
1.65
16.2
1.37
23.5
1.14
33.5
1.04
43.5
1.01
12.4
1.63
16.4
1.36
24.0
1.13
34.0
1.04
44.0
1.01
12.6
1.61
16.6
1.35
24.5
1.13
34.5
1.04
44.5
1.01
12.8
1.59
16.8
1.34
25.0
1.12
35.0
1.04
45.0
1.01
13.0
1.58
17.0
1.33
25.5
1.11
35.5
1.03
45.5
1.01
13.2
1.56
17.2
1.32
26.0
1.11
36.0
1.03
46.0
1.01
13.4
1.54
17.4
1.31
26.5
1.10
36.5
1.03
46.5
1.01
13.6
1.53
17.6
1.30
27.0
1.09
37.0
1.03
47.0
1.01
13.8
1.51
17.8
1.30
27.5
1.09
37.5
1.03
47.5
1.01
VSWR is sometimes stated as a ratio. For example: 1.2:1 “one point two to one” VSWR. The first number is the VSWR
value taken from the table or calculated using the formula. The second number is always 1.
17 | Keysight | Techniques for Precise Cable and Antenna Measurements in the FieldUsing FieldFox Handheld Analyzers - Application Note
Carry Precision With You.
Every piece of gear in your field kit had to prove its worth. Measuring up and
earning a spot is the driving idea behind Keysight’s FieldFox analyzers. They're
equipped to handle routine maintenance, in-depth troubleshooting and anything
in between. Better yet, FieldFox delivers Keysight-quality measurements - wherever you need to go. Add FieldFox to your kit and carry precision with you.
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18 | Keysight | Techniques for Precise Cable and Antenna Measurements in the Field Using FieldFox Handheld Analyzers - Application Note
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