null  null
NONRESIDENT
TRAINING
COURSE
October 1995
Electronics Technician
Volume 7—Antennas and Wave
Propagation
NAVEDTRA 14092
DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.
Although the words “he,” “him,” and
“his” are used sparingly in this course to
enhance communication, they are not
intended to be gender driven or to affront or
discriminate against anyone.
DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.
PREFACE
By enrolling in this self-study course, you have demonstrated a desire to improve yourself and the Navy.
Remember, however, this self-study course is only one part of the total Navy training program. Practical
experience, schools, selected reading, and your desire to succeed are also necessary to successfully round
out a fully meaningful training program.
COURSE OVERVIEW: In completing this nonresident training course, you should be able to: discuss
wave propagation in terms of the effects the earth’s atmosphere has on it and the options available to receive
optimum performance from equipment; identify communications and radar antennas using physical
characteristics and installation location, radiation patterns, and power and frequency-handling capabilities.
Be familiar with safety precautions for technicians working aloft; and discuss the different types of
transmission lines in terms of physical structure, frequency limitations, electronic fields, and radiation
losses.
THE COURSE: This self-study course is organized into subject matter areas, each containing learning
objectives to help you determine what you should learn along with text and illustrations to help you
understand the information. The subject matter reflects day-to-day requirements and experiences of
personnel in the rating or skill area. It also reflects guidance provided by Enlisted Community Managers
(ECMs) and other senior personnel, technical references, instructions, etc., and either the occupational or
naval standards, which are listed in the Manual of Navy Enlisted Manpower Personnel Classifications
and Occupational Standards, NAVPERS 18068.
THE QUESTIONS: The questions that appear in this course are designed to help you understand the
material in the text.
VALUE: In completing this course, you will improve your military and professional knowledge.
Importantly, it can also help you study for the Navy-wide advancement in rate examination. If you are
studying and discover a reference in the text to another publication for further information, look it up.
1995 Edition Prepared by
ETC Larry D. Simmons
and
ETC Floyd L. Ace III
Published by
NAVAL EDUCATION AND TRAINING
PROFESSIONAL DEVELOPMENT
AND TECHNOLOGY CENTER
NAVSUP Logistics Tracking Number
0504-LP-026-7580
i
Sailor’s Creed
“I am a United States Sailor.
I will support and defend the
Constitution of the United States of
America and I will obey the orders
of those appointed over me.
I represent the fighting spirit of the
Navy and those who have gone
before me to defend freedom and
democracy around the world.
I proudly serve my country’s Navy
combat team with honor, courage
and commitment.
I am committed to excellence and
the fair treatment of all.”
ii
CONTENTS
Page
CHAPTER
1. Wave Propagation
1-1
2. Antennas
2-1
3. Introduction to Transmission and Waveguides
3-1
APPENDIX
AI-1
AII-1
I. Glossary
II. References
Index-1
INDEX
iii
SUMMARY OF THE ELECTRONICS TECHNICIAN
TRAINING SERIES
This series of training manuals was developed to replace the Electronics Technician
3 & 2 TRAMAN. The content is directed to personnel working toward advancement to
Electronics Technician Second Class.
The nine volumes in the series are based on major topic areas with which the ET2 should
be familiar. Volume 1, Safety, provides an introduction to general safety as it relates to
the ET rating. It also provides both general and specific information on electronic tag-out
procedures, man-aloft procedures, hazardous materials (i.e., solvents, batteries, and vacuum
tubes), and radiation hazards. Volume 2, Administration, discusses COSAL updates, 3-M
documentation, supply paperwork, and other associated administrative topics. Volume 3,
Communication Systems, provides a basic introduction to shipboard and shore-based
communication systems. Systems covered include man-pat radios (i.e., PRC-104, PSC-3)
in the hf, vhf, uhf, SATCOM, and shf ranges. Also provided is an introduction to the
Communications Link Interoperability System (CLIPS). Volume 4, Radar Systems, is a
basic introduction to air search, surface search, ground controlled approach, and carrier
controlled approach radar systems. Volume 5, Navigation Systems, is a basic introduction
to navigation systems, such as OMEGA, SATNAV, TACAN, and man-pat systems. Volume
6, Digital Data Systems, is a basic introduction to digital data systems and includes discussions
about SNAP II, laptop computers, and desktop computers. Volume 7, Antennas and Wave
Propagation, is an introduction to wave propagation, as it pertains to Electronics Technicians,
and shipboard and shore-based antennas. Volume 8, Support Systems, discusses system
interfaces, troubleshooting, sub-systems, dry air, cooling, and power systems. Volume 9,
Electro-Optics, is an introduction to night vision equipment, lasers, thermal imaging, and
fiber optics.
iv
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v
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vi
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vii
CHAPTER 1
WAVE PROPAGATION
TROPOSPHERE
The eyes and ears of a ship or shore station depend
on sophisticated, highly computerized electronic
systems. The one thing all of these systems have in
common is that they lead to and from antennas. Ship’s
operators who must communicate, navigate, and be
ready to fight the ship 24 hours a day depend on you
to keep these emitters and sensors operational.
Almost all weather phenomena take place in the
troposphere. The temperature in this region decreases
rapidly with altitude. Clouds form, and there may be
a lot of turbulence because of variations in the
temperature, pressure, and density. These conditions
have a profound effect on the propagation of radio
waves, as we will explain later in this chapter.
In this volume, we will review wave propagation,
antenna characteristics, shore-based and shipboard
communications antennas, matching networks, antenna
tuning, radar antennas, antenna safety, transmission
lines, connector installation and weatherproofing,
waveguides, and waveguide couplings. When you
have completed this chapter, you should be able to
discuss the basic principles of wave propagation and
the atmosphere’s effects on wave propagation.
STRATOSPHERE
The stratosphere is located between the troposphere
and the ionosphere. The temperature throughout this
region is almost constant and there is little water vapor
present. Because it is a relatively calm region with
little or no temperature change, the stratosphere has
almost no effect on radio waves.
THE EARTH’S ATMOSPHERE
IONOSPHERE
While radio waves traveling in free space have
little outside influence to affect them, radio waves
traveling in the earth’s atmosphere have many
influences that affect them. We have all experienced
problems with radio waves, caused by certain
atmospheric conditions complicating what at first
seemed to be a relatively simple electronic problem.
These problem-causing conditions result from a lack
of uniformity in the earth’s atmosphere.
This is the most important region of the earth’s
atmosphere for long distance, point-to-point communications. Because the existence of the ionosphere is
directly related to radiation emitted from the sun, the
movement of the earth about the sun or changes in
the sun’s activity will result in variations in the
ionosphere. These variations are of two general types:
(1) those that more or less occur in cycles and,
therefore, can be predicted with reasonable accuracy;
and (2) those that are irregular as a result of abnormal
behavior of the sun and, therefore, cannot be predicted.
Both regular and irregular variations have important
effects on radio-wave propagation. Since irregular
variations cannot be predicted, we will concentrate
on regular variations.
Many factors can affect atmospheric conditions,
either positively or negatively. Three of these are
variations in geographic height, differences in
geographic location, and changes in time (day, night,
season, year).
To understand wave propagation, you must have
at least a basic understanding of the earth’s atmosphere.
The earth’s atmosphere is divided into three separate
regions, or layers. They are the troposphere, the
stratosphere, and the ionosphere. These layers are
illustrated in figure 1-1.
Regular Variations
The regular variations can be divided into four
main classes: daily, 27-day, seasonal, and 11-year.
We will concentrate our discussion on daily variations,
1-1
Figure 1.1—Atmospheric layers.
since they have the greatest effect on your job. Daily
variations in the ionosphere produce four cloud-like
layers of electrically-charged gas atoms called ions,
which enable radio waves to be propagated great
distances around the earth. Ions are formed by a
process called ionization.
of the ultraviolet energy that initially set them free
and form an ionized layer.
Since the atmosphere is bombarded by ultraviolet
waves of differing frequencies, several ionized layers
are formed at different altitudes. Ultraviolet waves
of higher frequencies penetrate the most, so they
produce ionized layers in the lower portion of the
ionosphere. Conversely, ultraviolet waves of lower
frequencies penetrate the least, so they form layers
in the upper regions of the ionosphere.
Ionization
In ionization, high-energy ultraviolet light waves
from the sun periodically enter the ionosphere, strike
neutral gas atoms, and knock one or more electrons
free from each atom. When the electrons are knocked
free, the atoms become positively charged (positive
ions) and remain in space, along with the negativelycharged free electrons. The free electrons absorb some
An important factor in determining the density
of these ionized layers is the elevation angle of the
sun. Since this angle changes frequently, the height
and thickness of the ionized layers vary, depending
1-2
on the time of day and the season of the year.
Another important factor in determining layer
density is known as recombination.
F layer is divided into two layers, designated F1 (the
lower level) and F2 (the higher level).
The presence or absence of these layers in the
ionosphere and their height above the earth vary
with the position of the sun. At high noon, radiation
in the ionosphere above a given point is greatest,
while at night it is minimum. When the radiation is
removed, many of the particles that were ionized
recombine. During the time between these two
conditions, the position and number of ionized layers
within the ionosphere change.
Recombination
Recombination is the reverse process of
ionization. It occurs when free electrons and positive
ions collide, combine, and return the positive ions to
their original neutral state.
Like ionization, the recombination process
depends on the time of day. Between early morning
and late afternoon, the rate of ionization exceeds the
rate of recombination. During this period the ionized
layers reach their greatest density and exert
maximum influence on radio waves. However, during
the late afternoon and early evening, the rate of
recombination exceeds the rate of ionization, causing
the densities of the ionized layers to decrease.
Throughout the night, density continues to decrease,
reaching its lowest point just before sunrise. It is
important to understand that this ionization and
recombination process varies, depending on the
ionospheric layer and the time of day. The following
paragraphs provide an explanation of the four
ionospheric layers.
Since the position of the sun varies daily,
monthly, and yearly with respect to a specific point
on earth, the exact number of layers present is
extremely difficult to determine. However, the
following general statements about these layers can
be made.
D LAYER.— The D layer ranges from about 30
to 55 miles above the earth. Ionization in the D layer
is low because less ultraviolet light penetrates to this
level. At very low frequencies, the D layer and the
ground act as a huge waveguide, making communication possible only with large antennas and highpower transmitters. At low and medium frequencies,
the D layer becomes highly absorptive, which limits
the effective daytime communication range to about
200 miles. At frequencies above about 3 MHz, the D
layer begins to lose its absorptive qualities.
Ionospheric Layers
The ionosphere is composed of three distinct
layers, designated from lowest level to highest level
(D, E, and F) as shown in figure 1-2. In addition, the
Figure 1-2.—Layers of the ionosphere.
1-3
Long-distance
communication
is
possible
at
frequencies as high as 30 MHz. Waves at frequencies
above this range pass through the D layer but are
attenuated. After sunset. the D layer disappears
because of the rapid recombination of ions. Lowfrequency and medium-frequency long-distance
communication becomes possible. This is why AM
behaves so differently at night. Signals passing
through the D layer normally are not absorbed but
are propagated by the E and F layers.
signals at frequencies as high as 100 MHz. During
minimum sunspot activity, the maximum usable
frequency can drop to as low as 10 MHz.
E LAYER.— The E layer ranges from approximately 55 to 90 miles above the earth. The rate of
ionospheric recombination in this layer is rather
rapid after sunset, causing it to nearly disappear by
midnight. The E layer permits medium-range
communications on the low-frequency through veryhigh-frequency bands. At frequencies above about 150
MHz, radio waves pass through the E layer.
REFRACTION
ATMOSPHERIC PROPAGATION
Within the atmosphere, radio waves can be
refracted, reflected, and diffracted. In the following
paragraphs, we will discuss these propagation
characteristics.
A radio wave transmitted into ionized layers is
always refracted, or bent. This bending of radio
waves is called refraction. Notice the radio wave
shown in figure 1-3, traveling through the earth’s
atmosphere at a constant speed. As the wave enters
the denser layer of charged ions, its upper portion
moves faster than its lower portion. The abrupt speed
increase of the upper part of the wave causes it to
bend back toward the earth. This bending is always
toward the propagation medium where the radio
wave’s velocity is the least.
Sometimes a solar flare will cause this layer to
ionize at night over specific areas. Propagation in this
layer during this time is called SPORADIC-E. The
range of communication in sporadic-E often exceeds
1000 miles, but the range is not as great as with F
layer propagation.
F LAYER.— The F layer exists from about 90 to
240 miles above the earth. During daylight hours, the
F layer separates into two layers, F1 and F2. During
the night, the F1 layer usually disappears, The F
layer produces maximum ionization during the
afternoon hours, but the effects of the daily cycle are
not as pronounced as in the D and E layers. Atoms in
the F layer stay ionized for a longer time after sunset,
and during maximum sunspot activity, they can stay
ionized all night long.
Figure 1-3.—Radio-wave refraction.
Since the F layer is the highest of the
ionospheric layers, it also has the longest propagation
capability. For horizontal waves, the single-hop F2
distance can reach 3000 miles. For signals to
propagate over greater distances, multiple hops are
required.
The amount of refraction a radio wave undergoes
depends on three main factors.
1. The ionization density of the layer
The F layer is responsible for most highfrequency, long-distance communications. The
maximum frequency that the F layer will return
depends on the degree of sunspot activity. During
maximum sunspot activity, the F layer can return
2. The frequency of the radio wave
3. The angle at which the radio wave enters the
layer
1-4
Figure 1-4.—Effects of ionospheric density on radio waves.
into space. For any given ionized layer, there is a
frequency, called the escape point, at which energy
transmitted directly upward will escape into
space. The maximum frequency just below the
escape point is called the critical frequency. In
this example, the 100-MHz wave’s frequency is
greater than the critical frequency for that ionized
layer.
Layer Density
Figure 1-4 shows the relationship between
radio waves and ionization density. Each ionized
layer has a middle region of relatively dense
ionization with less intensity above and below. As
a radio wave enters a region of increasing
ionization, a velocity increase causes it to bend
back toward the earth. In the highly dense
middle region, refraction occurs more slowly
because the ionization density is uniform. As the
wave enters the upper less dense region, the
velocity of the upper part of the wave decreases
and the wave is bent away from the earth.
Frequency
The lower the frequency of a radio wave, the
more rapidly the wave is refracted by a given
degree of ionization. Figure 1-5 shows three
separate waves of differing frequencies entering
the ionosphere at the same angle. You can see that
the 5-MHz wave is refracted quite sharply, while
the 20-MHz wave is refracted less sharply and
returns to earth at a greater distance than the 5MHz wave. Notice that the 100-MHz wave is lost
Figure 1-5.—Frequency versus refraction
and distance.
The critical frequency of a layer depends upon
the layer’s density. If a wave passes through a
1-5
particular layer, it may still be refracted by a
higher layer if its frequency is lower than the
higher layer’s critical frequency.
Angle of Incidence and Critical Angle
When a radio wave encounters a layer of the
ionosphere, that wave is returned to earth at the
same angle (roughly) as its angle of incidence.
Figure 1-6 shows three radio waves of the same
frequency entering a layer at different incidence
angles. The angle at which wave A strikes the
layer is too nearly vertical for the wave to be
refracted to earth, However, wave B is refracted
back to earth. The angle between wave B and the
earth is called the critical angle. Any wave, at a
given frequency, that leaves the antenna at an
incidence angle greater than the critical angle will
be lost into space. This is why wave A was not
refracted. Wave C leaves the antenna at the
smallest angle that will allow it to be refracted and
still return to earth. The critical angle for radio
waves depends on the layer density and the
wavelength of the signal.
Figure 1-6.—Incidence angles of radio waves.
As the frequency of a radio wave is increased,
the critical angle must be reduced for refraction to
occur. Notice in figure 1-7 that the 2-MHz wave
strikes the ionosphere at the critical angle for that
frequency and is refracted. Although the 5-MHz
line (broken line) strikes the ionosphere at a less
critical angle, it still penetrates the layer and is
lost As the angle is lowered, a critical angle is
finally reached for the 5-MHz wave and it is
refracted back to earth.
Figure 1-7.—Effect of frequency on the critical angle.
1-6
Skip Zone
SKIP DISTANCE AND ZONE
The skip zone is a zone of silence between the
point where the ground wave is too weak for
reception and the point where the sky wave is first
returned to earth. The outer limit of the skip zone
varies considerably, depending on the operating
frequency, the time of day, the season of the year,
sunspot activity, and the direction of transmission.
Recall from your previous study that a
transmitted radio wave separates into two parts,
the sky wave and the ground wave. With those
two components in mind, we will now briefly
discuss skip distance and skip zone.
Skip Distance
At very-low, low, and medium frequencies, a
skip zone is never present. However, in the highfrequency spectrum, a skip zone is often present.
As the operating frequency is increased, the skip
zone widens to a point where the outer limit of the
skip zone might be several thousand miles away.
At frequencies above a certain maximum, the
outer limit of the skip zone disappears completely,
and no F-layer propagation is possible.
Look at the relationship between the sky wave
skip distance, skip zone, and ground wave
coverage shown in figure 1-8. The skip distance is
the distance from the transmitter to the point
where the sky wave first returns to the earth. The
skip distance depends on the wave’s frequency and
angle of incidence, and the degree of ionization.
Occasionally, the first sky wave will return to
earth within the range of the ground wave. In this
case, severe fading can result from the phase
difference between the two waves (the sky wave
has a longer path to follow).
REFLECTION
Reflection occurs when radio waves are
“bounced” from a flat surface. There are basically
two types of reflection that occur in the
atmosphere: earth reflection and ionospheric
reflection. Figure 1-9 shows two
Figure 1-8.—Relationship between skip
zone, skip distance, and ground wave.
Figure 1-9.—Phase shift of reflected radio waves.
1-7
waves reflected from the earth’s surface. Waves A
and B bounce off the earth’s surface like light off of
a mirror. Notice that the positive and negative
alternations of radio waves A and B are in phase before
they strike the earth’s surface.
However, after
reflection the radio waves are approximately 180
degrees out of phase. A phase shift has occurred.
The amount of phase shift that occurs is not
constant. It varies, depending on the wave polarization
and the angle at which the wave strikes the surface.
Because reflection is not constant, fading occurs.
Normally, radio waves reflected in phase produce
stronger signals, while those reflected out of phase
produce a weak or fading signal.
Ionospheric reflection occurs when certain radio
waves strike a thin, highly ionized layer in the
ionosphere. Although the radio waves are actually
refracted, some may be bent back so rapidly that they
appear to be reflected. For ionospheric reflection to
occur, the highly ionized layer can be approximately
no thicker than one wavelength of the wave. Since
the ionized layers are often several miles thick,
ionospheric reflection mostly occurs at long wavelengths (low frequencies).
Figure 1-10.—Diffraction around an object.
ATMOSPHERIC EFFECTS
ON PROPAGATION
As we stated earlier, changes in the ionosphere
can produce dramatic changes in the ability to
communicate.
In some cases, communications
distances are greatly extended. In other cases,
communications distances are greatly reduced or
eliminated. The paragraphs below explain the major
problem of reduced communications because of the
phenomena of fading and selective fading.
DIFFRACTION
Fading
Diffraction is the ability of radio waves to turn
sharp corners and bend around obstacles. Shown in
figure 1-10, diffraction results in a change of direction
of part of the radio-wave energy around the edges of
an obstacle. Radio waves with long wavelengths
compared to the diameter of an obstruction are easily
propagated around the obstruction. However, as the
wavelength decreases, the obstruction causes more
and more attenuation, until at very-high frequencies
a definite shadow zone develops. The shadow zone
is basically a blank area on the opposite side of an
obstruction in line-of-sight from the transmitter to the
receiver.
The most troublesome and frustrating problem in
receiving radio signals is variations in signal strength,
most commonly known as FADING.
Several
conditions can produce fading. When a radio wave
is refracted by the ionosphere or reflected from the
earth’s surface, random changes in the polarization
of the wave may occur. Vertically and horizontally
mounted receiving antennas are designed to receive
vertically and horizontally polarized waves, respectively. Therefore, changes in polarization cause
changes in the received signal level because of the
inability of the antenna to receive polarization changes.
Diffraction can extend the radio range beyond the
horizon. By using high power and low-frequencies,
radio waves can be made to encircle the earth by
diffraction.
Fading also results from absorption of the rf energy
in the ionosphere. Most ionospheric absorption occurs
in the lower regions of the ionosphere where ionization
1-8
density is the greatest. As a radio wave passes into
the ionosphere, it loses some of its energy to the free
electrons and ions present there. Since the amount of
absorption of the radio-wave energy varies with the
density of the ionospheric layers, there is no fixed
relationship between distance and signal strength for
ionospheric propagation. Absorption fading occurs for
a longer period than other types of fading, since
absorption takes place slowly. Under certain
conditions, the absorption of energy is so great that
communication over any distance beyond the line of
sight becomes difficult.
Figure 1-11.—Multipath transmission.
Multipath fading may be minimized by practices
called SPACE DIVERSITY and FREQUENCY
DIVERSITY In space diversity, two or more receiving
antennas are spaced some distance apart. Fading
does not occur simultaneously at both antennas.
Therefore, enough output is almost always available
from one of the antennas to provide a useful signal.
Although fading because of absorption is the
most serious type of fading, fading on the ionospheric
circuits is mainly a result of multipath propagation.
Multipath Fading
MULTIPATH is simply a term used to describe
the multiple paths a radio wave may follow between
transmitter and receiver. Such propagation paths
include the ground wave, ionospheric refraction,
reradiation by the ionospheric layers, reflection from
the earth’s surface or from more than one ionospheric
layer, and so on. Figure 1-11 shows a few of the paths
that a signal can travel between two sites in a typical
circuit. One path, XYZ, is the basic ground wave.
Another path, XFZ, refracts the wave at the F layer
and passes it on to the receiver at point Z. At point Z,
the received signal is a combination of the ground
wave and the sky wave. These two signals, having
traveled different paths, arrive at point Z at different
times. Thus, the arriving waves may or may not be in
phase with each other. A similar situation may result
at point A. Another path, XFZFA, results from a
greater angle of incidence and two refractions from
the F layer. A wave traveling that path and one
traveling the XEA path may or may not arrive at
point A in phase. Radio waves that are received in
phase reinforce each other and produce a stronger
signal at the receiving site, while those that are
received out of phase produce a weak or fading
signal. Small alterations in the transmission path
may change the phase relationship of the two signals,
causing periodic fading.
In frequency diversity, two transmitters and two
receivers are used, each pair tuned to a different
frequency, with the same information being
transmitted simultaneously over both frequencies.
One of the two receivers will almost always produce a
useful signal.
Selective Fading
Fading resulting from multipath propagation
varies with frequency since each frequency arrives at
the
receiving point via a different radio path. When a
wide
band
of
frequencies
is
transmitted
simultaneously,
each frequency will vary in the amount of fading.
This variation is called SELECTIVE FADING. When
selective fading occurs, all frequencies of the
transmitted signal do not retain their original phases
and relative amplitudes. This fading causes severe
distortion of the signal and limits the total signal
transmitted.
Frequency shifts and distance changes because
of daily variations of the different ionospheric layers
are summarized in table 1-1.
1-9
Table 1-1.–Daily Ionospheric Communications
D LAYER: reflects vlf waves for long-range
communications; refracts lf and mf for
short-range communications; has little
effect on vhf and above; gone at night.
E LAYER: depends on the angle of the sun:
refracts hf waves during the day up to 20
MHz to distances of 1200 miles: greatly
reduced at night.
F LAYER:
the time
consists
into two
structure and density depend on
of day and the angle of the sun:
of one layer at night and splits
layers during daylight hours.
F1 LAYER: density depends on the angle of
the sun; its main effect is to absorb hf
waves passing through to the F2 layer.
F2 LAYER: provides long-range hf communications; very variable; height and density
change with time of day, season, and sunspot activity.
Figure 1-12.—Ionospheric
layers.
of these layers is greatest during the summer. The
F2 layer is just the opposite. Its ionization is greatest
during the winter, Therefore, operating frequencies
for F2 layer propagation are higher in the winter than
in the summer.
OTHER PHENOMENA THAT AFFECT
COMMUNICATIONS
Although daily changes in the ionosphere have
the greatest effect on communications, other phenomena also affect communications, both positively and
negatively. Those phenomena are discussed briefly
in the following paragraphs.
SUNSPOTS
One of the most notable occurrences on the surface
of the sun is the appearance and disappearance of dark,
irregularly shaped areas known as SUNSPOTS.
Sunspots are believed to be caused by violent eruptions
on the sun and are characterized by strong magnetic
fields.
These sunspots cause variations in the
ionization level of the ionosphere.
SEASONAL VARIATIONS IN THE
IONOSPHERE
Seasonal variations are the result of the earth’s
revolving around the sun, because the relative position
of the sun moves from one hemisphere to the other
with the changes in seasons. Seasonal variations of
the D, E, and F1 layers are directly related to the
highest angle of the sun, meaning the ionization density
Sunspots tend to appear in two cycles, every 27
days and every 11 years.
1-10
extend up to several hundred miles into the ionosphere.
This condition may be either harmful or helpful to
radio-wave propagation.
Twenty-Seven Day Cycle
The number of sunspots present at any one time
is constantly changing as some disappear and new ones
emerge. As the sun rotates on its own axis, these
sunspots are visible at 27-day intervals, which is the
approximate period for the sun to make one complete
revolution. During this time period, the fluctuations
in ionization are greatest in the F2 layer. For this
reason, calculating critical frequencies for long-distance
communications for the F2 layer is not possible and
allowances for fluctuations must be made.
On the harmful side, sporadic E may blank out
the use of higher more favorable layers or cause
additional absorption of radio waves at some frequencies. It can also cause additional multipath problems
and delay the arrival times of the rays of RF energy.
On the helpful side, the critical frequency of the
sporadic E can be greater than double the critical
frequency of the normal ionospheric layers. This may
permit long-distance communications with unusually
high frequencies. It may also permit short-distance
communications to locations that would normally be
in the skip zone.
Eleven-Year Cycle
Sunspots can occur unexpectedly, and the life span
variable.
The
is
sunspots
of individual
ELEVEN-YEAR SUN SPOT CYCLE is a regular
cycle of sunspot activity that has a minimum and
maximum level of activity that occurs every 11 years.
During periods of maximum activity, the ionization
density of all the layers increases. Because of this,
the absorption in the D layer increases and the critical
frequencies for the E, F1, and F2 layers are higher.
During these times, higher operating frequencies must
be used for long-range communications.
IRREGULAR
Sporadic E can appear and disappear in a short
time during the day or night and usually does not occur
at same time for all transmitting or receiving stations.
Sudden Ionospheric Disturbances
Commonly known as SID, these disturbances may
occur without warning and may last for a few minutes
to several hours. When SID occurs, long-range hf
communications are almost totally blanked out. The
radio operator listening during this time will believe
his or her receiver has gone dead.
VARIATIONS
Irregular variations are just that, unpredictable
changes in the ionosphere that can drastically affect
our ability to communicate. The more common
variations are sporadic E, ionospheric disturbances,
and ionospheric storms.
The occurrence of SID is caused by a bright solar
eruption producing an unusually intense burst of
ultraviolet light that is not absorbed by the F1, F2,
or E layers. Instead, it causes the D-layer ionization
density to greatly increase. As a result, frequencies
above 1 or 2 megahertz are unable to penetrate the
D layer and are completely absorbed.
Sporadic E
Irregular cloud-like patches of unusually high
ionization, called the sporadic E, often format heights
near the normal E layer. Their exact cause is not
known and their occurrence cannot be predicted.
However, sporadic E is known to vary significantly
with latitude. In the northern latitudes, it appears to
be closely related to the aurora borealis or northern
lights.
Ionospheric Storms
Ionospheric storms are caused by disturbances in
the earth’s magnetic field. They are associated with
both solar eruptions and the 27-day cycle, meaning
they are related to the rotation of the sun. The effects
of ionospheric storms are a turbulent ionosphere and
very erratic sky-wave propagation. The storms affect
mostly the F2 layer, reducing its ion density and
causing the critical frequencies to be lower than
The sporadic E layer can be so thin that radio
waves penetrate it easily and are returned to earth by
the upper layers, or it can be heavily ionized and
1-11
snowflake, the scattering and absorption losses are
difficult to compute, but will be less than those caused
by raindrops.
normal. What this means for communication purposes
is that the range of frequencies on a given circuit is
smaller than normal and that communications are
possible only at lower working frequencies.
HAIL.— Attenuation by hail is determined by the
size of the stones and their density. Attenuation of
radio waves by scattering because of hailstones is
considerably less than by rain.
Weather
Wind, air temperature, and water content of the
atmosphere can combine either to extend radio
communications or to greatly attenuate wave propagation. making normal communications extremely
difficult. Precipitation in the atmosphere has its
greatest effect on the higher frequency ranges.
Frequencies in the hf range and below show little effect
from this condition.
TEMPERATURE
INVERSION
When layers of warm air form above layers of
cold air, the condition known as temperature inversion
develops. This phenomenon causes ducts or channels
to be formed, by sandwiching cool air either between
the surface of the earth and a layer of warm air, or
between two layers of warm air. If a transmitting
antenna extends into such a duct, or if the radio wave
enters the duct at a very low angle of incidence, vhf
and uhf transmissions may be propagated far beyond
normal line-of-sight distances. These long distances
are possible because of the different densities and
refractive qualities of warm and cool air. The sudden
change in densities when a radio wave enters the warm
air above the duct causes the wave to be refracted back
toward earth. When the wave strikes the earth or a
warm layer below the duct, it is again reflected or
refracted upward and proceeds on through the duct
with a multiple-hop type of action. An example of
radio-wave propagation by ducting is shown in figure
1-14.
RAIN.— Attenuation because of raindrops is greater
than attenuation for any other form of precipitation.
Raindrop attenuation may be caused either by
absorption, where the raindrop acts as a poor dielectric,
absorbs power from the radio wave and dissipates the
power by heat loss; or by scattering (fig. 1-13).
Raindrops cause greater attenuation by scattering than
by absorption at frequencies above 100 megahertz.
At frequencies above 6 gigahertz, attenuation by
raindrop scatter is even greater.
Figure 1-13.–Rf energy losses from
scattering.
FOG.— Since fog remains suspended in the
atmosphere, the attenuation is determined by the
quantity of water per unit volume (density of the fog)
and by the size of the droplets. Attenuation because
of fog has little effect on frequencies lower than 2
gigahertz, but can cause serious attenuation by
absorption at frequencies above 2 gigahertz.
Figure 1-14.—Duct effect caused by temperature
inversion.
TRANSMISSION
LOSSES
All radio waves propagated over the ionosphere
undergo energy losses before arriving at the receiving
site. As we discussed earlier, absorption and lower
SNOW.— Since snow has about 1/8 the density
of rain, and because of the irregular shape of the
1-12
atmospheric levels in the ionosphere account for a
large part of these energy losses. There are two other
types of losses that also significantly affect
propagation. These losses are known as ground
reflection losses and freespace loss. The combined
effect of absorption ground reflection loss, and
freespace loss account for most of the losses of radio
transmissions propagated in the ionosphere.
GROUND REFLECTION LOSS
When propagation is accomplished via multihop
refraction, rf energy is lost each time the radio wave
is reflected from the earth’s surface. The amount of
energy lost depends on the frequency of the wave, the
angle of incidence, ground irregularities, and the
electrical conductivity of the point of reflection.
Figure 1-15.—Freespace loss principle.
MAXIMUM USABLE FREQUENCY
FREESPACE LOSS
The higher the frequency of a radio wave, the
lower the rate of refraction by the ionosphere.
Therefore, for a given angle of incidence and time of
day, there is a maximum frequency that can be used
for communications between two given locations. This
frequency is known as the MAXIMUM USABLE
FREQUENCY (muf).
Normally, the major loss of energy is because of
the spreading out of the wavefront as it travels from
the transmitter. As distance increases, the area of the
wavefront spreads out, much like the beam of a
flashlight. This means the amount of energy
contained within any unit of area on the wavefront
decreases as distance increases. By the time the
energy arrives at the receiving antenna, the
wavefront is so spread out that the receiving antenna
extends into only a small portion of the wavefront.
This is illustrated in figure 1-15.
Waves at frequencies above the muf are
normally refracted so slowly that they return to earth
beyond the desired location or pass on through the
ionosphere and are lost. Variations in the ionosphere
that can raise or lower a predetermined muf may
occur at anytime. his is especially true for the highly
variable F2 layer.
FREQUENCY SELECTION
You must have a thorough knowledge of radiowave propagation to exercise good judgment when
selecting transmitting and receiving antennas and
operating frequencies. Selecting a usable operating
frequency within your given allocations and
availability is of prime importance to maintaining
reliable communications.
LOWEST USABLE FREQUENCY
Just as there is a muf that can be used for
communications between two points, there is also a
minimum operating frequency that can be used
known as the LOWEST USABLE FREQUENCY (luf).
As the frequency of a radio wave is lowered, the rate
of refraction increases. So a wave whose frequency is
below the established luf is refracted back to earth at
a shorter distance than desired, as shown in figure 116.
For successful communication between any two
specified locations at any given time of the day, there
is a maximum frequency, a lowest frequency and an
optimum frequency that can be used.
1-13
properties
of
the
ionosphere,
absorption
considerations, and the amount of noise present.
OPTIMUM WORKING FREQUENCY
The most practical operating frequency is one
that you can rely onto have the least number of
problems. It should be high enough to avoid the
problems of multipath fading, absorption, and noise
encountered at the lower frequencies; but not so high
as to be affected by the adverse effects of rapid
changes in the ionosphere.
A frequency that meets the above criteria is
known as the OPTIMUM WORKING FREQUENCY
It is abbreviated “fot” from the initial letters of the
French words for optimum working frequency,
“frequence optimum de travail.” The fot is roughly
about 85% of the muf, but the actual percentage
varies and may be considerably more or less than 85
percent.
Figure 1-16.—Refraction of frequencies below
the lowest usable frequency (luf).
As a frequency is lowered, absorption of the radio
wave increases. A wave whose frequency is too low is
absorbed to such an extent that it is too weak for
reception. Atmospheric noise is also greater at lower
frequencies. A combination of higher absorption and
atmospheric noise could result in an unacceptable
signal-to-noise ratio.
In this chapter, we discussed the basics of radiowave propagation and how atmospheric conditions
determine the operating parameters needed to ensure
successful communications. In chapter 2, we will
discuss basic antenna operation and design to
complete your understanding
of
radio-wave
propagation.
For a given angle ionospheric conditions, of
incidence and set of the luf depends on the refraction
1-14
CHAPTER 2
ANTENNAS
As an Electronics Technician, you are responsible
for maintaining systems that both radiate and receive
electromagnetic energy. Each of these systems requires
some type of antenna to make use of this electromagnetic energy. In this chapter we will discuss antenna
characteristics, different antenna types, antenna tuning,
and antenna safety.
either vertically or horizontally. Marconi antennas
operate with one end grounded and are mounted
perpendicular to the earth or a surface acting as a
ground. The Hertz antenna, also referred to as a
dipole, is the basis for some of the more complex
antenna systems used today. Hertz antennas are
generally used for operating frequencies of 2 MHz
and above, while Marconi antennas are used for
operating frequencies below 2 MHz.
ANTENNA CHARACTERISTICS
An antenna may be defined as a conductor or group
of conductors used either for radiating electromagnetic
energy into space or for collecting it from space.
Electrical energy from the transmitter is converted
into electromagnetic energy by the antenna and radiated
into space. On the receiving end, electromagnetic
energy is converted into electrical energy by the
antenna and fed into the receiver.
All antennas, regardless of their shape or size, have
four basic characteristics: reciprocity, directivity, gain,
and polarization.
RECIPROCITY
RECIPROCITY is the ability to use the same
antenna for both transmitting and receiving. The
electrical characteristics of an antenna apply equally,
regardless of whether you use the antenna for
transmitting or receiving. The more efficient an
antenna is for transmitting a certain frequency, the
more efficient it will be as a receiving antenna for
the same frequency. This is illustrated by figure 2-1,
view A. When the antenna is used for transmitting,
maximum radiation occurs at right angles to its axis.
When the same antenna is used for receiving (view
B), its best reception is along the same path; that is,
at right angles to the axis of the antenna.
The electromagnetic radiation from an antenna
is made up of two components, the E field and the
H field. The total energy in the radiated wave remains
constant in space except for some absorption of energy
by the earth. However, as the wave advances, the
energy spreads out over a greater area. This causes
the amount of energy in a given area to decrease as
distance from the source increases.
The design of the antenna system is very important
in a transmitting station. The antenna must be able
to radiate efficiently so the power supplied by the
transmitter is not wasted. An efficient transmitting
antenna must have exact dimensions, determined by
the frequency being transmitted. The dimensions of
the receiving antenna are not critical for relatively low
frequencies, but their importance increases drastically
as the transmitted frequency increases.
DIRECTIVITY
The DIRECTIVITY of an antenna or array is a
measure of the antenna’s ability to focus the energy
in one or more specific directions. You can determine
an antenna’s directivity by looking at its radiation
pattern. In an array propagating a given amount of
energy, more radiation takes place in certain directions
than in others. The elements in the array can be
arranged so they change the pattern and distribute the
energy more evenly in all directions. The opposite
is also possible. The elements can be arranged so the
radiated energy is focused in one direction. The
Most practical transmitting antennas are divided
into two basic classifications, HERTZ ANTENNAS
(half-wave) and MARCONI (quarter-wave) ANTENNAS. Hertz antennas are generally installed some
distance above the ground and are positioned to radiate
2-1
Figure 2-2.—Horizontal and vertical polarization.
The radiation field is made up of magnetic and
electric lines of force that are always at right angles
to each other. Most electromagnetic fields in space
are said to be linearly polarized. The direction of
polarization is the direction of the electric vector. That
is, if the electric lines of force (E lines) are horizontal,
the wave is said to be horizontally polarized (fig. 2-2),
and if the E lines are vertical, the wave is said to be
vertically polarized. Since the electric field is parallel
to the axis of the dipole, the antenna is in the plane
of polarization.
Figure 2-1.—Reciprocity of antennas.
elements can be considered as a group of antennas
fed from a common source.
GAIN
As we mentioned earlier, some antennas are highly
directional. That is, they propagate more energy in
certain directions than in others. The ratio between
the amount of energy propagated in these directions
and the energy that would be propagated if the antenna
were not directional is known as antenna GAIN. The
gain of an antenna is constant. whether the antenna
is used for transmitting or receiving.
A horizontally placed antenna produces a horizontally polarized wave, and a vertically placed antenna
produces a vertically polarized wave.
In general, the polarization of a wave does not
change over short distances. Therefore, transmitting
and receiving antennas are oriented alike, especially
if they are separated by short distances.
POLARIZATION
Over long distances, polarization changes. The
change is usually small at low frequencies, but quite
drastic at high frequencies. (For radar transmissions,
a received signal is actually a wave reflected from
an object. Since signal polarization varies with the
type of object, no set position of the receiving antenna
is correct for all returning signals). Where separate
antennas are used for transmitting and receiving, the
receiving antenna is generally polarized in the same
direction as the transmitting antenna.
Energy from an antenna is radiated in the form
of an expanding sphere. A small section of this sphere
is called a wavefront. positioned perpendicular to the
direction of the radiation field (fig. 2-2). Within this
wavefront. all energy is in phase. Usually, all points
on the wavefront are an equal distance from the
antenna. The farther from the antenna the wave is,
the less curved it appears. At a considerable distance,
the wavefront can be considered as a plane surface
at right angles to the direction of propagation.
2-2
The distance the wave travels during the period of
1 cycle is known as the wavelength. It is found by
dividing the rate of travel by the frequency.
When the transmitting antenna is close to the
ground, it should be polarized vertically, because
vertically polarized waves produce a greater signal
strength along the earth’s surface. On the other hand,
when the transmitting antenna is high above the
ground, it should be horizontally polarized to get the
greatest signal strength possible to the earth’s surface.
Look at the current and voltage distribution on
the antenna in figure 2-4. A maximum movement
of electrons is in the center of the antenna at all times;
therefore, the center of the antenna is at a low
impedance.
RADIATION OF ELECTROMAGNETIC
ENERGY
Various factors in the antenna circuit affect the
radiation of electromagnetic energy. In figure 2-3,
for example, if an alternating current is applied to the
A end of wire antenna AB, the wave will travel along
the wire until it reaches the B end. Since the B end
is free, an open circuit exists and the wave cannot
travel further. This is a point of high impedance.
The wave bounces back (reflects) from this point of
high impedance and travels toward the starting point,
where it is again reflected. Theoretically, the energy
of the wave should be gradually dissipated by the
resistance of the wire during this back-and-forth motion
(oscillation). However, each time the wave reaches
the starting point, it is reinforced by an impulse of
energy sufficient to replace the energy lost during its
travel along the wire. This results in continuous
oscillations of energy along the wire and a high voltage
at the A end of the wire. These oscillations move
along the antenna at a rate equal to the frequency of
the rf voltage and are sustained by properly timed
impulses at point A.
Figure 2-4.—Standing waves of current and voltage on
an antenna.
Figure 2-3.—Antenna and rf source.
This condition is called a STANDING WAVE of
current. The points of high current and high voltage
are known as current and voltage LOOPS. The points
of minimum current and minimum voltage are known
as current and voltage NODES. View A shows a
current loop and two current nodes. View B shows
two voltage loops and a voltage node. View C shows
The rate at which the wave travels along the wire
is constant at approximately 300,000,000 meters per
second. The length of the antenna must be such that
a wave will travel from one end to the other and back
again during the period of 1 cycle of the rf voltage.
2-3
the resultant voltage and current loops and nodes.
The presence of standing waves describes the condition
of resonance in an antenna. At resonance, the waves
travel back and forth in the antenna, reinforcing each
other, and are transmitted into space at maximum
radiation. When the antenna is not at resonance, the
waves tend to cancel each other and energy is lost
in the form of heat.
RADIATION TYPES AND PATTERNS
A logical assumption is that energy leaving an
antenna radiates equally over 360 degrees. This is
not the case for every antenna.
The energy radiated from an antenna forms a field
having a definite RADIATION PATTERN. The
radiation pattern for any given antenna is determined
by measuring the radiated energy at various angles
at constant distances from the antenna and then plotting
the energy values on a graph. The shape of this pattern
depends on the type of antenna being used.
Some antennas radiate energy equally in all
directions.
Radiation of this type is known as
ISOTROPIC RADIATION. The sun is a good
example of an isotropic radiator. If you were to
measure the amount of radiated energy around the
sun’s circumference, the readings would all be fairly
equal (fig. 2-5).
Most radiators emit (radiate) energy more strongly
in one direction than in another. These radiators are
referred to as ANISOTROPIC radiators. A flashlight
is a good example of an anisotropic radiator (fig. 2-6).
The beam of the flashlight lights only a portion of
the space surrounding it. The area behind the flashlight
remains unlit, while the area in front and to either side
is illuminated.
Figure 2-5.—Isotropic radiation graphs.
you should learn to use the appropriate terminology,
In general, major lobes are those in which the greatest
amount of radiation occurs. Minor lobes are those
in which the least amount of radiation occurs.
MAJOR AND MINOR LOBES
The pattern shown in figure 2-7, view B, has
radiation concentrated in two lobes. The radiation
intensity in one lobe is considerably stronger than in
the other. The lobe toward point X is called a MAJOR
LOBE; the other is a MINOR LOBE. Since the
complex radiation patterns associated with antennas
frequently contain several lobes of varying intensity,
ANTENNA LOADING
There will be times when you may want to use
one antenna system to transmit on several different
frequencies. Since the antenna must always be in
resonance with the applied frequency, you must either
lengthen it or shorten it to produce the required
2-4
Figure 2-7.—Major and minor lobes.
antenna is too long for the transmitting frequency, it
will be resonant at a lower frequency and offers an
Inductive reactance can be
inductive reactance.
compensated for by introducing a lumped capacitive
reactance, as shown in view B. An antenna with
normal loading is represented in view C.
Figure 2-6.—Anisotropic radiator.
Changing the antenna dimensions
resonance.
physically is impractical, but changing them electrically
is relatively simple. To change the electrical length
of an antenna, you can insert either an inductor or a
capacitor in series with the antenna. This is shown
in figure 2-8, views A and B. Changing the electrical
length
by
this
method
is
known
as
LUMPED-IMPEDANCE TUNING or LOADING.
If the antenna is too short for the wavelength being
used, it will be resonant at a higher frequency.
Therefore, it offers a capacitive reactance at the
excitation frequency. This capacitive reactance can
be compensated for by introducing a lumped inductive
reactance, as shown in view A. Similarly, if the
Figure 2-8.—Electrical antenna loading.
GROUND EFFECTS
As we discussed earlier, ground losses affect
radiation patterns and cause high signal losses for some
frequencies. Such losses can be greatly reduced if
a good conducting ground is provided in the vicinity
of the antenna. This is the purpose of the GROUND
SCREEN (fig. 2-9, view A) and COUNTERPOISE
(fig. 2-9, view B).
2-5
COMMUNICATIONS ANTENNAS
Some antennas can be used in both shore-based
and ship-based applications. Others, however, are
designed to be used primarily in one application or
the other. The following paragraphs discuss, by
frequency range, antennas used for shore-based
communications.
VERY LOW FREQUENCY (VLF)
The main difficulty in vlf and lf antenna design
is the physical disparity between the maximum
practical size of the antenna and the wavelength of
the frequency it must propagate. These antennas must
be large to compensate for wavelength and power
handling requirements (0.25 to 2 MW), Transmitting
antennas for vlf have multiple towers 600 to 1500
feet high, an extensive flat top for capacitive loading, and a copper ground system for reducing ground
losses. Capacitive top-loading increases the bandwidth
characteristics, while the ground plane improves
radiation efficiency.
Representative antenna configurations are shown
in figures 2-10 through 2-12. Variations of these basic
antennas are used at the majority of the Navy vlf sites.
Figure 2-9.—Ground screen and
counterpoise.
The ground screen in view A is composed of a
series of conductors arranged in a radial pattern and
buried 1 or 2 feet below the surface of the earth.
These conductors, each usually 1/2 wavelength long,
reduce ground absorption losses in the vicinity of the
antenna.
A counterpoise (view B) is used when easy access
to the base of the antenna is necessary. It is also used
when the area below the antenna is not a good
conducting surface, such as solid rock or ground that
is sandy. The counterpoise serves the same purpose
as the ground screen but is usually elevated above the
earth. No specific dimensions are necessary for a
counterpoise, nor is the number of wires particularly
critical. The primary requirement is that the counterpoise be insulated from ground and form a grid of
reflector elements for the antenna system.
Figure 2-10.—Triatic-type antenna.
2-6
Figure 2-12.—Trideco-type antenna.
Figure 2-11.—Goliath-type antenna.
HIGH FREQUENCY (HF)
LOW FREQUENCY (LF)
High-frequency (hf) radio antenna systems are used
to support many different types of circuits, including
ship-to-shore,
point-to-point, and ground-to-air
broadcast. These diverse applications require the use
of various numbers and types of antennas that we will
review on the following pages.
Antennas for lf are not quite as large as antennas
for vlf, but they still occupy a large surface area. Two
examples of If antenna design are shown in figures
2-13 and 2-14. The Pan polar antenna (fig. 2-1 3) is
an umbrella top-loaded monopole. It has three loading
loops spaced 120 degrees apart, interconnected between
the tower guy cables. Two of the loops terminate at
ground, while the other is used as a feed. The NORD
antenna (fig. 2-14), based on the the folded-unipole
principle, is a vertical tower radiator grounded at the
base and fed by one or more wires connected to the
top of the tower. The three top loading wires extend
from the top of the antenna at 120-degree intervals
to three terminating towers. Each loading wire has
a length approximately equal to the height of the main
tower plus 100 feet. The top loading wires are
insulated from ground and their tower supports are
one-third the height of the transmitting antenna.
Yagi
The Yagi antenna is an end-fired parasitic array.
It is constructed of parallel and coplaner dipole
elements arranged along a line perpendicular to the
axis of the dipoles, as illustrated in figure 2-15. The
most limiting characteristic of the Yagi antenna is its
extremely narrow bandwidth. Three percent of the
center frequency is considered to be an acceptable
bandwidth ratio for a Yagi antenna. The width of
the array is determined by the lengths of the elements.
The length of each element is approximately one-half
2-7
Figure 2-13.—Pan polar antenna.
wavelength, depending on its intended use (driver,
reflector, or director). The required length of the array
depends on the desired gain and directivity. Typically,
the length will vary from 0.3 wavelength for
three-element arrays, to 3 wavelengths for arrays with
numerous elements. For hf applications, the maximum
practical array length is 2 wavelengths. The array’s
height above ground will determine its vertical
radiation angle. Normally, array heights vary from
0.25 to 2.5 wavelengths. The dipole elements are
usually constructed from tubing, which provides for
better gain and bandwidth characteristics and provides
sufficient mechanical rigidity for self-support. Yagi
arrays of four elements or less are not structurally
complicated. Longer arrays and arrays for lower
frequencies, where the width of the array exceeds 40
feet, require elaborate booms and supporting structures.
Yagi arrays may be either fixed-position or rotatable.
impedance and pattern characteristics to be repeated
periodically with the logarithm of the driving frequency
is called a LOG-PERIODIC ANTENNA (LPA). The
LPA, in general, is a medium-power, high-gain,
moderately-directive antenna of extremely broad
bandwidth. Bandwidths of up to 15:1 are possible,
with up to 15 dB power gain. LPAs are rather
complex antenna systems and are relatively expensive.
The installation of LPAs is normally more difficult
than for other hf antennas because of the tower heights
involved and the complexity of suspending the
radiating elements and feedlines from the towers.
Vertical Monopole LPA
The log-periodic vertical monopole antenna (fig.
2-16) has the plane containing the radiating elements
in a vertical field. The longest element is approximately one-quarter wavelength at the lower cutoff
frequency. The ground system for the monopole
arrangement provides the image equivalent of the other
quarter wavelength for the half-dipole radiating
elements. A typical vertical monopole designed to
LOG-PERIODIC ANTENNAS (LPAs)
An antenna arranged so the electrical length and
spacing between successive elements causes the input
2-8
Figure 2-14.—NORD antenna.
Figure 2-16.—Log-periodic vertical monopole
antenna.
Figure 2-15.—Yagi antenna.
2-9
cover a frequency range of 2 to 30 MHz requires one
tower approximately 140 feet high and an antenna
length of around 500 feet, with a ground system that
covers approximately 3 acres of land in the immediate
vicinity of the antenna.
Sector Log-Periodic Array
This version of a vertically polarized fixed-azimuth
LPA consists of four separate curtains supported by
a common central tower, as shown in figure 2-17.
Each of the four curtains operates independently,
providing antennas for a minimum of four transmit
or receive systems. and a choice of sector coverage.
The four curtains are also capable of radiating a rosette
pattern of overlapping sectors for full coverage, as
shown by the radiation pattern in figure 2-17. The
central supporting tower is constructed of steel and
may range to approximately 250 feet in height, with
the length of each curtain reaching 250 feet, depending
on its designed operating frequencies. A sector antenna
that uses a ground plane designed to cover the entire
hf spectrum takes up 4 to 6 acres of land area.
Figure 2-18.—Rotatable log-periodic antenna.
Rotatable LPA (RLPA)
RLPAs (fig. 2-18) are commonly used in
ship-to-shore-to-ship and in point-to-point ecm-u-nunications. Their distinct advantage is their ability to rotate
360 degrees. RLPAs are usually constructed with
either tubular or wire antenna elements. The RLPA
in figure 2-18 has wire elements strung on three
aluminum booms of equal length, spaced equally and
arranged radially about a central rotator on top of a
steel tower approximately 100 feet high.
The
frequency range of this antema is 6 to 32 MHz. The
gain is 12 dB with respect to isotropic antennas.
Power handling capability is 20 kw average, and vswr
is 2:1 over the frequency range.
INVERTED CONE ANTENNA
Inverted cone antennas are vertically polarized,
omnidirectional, and have an extremely broad
bandwidth. They are widely used for ship-to-shore
and ground-to-air communications. Inverted cone
antennas are installed over a radial ground plane
system and are supported by poles, as shown in figure
2-19. The equally-spaced vertical radiator wires
terminate in a feed ring assembly located at the bottom
center, where a 50-ohm coaxial transmission line feeds
the antenna. Inverted cones usually have gains of 1
to 5 dB above isotropic antennas, with a vswr not
Figure 2-17.—Sector LPA and its horizontal radiation
pattern.
2-10
Figure 2-19.—Inverted cone antenna.
greater than 2:1. They are considered medium- to
high-power radiators, with power handling capabilities
of 40 kW average power.
CONICAL MONOPOLE ANTENNA
Conical monopoles are used extensively in hf
communications. A conical monopole is an efficient
broadband, vertically polarized, omnidirectional antenna
in a compact size. Conical monopoles are shaped like
two truncated cones connected base-to-base. The basic
conical monopole configuration, shown in figure 2-20,
is composed of equally-spaced wire radiating elements
arranged in a circle around an aluminum center tower.
Usually, the radiating elements are connected to the
top and bottom discs, but on some versions, there is
a center waist disc where the top and bottom radiators
are connected. The conical monopole can handle up
to 40 kW of average power. Typical gain is -2 to +2
dB, with a vswr of up to 2.5:1.
RHOMBIC ANTENNA
Rhombic antennas can be characterized as
high-power, low-angle, high-gain, horizontallypolarized, highly-directive, broadband antennas of
simple, inexpensive construction. The rhombic antenna
(fig. 2-21) is a system of long-wire radiators that
depends on radiated wave interaction for its gain and
directivity. A properly designed rhombic antenna
presents to the transmission line an input impedance
insensitive to frequency variations up to 5:1. It
maintains a power gain above 9 dB anywhere within
At the design-center
a 2:1 frequency variation.
frequency, a gain of 17 dB is typical. The radiation
pattern produced by the four radiating legs of a
rhombic antenna is modified by reflections from the
earth under, and immediately in front of, the antenna.
Because of the importance of these ground
Figure 2-20.—Conical monopole antenna.
reflections in the proper formation of the main lobe,
the rhombic should be installed over reasonably smooth
and level ground. The main disadvantage of the
rhombic antenna is the requirement for a large land
area, usually 5 to 15 acres.
QUADRANT ANTENNA
The hf quadrant antenna (fig. 2-22) is a
used
in
antenna
receiving
special-purpose
ground-to-air-to-ground communications. It is unique
among horizontally-polarized antennas because its
2-11
Figure 2-21.—Three-wire rhombic antenna.
element arrangement makes possible a radiation pattern resembling that of a vertically-polarized,
omnidirectional antenna. Construction and installation
of this antenna is complex because of the physical
relationships between the individual elements and the
requirement for a separate transmission line for each
dipole. Approximately 2.2 acres of land are required
to accommodate the quadrant antenna.
2-12
Figure 2-22.—Quadrant antenna.
The self-supporting feature of the whip makes it
particularly useful where space is limited. Whips can
be tilted, a design feature that makes them suited for
use along the edges of aircraft carrier flight decks.
Aboard submarines, they can be retracted into the sail
structure.
WHIP ANTENNAS
Hf whip antennas (fig. 2-23) are vertically-polarized
omnidirectional monopoles that are used for
short-range, ship-to-shore and transportable communications systems. Whip antennas are made of tubular
metal or fiberglass, and vary in length from 12 feet
to 35 feet, with the latter being the most prevalent.
Although whips are not considered as highly efficient
antennas, their ease of installation and low cost provide
a compromise for receiving and low-to-medium power
transmitting installations.
Most whip antennas require some sort of tuning
system and a ground plane to improve their radiation
efficiency throughout the hf spectrum. Without an
antenna tuning system, whips generally have a narrow
bandwidth and are limited in their power handling
2-13
Figure 2-24.—Vertical fan antenna.
each cut for one-quarter wavelength at the lowest
frequency to be used. The wires are fanned 30 degrees
between adjacent wires. The fan antenna provides
satisfactory performance and is designed for use as
a random shipboard antenna in the hf range (2-30
MHz).
DISCAGE ANTENNA
The discage antenna (fig. 2-25) is a broadband
omnidirectional antenna. The diseage structure consists
of two truncated wire rope cones attached base-to-base
and supported by a central mast. The lower portion
of the structure operates as a cage monopole for the
4- to 12-MHz frequency range. The upper portion
operates as a discone radiator in the 10- to 30-MHz
frequency range. Matching networks limit the vswr
to not greater than 3:1 at each feed point.
Vinyl-covered phosphor bronze wire rope is used
for the wire portions. The support mast and other
portions are aluminum.
Figure 2-23.—Whip antennas.
capabilities. Power ratings for most whips range from
1 to 5 kW PEP.
VHF/UHF
WIRE-ROPE FAN ANTENNAS
At vhf and uhf frequencies, the shorter wavelength
makes the physical size of the antenna relatively small.
Aboard ship these antennas are installed as high as
Figure 2-24 shows a five-wire vertical fan antenna.
This is a broadband antenna composed of five wires,
2-14
.
Figure 2-25.—AS-2802/SCR discage antenna.
possible and away from any obstructions. The reason
for the high installation is that vertical conductors,
such as masts, rigging, and cables in the vicinity, cause
unwanted directivity in the radiation pattern.
For best results in the vhf and uhf ranges, both
transmitting and receiving antennas must have the same
polarization. Vertically polarized antennas (primarily
dipoles) are used for all ship-to-ship, ship-to-shore,
and air-to-ground vhf and uhf communications.
The following paragraphs describe the most
common uhf/vhf dipole antennas. All the examples
are vertically-polarized, omnidirectional, broadband
antennas.
Biconical Dipole
The biconical dipole antenna (fig. 2-26) is designed
for use at a normal rf power rating of around 250
watts, with a vswr not greater than 2:1. All major
components of the radiating and support structures
are aluminum. The central feed section is protected
and waterproofed by a laminated fiberglass cover.
Figure 2-26.—AS-2811/SCR biconical dipole
antenna.
2-15
AT-150/SRC (fig. 2-28, view A) has vertical radiating
elements and a balun arrangement that electrically
balances the antenna to ground.
Center-Fed Dipole
The center-fed dipole (fig. 2-27) is designed for
use at an average power rating of 100 watts. All major
components of the radiating and support structures
are aluminum. The central feed section and radiating
elements are protected by a laminated fiberglass cover.
Center-fed dipole antennas range from 29 to 47 inches
in height and have a radiator diameter of up to 3
inches.
Figure 2-28, view B, shows an AS-390/SRC
antenna assembly. This antenna is an unbalanced
broadband coaxial stub antenna. It consists of a
radiator and a ground plane. The ground plane (or
counterpoise) consists of eight elements bent downward
37 degrees from horizontal. The lower ends of the
elements form points of a circle 23 inches in diameter.
The lower section of the radiator assembly contains
a stub for adjusting the input impedance of the antenna.
The antenna is vertically polarized, with an rf power
rating of 200 watts, and a vswr not greater than 2:1.
Coaxial Dipole
Figure 2-28 shows two types of coaxial dipoles.
The coaxial dipole antenna is designed for use in the
uhf range, with an rf power rating of 200 watts. The
SATELLITE SYSTEMS
The Navy Satellite Communication System
links,
(SATCOM) provides communications
via satellites, between designated mobile units and
shore sites. These links supply worldwide communications coverage. The following paragraphs describe
some of the more common SATCOM antenna systems
to which you will be exposed.
AS-2815/SRR-1
The AS-2815/SSR-1 fleet broadcast receiving
antenna (fig. 2-29) has a fixed 360-degree horizontal
pattern with a maximum gain of 4 dB at 90 degrees
from the antenna’s horizontal plane. The maximum
loss in the antenna’s vertical pattern sector is 2 dB.
The vswr is less than 1.5:1, referenced to 50 ohms.
This antenna should be positioned to protect it from
interference and possible front end burnout from radar
and uhf transmitters.
ANTENNA GROUPS OE-82B/WSC-1(V)
AND OE-82C/WSC-1(V)
Designed primarily for shipboard installations, these
antenna groups interface with the AN/WSC-3
transceiver. The complete installation consists of an
antenna, bandpass amplifier-filter, switching unit, and
antenna control (figs. 2-30 and 2-31), Depending on
requirements, one or two antennas may be installed
to provide a view of the satellite at all times. The
antenna assembly is attached to a pedestal that permits
Figure 2-27.—AS-2809/RC center-fed dipole antenna.
2-16
Figure 2-28.—Coaxial dipole.
it to rotate 360 degrees and to elevate from near
horizontal to approximately 20 degrees beyond zenith
(elevation angles from +2 to +110 degrees). The
antenna tracks automatically in azimuth and manually
in elevation. Frequency bands are 248-272 MHz for
receive and 292-312 MHz for transmit. Polarization
is right-hand circular for both transmit and receive.
Antenna gain characteristics are nominally 12 dB in
transmit and 11 dB in receive.
AN/WSC-5(V) SHORE STATION
ANTENNA
The AN/WSC-5(V) shore station antenna (fig. 2-32)
consists of four OE-82A/WSC-1(V) backplane
assemblies installed on a pedestal. This antenna is
intended for use with the AN/WSC-5(V) transceiver
at major shore stations. The antenna is oriented
manually and can be locked in position to receive
maximum signal strength upon capture of the satellite
signal. Hemispherical coverage is 0 to 110 degrees
above the horizon. Polarization is right-hand circular
in both transmit and receive. The antenna’s operating
frequency range is 240 to 318 MHz. With its mount,
Figure 2-29.—AS-2815/SSR-1 fleet broadcast
satellite receiving antenna.
2-17
Figure 2-30.—OE-82/WSC-1(V) antenna group.
Figure 2-31.—OE-82C/WSC-1(V) antenna group.
2-18
Figure 2-32.—OE-82A/WSC-1(V)/AN/WSC-5(V) shore
station antenna.
the antenna weighs 2500 pounds and is 15 feet high,
10 feet wide, and 10 feet deep. The gain characteristics of this antenna are nominally 15 dB in transmit
and 18 dB in receive.
Figure 2-33.—Andrew 58622 shore antenna.
ANDREW 58622 SHORE ANTENNA
The Andrew 58622 antenna (fig. 2-33) is a bifilar,
16-turn helical antenna right-hand circularly polarized,
with gain varying between 11.2 and 13.2 dB in the
240-315 MKz frequency band. It has a 39-inch ground
plate and is about 9 feet, 7 inches long. It can be
adjusted manually in azimuth and elevation. This
antenna is used at various shore installations, other
than NCTAMS, for transmit and receive operations.
AN/WSC-6(V) SHF SATCOM
ANTENNA
Figure 2-34.—AN/WSC-6(V) attenuation
scale.
The antennas used on current shf SATCOM
shipboard terminals are parabolic reflectors with
casseegrain feeds. These antennas provide for LPI (low
probability of intercept), with beamwidths less than
2.5 degrees (fig. 2-34). The reflectors are mounted
on three-axis pedestals and provide autotracking of
a beacon or communication signal by conical scanning
techniques. The antennas are radome enclosed and
include various electronic components. Both a 7-foot
model (fig. 2-35) and a 4-foot model (fig. 2-36) are
operational in the fleet.
2-19
Figure 2-35.—Seven-foot shf SATCOM antenna.
MATCHING NETWORKS
An antenna matching network consists of one or
more parts (such as coils, capacitors, and lengths of
transmission line) connected in series or parallel with
the transmission line to reduce the standing wave ratio
on the line. Matching networks are usually adjusted
when they are installed and require no further
adjustment for proper operation. Figure 2-37 shows
a matching network outside of the antenna feedbox,
with a sample matching network schematic.
Matching networks can also be built with variable
components so they can be used for impedance
matching over a range of frequencies. These networks
are called antenna tuners.
Antenna tuners are usually adjusted automatically
or manually each time the operating frequency is
changed. Standard tuners are made with integral
enclosures. Installation consists simply of mounting
Figure 2-36.—Four-foot shf SATCOM antenna.
2-20
means that the antenna does not physically change
length; instead, it is adapted electrically to the output
frequency of the transmitter and “appears” to change
its physical length. Antenna tuning is done by using
antenna couplers, tuners, and multicouplers.
Antenna couplers and tuners are used to match
a single transmitter or receiver to one antenna whereas
antenna multicouplers are used to match more than
one transmitter or receiver to o n e antenna for
simultaneous operation. Some of the many antenna
couplers that are in present use are addressed in the
following paragraphs. For specific information on
a particular coupler, refer to the appropriate equipment
technical manual.
Antenna Coupler Group AN/URA-38
Antenna Coupler Group AN/URA-38 is an
automatic antenna tuning system intended primarily
for use with the AN/URT-23(V) operating in the
high-frequency range. The equipment also includes
provisions for manual and semiautomatic tuning,
making the system readily adaptable for use with other
radio transmitters. The manual tuning feature is useful
when a failure occurs in the automatic tuning circuitry.
Tuning can also be done without the use of rf power
(silent tuning). This method is useful in installations
where radio silence must be maintained except for
brief transmission periods.
Figure 2-37.—Matching network.
the tuner, assembling the connections with the antenna
and transmission line, and pressurizing the tuner,
if necessary. Access must be provided to the pressure
gauge and pressurizing and purging connections.
ANTENNA TUNING
For every frequency in the frequency spectrum,
there is an antenna that is perfect for radiating at that
frequency. By that we mean that all of the power
being transmitted from the transmitter to the antenna
will be radiated into space. Unfortunately, this is the
ideal and not the rule. Normally, some power is lost
between the transmitter and the antenna. This power
loss is the result of the antenna not having the perfect
dimensions and size to radiate perfectly all of the
power delivered to it from the transmitter. Naturally,
it would be unrealistic to carry a separate antenna for
every frequency that a communications center is
capable of radiating; a ship would have to have
millions of antennas on board, and that would be
impossible.
The antenna coupler matches the impedance of
a 15-, 25-, 28-, or 35-foot whip antenna to a 50-ohm
transmission line, at any frequency in the 2-to 30-MHz
When the coupler is used with the
range.
AN/URT-23(V), control signals from the associated
antenna coupler control unit automatically tune the
coupler’s matching network in less than 5 seconds.
During manual and silent operation, the operator uses
the controls mounted on the antenna coupler control
unit to tune the antenna. A low-power (less than 250
watts) cw signal is required for tuning. Once tuned,
the CU 938A/URA-38 is capable of handling 1000
watts PEP.
To overcome this problem, we use ANTENNA
TUNING to lengthen and shorten antennas electrically
to better match the frequency on which we want to
transmit. The rf tuner is connected electrically to the
antenna and is used to adjust the apparent physical
length of the antenna by electrical means. This simply
Antenna Coupler Groups
AN/SRA-56, -57, and -58
Antenna coupler groups AN/SRA-56, -57, and
-58 are designed primarily for shipboard use. Each
2-21
The OA-9123/SRC consists of a cabinet assembly,
control power supply assembly, and four identical filter
assemblies. This multicoupler is a state-of-the-art
replacement for the AN/SRA-33 and only requires
about half of the space.
coupler group permits several transmitters to operate
simultaneously into a single, associated, broadband
antenna, thus reducing the total number of antennas
required in the limited space aboard ship.
These antenna coupler groups provide a coupling
path of prescribed efficiency between each transmitter
and the associated antenna. They also provide isolation
between transmitters, tunable bandpass filters to
suppress harmonic and spurious transmitter outputs,
and matching networks to reduce antenna impedances.
RECEIVING ANTENNA
DISTRIBUTION SYSTEMS
Receiving antenna distribution systems operate
at low power levels and are designed to prevent
multiple signals from being received. The basic
distribution system has several antenna transmission
lines and several receivers, as shown in figure 2-38.
The system includes two basic patch panels, one that
terminates the antenna transmission lines, and the other
that terminates the lines leading to the receivers. Thus,
any antenna can be patched to any receiver via patch
cords.
The three antenna coupler groups (AN/SRA-56,
-57, -58) are similar in appearance and function, but
they differ in frequency ranges. Antenna coupler group
AN/SRA-56 operates in the 2- to 6-MHz frequency
range. The AN/SRA-57 operates from 4 to 12 MHz,
and the AN/SRA-58 operates from 10 to 30 MHz.
When more than one coupler is used in the same
frequency range, a 15 percent frequency separation
must be maintained to avoid any interference.
Antenna Coupler Group AN/SRA-33
Antenna coupler group AN/SRA-33 operates in
the uhf (225-400 Mhz) frequency range. It provides
isolation between as many as four transmitter and
receiver combinations operating simultaneously into
a common uhf antenna without degrading operation.
The AN/SRA-33 is designed for operation with
shipboard radio set AN/WSC-3. The AN/SRA-33
consists of four antenna couplers (CU-1131/SRA-33
through CU-1134/SRA-33), a control power supply
C-4586/SRA-33, an electronic equipment cabinet
CY-3852/SRA-33, and a set of special-purpose cables.
OA-9123/SRC
The OA-9123/SRC multicoupler enables up to four
uhf transceivers, transmitters, or receivers to operate
on a common antenna. The multicoupler provides
low insertion loss and highly selective filtering in each
of the four ports. The unit is interface compatible
with the channel select control signals from radio sets
AN/WSC-3(V) (except (V)1). The unit is selfcontained and is configured to fit into a standard
19-inch open equipment rack.
Figure 2-38.—Receive signal distribution system.
2-22
radio waves behave. A point source, such as an
omnidirectional antenna produces a spherical radiation
pattern, or spherical wavefront. As the sphere expands,
the energy contained in a given surface area decreases
rapidly. At a relatively short distance from the
antenna, the energy level is so small that its reflection
from a target would be useless in a radar system.
Some distribution systems will be more complex.
That is, four antennas can be patched to four receivers,
or one antenna can be patched to more than one
receiver via the multicouplers.
RECEIVING MULTICOUPLER
AN/SRA-12
A solution to this problem is to form the energy
into a PLANE wavefront, In a plane wavefront, all
of the energy travels in the same direction, thus
providing more energy to reflect off of a target. To
concentrate the energy even further, a parabolic
reflector is used to shape the plane wavefront’s energy
into a beam of energy. This concentration of energy
provides a maximum amount of energy to be reflected
off of a target, making detection of the target much
more probable.
The AN/SRA-12 filter assembly multicoupler
provides seven radio frequency channels in the 14-kHz
to 32-MHz frequency range. Any of these channels
may be used independently of the other channels, or
they may operate simultaneously. Connections to the
receiver are made by coaxial patch cords, which are
short lengths of cable with a plug attached to each
end.
ANTENNA COUPLER GROUPS
AN/SRA-38, AN/SRA-39, AN/SRA-40,
AN/SRA-49, AN/SRA-49A, and AN/SRA-50
How does the parabolic reflector focus the radio
waves? Radio waves behave much as light waves do.
Microwaves travel in straight lines as do light rays.
They may be focused or reflected, just as light rays
may be. In figure 2-39, a point-radiation source is
placed at the focal point F. The field leaves this
antema with a spherical wavefront. As each part of
the wavefront moving toward the reflector reaches
the reflecting surface, it is shifted 180 degrees in phase
and sent outward at angles that cause all parts of the
field to travel in parallel paths. Because of the shape
of a parabolic surface, all paths from F to the reflector
and back to line XY are the same length. Therefore,
all parts of the field arrive at line XY at the same time
after reflection.
These groups are designed to connect up to 20
mf and hf receivers to a single antenna, with a highly
selective degree of frequency isolation. Each of the
six coupler groups consists of 14 to 20 individual
antenna couplers and a single-power supply module,
all are slide-mounted in a special electronic equipment
rack. An antenna input distribution line termination
(dummy load) is also supplied. In addition, there are
provisions for patching the outputs from the various
antenna couplers to external receivers.
RADAR ANTENNAS
Radar antennas are usually directional antennas
that radiate energy in one lobe or beam. The two most
important characteristics of directional antennas are
directivity and power gain. Most radar systems use
parabolic antennas. These antennas use parabolic
reflectors in different variations to focus the radiated
energy into a desired beam pattern.
While most radar antennas are parabolic, other
types such as the corner reflector, the broadside array,
and horn radiators may also be used.
Figure 2-39.—Parabolic reflector radiation.
PARABOLIC REFLECTORS
Energy that is not directed toward the paraboloid
(dotted lines in fig. 2-39) has a wide-beam characteris-
To understand why parabolic reflectors are used
for most radar antennas, you need to understand how
2-23
tic that would destroy the narrow pattern from the
parabolic reflector. This destruction is prevented by
the use of a hemispherical shield (not shown) that
directs most of what would otherwise be spherical
radiation toward the parabolic surface. Without the
shield, some of the radiated field would leave the
radiator directly, would not be reflected, and would
serve no useful purpose.
The shield makes the
beamsharper, and concentrates the majority of the
power in the beam. The same results can be obtained
by using either a parasitic array to direct the radiated
field back to the reflector, or a feed horn pointed at
the paraboloid.
The radiation pattern of the paraboloid contains
a major lobe, which is directed along the axis of the
paraboloid, and several minor lobes, as shown in figure
2-40. Very narrow beams are possible with this type
of reflector. View A of figure 2-41 illustrates the
parabolic reflector.
Figure 2-40.—Parabolic radiation pattern.
produce differently shaped beams. View B of figure
2-41 shows a horizontally truncated, or vertically
shortened, paraboloid.
This type of reflector is
designed to produce a beam that is narrow horizontally
but wide vertically. Since the beam is wide vertically,
it will detect aircraft at different altitudes without
changing the tilt of the antenna. It also works well
for surface search radars to overcome the pitch and
roll of the ship.
Truncated Paraboloid
While the complete parabolic reflector produces
a pencil-shaped beam, partial parabolic reflectors
Figure 2-41.—Reflector shapes.
2-24
BROADSIDE ARRAY
The truncated paraboloid reflector may be used
in height-finding systems if the reflector is rotated
90 degrees, as shown in view C of figure 2-41. This
type of reflector produces a beam that is wide
horizontally but narrow vertically. The beam pattern
is spread like a horizontal fan. Such a fan-shaped
beam can be used to determine elevation very
accurately.
Desired beam widths are provided for some vhf
radars by a broadside array, such as the one shown
in figure 2-42. The broadside array consists of two
or more half-wave dipole elements and a flat reflector.
The elements are placed one-half wavelength apart
and parallel to each other. Because they are excited
in phase, most of the radiation is perpendicular or
broadside to the plane of the elements. The flat
reflector is located approximately one-eighth wavelength behind the dipole elements and makes possible
the unidirectional characteristics of the antenna system.
Orange-Peel Paraboloid
A section of a complete circular paraboloid, often
called an ORANGE-PEEL REFLECTOR because of
its shape, is shown in view D of figure 2-41. Since
the reflector is narrow in the horizontal plane and wide
in the vertical, it produces a beam that is wide in the
horizontal plane and narrow in the vertical. In shape,
the beam resembles a huge beaver tail. This type of
antenna system is generally used in height-finding
equipment.
HORN RADIATORS
Horn radiators, like parabolic reflectors, may be
used to obtain directive radiation at microwave
frequencies. Because they do not involve resonant
elements, horns have the advantage of being usable
over a wide frequency band.
Cylindrical Paraboloid
The operation of a horn as an electromagnetic
directing device is analogous to that of acoustic horns.
However, the throat of an acoustic horn usually has
dimensions much smaller than the sound wavelengths
for which it is used, while the throat of the electromagnetic horn has dimensions that are comparable to the
wavelength being used.
When a beam of radiated energy noticeably wider
in one cross-sectional dimension than in the other is
desired, a cylindrical paraboloid section approximating
a rectangle can be used. View E of figure 2-41
illustrates this antenna. A parabolic cross section is
in one dimension only; therefore, the reflector is
directive in one plane only. The cylindrical paraboloid
reflector is either fed by a linear array of dipoles, a
slit in the side of a waveguide, or by a thin waveguide
radiator. Rather than a single focal point, this type
of reflector has a series of focal points forming a
straight line. Placing the radiator, or radiators, along
this focal line produces a directed beam of energy.
As the width of the parabolic section is changed,
different beam shapes are produced. This type of
antenna system is used in search systems and in ground
control approach (gca) systems.
Horn radiators are readily adaptable for use with
waveguides because they serve both as an impedance-
CORNER REFLECTOR
The corner-reflector antenna consists of two flat
conducting sheets that meet at an angle to form a
corner, as shown in view F of figure 2-41. This
reflector is normally driven by a half-wave radiator
located on a line that bisects the angle formed by the
sheet reflectors.
Figure 2-42.—Broadside array.
2-25
matching device and as a directional radiator. Horn
radiators may be fed by coaxial or other types of lines.
Horns are constructed in a variety of shapes as
illustrated in figure 2-43. The shape of the horn and
the dimensions of the length and mouth largely
determine the field-pattern shape. The ratio of the
horn length to mouth opening size determines the beam
angle and, thus, the directivity. In general, the larger
the opening of the horn, the more directive is the
resulting field pattern.
Figure 2-44.—Offset feedhorn.
AN/GPN-27(ASR-8) AIR
SURVEILLANCE RADAR
The AN/GPN-27(ASR-8) (fig. 2-45) antenna
radiates a beam 1.5 degrees in azimuth and shaped
in elevation to produce coverage of up to approximately 32 degrees above the horizon. This provides
a maplike presentation of aircraft within 55 nautical
miles of an airport terminal. The antenna azimuth
Figure 2-43.—Horn radiators.
FEEDHORNS
A waveguide horn, called a FEEDHORN, may
be used to feed energy into a parabolic dish. The
directivity of this feedhorn is added to that of the
parabolic dish. The resulting pattern is a very narrow
and concentrated beam. In most radars, the feedhorn
is covered with a window of polystyrene fiberglass
to prevent moisture and dirt from entering the open
end of the waveguide.
One problem associated with feedhorns is the
SHADOW introduced by the feedhorn if it is in the
path of the beam. (The shadow is a dead spot directly
in front of the feedhorn.) To solve this problem the
feedhorn can be offset from center. This location
change takes the feedhorn out of the path of the rf
beam and eliminates the shadow. An offset feedhorn
is shown in figure 2-44.
RADAR SYSTEMS
Now that you have a basic understanding of how
radar antennas operate, we will introduce you to a few
of the radar systems currently in use.
Figure 2-45.—AN/GPN-27(ASR-8) air
surveillance radar.
2-26
pedestal assembly, the feedhorn and feedhorn support
boom, and the reflector assembly.
pulse generator (APG), located in the rotary joint,
transmits to the radar indicator azimuth information
corresponding to beam direction. Polarization of the
radiated energy can be remotely switched to either
linear or circular polarization. The reflector has a
modified parabolic shape designed to produce an
approximately cosecant squared beam in the elevation
plane. The reflector surface, covered with expanded
aluminum screen, is 16.1 feet wide and 9 feet high.
The antenna feedhorn, which mounts on the polarizer,
provides impedance matching between the waveguide
system and free space, and produces the desired feed
pattern to illuminate the reflector. A radome over
the horn aperture excludes moisture and foreign matter,
and provides a pressure seal.
The base assembly provides a surface for mounting
the antenna to the ship. It also contains the azimuth
drive gearbox. The gearbox is driven by the azimuth
drive motor, which drives the pedestal in azimuth
through a pinion gear mated to a ring gear located
at the bottom of the cone-shaped pedestal assembly,
The azimuth drive circuits rotate the antenna through
360 degrees at speeds of 6 rpm and 12 rpm.
The reflector and the feedhorn support boom are
mounted on a trunnion, allowing the elevation angle
of the rf beam to be controlled by a jackscrew located
behind the reflector. The jackscrew is rotated by the
elevation drive gearbox, which is connected to two
dc motors. The rf energy to the feedhorn is routed
through elevation and azimuth rotary joints located
within the pedestal.
AS-3263/SPS-49(V)
The AS-3263/SPS-49(V) antenna (fig. 2-46)
consists of three major sections: the antenna base and
Figure 2-46.—AS-3263/SPS-49(V) antenna.
2-27
The antenna consists of two waveguide slotted
arrays mounted back-to-back. One array provides
linear polarization, while the other provides circular
polarization. The array used is selected by means of
a remotely controlled waveguide switch located on
the pedestal. Linear polarization is used for most
conditions. Circular polarization is used to reduce
return echoes from precipitation. Each antenna forms
a fan beam that is narrow in the azimuth plane and
moderately broad in the elevation plane.
The reflector is 24 feet wide and has a
double-curved surface composed of a series of
horizontal members that form a reflecting surface for
the horizontally polarized C-band energy. The antenna
has a 28-dB gain, with a beamwidth of 9 degrees
minimum vertically and approximately 3.3 degrees
horizontally. Antenna roll and pitch stabilization limits
are plus or minus 25 degrees, Stabilization accuracy
is plus or minus 1 degree with the horizontal plane.
The antenna is equipped with a safety switch
located near the antenna pedestal area. The safety
switch disables the azimuth and elevation functions
in the antenna and the radiate function in the transmitter to provide protection for personnel working on
the antenna.
Figure 2-47 shows a cross-section of the SPS-55
antenna. During transmission, the rf signal enters the
antenna through a feed waveguide and then enters a
feed manifold region of 80 periodic narrow-wall slots.
The slots are skewed in angle and alternated in
direction of skew. They are separated by approximately one-half wavelength, resulting in broadside
radiation into the sectoral horn region of the antenna.
The horizontally polarized radiation from the manifold
travels in the horn region toward the aperture, where
it encounters an array of vertical sheet metal slats.
OE-172/SPS-55
The OE-172/SPS-55 antenna group consists of the
antenna and the antenna pedestal. The antenna group
is mast-mounted by means of four bolt holes on the
base of the pedestal.
Figure 2-47.—SPS-55 antenna cross section.
2-28
azimuth course line to a transition point approximately
2 miles from the ramp of the flight deck.
This array is a polarizing filter, which ensures that
only horizontally polarized energy travels from the
horn region. The antenna scans at a rate of 16 rpm
and produces an absolute gain of 31 dB at midband.
The azimuth antenna, AS-1292/TPN-8, functions
in the azimuth rf line for radiation and reception of
X-band rf pulses. The azimuth antenna comprises
a truncated paraboloid-type reflector with an offset
feedhorn and a polarizer assembly that provides
remote-controlled selection of either horizontal or
circular polarization. The antenna is located above
the azimuth drive assembly on the stabilized yoke.
The azimuth drive can rotate the antenna in either 360
degrees or in limited-sector modes of operation in the
horizontal plane.
AN/SPN-35A AIRCRAFT CONTROL
APPROACH RADAR
The
AN/SPN-35A
(fig
2-48)
is
a
carrier-controlled-approach (CCA) radar set used for
precision landing approaches during adverse weather
conditions. The radar displays both azimuth and
elevation data, which enables the radar operator to
direct aircraft along a predetermined glide path and
Figure 2-48.—AN/SPN-3SA aircraft control approach radar.
2-29
equipment, or, when working aloft, to fall from the
elevated work area. Take care to ensure that all
transmission lines or antennas are de-energized before
working on or near them.
The elevation antenna, AS-1669/SPN-35, is a
truncated paraboloid-type reflector with a dual-channel
feedhorn and a
polarizer assembly providing
monopulse-type radiation and reception of X-band
rf pulses. The horizontal shape of the laminated
fiberglass reflector is cosecanted. The dual-channel
feedhorn and polarizer are fixed in circular polarization
by an external grid device. The elevation antenna is
stabilized-yoke mounted on the elevation drive
assembly adjacent to the azimuth antenna. The
elevation drive provides the required motion for the
elevation antenna and locks electrically with the search
drive when the radar set operates in the precision
mode.
When working aloft aboard ship, be sure to use
a working aloft chit. This will ensure that all radiators,
not only those on your own ship but also those nearby
are secured while you are aloft.
ALWAYS obey rf radiation warning signs and
keep a safe distance from radiating antennas. The
six types of warning signs for rf radiation hazards are
shown in figure 2-49.
The two primary safety concerns associated with
rf fields are rf burns and injuries caused by dielectric
heating.
The radar operates in three modes, final, surveillance, and simultaneous, with each antenna acting
independently. In the final (precision) mode, the
azimuth antenna scans a 30-degree sector (60-degree
sector optional) while the elevation antenna scans a
10-degree sector (35-degree sector optional). In the
surveillance mode the azimuth antenna rotates through
the full 360-degree search pattern at 16 rpm while
the elevation antenna scans a 10-degree sector. In
the simultaneous mode, the azimuth antenna rotates
through the full 360-degrees search pattern in
60-degree increments while the elevation antenna scans
a 10-degree sector. The data rate in this mode is
approximately 16 azimuth sweeps and 24 elevation
sweeps every 60 seconds.
RF BURNS
Close or direct contact with rf transmission lines
or antennas may result in rf burns caused by induced
voltages. These burns are usually deep, penetrating,
third-degree burns. To heal properly, rf burns must
heal from the inside toward the skin’s surface. Do
NOT take rf burns lightly. To prevent infection, you
must give proper attention to ALL rf burns, including
the small pinhole burns. ALWAYS seek treatment
for any rf burn or shock and report the incident to
your supervisor so appropriate action can be taken
to correct the hazard.
The antenna pedestal control stabilizes the azimuth
and elevation antennas for plus or minus 3 degrees
of pitch and plus or minus 10 degrees of roll.
DIELECTRIC HEATING
RF SAFETY PRECAUTIONS
While the severity of rf burns may vary from minor
to major, burns or other damage done by DIELECTRIC HEATING may result in long-term injury, or
even death. Dielectric heating is the heating of an
insulating material caused by placing it in a
high-frequency electric field. The heat results from
the rapid reversal of molecular polarization dielectric
material.
Although radio frequency emissions are usually
harmless, there are still certain safety precautions you
should follow whenever you are near high-power rf
sources. Normally, electromagnetic radiation from
transmission lines and antennas isn’t strong enough
to electrocute personnel. However, it may lead to other
accidents and can compound injuries. Voltages may
be induced into metal objects both above and below
ground, such as wire guys, wire cable, hand rails, and
ladders. If you come into contact with these objects,
you may receive a shock or an rf burn. The shock
can cause you to jump involuntarily, to fall into nearby
When a human is in an rf field, the body acts as
the dielectric. If the power in the rf field exceeds 10
milliwatts per centimeter, the individual will have a
noticeable rise in body temperature. Basically, the
body is “cooking” in the rf field. The vital organs
2-30
Figure 2-49.—Rf radiation warning signs
2-31
the switches tagged and locked open before you begin
working on or near the antenna.
of the body are highly susceptible to dielectric heating.
The eyes are also highly susceptible to dielectric
heating. Do NOT look directly into devices radiating
rf energy. Remember, rf radiation can be dangerous.
For your own safety, you must NOT stand directly
in the path of rf radiating devices.
When working near a stack, draw and wear the
recommended oxygen breathing apparatus. Among
other toxic substances, stack gas contains carbon
monoxide. Carbon monoxide is too unstable to build
up to a high concentration in the open, but prolonged
exposure to even small quantities is dangerous.
PRECAUTIONS WHEN WORKING
ALOFT
For more detailed information concerning the
dangers and hazards of rf radiation, refer to the
NAVELEX technical manual, Electromagnetic
Radiation Hazards. NAVELEX 0967-LP-624-6010.
As we mentioned earlier, it is extremely important
to follow all safety precautions when working aloft.
Before you work on an antenna, ensure that it is tagged
out properly at the switchboard to prevent it from
becoming operational. Always be sure to secure the
motor safety switches for rotating antennas. Have
This completes chapter 2. In chapter 3, we will
discuss transmission lines and waveguides.
2-32
CHAPTER 3
INTRODUCTION TO
TRANSMISSION LINES AND WAVEGUIDES
A TRANSMISSION LINE is a device designed
to guide electrical energy from one point to another.
It is used, for example, to transfer the output rf energy
of a transmitter to an antenna. This energy will not
travel through normal electrical wire without great
losses.
Although the antenna can be connected
directly to the transmitter, the antenna is usually
located some distance away from the transmitter. On
board ship, the transmitter is located inside a radio
room, and its associated antenna is mounted on a mast.
A transmission line is used to connect the transmitter
and the antenna.
flow that may be expected through the insulation,
If the line is uniform (all values equal at each unit
length), then one small section of the line may
represent several feet. This illustration of a two-wire
transmission line will be used throughout the discussion
of transmission lines; but, keep in mind that the
principles presented apply to all transmission lines.
We will explain the theories using LUMPED CONSTANTS and DISTRIBUTED CONSTANTS to further
simplify these principles.
The transmission line has a single purpose for both
the transmitter and the antenna. This purpose is to
transfer the energy output of the transmitter to the
antenna with the least possible power loss. How well
this is done depends on the special physical and
electrical characteristics (impedance and resistance)
of the transmission line.
A transmission line has the properties of inductance, capacitance, and resistance just as the more
conventional circuits have. Usually, however, the
constants in conventional circuits are lumped into a
single device or component. For example, a coil of
wire has the property of inductance. When a certain
amount of inductance is needed in a circuit, a coil of
the proper dimensions is inserted. The inductance
of the circuit is lumped into the one component. Two
metal plates separated by a small space, can be used
to supply the required capacitance for a circuit. In
such a case, most of the capacitance of the circuit is
lumped into this one component. Similarly, a fixed
resistor can be used to supply a certain value of circuit
resistance as a lumped sum. Ideally, a transmission
line would also have its constants of inductance,
capacitance, and resistance lumped together, as shown
in figure 3-1. Unfortunately, this is not the case.
Transmission line constants are as described in the
following paragraphs.
LUMPED CONSTANTS
TRANSMISSION LINE THEORY
The electrical characteristics of a two-wire
transmission line depend primarily on the construction
of the line. The two-wire line acts like a long
capacitor. The change of its capacitive reactance is
noticeable as the frequency applied to it is changed.
Since the long conductors have a magnetic field about
them when electrical energy is being passed through
them, they also exhibit the properties of inductance.
The values of inductance and capacitance presented
depend on the various physical factors that we
discussed earlier. For example, the type of line used,
the dielectric in the line, and the length of the line
must be considered. The effects of the inductive and
capacitive reactance of the line depend on the
frequency applied. Since no dielectric is perfect,
electrons manage to move from one conductor to the
other through the dielectric. Each type of two-wire
transmission line also has a conductance value. This
conductance value represents the value of the current
DISTRIBUTED CONSTANTS
Transmission line constants, called distributed
constants, are spread along the entire length of the
transmission line and cannot be distinguished separately. The amount of inductance, capacitance, and
resistance depends on the length of the line, the size
of the conducting wires, the spacing between the
3-1
Figure 3-3.—Distributed capacitance.
Figure 3-1.—Two-wire transmission line.
wires, and the dielectric (air or insulating medium)
between the wires. The following paragraphs will
be useful to you as you study distributed constants
on a transmission line.
Resistance of a Transmission Line
The transmission line shown in figure 3-4 has
electrical resistance along its length. This resistance
is usually expressed in ohms per unit length and is
shown as existing continuously from one end of the
line to the other.
Inductance of a Transmission Line
When current flows through a wire, magnetic lines
of force are set up around the wire. As the current
increases and decreases in amplitude, the field around
the wire expands and collapses accordingly. The
energy produced by the magnetic lines of force
collapsing back into the wire tends to keep the current
flowing in the same direction. This represents a certain
amount of inductance, which is expressed in
microhenrys per unit length. Figure 3-2 illustrates
the inductance and magnetic fields of a transmission
line.
Figure 3-4.—Distributed resistance.
Leakage Current
Since any dielectric, even air, is not a perfect
insulator, a small current known as LEAKAGE
CURRENT flows between the two wires. In effect,
the insulator acts as a resistor, permitting current to
pass between the two wires. Figure 3-5 shows this
leakage path as resistors in parallel connected between
the two lines. This property is called CONDUCTANCE (G) and is the opposite of resistance.
Conductance in transmission lines is expressed as the
reciprocal of resistance and is usually given in
micromhos per unit length.
Capacitance of a Transmission Line
Capacitance also exists between the transmission
line wires, as illustrated in figure 3-3. Notice that
the two parallel wires act as plates of a capacitor and
that the air between them acts as a dielectric. The
capacitance between the wires is usually expressed
in picofarads per unit length. This electric field
between the wires is similar to the field that exists
between the two plates of a capacitor.
Figure 3-2.—Distributed inductance.
Figure 3-5.—Leakage in a transmission line.
3-2
ELECTROMAGNETIC FIELDS
CHARACTERISTIC IMPEDANCE
The distributed constants of resistance, inductance,
and capacitance are basic properties common to all
transmission lines and exist whether or not any current
flow exists. As soon as current flow and voltage exist
in a transmission line, another property becomes quite
evident. This is the presence of an electromagnetic
field, or lines of force, about the wires of the
transmission line. The lines of force themselves are
not visible; however, understanding the force that an
electron experiences while in the field of these lines
is very important to your understanding of energy
transmission.
You can describe a transmission line in terms of
its impedance. The ratio of voltage to current (Ein/Iin)
at the input end is known as the INPUT IMPEDANCE
(Zin). This is the impedance presented to the transmitter by the transmission line and its load, the antenna.
The ratio of voltage to current at the output (E OUT/IOUT)
end is known as the OUTPUT IMPEDANCE (ZOUT ).
This is the impedance presented to the load by the
transmission line and its source. If an infinitely long
transmission line could be used, the ratio of voltage
to current at any point on that transmission line would
be some particular value of impedance. This impedance is known as the CHARACTERISTIC IMPEDANCE.
There are two kinds of fields; one is associated
with voltage and the other with current. The field
associated with voltage is called the ELECTRIC (E)
FIELD. It exerts a force on any electric charge placed
in it. The field associated with current is called a
MAGNETIC (H) FIELD, because it tends to exert
a force on any magnetic pole placed in it. Figure 3-6
illustrates the way in which the E fields and H fields
tend to orient themselves between conductors of a
typical two-wire transmission line. The illustration
shows a cross section of the transmission lines. The
E field is represented by solid lines and the H field
by dotted lines. The arrows indicate the direction of
the lines of force. Both fields normally exist together
and are spoken of collectively as the electromagnetic
field.
The maximum (and most efficient) transfer of
electrical energy takes place when the source impedance is matched to the load impedance. This fact is
very important in the study of transmission lines and
antennas. If the characteristic impedance of the
transmission line and the load impedance are equal,
energy from the transmitter will travel down the
transmission line to the antenna with no power loss
caused by reflection.
LINE LOSSES
The discussion of transmission lines so far has not
directly addressed LINE LOSSES; actually some losses
occur in all lines. Line losses may be any of three
types—COPPER, DIELECTRIC, and RADIATION
or INDUCTION LOSSES.
NOTE: Transmission lines are sometimes referred
to as rf lines. In this text the terms are used interchangeably.
Copper Losses
One type of copper loss is I 2R LOSS. In rf lines
the resistance of the conductors is never equal to zero.
Whenever current flows through one of these conductors, some energy is dissipated in the form of heat.
This heat loss is a POWER LOSS. With copper braid,
which has a resistance higher than solid tubing, this
power loss is higher.
Figure 3-6.—Fields between conductors.
3-3
The atomic structure of rubber is more difficult
to distort than the structure of some other dielectric
materials. The atoms of materials, such as polyethylene, distort easily. Therefore, polyethylene is often
used as a dielectric because less power is consumed
when its electron orbits are distorted.
Another type of copper loss is due to SKIN
EFFECT. When dc flows through a conductor, the
movement of electrons through the conductor’s cross
section is uniform, The situation is somewhat different
when ac is applied. The expanding and collapsing
fields about each electron encircle other electrons.
This phenomenon, called SELF INDUCTION, retards
the movement of the encircled electrons. The flux
density at the center is so great that electron movement
at this point is reduced. As frequency is increased,
the opposition to the flow of current in the center of
the wire increases. Current in the center of the wire
becomes smaller and most of the electron flow is on
the wire surface. When the frequency applied is 100
megahertz or higher, the electron movement in the
center is so small that the center of the wire could
be removed without any noticeable effect on current.
You should be able to see that the effective crosssectional area decreases as the frequency increases.
Since resistance is inversely proportional to the
cross-sectional area, the resistance will increase as the
frequency is increased.
Also, since power loss
increases as resistance increases, power losses increase
with an increase in frequency because of skin effect.
Radiation and Induction Losses
RADIAION and INDUCTION LOSSES are
similar in that both are caused by the fields surrounding the conductors. Induction losses occur when the
electromagnetic field about a conductor cuts through
any nearby metallic object and a current is induced
in that object. As a result, power is dissipated in the
object and is lost.
Radiation losses occur because some magnetic lines
of force about a conductor do not return to the
conductor when the cycle alternates. These lines of
force are projected into space as radiation, and this
results in power losses. That is, power is supplied
by the source, but is not available to the load.
VOLTAGE CHANGE
Copper losses can be minimized and conductivity
increased in an rf line by plating the line with silver.
Since silver is a better conductor than copper, most
of the current will flow through the silver layer. The
tubing then serves primarily as a mechanical support.
In an electric circuit, energy is stored in electric
and magnetic fields. These fields must be brought
to the load to transmit that energy. At the load, energy
contained in the fields is converted to the desired form
of energy.
Dielectric Losses
Transmission of Energy
DIELECTRIC LOSSES result from the heating
effect on the dielectric material between the conductors.
Power from the source is used in heating the dielectric.
The heat produced is dissipated into the surrounding
When there is no potential difference
medium.
between two conductors, the atoms in the dielectric
material between them are normal and the orbits of
the electrons are circular. When there is a potential
difference between two conductors, the orbits of the
electrons change. The excessive negative charge on
one conductor repels electrons on the dielectric toward
the positive conductor and thus distorts the orbits of
the electrons. A change in the path of electrons
requires more energy, introducing a power loss.
When the load is connected directly to the source
of energy, or when the transmission line is short,
problems concerning current and voltage can be solved
by applying Ohm’s law. When the transmission line
becomes long enough so the time difference between
a change occurring at the generator and a change
appearing at the load becomes appreciable, analysis
of the transmission line becomes important.
Dc Applied to a Transmission Line
In figure 3-7, a battery is connected through a
relatively long two-wire transmission line to a load
at the far end of the line. At the instant the switch
3-4
is closed, neither current nor voltage exists on the line.
When the switch is closed, point A becomes a positive
potential, and point B becomes negative. These points
of difference in potential move down the line.
However, as the initial points of potential leave points
A and B, they are followed by new points of difference
in potential, which the battery adds at A and B. This
is merely saying that the battery maintains a constant
potential difference between points A and B. A short
time after the switch is closed, the initial points of
difference in potential have reached points A’ and B’;
the wire sections from points A to A’ and points B
to B’ are at the same potential as A and B, respectively. The points of charge are represented by plus
(+) and minus (-) signs along the wires, The directions
of the currents in the wires are represented by the
arrowheads on the line, and the direction of travel is
indicated by an arrow below the line. Conventional
lines of force represent the electric field that exists
between the opposite kinds of charge on the wire
sections from A to A’ and B to B’. Crosses (tails of
arrows) indicate the magnetic field created by the
electric field moving down the line. The moving
electric field and the accompanying magnetic field
constitute an electromagnetic wave that is moving from
the generator (battery) toward the load. This wave
travels at approximately the speed of light in free
space. The energy reaching the load is equal to that
developed at the battery (assuming there are no losses
in the transmission line). If the load absorbs all of
the energy, the current and voltage will be evenly
distributed along the line.
Figure 3-8.—Ac voltage applied to a line.
line at the speed of light. The action is similar to the
wave created by the battery, except the applied voltage
is sinusoidal instead of constant. Assume that the
switch is closed at the moment the generator voltage
is passing through zero and that the next half cycle
makes point A positive. At the end of one cycle of
generator voltage, the current and voltage distribution
will be as shown in figure 3-8.
In this illustration the conventional lines of force
represent the electric fields. For simplicity, the
magnetic fields are not shown. Points of charge are
indicated by plus (+) and minus (-) signs, the larger
signs indicating points of higher amplitude of both
voltage and current. Short arrows indicate direction
of current (electron flow). The waveform drawn below
the transmission line represents the voltage (E) and
current (I) waves. The line is assumed to be infinite
in length so there is no reflection. Thus, traveling
sinusoidal voltage and current waves continually travel
in phase from the generator toward the load, or far
end of the line. Waves traveling from the generator
to the load are called INCIDENT WAVES. Waves
traveling from the load back to the generator are called
REFLECTED WAVES and will be explained in later
paragraphs.
Ac Applied to a Transmission Line
When the battery of figure 3-7 is replaced by an
ac generator (fig. 3-8), each successive instantaneous
value of the generator voltage is propagated down the
STANDING-WAVE RATIO
The measurement of standing waves on a transmission line yields information about equipment operating
Figure 3-7.—Dc voltage applied to a line.
3-5
conditions. Maximum power is absorbed by the load
when Z L = Z0 . If a line has no standing waves, the
termination for that line is correct and maximum power
transfer takes place.
voltage. Since power is proportional to the square
of the voltage, the ratio of the square of the maximum
and minimum voltages is called the power standing-wave ratio. In a sense, the name is misleading
because the power along a transmission line does not
vary.
You have probably noticed that the variation of
standing waves shows how near the rf line is to being
terminated in Z0. A wide variation in voltage along
the length means a termination far from Z0. A small
variation means termination near Z0. Therefore, the
ratio of the maximum to the minimum is a measure
of the perfection of the termination of a line. This
ratio is called the STANDING-WAVE RATIO (swr)
and is always expressed in whole numbers. For
example, a ratio of 1:1 describes a line terminated in
its characteristic impedance (Z0).
Current Standing-Wave Ratio
The ratio of maximum to minimum current along
a transmission line is called CURRENT STANDING- WAVE RATIO (iswr). Therefore:
This ratio is the same as that for voltages. It can be
used where measurements are made with loops that
sample the magnetic field along a line. It gives the
same results as vswr measurements.
Voltage Standing-Wave Ratio
The ratio of maximum voltage to minimum voltage
on a line is called the VOLTAGE STANDING-WAVE
RATIO (vswr). Therefore:
TRANSMISSION MEDIUMS
The Navy uses many different types of TRANSMISSION MEDIUMS in its electronic applications.
Each medium (line or waveguide) has a certain
characteristic impedance value, current-carrying
capacity, and physical shape and is designed to meet
a particular requirement.
The vertical lines in the formula indicate that the
enclosed quantities are absolute and that the two values
are taken without regard to polarity, Depending on
the nature of the standing waves, the numerical value
of vswr ranges from a value of 1 (ZL = Z0, no standing
waves) to an infinite value for theoretically complete
reflection. Since there is always a small loss on a
line, the minimum voltage is never zero and the vswr
is always some finite value. However, if the vswr
is to be a useful quantity. the power losses along the
line must be small in comparison to the transmitted
power.
The five types of transmission mediums that we
will discuss in this topic include PARALLEL-LINE,
TWISTED PAIR, SHIELDED PAIR, COAXIAL
LINE, and WAVEGUIDES. The use of a particular
line depends, among other things, on the applied
frequency, the power-handling capabilities, and the
type of installation.
Power Standing-Wave Ratio
Parallel Line
The square of the vswr is called the POWER
STANDING-WAVE RATIO (pswr). Therefore:
One type of parallel line is the TWO-WIRE OPEN
LINE, illustrated in figure 3-9. This line consists of
two wires that are generally spaced from 2 to 6 inches
apart by insulating spacers. This type of line is most
often used for power lines, rural telephone lines, and
telegraph lines. It is sometimes used as a transmission
line between a transmitter and an antenna or between
an antenna and a receiver. An advantage of this type
This ratio is useful because the instruments used to
detect standing waves react to the square of the
3-6
Figure 3-11.—Twisted pair.
Figure 3-9.—Two-wire open line.
of line is its simple construction. The principal
disadvantages of this type of line are the high radiation
losses and electrical noise pickup because of the lack
of shielding. Radiation losses are produced by the
changing fields created by the changing current in each
conductor.
Shielded Pair
The SHIELDED PAIR, shown in figure 3-12,
consists of parallel conductors separated from each
other and surrounded by a solid dielectric. The
conductors are contained within a braided copper
tubing that acts as an electrical shield. The assembly
is covered with a rubber or flexible composition
coating that protects the line from moisture and
mechanical damage. Outwardly, it looks much like
the power cord of a washing machine or refrigerator.
Another type of parallel line is the TWO-WIRE
RIBBON (TWIN LEAD) LINE, illustrated in figure
3-10. This type of transmission line is commonly used
to connect a television receiving antenna to a home
television set. This line is essentially the same as the
two-wire open line except that uniform spacing is
assured by embedding the two wires in a low-loss
dielectric, usually polyethylene. Since the wires are
embedded in the thin ribbon of polyethylene, the
dielectric space is partly air and partly polyethylene.
Twisted Pair
The TWISTED PAIR transmission line is illustrated
in figure 3-11. As the name implies, the line consists
of two insulated wires twisted together to form a
flexible line without the use of spacers. It is not used
for transmitting high frequency because of the high
dielectric losses that occur in the rubber insulation.
When the line is wet, the losses increase greatly.
Figure 3-12.—Shielded pair.
The principal advantage of the shielded pair is that
the conductors are balanced to ground; that is, the
capacitance between the wires is uniform throughout
the length of the line. This balance is due to the
uniform spacing of the grounded shield that surrounds
the wires along their entire length. The braided copper
shield isolates the conductors from stray magnetic
fields.
Coaxial Lines
There are two types of COAXIAL LINES, RIGID
(AIR) COAXIAL LINE and FLEXIBLE (SOLID)
COAXIAL LINE. The physical construction of both
types is basically the same; that is, each contains two
concentric conductors.
Figure 3-10.—Two-wire ribbon line.
3-7
Flexible coaxial lines (fig. 3-14) are made with
The rigid coaxial line consists of a central, insulated
wire (inner conductor) mounted inside a tubular outer
conductor. This line is shown in figure 3-13. In some
applications, the inner conductor is also tubular. The
inner conductor is insulated from the outer conductor
by insulating spacers or beads at regular intervals.
The spacers are made of Pyrex, polystyrene, or some
other material that has good insulating characteristics
and low dielectric losses at high frequencies.
an inner conductor that consists of flexible wire
insulated from the outer conductor by a solid,
continuous insulating material. The outer conductor
is made of metal braid, which gives the line flexibility.
Early attempts at gaining flexibility involved using
rubber insulators between the two conductors.
However, the rubber insulators caused excessive losses
at high frequencies.
Figure 3-14.—Flexible coaxial line.
Figure 3-13.—Air coaxial line.
Because of the high-frequency losses associated
with rubber insulators, polyethylene plastic was
developed to replace rubber and eliminate these losses.
Polyethylene plastic is a solid substance that remains
flexible over a wide range of temperatures. It is
unaffected by seawater, gasoline, oil, and most other
liquids that may be found aboard ship. The use of
polyethylene as an insulator results in greater
high-frequency losses than the use of air as an
insulator. However, these losses are still lower than
the losses associated with most other solid dielectric
materials.
The chief advantage of the rigid line is its ability
to minimize radiation losses. The electric and magnetic
fields in a two-wire parallel line extend into space for
relatively great distances and radiation losses occur.
However, in a coaxial line no electric or magnetic
fields extend outside of the outer conductor. The fields
are confined to the space between the two conductors,
resulting in a perfectly shielded coaxial line. Another
advantage is that interference from other lines is
reduced.
The rigid line has the following disadvantages:
(1) it is expensive to construct; (2) it must be kept
dry to prevent excessive leakage between the two
conductors; and (3) although high-frequency losses
are somewhat less than in previously mentioned lines,
they are still excessive enough to limit the practical
length of the line.
This concludes our study of transmission lines.
The rest of this chapter will be an introduction into
the study of waveguides.
WAVEGUIDE THEORY
The two-wire transmission line used in conventional
circuits is inefficient for transferring electromagnetic
energy at microwave frequencies. At these frequencies,
energy escapes by radiation because the fields are not
confined in all directions, as illustrated in figure 3-15.
Coaxial lines are more efficient than two-wire lines
for transferring electromagnetic energy because the
fields are completely confined by the conductors, as
illustrated in figure 3-16. Waveguides are the most
Leakage caused by the condensation of moisture
is prevented in some rigid line applications by the use
of an inert gas, such as nitrogen, helium, or argon.
It is pumped into the dielectric space of the line at
a pressure that can vary from 3 to 35 pounds per
square inch. The inert gas is used to dry the line when
it is first installed and pressure is maintained to ensure
that no moisture enters the line.
3-8
efficient way to transfer electromagnetic energy.
WAVEGUIDES are essentially coaxial lines without
They are constructed from
center conductors.
conductive material and may be rectangular, circular,
or elliptical in shape, as shown in figure 3-17.
Figure 3-17.—Waveguide shapes.
of a coaxial cable is large, but the surface area of the
inner conductor is relatively small. At microwave
frequencies, the current-carrying area of the inner conductor is restricted to a very small layer at the
surface of the conductor by an action called SKIN
EFFECT.
Figure 3-15.—Fields confined in two directions only.
Skin effect tends to increase the effective resistance
of the conductor. Although energy transfer in coaxial
cable is caused by electromagnetic field motion, the
magnitude of the field is limited by the size of the
current-carrying area of the inner conductor. The small
size of the center conductor is even further reduced
by skin effect, and energy transmission by coaxial
cable becomes less efficient than by waveguides.
DIELECTRIC LOSSES are also lower in waveguides
than in two-wire and coaxial transmission lines.
Dielectric losses in two-wire and coaxial lines are
caused by the heating of the insulation between the
conductors. The insulation behaves as the dielectric
of a capacitor formed by the two wires of the
transmission line. A voltage potential across the two
wires causes heating of the dielectric and results in
a power loss. In practical applications, the actual
breakdown of the insulation between the conductors
of a transmission line is more frequently a problem
than is the dielectric loss.
Figure 3-16.—Fields confined in all directions.
WAVEGUIDE ADVANTAGES
Waveguides have several advantages over two-wire
and coaxial transmission lines. For example, the large
surface area of waveguides greatly reduces COPPER
(12R) LOSSES. Two-wire transmission lines have large
copper losses because they have a relatively small
surface area. The surface area of the outer conductor
This breakdown is usually caused by stationary
voltage spikes or “nodes,” which are caused by
standing waves. Standing waves are stationary and
occur when part of the energy traveling down the line
3-9
WAVEGUIDE DISADVANTAGES
is reflected by an impedance mismatch with the load.
The voltage potential of the standing waves at the
points of greatest magnitude can become large enough
to break down the insulation between transmission
line conductors.
Physical size is the primary lower-frequency
limitation of waveguides. The width of a waveguide
must be approximately a half wavelength at the
frequency of the wave to be transported. For example,
a waveguide for use at 1 megahertz would be about
700 feet wide. This makes the use of waveguides at
The dielectric in waveguides is air, which has a
much lower dielectric loss than conventional insulating
materials. However, waveguides are also subject to
dielectric breakdown caused by standing waves.
Standing waves in waveguides cause arcing, which
decreases the efficiency of energy transfer and can
severely damage the waveguide. Also since the
frequencies below 1000 megahertz increasingly
impractical. The lower frequency range of any system
using waveguides is limited by the physical dimensions
of the waveguides.
electromagnetic fields are completely contained within
the waveguide, radiation losses are kept very low.
Waveguides are difficult to install because of their
rigid, hollow-pipe shape. Special couplings at the
joints are required to assure proper operation. Also,
the inside surfaces of waveguides are often plated with
silver or gold to reduce skin effect losses. These
requirements increase the costs and decrease the
practicality of waveguide systems at any other than
microwave frequencies.
Power-handling capability is another advantage
of waveguides. Waveguides can handle more power
than coaxial lines of the same size because
power-handling capability is directly related to the
distance between conductors. Figure 3-18 illustrates
the greater distance between conductors in a
waveguide.
DEVELOPING THE WAVEGUIDE
FROM PARALLEL LINES
You may better understand the transition from
ordinary transmission line concepts to waveguide
theories by considering the development of a
waveguide from a two-wire transmission line. Figure
3-19 shows a section of a two-wire transmission line
supported on two insulators. At the junction with the
line, the insulators must present a very high impedance
to ground for proper operation of the line. A low
impedance insulator would obviously short-circuit the
line to ground, and this is what happens at very high
frequencies. Ordinary insulators display the characteristics of the dielectric of a capacitor formed by the
wire and ground. As the frequency increases, the
Figure 3-18.—Comparison of spacing in coaxial cable
and a circular waveguide.
overall impedance decreases. A better high-frequency
insulator is a quarter-wave section of transmission
line shorted at one end. Such an insulator is shown
in figure 3-20. The impedance of a shorted quarter-wave section is very high at the open-end junction
with the two-wire transmission line. This type of
insulator is known as a METALLIC INSULATOR
and may be placed anywhere along a two-wire line.
In view of the advantages of waveguides, you
would think that waveguides should be the only type
of transmission lines used. However, waveguides have
certain disadvantages that make them practical for use
only at microwave frequencies.
3-10
Figure 3-19.—Two-wire transmission line.
Figure 3-21.—Metallic insulator on each side of a
two-wire line.
Figure 3-20.—Quarter-wave section of transmission
line shorted at one end.
Note that quarter-wave sections are insulators at only
one frequency. This severely limits the bandwidth,
efficiency, and application of this type of two-wire
line.
Figure 3-21 shows several metallic insulators on
each side of a two-wire transmission line. As more
insulators are added, each section makes contact with
the next, and a rectangular waveguide is formed. The
lines become part of the walls of the waveguide, as
illustrated in figure 3-22.
The energy is then
conducted within the hollow waveguide instead of
along the two-wire transmission line.
Figure 3-22.—Forming a waveguide by adding
quarter-wave sections.
like a two-wire line that is completely shunted by
quarter-wave sections. If it did, the use of a waveguide would be limited to a single-frequency wave
length that was four times the length of the quarterwave sections. In fact, waves of this length cannot
pass efficiently through waveguides. Only a small
range of frequencies of somewhat shorter wavelength
(higher frequency) can pass efficiently.
The comparison of the way electromagnetic fields
work on a transmission line and in a waveguide is
not exact. During the change from a two-wire line
to a waveguide, the electromagnetic field configurations
also undergo many changes. As a result of these
changes, the waveguide does not actually operate
3-11
As shown in figure 3-23, the widest dimension
of a waveguide is called the “a” dimension and
determines the range of operating frequencies. The
narrowest dimension determines the power-handling
capability of the waveguide and is called the “b”
dimension.
ENERGY PROPAGATION IN
WAVEGUIDES
Since energy is transferred through waveguides
by electromagnetic fields, you need a basic understanding of field theory. Both electric (E FIELD) and
magnetic fields (H FIELD) are present in waveguides,
and the interaction of these fields causes energy to
travel through the waveguide. This action is best
understood by first looking at the properties of the
two individual fields.
E Field
An electric field exists when a difference of
potential causes a stress in the dielectric between two
points. The simplest electric field is one that forms
between the plates of a capacitor when one plate is
made positive compared to the other, as shown in view
A of figure 3-24. The stress created in the dielectric
is an electric field.
Figure 3-23.—Labeling waveguide dimensions,
NOTE: This method of labeling waveguides is
not standard in all texts, Different methods may be
used in other texts on microwave principles, but this
method is in accordance with Navy Military Standards
(MIL-STDS).
Electric fields are represented by arrows that point
from the positive toward the negative potential. The
number of arrows shows the relative strength of the
field. In view B, for example, evenly spaced arrows
indicate the field is evenly distributed. For ease of
explanation, the electric field is abbreviated E field,
and the lines of stress are called E lines.
In theory, a waveguide could function at an infinite
number of frequencies higher than the designed
frequency; however, in practice, an upper frequency
limit is caused by modes of operation, which will be
discussed later.
H Field
If the frequency of a signal is decreased so much
that two quarter-wavelengths are longer than the wide
dimension of a waveguide, energy will no longer pass
through the waveguide. This is the lower frequency
limit, or CUTOFF FREQUENCY of a given
In practical applications, the wide
waveguide.
dimension of a waveguide is usually 0.7 wavelength
at the operating frequency. This allows the waveguide
to handle a small range of frequencies both above and
below the operating frequency. The “b” dimension
is governed by the breakdown potential of the
dielectric, which is usually air. Dimensions ranging
from 0.2 to 0.5 wavelength are common for the “b”
sides of a waveguide.
The magnetic field in a waveguide is made up of
magnetic lines of force that are caused by current flow
through the conductive material of the waveguide.
Magnetic lines of force, called H lines, are continuous
closed loops, as shown in figure 3-25. All of the H
lines associated with current are collectively called
a magnetic field or H field. The strength of the H
field, indicated by the number of H lines in a given
area, varies directly with the amount of current.
Although H lines encircle a single, straight wire,
they behave differently when the wire is formed into
a coil, as shown in figure 3-26. In a coil the individual
H lines tend to form around each turn of wire. Since
3-12
Figure 3-24.—Simple electric fields.
waveguide is confined to the physical limits of the
guide. Two conditions, known as BOUNDARY
CONDITIONS, must be satisfied for energy to travel
through a waveguide.
The first boundary condition (illustrated in fig.
3-27, view A can be stated as follows:
For an electric field to exist at the surface
of a conductor, it must be perpendicular
to the conductor.
Figure 3-25.—Magnetic field on a single wire.
the H lines take opposite directions between adjacent
turns, the field between the turns is canceled. Inside
and outside the coil, where the direction of each H
field is the same, the fields join and form continuous
H lines around the entire coil. A similar action takes
place in a waveguide.
Figure 3-27.—E field boundary condition.
The opposite of this boundary condition, shown
in view B, is also true. An electric field CANNOT
exist parallel to a perfect conductor.
Figure 3-26.—Magnetic field on a coil.
The second boundary condition, which is illustrated
in figure 3-28, can be stated as follows:
BOUNDARY CONDITIONS IN
A WAVEGUIDE
For a varying magnetic field to exist, it must
form closed loops in parallel with the
conductors and be perpendicular to the
electric field.
The travel of energy down a waveguide is similar,
but not identical, to the travel of electromagnetic waves
in free space. The difference is that the energy in a
3-13
intervals, as illustrated in figure 3-30. Angle
is
the direction of travel of the wave with respect to some
reference axis.
Figure 3-28.—H field boundary condition.
Since an E field causes a current flow that in turn
produces an H field, both fields always exist at the
same time in a waveguide. If a system satisfies one
of these boundary conditions, it must also satisfy the
other since neither field can exist alone.
Figure 3-30.—Wavefronts in space.
WAVEFRONTS WITHIN A
WAVEGUIDE
The reflection of a single wavefront off the “b”
wall of a waveguide is shown in figure 3-31. The
wavefront is shown in view A as small particles, In
views B and C particle 1 strikes the wall and is
bounced back from the wall without losing velocity.
If the wall is perfectly flat, the angle at which it the
is the same
wall, known as the angle of incidence
An instant after particle
as the angle of reflection
1 strikes the wall, particle 2 strikes the wall, as shown
Electromagnetic energy transmitted into space
consists of electric and magnetic fields that are at right
angles (90 degrees) to each other and at right angles
to the direction of propagation. A simple analogy to
establish this relationship is by use of the right-hand
rule for electromagnetic energy, based on the
POYNTING VECTOR. It indicates that a screw
(right-hand thread) with its axis perpendicular to the
electric and magnetic fields will advance in the
direction of propagation if the E field is rotated to
the right (toward the H field). This rule is illustrated
in figure 3-29.
Figure 3-29.—The Poynting vector.
The combined electric and magnetic fields form
a wavefront that can be represented by alternate
negative and positive peaks at half-wavelength
Figure 3-31.—Reflection of a single wavefront.
3-14
in view C, and reflects in the same manner. Because
all the particles are traveling at the same velocity,
particles 1 and 2 do not change their relative position
with respect to each other. Therefore, the reflected
wave has the same shape as the original.
The
remaining particles as shown in views D, E, and F
reflect in the same manner. This process results in
a reflected wavefront identical in shape, but opposite
in polarity, to the incident wave.
The velocity of propagation of a wave along a
waveguide is less than its velocity through free space
(speed of light). This lower velocity is caused by the
zigzag path taken by the wavefront.
The
forward-progress velocity of the wavefront in a
waveguide is called GROUP VELOCITY and is
somewhat slower than the speed of light.
The group velocity of energy in a waveguide is
determined by the reflection angle of the wavefronts
off the “b” walls. The reflection angle is determined
by the frequency of the input energy. This basic
principle is illustrated in figure 3-33. As frequency
is decreased. the reflection angle increases, causing
the group velocity to decrease. The opposite is also
true; increasing frequency increases the group velocity.
Figure 3-32, views A and B, each illustrate the
direction of propagation of two different electromagnetic wavefronts of different frequencies being radiated
into a waveguide by a probe. Note that only the
direction of propagation is indicated by the lines and
arrowheads. The wavefronts are at right angles to
the direction of propagation. The angle of incidence
and the angle of reflection
of the wavefronts
vary in size with the frequency of the input energy,
but the angles of reflection are equal to each other
in a waveguide. The CUTOFF FREQUENCY in a
waveguide is a frequency that would cause angles of
incidence and reflection to be perpendicular to the
walls of the guide. At any frequency below the cutoff
frequency, the wavefronts will be reflected back and
forth across the guide (setting up standing waves) and
no energy will be conducted down the waveguide.
Figure 3-33.—Reflection angle at various frequencies.
WAVEGUIDE MODES OF
OPERATION
The waveguide analyzed in the previous paragraphs
yields an electric field configuration known as the
half-sine electric distribution. This configuration,
called a MODE OF OPERATION, is shown in figure
3-34. Recall that the strength of the field is indicated
by the spacing of the lines; that is, the closer the lines,
the stronger the field. The regions of maximum
voltage in this field move continuously down the
waveguide in a sine-wave pattern. To meet boundary
conditions. the field must always be zero at the “b”
walls.
Figure 3-32.—Different frequencies in a waveguide.
3-15
Figure 3-34.—Half-sine E field distribution.
The half-sine field is only one of many field
configurations, or modes, that can exist in a rectangular
waveguide. A full-sine field can also exist in a
rectangular waveguide because, as shown in figure
3-35, the field is zero at the “b” walls.
Figure 3-36.—Magnetic field caused by a half-sine
E field.
Of the possible modes of operation available for
a given waveguide, the dominant mode has the lowest
cutoff frequency. The high-frequency limit of a
rectangular waveguide is a frequency at which its “a”
dimension becomes large enough to allow operation
in a mode higher than that for which the waveguide
has been designed.
Circular waveguides are used in specific areas of
radar and communications systems, such as rotating
joints used at the mechanical point where the antennas
rotate. Figure 3-37 illustrates the dominant mode of
a circular waveguide. The cutoff wavelength of a
circular guide is 1.71 times the diameter of the
waveguide. Since the “a” dimension of a rectangular
waveguide is approximately one half-wavelength at
the cutoff frequency, the diameter of an equivalent
circular waveguide must be 2/1.71, or approximately
Figure 3-35.—Full-sine E field distribution.
The magnetic field in a rectangular waveguide is
in the form of closed loops parallel to the surface of
the conductors. The strength of the magnetic field
is proportional to the electric field. Figure 3-36
illustrates the magnetic field pattern associated with
a half-sine electric field distribution. The magnitude
of the magnetic field varies in a sine-wave pattern
down the center of the waveguide in “time phase” with
the electric field. TIME PHASE means that the peak
H lines and peak E lines occur at the same instant in
time, although not necessarily at the same point along
the length of the waveguide.
The dominant mode is the most efficient mode.
Waveguides are normally designed so that only the
dominant mode will be used. To operate in the
dominant mode, a waveguide must have an “a” (wide)
dimension of at least one half-wavelength of the
frequency to be propagated. The “a” dimension of
the waveguide must be kept near the minimum
allowable value to ensure that only the dominant mode
will exist. In practice, this dimension is usually 0.7
wavelength.
Figure 3-37.—Dominant mode in a circular
waveguide.
3-16
are no E-field patterns across the “b” dimension, so
The complete mode
the second subscript is 0.
description of the dominant mode in rectangular
waveguides is TE 1,0 . Subsequent description of
waveguide operation in this text will assume the
dominant (TE1,0) mode unless otherwise noted.
1.17 times the “a” dimension of a rectangular
waveguide.
MODE NUMBERING SYSTEMS
So far, only the most basic types of E and H field
arrangements have been shown. More complicated
arrangements are often necessary to make possible
coupling, isolation, or other types of operation. The
field arrangements of the various modes of operation
are divided into two categories: TRANSVERSE
ELECTRIC (TE) and TRANSVERSE MAGNETIC
(TM).
A similar system is used to identify the modes of
circular waveguides. The general classification of TE
and TM is true for both circular and rectangular
waveguides. In circular waveguides the subscripts
have a different meaning. The first subscript indicates
the number of fill-wave patterns around the circumference of the waveguide. The second subscript indicates
the number of half-wave patterns across the diameter.
In the transverse electric (TE) mode, the entire
electric field is in the transverse plane, which is
perpendicular to the waveguide, (direction of energy
travel). Part of the magnetic field is parallel to
the length axis.
In the circular waveguide in figure 3-39, the E
field is perpendicular to the length of the waveguide
with no E lines parallel to the direction of propagation.
Thus, it must be classified as operating in the TE
mode. If you follow the E line pattern in a counterclockwise direction starting at the top, the E lines
go from zero, through maximum positive (tail of
arrows), back to zero, through maximum negative
(head of arrows), and then back to zero again. This
is one full wave, so the first subscript is 1. Along
the diameter, the E lines go from zero through
maximum and back to zero, making a half-wave
variation. The second subscript, therefore, is also 1.
TE1,1 is the complete mode description of the dominant
mode in circular waveguides. Several modes are
possible in both circular and rectangular waveguides.
Figure 3-40 illustrates several different modes that
can be used to verify the mode numbering system.
In the transverse magnetic (TM) mode, the
entire magnetic field is in the transverse plane and
has no portion parallel to the length axis.
Since there are several TE and TM modes,
subscripts are used to complete the description of the
field pattern. In rectangular waveguides, the first
subscript indicates the number of half-wave patterns
in the “a” dimension, and the second subscript indicates
the number of half-wave patterns in the “b” dimension.
The dominant mode for rectangular waveguides
is shown in figure 3-38. It is designated as the TE
mode because the E fields are perpendicular to the
“a” walls. The first subscript is 1, since there is only
one half-wave pattern across the “a” dimension. There
Figure 3-39.—Counting wavelengths in a circular
waveguide.
Figure 3-38.—Dominant mode in a rectangular
waveguide.
3-17
Figure 3-40.—Various modes of operation for rectangular and circular waveguides.
WAVEGUIDE INPUT/OUTPUT
METHODS
In many applications a lesser degree of energy
transfer, called loose coupling, is desirable. The
amount of energy transfer can be reduced by decreasing
the length of the probe, by moving it out of the center
of the E field, or by shielding it. Where the degree
of coupling must be varied frequently, the probe is
made retractable so the length can be easily changed.
A waveguide, as explained earlier in this topic,
operates differently from an ordinary transmission line.
Therefore, special devices must be used to put energy
into a waveguide at one end and remove it from the
other end.
The size and shape of the probe determines its
frequency, bandwidth, and power-handling capability.
As the diameter of a probe increases, the bandwidth
The three devices used to injector remove energy
from waveguides are PROBES, LOOPS, and SLOTS.
Slots may also be called APERTURES or WINDOWS.
increases. A probe similar in shape to a door knob
is capable of handling much higher power and a larger
bandwidth than a conventional probe. The greater
power-handling capability is directly related to the
increased surface area.
Two examples of
broad-bandwidth probes are illustrated in figure 3-41,
view D. Removal of energy from a waveguide is
When a small probe is inserted into a waveguide
and supplied with microwave energy, it acts as a
quarter-wave antenna. Current flows in the probe and
sets up an E field such as the one shown in figure
3-41, view A. The E lines detach themselves from
the probe. When the probe is located at the point of
highest efficiency, the E lines set up an E field of
considerable intensity.
simply a reversal of the injection process using the
same type of probe.
Another way of injecting energy into a waveguide
is by setting up an H field in the waveguide. This
can be accomplished by inserting a small loop that
carries a high current into the waveguide, as shown
in figure 3-42, view A. A magnetic field builds up
The most efficient place to locate the probe is in
the center of the “a” wall, parallel to the “b” wall, and
one quarter-wavelength from the shorted end of the
waveguide, as shown in figure 3-41, views B and
C. This is the point at which the E field is maximum
in the dominant mode. Therefore, energy transfer
(coupling) is maximum at this point. Note that the
quarter-wavelength spacing is at the frequency required
to propagate the dominant mode.
around the loop and expands to fit the waveguide, as
shown in view B. If the frequency of the current in
the loop is within the bandwidth of the waveguide,
energy will be transferred to the waveguide.
3-18
Figure 3-41.—Probe coupling in a rectangular waveguide.
For the most efficient coupling to the waveguide,
the loop is inserted at one of several points where the
magnetic field will be of greatest strength. Four of
those points are shown in figure 3-42, view C.
When less efficient coupling is desired, you can
rotate or move the loop until it encircles a smaller
number of H lines. When the diameter of the loop
is increased, its power-handling capability also
increases.
The bandwidth can be increased by
increasing the size of the wire used to make the loop.
When a loop is introduced into a waveguide in
which an H field is present, a current is induced in
the loop. When this condition exists, energy is
removed from the waveguide.
Slots or apertures are sometimes used when very
loose (inefficient) coupling is desired, as shown in
figure 3-43. In this method energy enters through
a small slot in the waveguide and the E field expands
into the waveguide. The E lines expand first across
the slot and then across the interior of the waveguide.
Figure 3-42.—Loop coupling in a rectangular
waveguide.
3-19
WAVEGUIDE IMPEDANCE
MATCHING
Waveguide transmission systems are not always
perfectly impedance matched to their load devices.
The standing waves that result from a mismatch cause
a power loss, a reduction in power-handling capability,
and an increase in frequency sensitivity. Impedance-changing devices are therefore placed in the
waveguide to match the waveguide to the load. These
devices are placed near the source of the standing
waves.
Figure 3-44 illustrates three devices, called irises,
that are used to introduce inductance or capacitance
into a waveguide. An iris is nothing more than a metal
plate that contains an opening through which the waves
may pass. The iris is located in the transverse plane
of either the magnetic or electric field.
Figure 3-43.—Slot coupling in a waveguide.
Minimum reflections occur when energy is injected
or removed if the size of the slot is properly proportioned to the frequency of the energy.
After learning how energy is coupled into and out
of a waveguide with slots, you might think that leaving
the end open is the most simple way of injecting or
removing energy in a waveguide. This is not the case,
however, because when energy leaves a waveguide,
fields form around the end of the waveguide. These
fields cause an impedance mismatch which, in turn,
causes the development of standing waves and a drastic
loss in efficiency. Various methods of impedance
matching and terminating waveguides will be covered
in the next section.
An inductive iris and its equivalent circuit are
illustrated in figure 3-44, view A. The iris places a
shunt inductive reactance across the waveguide that
is directly proportional to the size of the opening.
Notice that the inductive iris is in the magnetic plane.
The shunt capacitive reactance, illustrated in view
B, basically acts the same way. Again, the reactance
is directly proportional to the size of the opening, but
the iris is placed in the electric plane. The iris,
illustrated in view C, has portions in both the magnetic
Figure 3-44.—Waveguide irises.
3-20
and electric transverse planes and forms an equivalent
parallel-LC circuit across the waveguide. At the
resonant frequency, the iris acts as a high shunt
resistance. Above or below resonance, the iris acts
as a capacitive or inductive reactance.
POSTS and SCREWS made from conductive
material can be used for impedance-changing devices
in waveguides. Views A and B of figure 3-45,
illustrate two basic methods of using posts and screws.
A post or screw that only partially penetrates into the
waveguide acts as a shunt capacitive reactance. When
the post or screw extends completely through the
waveguide, making contact with the top and bottom
walls, it acts as an inductive reactance. Note that when
screws are used, the amount of reactance can be varied.
Figure 3-46.—Waveguide horns.
As you may have noticed, horns are really simple
antennas. They have several advantages over other
impedance-matching devices, such as their large
bandwidth and simple construction.
A waveguide may also be terminated in a resistive
load that is matched to the characteristic impedance
of the waveguide. The resistive load is most often
called a DUMMY LOAD, because its only purpose
is to absorb all the energy in a waveguide without
causing standing waves.
Figure 3-45.—Conducting posts and screws.
WAVEGUIDE TERMINATIONS
There is no place on a waveguide to connect a
fixed termination resistor; therefore, several special
arrangements are used to terminate waveguides. One
method is to fill the end of the waveguide with a
graphite and sand mixture, as illustrated in figure 3-47,
view A. When the fields enter the mixture, they
induce a current flow in the mixture that dissipates
the energy as heat. Another method (view B) is to
use a high-resistance rod placed at the center of the
E field. The E field causes current to flow in the rod,
and the high resistance of the rod dissipates the energy
as a power loss, again in the form of heat.
Electromagnetic energy is often passed through
a waveguide to transfer the energy from a source into
space. As previously mentioned, the impedance of
a waveguide does not match the impedance of space,
and without proper impedance matching standing waves
cause a large decrease in the efficiency of the
waveguide.
Any abrupt change in impedance causes standing
waves, but when the change in impedance at the end
of a waveguide is gradual, almost no standing waves
are formed. Gradual changes in impedance can be
obtained by terminating the waveguide with a
funnel-shaped HORN, such as the three types illustrated
in figure 3-46. The type of horn used depends upon
the frequency and the desired radiation pattern.
Still another method for terminating a waveguide
is the use of a wedge of highly resistive material, as
shown in view C of figure 3-47. The plane of the
wedge is placed perpendicular to the magnetic lines
3-21
to carry liquids or other substances. The design of
a waveguide is determined by the frequency and power
level of the electromagnetic energy it will carry. The
following paragraphs explain the physical factors
involved in the design of waveguides.
Waveguide Bends
The size, shape, and dielectric material of a
waveguide must be constant throughout its length for
energy to move from one end to the other without
reflections. Any abrupt change in its size or shape
can cause reflections and a loss in overall efficiency.
When such a change is necessary, the bends, twists,
and joints of the waveguides must meet certain
conditions to prevent reflections.
Waveguides maybe bent in several ways that do
not cause reflections. One way is the gradual bend
shown in figure 3-48. This gradual bend is known
as an E bend because it distorts the E fields. The E
bend must have a radius greater than two wavelengths
to prevent reflections.
Figure 3-47.—Terminating waveguides.
of force. When the H lines cut through the wedge,
current flows in the wedge and causes a power loss.
As with the other methods, this loss is in the form
of heat. Since very little energy reaches the end of
the waveguide, reflections are minimum.
Figure 3-48.—Gradual E bend.
All of the terminations discussed so far are
designed to radiate or absorb the energy without
reflections. In many instances, however, all of the
energy must be reflected from the end of the
waveguide. The best way to accomplish this is to
permanently weld a metal plate at the end of the
waveguide, as shown in view D of figure 3-47.
Another common bend is the gradual H bend (fig.
3-49). It is called an H bend because the H fields
are distorted when a waveguide is bent in this manner.
Again, the radius of the bend must be greater than
two wavelengths to prevent reflections. Neither the
E bend in the “a” dimension nor the H bend in the
“b” dimension changes the normal mode of operation.
WAVEGUIDE PLUMBING
Since waveguides are really only hollow metal
pipes, the installation and the physical handling of
waveguides have many similarities to ordinary
plumbing. In light of this fact, the bending, twisting,
joining, and installation of waveguides is commonly
called waveguide plumbing. Naturally, waveguides
are different in design from pipes that are designed
Figure 3-49.—Gradual H bend.
3-22
A sharp bend in either dimension may be used
if it meets certain requirements. Notice the two
45-degree bends in figure 3-50; the bends are 1/4λ
apart. The reflections that occur at the 45-degree bends
cancel each other, leaving the fields as though no
reflections have occurred.
Figure 3-52.—Flexible waveguide.
constructed in sections and the sections connected with
joints. The three basic types of waveguide joints are
the PERMANENT, the SEMIPERMANENT, and the
ROTATING JOINTS. Since the permanent joint is
a factory-welded joint that requires no maintenance,
only the semipermanent and rotating joints will be
discussed.
Figure 3-50.—Sharp bends.
Sometimes the electromagnetic fields must be
rotated so that they are in the proper phase to match
the phase of the load. This may be accomplished by
twisting the waveguide as shown in figure 3-51. The
twist must be gradual and greater than
Sections of waveguide must be taken apart for
maintenance and repair. A semipermanent joint, called
a CHOKE JOINT, is most commonly used for this
purpose. The choke joint provides good electromagnetic continuity between the sections of the waveguide
with very little power loss.
A cross-sectional view of a choke joint is shown
in figure 3-53. The pressure gasket shown between
the two metal surfaces forms an airtight seal. Notice
from the “a”
in view B that the slot is exactly
deep,
wall of the waveguide. The slot is also
as shown in view A, and because it is shorted at point
1, a high impedance results at point 2. Point 3 is
from point 2. The high impedance at point 2 results
in a low impedance, or short, at point 3. This effect
creates a good electrical connection between the two
sections that permits energy to pass with very little
reflection or loss.
Figure 3-51.—Waveguide twist.
The flexible waveguide (fig. 3-52) allows special
bends, which some equipment applications might
require. It consists of a specially wound ribbon of
conductive material, the most commonly used is brass,
with the inner surface plated with chromium. Power
losses are greater in the flexible waveguide because
the inner surfaces are not perfectly smooth. Therefore,
it is only used in short sections where no other
reasonable solution is available.
Whenever a stationary rectangular waveguide is
to be connected to a rotating antenna, a rotating joint
must be used. A circular waveguide is normally used
in a rotating joint. Rotating a rectangular waveguide
would cause field pattern distortion. The rotating
section of the joint, illustrated in figure 3-54, uses a
choke joint to complete the electrical connection with
the stationary section. The circular waveguide is
designed so that it will operate in the TM0,1 mode.
Waveguide Joints
Since an entire waveguide system cannot possibly
be molded into one piece, the waveguide must be
3-23
The rectangular sections are attached as shown in the
illustration to prevent the circular waveguide from
operating in the wrong mode. Distance “O” is
so that a high impedance will be presented to any
unwanted modes. This is the most common design
used for rotating joints, but other types may be used
in specific applications.
WAVEGUIDE
MAINTENANCE
The installation of a waveguide system presents
problems that are not normally encountered when
dealing with other types of transmission lines. These
problems often fall within the technician’s area of
responsibility.
A brief discussion of waveguide
handling, installation, and maintenance will help
prepare you for this maintenance responsibility,
Detailed information concerning waveguide maintenance in a particular system may be found in the
technical manuals for the system.
Since a waveguide naturally has a low loss ratio,
most losses in a waveguide system are caused by other
factors. Improperly connected joints or damaged inner
surfaces can decrease the efficiency of a system to
the point that it will not work at all. Therefore, you
must take great care when working with waveguides
to prevent physical damage. Since waveguides are
made from a soft, conductive material, such as copper
or aluminum, they are very easy to dent or deform.
Even the slightest damage to the inner surface of a
waveguide will cause standing waves and, often,
internal arcing. Internal arcing causes further damage
to the waveguide in an action that is often
self-sustaining until the waveguide is damaged beyond
use. Part of your job as a technician will be to inspect
the waveguide system for physical damage. The
previously mentioned dents are only one type of
physical damage that can decrease the efficiency of
Another problem occurs because
the system.
waveguides are made from a conductive material such
as copper while the structures of most ships are made
from steel. When two dissimilar metals, such as
copper and steel, are in direct contact, an electrical
action called ELECTROLYSIS takes place that causes
very rapid corrosion of the metals. Waveguides can
be completely destroyed by electrolytic corrosion in
a relatively short period of time if they are not isolated
from direct contact with other metals. Any inspection
Figure 3-53.—Choke joint.
Figure 3-54.—Rotating joint.
3-24
for measurement or use in another circuit. Most
couplers sample energy traveling in one direction only.
However, directional couplers can be constructed that
sample energy in both directions. These are called
BIDIRECTIONAL couplers and are widely used in
radar and communications systems.
of a waveguide system should include a detailed
inspection of all support points to ensure that electrolytic corrosion is not taking place. Any waveguide
that is exposed to the weather should be painted and
all joints sealed. Proper painting prevents natural
corrosion, and sealing the joints prevents moisture from
entering the waveguide.
Directional couplers may be constructed in many
ways.
The coupler illustrated in figure 3-55 is
constructed from an enclosed waveguide section of
the same dimensions as the waveguide in which the
energy is to be sampled. The “b” wall of this enclosed
section is mounted to the “b” wall of the waveguide
from which the sample will be taken. There are two
holes in the “b” wall between the sections of the
coupler. These two holes are
apart. The upper
section of the directional coupler has a wedge -o f
energy-absorbing material at one end and a pickup
probe connected to an output jack at the other end.
The absorbent material absorbs the energy not directed
at the probe and a portion of the overall energy that
enters the section.
Moisture can be one of the worst enemies of a
waveguide system. As previously discussed, the
dielectric in waveguides is air, which is an excellent
dielectric as long as it is free of moisture. Wet air,
however, is a very poor dielectric and can cause serious
internal arcing in a waveguide system. For this reason,
care is taken to ensure that waveguide systems are
pressurized with air that is dry. Checking the pressure
and moisture content of the waveguide air may be one
of your daily system maintenance duties.
More detailed waveguide installation and maintenance information can be found in the technical
manuals that apply to your particular system. Another
good source is the Electronics Installation and
Maintenance Handbooks (EIMB) published by Naval
Sea Systems Command. Installation Standards (EIMB)
Handbook, NAVSEA 0967-LP-000-0110, is the volume
that deals with waveguide installation and maintenance.
WAVEGUIDE DEVICES
The discussion of waveguides, up to this point,
has been concerned only with the transfer of energy
from one point to another. Many waveguide devices
have been developed, however, that modify the energy
in some fashion during the transmission. Some devices
do nothing more than change the direction of the
energy. Others have been designed to change the basic
characteristics or power level of the electromagnetic
energy.
Figure 3-55.—Directional coupler.
Figure 3-56 illustrates two portions of the incident
wavefront in a waveguide. The waves travel down
the waveguide in the direction indicated and enter the
coupler section through both holes. Since both portions
of the wave travel the same distance, they are in phase
when they arrive at the pickup probe. Because the
waves are in phase, they add together and provide a
sample of the energy traveling down the waveguide.
The sample taken is only a small portion of the energy
that is traveling down the waveguide. The magnitude
of the sample, however, is proportional to the
magnitude of the energy in the waveguide. The
absorbent material is designed to ensure that the ratio
This section will explain the basic operating
principles of some of the more common waveguide
devices, such as DIRECTIONAL COUPLERS,
CAVITY RESONATORS, and HYBRID JUNCTIONS.
Directional Couplers
The directional coupler is a device that provides
a method of sampling energy from within a waveguide
3-25
between the sample energy and the energy in the
waveguide is constant. Otherwise, the sample would
contain no useful information. The ratio is usually
stamped on the coupler in the form of an attenuation
factor.
and the probe are in opposite positions from the
directional coupler designed to sample the incident
energy. This positioning causes the two portions of
the reflected energy to arrive at the probe in phase,
providing a sample of the reflected energy. The
transmitted energy is absorbed by the absorbent
material.
Figure 3-56.—Incident wave in a directional coupler
designed to sample incident waves.
Figure 3-58.—Directional coupler designed to sample
retlected energy.
The effect of a directional coupler on any reflected
energy is illustrated in figure 3-57. Note that these
two waves do not travel the same distance to the
pickup probe. The wave represented by the dotted
further and arrives at the probe 180
line travels
degrees out of phase with the wave, represented by
the solid line. Because the waves are 180 degrees
out of phase at the probe, they cancel each other and
no energy is induced into the pickup probe. When
the reflected energy arrives at the absorbent material,
it adds and is absorbed by the material.
A simple bidirectional coupler for sampling both
transmitted and reflected energy can be constructed
by mounting two directional couplers on opposite sides
of a waveguide, as shown in figure 3-59.
Figure 3-59.—Bidirectional coupler.
Cavity Resonators
Figure 3-57.—Reflected wave in a directional
coupler.
By definition, a resonant cavity is any space
completely enclosed by conducting - walls that can
contain oscillating electromagnetic fields and possess
resonant properties. The cavity has many advantages
A directional coupler designed to sample reflected
energy is shown in figure 3-58. The absorbent material
3-26
and uses at microwave frequencies. Resonant cavities
have a very high Q and can be built to handle
relatively large amounts of power. Cavities with a
Q value in excess of 30,000 are not uncommon. The
high Q gives these devices a narrow bandpass and
allows very accurate tuning. Simple, rugged construction is an additional advantage.
Although cavity resonators, built for different
frequency ranges and applications, have a variety of
shapes, the basic principles of operation are the same
for all.
One example of a cavity resonator is the rectangular
box shown in figure 3-60, view A. It may be thought
of as a section of rectangular waveguide closed at both
ends by conducting plates. The frequency at which
of the distance
the resonant mode occurs is
between the end plates. The magnetic field patterns
in the rectangular cavity are shown in view B.
There are two variables that determine the primary
frequency of any resonant cavity. The first variable
is PHYSICAL SIZE. In general, the smaller the
cavity, the higher its resonant frequency. The second
controlling factor is the SHAPE of the cavity. Figure
3-61 illustrates several cavity shapes that are commonly
used. Remember from the previously stated definition
of a resonant cavity that any completely enclosed
conductive surface, regardless of its shape, can act
as a cavity resonator.
Energy can be inserted or removed from a cavity
by the same methods that are used to couple energy
into and out of waveguides. The operating principles
of probes, loops, and slots are the same whether used
in a cavity or a waveguide. Therefore, any of the three
methods can be used with cavities to inject or remove
energy.
Figure 3-60.—Rectangular waveguide cavity
resonator.
The resonant frequency of a cavity can be varied
by changing any of the three parameters: cavity
volume, cavity capacitance, or cavity inductance.
Changing the frequencies of a cavity is known as
TUNING. The mechanical methods of tuning a cavity
may vary with the application, but all methods use
the same electrical principles.
Waveguide Junctions
You may have assumed that when energy traveling
down a waveguide reaches a junction it simply divides
and follows the junction. This is not strictly true.
3-27
Figure 3-61.—Types of cavities.
Different types of junctions affect the energy in
different ways. Since waveguide junctions are used
extensively in most systems, you need to understand
the basic operating principles of those most commonly
used.
The T JUNCTION is the most simple of the
commonly used waveguide junctions. T junctions are
divided into two basic types, the E TYPE and the H
TYPE. HYBRID JUNCTIONS are more complicated
developments of the basic T junctions. The MAGIC-T
and the HYBRID RING are the two most commonly
used hybrid junctions.
E-TYPE T JUNCTION.— An E-type T junction
is illustrated in figure 3-62, view A.
Figure 3-62.—E fields in an E-type T junction.
3-28
2 have the same phase and amplitude. No difference
of potential exists across the entrance to the b arm,
and no energy will be coupled out. However, when
the two signals fed into the a and c arms are 180
degrees out of phase, as shown in view M, points
1 and 2 have a difference of potential. This difference
of potential induces an E field from point 1 to point
2 in the b arm, and energy is coupled out of this arm.
Views N and P illustrate two methods of obtaining
two outputs with only one input.
It is called an E-type T junction because the junction
arm extends from the main waveguide in the same
direction as the E field in the waveguide.
Figure 3-62, view B, illustrates cross-sectional
views of the E-type T junction with inputs fed into
the various arms. For simplicity, the magnetic lines
that are always present with an electric field have been
omitted. In view K, the input is fed into arm b and
the outputs are taken from the a and c arms. When
the E field arrives between points 1 and 2, point 1
becomes positive and point 2 becomes negative. The
positive charge at point 1 then induces a negative
charge on the wall at point 3. The negative charge
at point 2 induces a positive charge at point 4. These
charges cause the fields to form 180 degrees out of
phase in the main waveguide; therefore, the outputs
will be 180 degrees out of phase with each other.
In view L, two in-phase inputs of equal amplitude are
fed into the a and c arms. The signals at points 1 and
H-TYPE T JUNCTION.— An H-type T junction
is illustrated in figure 3-63, view A. It is called an
H-type T junction because the long axis of the “b”
arm is parallel to the plane of the magnetic lines of
force in the waveguide. Again, for simplicity, only
the E lines are shown in this figure. Each X indicates
an E line moving away from the observer. Each dot
indicates an E line moving toward the observer.
Figure 3-63.—E field in an H-type T junction.
3-29
In view 1 of figure 3-63, view B, the signal is fed
into arm b and in-phase outputs are obtained from
the a and c arms. In view 2, in-phase signals are fed
into arms a and c and the output signal is obtained
from the b arm because the fields add at the junction
and induce E lines into the b arm. If
180-degree-out-of-phase signals are fed into arms a
and c, as shown in view 3, no output is obtained from
the b arm because the opposing fields cancel at the
junction. If a signal is fed into the a arm, as shown
in view 4 , outputs will be obtained from the b and
c arms. The reverse is also true. If a signal is fed
into the c arm, outputs will be obtained from the a
and b arms.
MAGIC-T HYBRID JUNCTION.— A simplified version of the magic-T hybrid junction is shown
in figure 3-64. The magic-T is a combination of the
H-type and E-type T junctions. The most common
application of this type of junction is as the mixer
section for microwave radar receivers.
Figure 3-65.—Magic-T with input to arm b.
In summary, when an input is applied to arm b
of the magic-T hybrid junction, the output signals from
arms a and c are 180 degrees out of phase with each
other, and no output occurs at the d arm.
Figure 3-64.—Magic-T hybrid junction.
If a signal is fed into the b arm of the magic-T,
it will divide into two out-of-phase components. As
shown in figure 3-65, view A, these two components
will move into the a and c arms. The signal entering
the b arm will not enter the d arm because of the zero
potential existing at the entrance of the d arm. The
potential must be zero at this point to satisfy the
boundary conditions of the b arm. This absence of
potential is illustrated in views B and C where the
magnitude of the E field in the b arm is indicated by
the length of the arrows. Since the E lines are at
maximum in the center of the b arm and minimum
at the edge where the d arm entrance is located, no
potential difference exists across the mouth of the d
arm.
The action that occurs when a signal is fed into
the d arm of the magic-T is illustrated in figure 3-66.
As with the H-type T junction, the signal entering the
d arm divides and moves down the a and c arms as
outputs that are in phase with each other and with the
input. The shape of the E fields in motion is shown
by the numbered curved slices. As the E field moves
down the d arm, points 2 and 3 are at an equal
potential. The energy divides equally into arms a and
c, and the E fields in both arms become identical in
shape. Since the potentials on both sides of the b arm
are equal, no potential difference exists at the entrance
to the b arm, resulting in no output.
3-30
destroy the shape of the junctions. One method is
shown in figure 3-68. A post is used to match the
H plane, and an iris is used to match the E plane.
Even though this method reduces reflections, it
lowers the power-handling capability even further.
Figure 3-66.—Magic-T with input to arm d.
When an input signal is fed into the a arm as
shown in figure 3-67, a portion of the energy is
coupled into the b arm as it would be in an E-type
T junction. An equal portion of the signal is
coupled through the d arm because of the action of
the H-type junction. The c arm has two fields
across it that are out of phase with each other.
Therefore, the fields cancel, resulting in no output
at the c arm. The reverse of this action takes place
if a signal is fed into the c arm, resulting in
outputs at the b and d arms and no output at the a
arm.
Figure 3-68.—Magic-T impedance matching.
HYBRID RING.— A type of hybrid junction
that overcomes the power limitation of the magicT is the hybrid ring, also called a RAT RACE. The
hybrid ring, illustrated in figure 3-69, view A, is
actually a modification of the magic-T. It is
constructed of rectangular waveguides molded
into a circular pattern. The arms are joined to the
circular waveguide to form E-type T junctions.
View B shows, in wavelengths, the dimensions
required for a hybrid ring to operate properly.
The hybrid ring is used primarily in highpowered radar and communications systems to
perform two functions. During the transmit
period, the hybrid ring couples microwave energy
from the transmitter to the antenna and allows no
energy to reach the receiver. During the receive
cycle, the hybrid ring couples energy from the
antenna to the receiver and allows no energy to
reach the transmitter. Any device that performs
both of these functions is called a DUPLEXER. A
duplexer permits a system to use the same
antenna for both transmitting and receiving.
Figure 3-67.—Magic-T with input to arm a.
Unfortunately, when a signal is applied to any
arm of a magic-T, the flow of energy in the output
arms is affected by reflections. Reflections are
caused by impedance mismatching at the
junctions. These reflections are the cause of the
two major disadvantages of the magic-T. First, the
reflections represent a power loss since all the
energy fed into the junction does not reach the
load that the arms feed. Second, the reflections
produce standing waves that can result in internal
arcing. Thus, the maximum power a magic-T can
handle is greatly reduced.
Reflections can be reduced by using some
means of impedance matching that does not
SUMMARY
This concludes our discussion on transmission
lines and waveguides. In this volume you have
been given a basic introduction on wave
propagation from the time it leaves the
transmitter to the point of reception. In volume 8
you will be introduced to a variety of electronic
support systems.
3-31
Figure 3-69.—Hybrid ring with wavelength
measurements.
3-32
APPENDIX I
GLOSSARY
BEVERAGE ANTENNA—A horizontal, longwire
antenna designed for reception and transmission
of low-frequency, vertically polarized ground
waves. Also known as WAVE ANTENNA.
ABSORPTION—(1) Absorbing light waves. Does
not allow any reflection or refraction; (2)
Atmospheric absorption of rf energy with no
reflection or refraction (adversely affects longdistance communications).
BIDIRECTIONAL ARRAY—An array that radiates
in opposite directions along the line of maximum
radiation.
ACOUSTICS—The science of sound.
AMPLITUDE—The portion of a cycle measured from
a reference line to a maximum value above (or
to a maximum value below) the line.
BROADSIDE ARRAY—An array in which the
direction of maximum radiation is perpendicular
to the plane containing the elements.
ANGLE OF INCIDENCE—The angle between the
incident wave and the normal.
BOUNDARY CONDITIONS—The two conditions
that the E-field and H-field within a waveguide
must meet before energy will travel down the
waveguide. The E-field must be perpendicular
to the walls and the H-field must be in closed
loops, parallel to the walls, and perpendicular to
the E-field.
ANGLE OF REFLECTION—The angle between
the reflected wave and the normal.
ANGLE OF REFRACTION—The angle between
the normal and the path of a wave through the
second medium.
CAVITY RESONATOR—A space totally enclosed
by a metallic conductor and supplied with energy
in such a way that it becomes a source of
electromagnetic oscillations. The size and shape
of the enclosure determine the resonant frequency.
ANGSTROM UNIT—The unit used to define the
wavelength of light waves.
ANISOTROPIC—The property of a radiator to emit
strong radiation in one direction.
CENTER-FEED METHOD—Connecting the center
of an antenna to a transmission line, which is then
connected to the final (output) stage of the
transmitter. Also known as CURRENT-FEED
METHOD.
ANTENNA—A conductor or set of conductors used
either to radiate rf energy into space or to collect
rf energy from space.
APERTURE—See SLOT.
CHARACTERISTIC IMPEDANCE—The ratio of
voltage to current at any given point on a
transmission line. Represented by a value of
ARRAY OF ARRAYS—See COMBINATION
ARRAY.
impedance.
BAY—Part of an antenna array.
CHOKE JOINT—A joint between two sections of
waveguide that provides a good electrical
connection without power losses or reflections.
BEARING—An angular measurement that indicates
the direction of an object in degrees from true
north. Also called azimuth.
AI-1
CRITICAL FREQUENCY—The maximum fre-
COAXIAL LINE—A type of transmission line that
quency at which a radio wave can be transmitted
contains two concentric conductors.
vertically and still be refracted back to earth.
COLLINEAR ARRAY—An array with all the
CURRENT-FEED METHOD—See CENTER-FEED
METHOD.
elements in a straight line. Maximum radiation
is perpendicular to the axis of the elements.
CURRENT STANDING-WAVE RATIO
(ISWR)—The ratio of maximum to minimum
current along a transmission line.
COMBINATION ARRAY—An array system that
uses the characteristics of more than one array.
Also known as ARRAY OF ARRAYS.
CUTOFF FREQUENCY—The frequency at which
the attenuation of a waveguide increases sharply
and below which a traveling wave in a given
mode cannot be maintained. A frequency with
a half wavelength that is greater than the wide
dimension of a waveguide.
COMPLEX WAAE—A wave produced by combining
two or more pure tones at the same time.
CONDUCTANCE—The opposite of resistance in
The minute amount of
transmission lines.
resistance that is present in the insulator of a
CYCLE—One complete alternation of a sine wave
that has a maximum value above and a maximum
value below the reference line.
transmission line.
CONNECTED ARRAY—see DRIVEN ARRAY
DAMPING—Reduction of energy by absorption.
COPPER LOSS—Power loss in copper conductors
DENSITY—(1) The compactness of a substance;
(2) Mass per unit volume.
caused by the internal resistance of the conductors
2
to current flow. Also know as 1 R LOSS.
DETECTOR—The device that responds to a wave
or disturbance.
CORNER-REFLECTOR ANTENNA—A half-wave
antenna with a reflector consisting of two flat
metal surfaces meeting at an angle behind the
DIELECTRIC HEATING—The heating of an
insulating material by placing it in a high
frequency electric field.
radiator.
COUNTERPOISE—A network of wire that is
DIELECTRIC LOSSES—The losses resulting from
the heating effect on the dielectric material
between conductors.
connected to a quarter-wave antenna at one end
and provides the equivalent of an additional ¼
wavelength.
DIELECTRIC CONSTANT—The ratio of a given
dielectric to the dielectric value of a vacuum.
COUPLING DEVICE—A coupling coil that connects the transmitter to the feeder.
DIFFRACTION—The bending of the paths of waves
when the waves meet some form of obstruction.
CREST (TOP)—The peak of the positive alternation
(maximum value above the line) of a wave.
DIPOLE—A common type of half-wave antenna
made from a straight piece of wire cut in half.
Each half operates at a quarter wavelength of the
output.
CRITICAL ANGLE—The maximum angle at which
radio waves can be transmitted and still be
refracted back to earth.
AI-2
DIRECTIONAL.—Radiation that varies with direction.
waveguide in the same direction as the E-field
in the waveguide.
DIRECTIONAL COUPLER—A device that samples
the energy traveling in a waveguide for use in
another circuit.
ECHO—The reflection of the original sound wave
as it bounces off a distant surface.
ELECTROMAGNETIC FIELD—The combination
of an electric (E) field and a magnetic (H) field.
DIRECTOR—The parasitic element of an array that
reinforces energy coming from the driver toward
itself.
ELECTROMAGNETIC INTERFERENCE—Manmade or natural interference that degrades the
quality of reception of radio waves.
DIRECTIVITY—The property of an array that causes
more radiation to take place in certain directions
than in others.
ELECTROMAGNETIC RADIATION—The
radiation of radio waves into space.
DISTRIBUTED CONSTANTS—The constants of
inductance, capacitance, and resistance in a
transmission line. The constants are spread along
the entire length of the line and cannot be
distinguished separately.
ELECTRIC FIELD—See E-FIELD.
ELEMENT—A part of an antenna that can be either
an active radiator or a parasitic radiator.
DOMINANT MODE—The easiest mode to produce
in a waveguide, and also, the most efficient mode
in terms of energy transfer.
END-FEED METHOD—Connecting one end of an
antenna through a capacitor to the final output
Also known as
stage of a transmitter.
VOLTAGE-FEED METHOD.
DOPPLER EFFECT—The apparent change in
frequency or pitch when a sound source moves
either toward or away from a listener.
END-FIRE ARRAY—An array in which the direction
of radiation is parallel to the axis of the array.
DOUBLET—Another name for the dipole antenna.
ELEVATION ANGLE—The angle between the line
of sight to an object and the horizontal plane.
DRIVEN ARRAY—An array in which all of the
elements are driven. Also known as CONNECTED ARRAY
FADING—Variations in signal strength by atmospheric conditions.
DRIVEN ELEMENT—An element of an antenna
(transmitting or receiving) that is connected
directly to the transmission line.
FEEDER—A transmission line that carries energy
to the antenna.
FLAT LINE—A transmission line that has no
standing waves. This line requires no special
tuning device to transfer maximum power.
DUMMY LOAD—A device used at the end of a
transmission line or waveguide to convert
transmitted energy into heat so no energy is
radiated outward or reflected back.
FLEXIBLE COAXIAL LINE— coaxial line made
with a flexible inner conductor insulated from
the outer conductor by a solid, continuous
insulating material.
E-FIELD—Electric field that exists when a difference
in electrical potential causes a stress in the
dielectric between two points. AlSO known as
ELECTRIC FIELD.
FOLDED DIPOLE—An ordinary half-wave antenna
(dipole) that has one or more additional conductors connected across the ends parallel to each
other.
E-TYPE T-JUNCTION—A waveguide junction in
which the junction arm extends from the main
AI-3
FOUR-ELEMENT ARRAY—An array with three
parasitic elements and one driven element.
GROUP VELOCITY—The forward progress velocity
of a wave front in a waveguide.
FREE-SPACE LOSS—The loss of energy of a radio
wave because of the spreading of the wavefront
as it travels from the transmitter.
H-FIELD—Any space or region in which a magnetic
force is exerted. The magnetic field may be
produced by a current-carrying coil or conductor,
by a permanent magnet, or by the earth itself.
Also known as MAGNETIC FIELD.
FREQUENCY—The number of cycles that occur in
one second. Usually expressed in Hertz.
H-TYPE T-JUNCTION—A waveguide junction in
which the junction arm is parallel to the magnetic
lines of force in the main waveguide.
FREQUENCY DIVERSITY—Transmitting (and
receiving) of radio waves on two different
frequencies simultaneously.
HALF-WAVE DIPOLE ANTENNA—An antenna
consisting of two rods (¼ wavelength h) in a
straight line, that radiates electromagnetic energy.
FRONT-TO-BACK RATIO—The ratio of the energy
radiated in the principal direction to the energy
radiated in the opposite direction.
FUNDAMENTAL FREQUENCY—The
frequency or first harmonic frequency.
GAIN—The ratio between the amount
propagated from an antenna that is
to the energy from the same antenna
be propagated if the antenna were not
HARMONIC—A frequency that is a whole number
multiple of a smaller base frquency.
basic
HERTZ ANTENNA—A half-wave antenna installed
some distance above ground and positioned either
vertically or horizontally.
of energy
directional
that would
directional.
HORN—A funnel-shaped section of waveguide used
as a termination device and as a radiating antenna.
GENERATOR END—See INPUT END
HORIZONTAL AXIS—On a graph, the straight line
axis plotted from left to right.
GROUND PLANE—The portion of a groundplane
antenna that acts as ground.
HORIZONTAL PATTERN—The part of a radiation
pattern that is radiated in all directions along the
horizontal plane.
GROUND-PLANE ANTENNA—A type of antenna
that uses a ground plane as a simulated ground
to produce low-angle radiation.
HORIZONTALLY POLARIZED—Waves that are
radiated with their E-field component parallel
to the earth’s surface.
GROUND REFLECTION LOSS—The loss of rf
energy each time a radio wave is reflected from
the earth’s surface.
HYBRID JUNCTION—A waveguide junction that
combines two or more basic T-junctions.
GROUND SCREEN—A series of conductors buried
below the surface of the earth and arranged in
a radial pattern. Used to reduce losses in the
ground.
HYBRID RING—A hybrid-waveguide junction that
combines a series of E-type T-junctions in a ring
configuration.
GROUND WAVES—Radio waves that travel near the
surface of the earth.
I 2R LOSS—See COPPER LOSS.
AI-4
LOAD END—See OUTPUT END.
INCIDENT WAVE—(1) The wave that strikes the
surface of a medium; (2) The wave that travels
from the sending end to the receiving end of a
transmission line.
LOAD ISOLATOR—A passive attenuator in which
the loss in one direction is much greater than that
in the opposite direction. An example is a ferrite
isolator for waveguides that allow energy to travel
INDUCTION FIELD—The electromagnetic field
produced about an antenna when current and
voltage are present on the same antenna.
in only one direction.
LOADING—See LUMPED-IMPEDANCE TUNING.
INDUCTION LOSSES—The losses that occur when
the electromagnetic field around a conductor cuts
through a nearby metallic object and induces a
current into that object.
LOBE—An area of a radiation pattern plotted on a
polar-coordinate graph that represents maximum
radiation.
INPUT END—The end of a two-wire transmission
line that is connected to a source. Also known
as a GENERATOR END or a TRANSMITTER
END.
LONG-WIRE ANTENNA—An antenna that is a
wavelength or more long at its operating frequency.
LONGITUDINAL WAVES—Waves in which the
disturbance (back and forth motion) takes place
in the direction of propagation. Sometimes called
INPUT IMPEDANCE—The impedance presented
to the transmitter by the transmission line and
its load.
compression waves.
INTERFERENCE—Any disturbance that produces
an undesirable response or degrades a wave.
LOOP—(1) The curves of a standing wave or antenna
that represent amplitude of current or voltage;
IONOSPHERE—The most important region of the
atmosphere extending from 31 miles to 250 miles
above the earth. Contains four cloud-like layers
that affect radio waves.
(2) A curved conductor that connects the ends
of a coaxial cable or other transmission line and
projects into a waveguide or resonant cavity for
the purpose of injecting or extracting energy.
IONOSPHERIC STORMS—Disturbances in the
earth’s magnetic field that make communications
practical only at lower frequencies.
LOWEST USABLE FREQUENCY—The minimum
operating frequency that can be used for communications between two points.
IONIZATION—The process of upsetting electrical
neutrality.
LUMPED CONSTANTS—The properties of
inductance, capacitance, and resistance in a
transmission line.
IRIS—A metal plate with an opening through which
electromagnetic waves may pass. Used as an
impedance matching device in waveguides.
LUMPED-IMPEDANCE TUNING—The insertion of an inductor or capacitor in series with an
antenna to lengthen or shorten the antenna
electrically. Also known as LOADING.
ISOTROPIC RADIATION—The radiation of energy
equally in all directions.
LOOSE COUPLING—Inefficient coupling of energy
LEAKAGE CURRENT—The small amount of
current that flows between the conductors of a
transmission line through the dielectric.
from one circuit to another that is desirable in
some applications. Also called weak coupling.
AI-5
NONDIRECTIONAL—See OMNIDIRECTIONAL,
MAGIC-T JUNCTION—A combination of the
H-type and E-type T-junctions.
NONRESONANT LINE—A transmission line that
MAGNETIC FIELD—See H-FIELD.
has no standing waves of current or voltage.
MAJOR LOBE—The lobe in which the greatest
amount of radiation occurs.
NORMAL—The imaginary line perpendicular to the
point at which the incident wave strikes the
reflecting surface. Also called the perpendicular.
MARCONI ANTENNA—A quarter-wave antenna
oriented perpendicular to the earth and operated
Also known as
with one end grounded.
QUARTER-WAVE ANTENNA.
NULL—On a polar-coordinate graph, the area that
represents minimum or 0 radiation.
OMNIDIRECTIONAL—Transmitting in all directions. Also known as NONDIRECTIONAL.
MAXIMUM USABLE FREQUENCY—Maximum
frequency that can be used for communications
between two locations for a given time of day
and a given angle of incidence.
OPEN-ENDED LINE—A transmission line that has
an infinitely large terminating impedance.
OPTIMUM WORKING FREQUENCY—The most
practical operating frequency that can be used
with the least amount of problems; roughly 85
percent of the maximum usable frequency.
MEDIUM—The substance through which a wave
travels from one point to the next. Air, water,
wood, etc., are examples of a medium.
METALLIC INSULATOR—A shorted quarter-wave
section of transmission line.
ORIGIN—The point on a graph where the vertical
and horizontal axes cross each other.
MICROWAVE REGION—The portion of the
electromagnetic spectrum from 1,000 megahertz
to 100,000 megahertz.
OUTPUT END—The end of a transmission line that
is opposite the source. Also known as RECEIVING END.
MINOR LOBE—The lobe in which the radiation
intensity is less than a major lobe.
OUTPUT IMPEDANCE—The impedance presented
to the load by the transmission line and its source.
MULTIELEMENT ARRAY—An array consisting
of one or more arrays and classified as to
directivity.
PARALLEL RESONANT CIRCUIT—A circuit that
acts as a high impedance at resonance.
PARALLEL-WIRE—A type of transmission line
consisting of two parallel wires.
MULTIELEMENT PARASITIC ARRAY—An array
that contains two or more parasitic elements and
a driven element.
PARASITIC ARRAY—An array that has one or
more parasitic elements.
MULTIPATH—The multiple paths a radio wave may
follow between transmitter and receiver.
NEGATIVE ALTERNATION—The portion of a
sine wave below the reference line.
PARASITIC ELEMENT—The passive element of
an antenna array that is connected to neither the
transmission line nor the driven element.
NODE—The fixed minimum points of voltage or
current on a standing wave or antenna.
PERIOD—The amount of time required for completion of one full cycle.
AI-6
RADIATION RESISTANCE—The resistance, which
PHASE SHIFTER—A device used to change the
phase relationship between two ac signals.
if inserted in place of an antenna, would consume
the same amount of power as that radiated by
PLANE OF POLARIZATION—The plane (vertical
or horizontal) with respect to the earth in which
the E-field propagates.
the antenna.
RADIO FREQUENCIES—Electromagnetic frequencies that fall between 3 kilohertz and 300
POSITIVE ALTERNATION—The portion of a
sine wave above the reference line.
gigahertz and are used for radio communications.
RADIO HORIZON—The boundary beyond the
POWER GAIN—The ratio of the radiated power
of an antenna compared to the output power of
A measure of antenna
a standard antenna.
efficiency usually expressed in decibels. Also
referred to as POWER RATIO.
natural horizon in which radio waves cannot be
propagated over the earth’s surface.
RADIO WAVE—(1) A form of radiant energy that
can neither be seen nor felt; (2) An electromag-
POWER LOSS—The heat loss in a conductor as
current flows through it.
netic wave generated by a transmitter.
RAREFIED WAVE—A longitudinal wave that has
POWER RATIO—See POWER GAIN.
been expanded or rarefied (made less dense) as
it moves away from the source.
POWER STANDING—WAVE RATIO
(PSWR)—The ratio of the square of the maximum and minimum voltages of a transmission
line.
RECEIVER—The object that responds to a wave or
disturbance. Same as detector.
PROPAGATION—Waves traveling through a
medium.
RECEIVING ANTENNA—The device used to pick
up an rf signal from space.
PROBE—A metal rod that projects into, but is
insulated from, a waveguide or resonant cavity
and used to inject or extract energy.
RECEIVING END—See OUTPUT END.
RECIPROCITY—The ability of an antenna to both
QUARTER-WAVE ANTENNA—See MARCONI
ANTENNA.
transmit and receive electromagnetic energy with
RADIATION FIELD—The electromagnetic field that
detaches itself from an antenna and travels
through space.
REFLECTED WAVE—(1) The wave that reflects
equal efficiency.
back from a medium; (2) Waves traveling from
the load back to the generator on a transmission
line; (3) The wave moving back to the sending
RADIATION LOSSES—The losses that occur when
magnetic lines of force about a conductor are
projected into space as radiation and are not
returned to the conductor as the cycle alternates.
end of a transmission line after reflection has
occurred.
REFLECTION WAVES—Waves that are neither
transmitted nor absorbed, but are reflected from
RADIATION PATTERN—A plot of the radiated
energy from an antenna.
the surface of the medium they encounter.
AI-7
SERIES RESONANT CIRCUIT—A circuit that
acts as a low impedance at resonance.
REFLECTOR—The parasitic element of an array
that causes maximum energy radiation in a
direction toward the driven element.
SHIELDED PAIR—A line consisting of parallel
conductors separated from each other and
surrounded by a solid dielectric.
REFRACTION—The changing of direction as a wave
leaves one medium and enters another medium
of a different density.
SHORT-CIRCUITED LINE—A transmission line
that has a terminating impedance equal to 0.
REFRACTIVE INDEX—The ratio of the phase
velocity of a wave in free space to the phase
velocity of the wave in a given substance
(dielectric).
SKIN EFFECT—The tendency for alternating current
to concentrate in the surface layer of a conductor.
The effect increases with frequency and serves
to increase the effective resistance of the conductor.
RERADIATION—The reception and retransmission
of radio waves caused by turbulence in the
troposphere.
SKIP DISTANCE—The distance from a transmitter
to the point where the sky wave is first returned
to earth.
RESONANCE—The condition produced when the
frequency of vibrations are the same as the natural
frequency (of a cavity), The vibrations reinforce
each other.
SKIP ZONE—A zone of silence between the point
where the ground wave becomes too weak for
reception and the point where the sky wave is
first returned to earth.
RESONANT LINE—A transmission line that has
standing waves of current and voltage.
SKY WAVES—Radio waves reflected back to earth
from the ionosphere.
RHOMBIC ANTENNA—A diamond-shaped antenna
used widely for long-distance, high-frequency
transmission and reception.
SLOT—Narrow opening in a waveguide wall used
to couple energy in or out of the waveguide. Also
called an APERTURE or a WINDOW.
RIGID COAXIAL LINE—A coxial line consisting of a central, insulated wire (inner conductor)
mounted inside a tubular outer conductor.
SOURCE—(1) The object that produces waves or
disturbance; (2) The name given to the end of
a two-wire transmission line that is connected
to a source.
ROTATING JOINT—A joint that permits one section of a transmission line or waveguide to rotate
continuously with respect to another while passing
energy through the joint. Also called a rotary
coupler.
SPACE DIVERSITY—Reception of radio waves by
two or more antennas spaced some distance apart,
SCATTER ANGLE—The angle at which the
receiving antenna must be aimed to capture the
scattered energy of tropospheric scatter.
SPACE WAVE—A radio wave that travels directly
from the transmitter to the receiver and remains
in the troposphere.
SELF-INDUCTION—The phenomenon caused by
the expanding and collapsing fields of an electron
that encircles other electrons and retards the
movement of the encircled electrons.
SPECTRUM—(1) The entire range of electromagnetic
waves; (2) VISIBLE. The range of electromagnetic waves that stimulate the sense of sight;
AI-8
TRANSMISSION LINE—A device designed to guide
(3) ELECTROMAGNETIC. The entire range of
electromagnetic waves arranged in order of their
frequencies.
electrical energy from one point to another.
TRANSMITTING ANTENNA—The device used
SPORADIC E LAYER—Irregular cloud-like patches
of unusually high ionization. Often forms at
heights near the normal E-layer.
to send the transmitted signal energy into space.
TRANSMISSION MEDIUMS—The various types
of lines and waveguides used as transmission
SPREADER—Insulator used with transmission lines
and antennas to keep the parallel wires separated.
lines.
STANDING WAVE—The distribution of voltage and
current formed by the incident and reflected
waves, which have minimum and maximum
points on a resultant wave that appears to stand
still.
TRANSMITTER END—See INPUT END.
TRANSVERSE WAVE MOTION—The up and
down motion of a wave as the wave moves
outward.
STANDING-WAVE RATIO (SWR)—The ratio of
the maximum to the minimum amplitudes of
corresponding components of a field, voltage,
or current along a transmission line or waveguide
in the direction of propagation measured at a
given frequency. Measures the perfection of the
termination of the line.
TRANSVERSE ELECTRIC MODE—The entire
electric field in a waveguide is perpendicular to
the wide dimension and the magnetic field is
parallel to the length. Also called the TE mode.
TRANSVERSE MAGNETIC MODE—The entire
magnetic field in a waveguide is perpendicular
STRATOSPHERE—Located between the troposphere
and the ionosphere. Has little effect on radio
waves.
to the wide dimension (“a” wall) and some portion
of the electric field is parallel to the length. Also
called the TM mode.
STUB—Short section of a transmission line used to
match the impedance of a transmission line to
an antenna. Can also be used to produce desired
phase relationships between connected elements
of an antenna.
TROPOSPHERE—The portion of the atmosphere
closest to the earth’s surface, where all weather
phenomena take place.
TROPOSPHERIC SCATTER—The propagation
SUDDEN IONOSPHERIC DISTURBANCE—An
irregular ionospheric disturbance that can totally
blank out hf radio communications.
of radio waves in the troposphere by means of
scatter.
TROUGH (BOTTOM)—The peak of the negative
SURFACE WAVE—A radio wave that travels along
the contours of the earth, thereby being highly
attenuated.
alternation (maximum value below the line).
TUNED LINE—Another name for the resonant line.
This line uses tuning devices to eliminate the
TEMPERATURE INVERSION—The condition in
which warm air is formed above a layer of cool
air that is near the earth’s surface.
reactance and to transfer maximum power from
the source to the line.
THREE-ELEMENT ARRAY—An array with two
parasitic elements (reflector and director) and a
driven element.
TURNSTILE ANTENNA—A type of antenna used
in vhf communications that is omnidirectional
AI-9
and consists of two horizontal half-wave antennas
mounted at right angles to each other in the same
horizontal plane.
VOLTAGE STANDING-WAVE RATIO
(VSWR)—The ratio of maximum to minimum
voltage of a transmission line.
TWISTED PAIR—A line consisting of two insulated
wires twisted together to form a flexible line
without the use of spacers.
WAVE ANTENNA—See BEVERAGE ANTENNA.
WAVE MOTION—A recurring disturbance advancing through space with or without the use of a
physical medium.
TWO-WIRE OPEN LINE—A parallel line consisting
of two wires that are generally spaced from 2
to 6 inches apart by insulating spacers.
WAVE TRAIN—A continuous series of waves with
the same amplitude and wavelength.
TWO-WIRE RIBBON (TWIN LEAD)—A parallel
line similar to a two-wire open line except that
uniform spacing is assured by embedding the two
wires in a low-loss dielectric.
WAVEFRONT—A small section of an expanding
sphere of electromagnetic radiation, perpendicular
to the direction of travel of the energy.
WAVEGUIDE—A rectangular, circular, or elliptical
metal pipe designed to transport electro-magnetic
waves through its interior.
UNIDIRECTIONAL ARRAY—An array that radiates
in only one general direction.
UNTUNED LINE—Another name for the flat or
nonresonant line.
WAVEGUIDE MODE OF OPERATION—
Particular field configuration in a waveguide that
satisfies the boundary conditions. Usually divided
into two broad types: the transverse electric (TE)
and the transverse magnetic (TM).
V ANTENNA—A bidirectional antenna, shaped like
a V, which is widely used for communications.
VELOCITY—The rate at which a disturbance travels
through a medium.
WAVEGUIDE POSTS—A rod of conductive material
used as impedance-changing devices in
waveguides.
VERTICAL AXIS—On a graph, the straight line axis
oriented from bottom to top.
WAVEGUIDE SCREW—A screw that projects into
a waveguide for the purpose of changing the
impedance.
VERTICAL PATTERN—The part of a radiation
pattern that is radiated in the vertical plane.
WAVELENGTH—(1) The distance in space occupied
by 1 cycle of a radio wave at any given instant;
(2) The distance a disturbance travels during one
period of vibration.
VERTICAL PLANE—An imaginary plane that is
perpendicular to the horizontal plane.
VERTICALLY POLARIZED—Waves radiated with
the E-field component perpendicular to the earth’s
surface.
WINDOW—See Slot.
YAGI ANTENNA—A multielement parasitic array.
Elements lie in the same plane as those of the
end-fire array.
VOLTAGE-FEED METHOD—See END-FEED
METHOD.
AI-10
APPENDIX II
REFERENCES USED TO DEVELOP THIS TRAMAN
Shipboard Antenna Systems, Vol 1, Communications Antenna Fundamentals, NAVSEA 0967-LP-177-3010, Naval Sea
Systems Command, Washington, DC, 1972.
Shipboard Antenna Systems, Vol 2, Instatallation Details Communications Antenna Systems, NAVSEA
0967-LP-177-3020, Naval Sea Systems Command, Washington, DC, 1973.
Shipboard Antenna Systems, Vol 3, Antenna Couplers Communications Antenna Systems, NAVSEA
0967-LP-177-3030, Naval Sea Systems Command, Washington, DC, 1973.
Shipboard Antenna Systems, Vol 4, Testing and Maintenance Communications Antenna Systems, NAVSEA
0967-LP-177-3040, Naval Sea Systems Command, Washington, DC, 1972.
Navy Electricity and Electronics Training Series, Module 10, Introduction to Wave Propagation, Transmission Lines,
and Antennas, NAVEDTRA B72-10-00-93, Naval Education and Training Program Management Support
Activity, Pensacola FL, 1993.
Navy Electricity and Electronics Training Series, Module 11, Microwave Principles, NAVEDTRA 172-11-00-87,
Naval Education and Training Program Management Support Activity, Pensacola FL, 1987.
Navy UHF Satellite Communication System Description, FSCS-200-83-1, Naval Ocean Systems Center, San Diego,
CA, 1991.
AII-1
INDEX
A
Antennas/antenna radiation
anisotropic radiation, 2-4
characteristics, 2-1
counterpoise, 2-5, 2-6
directivity, 2-1
gain, 2-2
ground screen, 2-5, 2-6
Hertz antennas, 2-1
isotropic radiation, 2-4
loading, 2-4
lobe, 2-4
loop, 2-3
low probability of intercept (LPI), 2-19
lumped-impedance tuning, 2-5
major lobe, 2-4
Marconi antennas, 2-1
minor lobe, 2-4
node, 2-3, 3-9
period, 2-3
polarization, 2-2
reciprocity, 2-1
standing wave, 2-3
wavelength, 2-3
Atmosphere, 1-1
ionosphere, 1-1
stratosphere, 1-1
temperature inversion, 1-12
troposphere, 1-1
weather, 1-12
log-periodic (LPA), 2-8, 2-16
long-wire, 2-11
low frequency (lf), 2-7
NORD, 2-9
pan polar, 2-8
parasitic array, 2-7
quadrant, 2-11, 2-13
rhombic, 2-11, 2-12
rotatable LPA (RLPA), 2-10
sector log-periodic array, 2-10
Trideco, 2-7
tuning system, 2-13
ultra high frequency (uhf), 2-14
vertical monopole LPA, 2-8, 2-9
very high frequency (vhf), 2-14
very low frequency (vlf), 2-6
whip, 2-13, 2-14
wire rope fan, 2-14
Yagi, 2-7, 2-9
Coupler groups, 2-21 to 23
coupler group AN/SRA-33, 2-22
coupler group AN/SRA-39, 2-23
coupler group AN/SRA-40, 2-23
coupler group AN/SRA-49, 2-23
coupler group AN/SRA-49A, 2-23
coupler group AN/SRA-50, 2-23
coupler group AN/SRA-56, 2-21, 2-22
coupler group AN/SRA-57, 2-21, 2-22
coupler group AN/SRA-58, 2-21, 2-22
coupler group AN/URA-38, 2-21, 2-23
multicoupler (receive filter) AN/SR4-12, 2-23
multicoupler OA-9123/SRC, 2-22
C
I
Communications antennas, 2-6
biconical dipole, 2-15
boom, 2-10
center-fed dipole, 2-16
coaxial dipole, 2-16, 2-17
conical monopole, 2-11
discage, 2-14, 2-15
Goliath, 2-7
ground plane, 2-10, 2-13
high frequency (hf), 2-7
inverted cone, 2-10, 2-11
Ionosphere, 1-1
D layer, 1-3
E layer, 1-4
F/F1/F2 layer, 1-4
ionization 1-2
ionized layers, 1-2, 1-3
ionospheric storms, 1-11
ions, 1-2
regular variations, 1-1
seasonal variations, 1-10
INDEX-1
solar flare, 1-4
sporadic E, 1-4, 1-11
sudden ionospheric disturbances (SID), 1-11
sunspot activity. 1-4, 1-10
sunspots cycles, 1-11
horn radiator, 2-25, 2-26
OE-172/SPS-55, 2-28
orange-peel paraboloid, 2-24, 2-25
parabolic reflector, 2-23
paraboloid, 2-24
truncated paraboloid, 2-24
AS-1669/SPN-35, 2-29, 2-30
M
Matching networks, 2-20
antenna couplers, 2-21
antenna tuners, 2-20
antenna tuning, 2-21
receive distribution system, 2-22
RF safety, 2-30
dielectric heating, 2-30
radiation warning signs, 2-31
rf burns, 2-30
working aloft, 2-32
P
S
Propagation, 1-4
angle of incidence
1-6, 3-14
3-14
angle of reflection
critical angle, 1-6
critical frequency, 1-5, 1-11
escape point, 1-5
fading, 1-7
frequency diversity. 1-8
layer density, 1-5
multipath fading, 1-8
reflection, 1-7
refraction, 1-4
selective fading, 1-8
skip distance, 1-7
skip zone, 1-7
sky wave, 1-7
space diversity, 1-8
Satellite communications antennas, 2-16
AN/WSC-6(V), 2-19, 2-20
Andrew 58622, 2-19
AS-2815/SRR-1, 2-16, 2-17
backplane, 2-17, 2-19
OE-82A/WSC-1(V), 2-17, 2-19
OE-82B/WSC-1(V), 2-16, 2-18
0E-82C/WSC-1(V), 2-16, 2-18
R
Radar antennas, 2-23
AN/GPN-27 (ASR-8), 2-26
AN/SPN-35A, 2-29
AS-1292/TPN-8, 2-29
AS-32631SPS-49(V), 2-27
azimuth pulse generator (APG), 2-27
broadside array, 2-25
carrier-controlled approach (CCA), 2-29
corner reflector, 2-24, 2-25
cylindrical paraboloid, 2-24, 2-25
feedhorn, 2-26
focal point, 2-23
height-finding, 2-25
T
Transmission, 1-12
absorption, 1-14
freespace losses, 1-13
frequency selection, 1-13
ground reflection losses, 1-13
lowest usable frequency (luf), 1-13
maximum usable frequency (muf), 1-13
optimum working frequency (fot), 1-14
plane wavefront, 2-23
wavefront, 1-13, 2-2, 2-23, 3-14
Transmission line, 3-1
ac, 3-5
capacitance, 3-1, 3-2
characteristic impedance (Z0), 3-3
coaxial line (flexible/rigid), 3-6, 3-7
conductance (G), 3-1, 3-2
copper losses (I2R), 3-3, 3-9
current standing-wave ratio (iswr), 3-6
dc, 3-4, 3-5
dielectric losses, 3-3, 3-4, 3-9
distributed constants, 3-1
INDEX-2
electric (E) field, 3-3, 3-12
electromagnetic fields, 3-3
incident wave, 3-5
inductance, 3-1, 3-2
induction losses, 3-3, 3-4
input impedance (Zin), 3-3
leakage current, 3-2
line losses, 3-3
lumped constants, 3-1
magnetic (H) field, 3-3, 3-12
output impedance (Z out ), 3-3
parallel line, 3-6
power loss, 3-3
power standing-wave ratio (pswr), 3-6
radiation losses, 3-3, 3-4
reflected wave, 3-5
resistance, 3-1
self-induction, 3-4
shielded pair, 3-6, 3-7
skin effect, 3-4, 3-9
standing-wave ratio (SWR), 3-5
twisted pair, 3-6, 3-7
two-wire, 3-1
two-wire open line, 3-6
two-wire ribbon line, 3-7
voltage standing-wave ratio, (vswr), 3-6
waveguides, 3-6
W
Waveguide input/output, 3-18
apertures, 3-18, 3-19
bidirectional coupler, 3-25, 3-26
cavity resonators, 3-25, 3-26, 3-27
directional coupler, 3-25, 3-26
dummy load, 3-21
duplexer, 3-31
horn, 3-21
hybrid junctions, 3-25, 3-28, 3-30
hybrid ring, 3-28, 3-31, 3-32
impedance matching, 3-19
iris, 3-20
junctions, 3-27
loop, 3-19
magic-T, 3-28, 3-30
posts, 3-20
probes, 3-18
resistive load, 3-21, 3-22
screws, 3-21
slots, 3-18, 3-19
T junction (E and H type), 3-28, 3-29
terminations, 3-20
windows, 3-18
Waveguides, 3-6
“a” dimension, 3-12
“b” dimension, 3-12
angle of incidence
3-14, 3-15
3-14, 3-15
angle of reflection
arcing, 3-10
bends, 3-22
boundary conditions, 3-13
choke joint, 3-23, 3-24
circular, 3-16
cutoff frequency, 3-12, 3-15
dominant mode, 3-16
E bend, 3-22
electrolysis, 3-24
group velocity, 3-15
H bend, 3-22
joints, 3-23
metallic insulator, 3-10, 3-11
mode numbering, 3-17
mode of operation, 3-12, 3-15
plumbing, 3-22
Poynting vector, 3-14
rotating joint, 3-23, 3-24
sharp bend, 3-23
size, 3-10
transverse electric (TE), 3-17
transverse magnetic (TM), 3-17
twist, 3-23
INDEX-3
Assignment Questions
Information: The text pages that you are to study are
provided at the beginning of the assignment questions.
ASSIGNMENT
Textbook
1-1.
Propagation,” chapter
1-6.
Geographic height
Geographic
location
Changes in time
All of the above
1-7.
Ionosphere
Stratosphere
Troposphere
Hydrosphere
1-5.
1-1
through
1-14.
Negative
Positive
Neutral
Inverted
The frequency of ultraviolet
light passing through the
atmosphere has what
relationship to the ionospheric
layer it ionizes?
2.
3.
4.
True
False
It is inversely
proportional
It is directly proportional
It is inversely
proportional during the day
and directly proportional
at night
It is directly proportional
during the day and
inversely proportional at
night
Variations in the ionosphere
resulting from changes in the
sun’s activity are known as
What term best describes the
process that returns positive
ions to their original neutral
state?
1.
2.
3.
4.
1.
2.
3.
4.
1-8.
1-4.
pages
In ionization, when an electron
is knocked free from a neutral
gas atom, what is the overall
charge of the atom?
1.
Because the stratosphere is a
relatively calm region with
little or no temperature
change, it will have almost no
effect on radio wave
propagation.
1.
2.
1,
1.
2.
3.
4.
In what portion of the
does the majority
atmosphere
of weather phenomena take
place?
1.
2.
3.
4.
1-3.
“Wave
Which of the following factors
can affect atmospheric
conditions?
1.
2.
3.
4.
1-2.
Assignment:
1
regular variations
irregular variations
both 1 and 2 above
seasons
1-9.
The regular variations in the
ionosphere can be separated
into how many classes?
1.
2.
3.
4.
One
Two
Three
Four
At what approximate time of
is the density of the
ionospheric layers at its
lowest level?
1.
2.
3.
4.
1–10.
Just before
Mid-morning
Afternoon
Sunset
One
Two
Three
Four
day
sunrise
How many distinct layers
ionosphere?
up t h e
1.
2.
3.
4.
1
Refraction
Recombination
Ionization
Polarization
make
1-11.
At what frequencies does the
combination of the earth’s
surface and the D layer act as
a waveguide?
1-17.
Which of the following is NOT a
factor for radio wave
refraction?
1.
1.
2.
3.
4.
1-12.
2.
3.
4.
The D layer loses its
absorptive qualities at
frequencies above what level?
1.
2.
3.
4.
1-13.
Vlf
Lf
Mf
Hf
30
20
10
3
1-18.
MHz
MHz
MHz
MHz
What is the approximate range
of the E layer above the
earth’s
surface?
1.
2.
3.
4.
30–54
55–90
91–130
131–160
For any given ionized layer,
the critical frequency is just
below the escape point.
1.
2.
1-19.
2.
3.
4.
1-14.
Frequencies above what level
pass through the E layer
unaffected?
1.
2.
3.
4.
1-15.
MHz
MHz
MHz
MHz
Five
Two
Three
Four
1-21.
1.
2.
3.
4.
D
E
F
H
1-22.
Frequency
Sunspot activity
Angle of transmission
All of the above
Radio waves reflecting from the
earth’s surface or the
ionosphere, 180 degrees out of
phase, have what effect, if
any, at the receiving station?
1.
2.
3.
4.
2
Ground wave
Skip zone
Skip distance
Ace area
Which of the following factors
will affect the outer limits of
the skip zone?
1.
2.
3.
4.
Most high–frequency, long-range
communications occur in what
layer(s) of the ionosphere?
Angle of incidence and
layer density only
Layer density and
wavelength only
Angle of incidence and
wavelength only
Wavelength and antenna
height only
What term best describes the
area located between the
transmitting antenna and the
point where the sky wave first
returns to the earth?
1.
2.
3.
4.
During daylight hours, the F
layer will divide into how many
separate
layers?
1.
2.
3.
4.
1-16.
50
100
150
200
1-20.
True
False
The critical angle for radio
wave propagation depends on
what two factors?
1.
miles
miles
miles
miles
Ionization density of the
layer
Frequency of the radio wave
Angle of incidence
Transmitter power
The signal will be weak or
faded
The signal will be stronger
The signal will be garbled
None
1-23.
1.
2.
3.
4.
1-24.
2.
3.
4.
The area of complete
coverage at vlf frequencies
The area within the
diameter of an obstruction
The area ranging the height
of the obstruction
The area on the opposite
side of the obstruction, in
line-of-site from the
transmitter to the receiver
1-29.
1-30.
4.
Two or more receiving
antennas spaced apart to
produce a usable signal
B.
Two or more receiving
antennas of varying heights
located together
C.
The use of two separate
transmitters
and
receivers
on different frequencies
transmitting the same
information
D.
The use of two separate
transmitters
and
receivers
on the same frequency
transmitting the same
information
Space
1. A
2. B
3. C
4. D
1-31.
Phase shift
Absorption
Multipath
Diffraction
2.
3.
4.
1-28.
Fading on the majority of the
ionospheric circuits is a
result of what particular type
of fading?
1.
2.
3.
4.
1-32.
3
A
B
C
D
It affects various
frequencies
It can cause changes
phase and amplitude
It can cause severe
distortion and limit
signal
strength
All of the above
Which
dense
1.
2.
3.
4.
Selective
Absorption
Multipath
Weather
diversity.
A wide band of frequencies is
transmitted
and
selective
fading occurs.
Which of the
following statements best
describes the effect of the
fading on the signal?
1.
Groundwaves
Ionospheric
refractions
Reflection from the earth’s
surface
All of the above
diversity.
Frequency
1.
2.
3.
4.
Which of the following are
examples of multipath radio
wave
transmissions?
1.
2.
3.
A.
a
What type of fading occurs for
the longest amount of time?
1.
2.
3.
4.
1-27.
reflection
refraction
diffraction
waveshaping
Which of the following
definitions best describes
shadow zone?
1.
1-26.
One
Two
Three
Four
The ability of radio waves to
turn sharp corners and bend
around obstacles is known as
1.
2.
3.
4.
1-25.
IN ANSWERING QUESTIONS 1-29 AND 1-30,
SELECT FROM THE FOLLOWING LIST THE
DEFINITION OF THE INDICATED TERM.
For ionospheric reflection to
occur, the ionized layer must
not be thicker than how many
wavelengths of the transmitted
frequency?
E
D
F2
F1
ionospheric
during the
in
total
layer is
winter?
most
1-33.
1-36.
During the 27–day sunspot
cycle, which ionospheric layer
experiences the greatest
fluctuations in density?
1.
2.
3.
4.
1.
2.
3.
4.
D
E
F1
F2
1-37.
Depends on the angle of the
sun; refracts hf waves
during the day, up to 20
MHz, to distances of 1200
miles; greatly reduced at
night
1-38.
B.
Reflects vlf waves for
long–range
communications;
refracts lf and mf for
short–range
communications;
has little effect on vhf
and above; gone at night
1-39.
C.
Density depends on the
angle of the sun; its main
effect is absorption of hf
waves passing through to
the F2 layer
D.
1.
2.
3.
1–34.
1-41.
A
B
C
D
1-42.
E layer.
1.
2.
3.
4.
A
B
C
E
D
E
F1
F2
What effect do ionospheric
storms have on (a) the range of
frequencies and (b) the working
frequency used for
communications?
1.
2.
3.
4.
4
Causes
increased
multipath
problems
Provides additional
absorption
Blanks out more favorable
layers
Increased static in line of
sight communications
When sudden ionospheric
disturbances (SID) occurs,
which ionospheric layer is
affected the most?
1.
2.
3.
4.
D layer.
1.
2.
3.
4.
1-35.
Structure and density
depend on the time of day
and the angle of the sun;
consists of one layer at
night and two layers during
the day
True
False
Which of the following problems
is NOT a negative side effect
of the sporadic E layer?
4.
E.
A
C
D
E
During periods of maximum
sunspot activity within the
eleven-year
cycle,
critical
frequencies for all layers
increase.
1.
2.
Provides long-range hf
communications;
very
variable; height and
density change with time of
day, season, and sunspot
activity
A
B
C
D
F2 layer.
1.
2.
3.
4.
1-40.
B
C
D
E
F1 layer.
1.
2.
3.
4.
IN ANSWERING QUESTIONS 1–34 THROUGH
1-38, SELECT FROM THE FOLLOWING LIST
THE DEFINITION OF THE INDICATED TERM.
A.
F layer.
(a)
(a)
(a)
(a)
Increase
Decrease
Increase
Decrease
(b)
(b)
(b)
(b)
increase
decrease
decrease
increase
1-43.
What form of precipitation has
the greatest absorption effect
on RF energy?
1.
2.
3.
4.
1-47.
Fog
Snow
Rain
Hail
Radio waves above the MUF will
experience what effect when
refracted from the ionosphere?
1.
2.
3.
1-44.
The duct effect produced by
temperature
inversion
allows
for long-distance
communications over what
frequency
band?
1.
2.
3.
4.
1-45.
1-48.
Vlf
Lf
Hf
Vhf
4.
1-49.
Angle of incidence
Ground
irregularities
Electrical
conductivity
at
the point of reflection
All of the above
3.
4.
1-50.
5
Increased
absorption
Higher levels of
atmospheric noise
Higher rate of refraction
All of the above
The frequency that will avoid
the problems of multipath
fading, absorption, noise, and
rapid changes in the ionosphere
is known by what term?
1.
2.
3.
4.
Absorption
Ground reflection
Freespace
Spread
F1
F2
D
E
Radio waves that are propagated
below the LUF are affected by
what problem(s)?
1.
2.
As an RF wave increases in
distance, the wavefront spreads
out, reducing the amount of
energy available within any
This
given unit of area.
action produces what type of
energy loss?
1.
2.
3.
4.
Variations in the ionosphere
may change a preexisting muf.
This is especially true because
of the volatility of which of
the following layers?
1.
2.
3.
4.
Which of the following factors
affect(s) the amount of ground
reflection loss when a radio
wave is reflected from the
earth’s surface?
1.
2.
3.
1-46.
4.
They will fall short of the
desired location
They will overshoot the
desired location
They will be absorbed by
lower layers
They will experience
multipath fading
LUF
MUF
FOT
LOS
ASSIGNMENT
Textbook
2-1.
Assignment:
“Antennas,“
chapter
4.
pages
2-7.
Electromagnetic radiation from
an antenna is made up of what
two
components?
1.
2.
3.
2,
E and H fields
Ground and sky waves
Vertical and horizontal
wavefronts
Reflected and refracted
energy
What determines the size
transmitting
antenna?
1.
2.
3.
4.
2-3.
2-9.
T
F
Quarter–wave
Half–wave
Three quarter–wave
Full-wave
10
6
4
2
2-11.
All antennas regardless of
their shape or size have how
many
basic
characteristics?
1.
2.
3.
4.
6
Signal
Strength
Reciprocity
Directivity
Polarization
9
6
4
3
dB
dB
dB
dB
Which, if any, of the
following components of a
radiated
electromagnetic
field
determines its direction of
polarization?
H lines
E lines
Angle of Propagation
None of the above
Over long distances the
polarization of a radiated
wave changes, at what
frequencies will this change
be the most dramatic?
1.
2.
3.
4.
1
2
3
4
Gain
Reciprocity
Directivity
Polarization
The gain of a transmitting
antenna is 9 dB, what will the
gain be for the same antenna
used for receiving?
1.
2.
3.
4.
MHz
MHz
MHz
MHz
2–32.
The ability of an antenna or
array to focus energy in one
or more specific directions is
represented by a measurement
of what antenna property?
1.
2.
3.
4.
2-10.
through
The ability to use the same
antenna for both transmitting
and receiving is known by what
term?
1.
2.
3.
4.
Marconi antennas are used for
operating
frequencies
below
what level?
1.
2.
3.
4.
2-6.
Transmitter power
Available space
Operating
frequency
Distance to be
transmitted
Hertz antennas are designed to
operate at what wavelength in
relationship to their
operating frequency?
1.
2.
3.
4.
2–5.
a
Most
practical
transmitting
antennas are divided into two
classifications,
Hertz
and
Marconi.
1.
2.
2-4.
of
2-1
1.
2.
3.
4.
2-8.
2-2.
2
VLF
LF
MF
HF
2-12.
1.
2.
3.
4.
2-13.
2-18.
A transmitting antenna at
ground level should be
polarized in what manner to
achieve best signal strength?
Horizontally
Vertically
Circularly
Linearly
1.
2.
3.
4.
What term describes the
distance a wave travels during
the period of one cycle?
1.
2.
3.
4.
2-19.
Wavelength
Frequency
Travel time
Radiation rate
3.
4.
The points of high current and
voltage are best described by
which of the following terms?
2-20.
1.
2.
3.
4.
2-15.
2-16.
Peaks
Crescents
Loops
Highs
1.
2.
1.
2.
3.
4.
4.
3.
2-21.
An antenna at resonance will
transmit at maximum
efficiency; an antenna that is
not at resonance will lose
power in which of the
following
ways?
1.
2.
2-17.
Skin effect loss
Heat loss
Ground absorption
Wave scattering
4.
2-22.
An antenna that radiates
energy in all directions is
said to have what type of
radiation pattern?
1.
2.
3.
4.
7
Lumped resistance
Lumped capacitive
reactance
Lumped inductive
reactance
Less power
A ground screen is a series of
conductors buried 1 or 2 feet
below the surface in a radial
pattern and is usually of what
length in comparison to the
wavelength being used?
1.
2.
3.
4.
Isotropic
Anisotropic
Bysotropic
Circumstropic
Lumped resistance
Lumped capacitive
reactance
Lumped inductive
reactance
More power
If an antenna is too long for
the wavelength being used,
what electrical compensation
must be introduced for the
antenna to achieve resonance?
3.
1.
2.
3.
4.
High and low probes
Maximum and minimum
points
Major and minor lobes
Positive and negative
lobes
If an antenna is too short for
the wavelength being used,
what electrical compensation
must be introduced for the
antenna to achieve resonance?
The points of minimum voltage
and minimum current are
represented by which of the
following terms?
Lows
Valleys
Descents
Nodes
Isotropic
Anisotropic
Bysotropic
Circumstropic
When viewing a radiation
pattern graph, you can expect
the areas of maximum and
minimum radiation be
identified by which of the
following
terms?
1.
2.
IN ANSWERING QUESTIONS 2-14 AND 2-15,
REFER TO FIGURE 2–4 OF THE TEXT.
2-14.
An antenna that radiates
energy more strongly in one
direction than another is said
to have what type of radiation
pattern?
One-quarter
wavelength
One–half
wavelength
Three-quarter
wavelength
Full wavelength
2-23.
When would
used?
1.
2.
3.
4.
a
counterpoise
2-29.
be
When easy access to the
antenna base is necessary
When the surface below is
solid rock
When the surface below is
sandy ground
All the above
The most distinct advantage
the
rotatable
log–periodic
antenna is its ability to
perform what function?
1.
2.
3.
4.
2-24.
Capacitive
top-loading
helps
to increase which of the
following antenna
characteristics?
1.
2.
3.
4.
2-25.
2-26.
Bandwidth
Power-handling
Directivity
Radiation
efficiency
What is the most
characteristic of
antenna?
1.
2.
3.
4.
2-30.
2–31.
Power-handling
Narrow bandwidth
Physical size
Lack of directivity
1.
2.
3.
4.
2-32.
Medium power handling
capabilities
High gain
Extremely broad bandwidth
All the above
A typical vertical monopole
log periodic antenna designed
to cover a frequency range of
2 to 30 MHz will require
approximately how many acres
of land for its ground plane
system?
2-28.
1
2
3
4
1.
2.
acre
acres
acres
acres
4.
A sector log–periodic array
can act as an antenna for a
minimum of what number of
transmit or receive systems?
1.
2.
3.
4.
2-34.
1
2
3
4
T
F
To prevent radiation
hazard to personnel
To prevent radiation
hazard to ordinance
To increase power
handling
capabilities
To prevent unwanted
directivity in the
radiation pattern from
mast
structures
The central feed section for
both the biconical and centerfed dipole are protected by
what type of covering?
1.
2.
3.
4.
8
Transmitter power
Antenna
height
Radiated wave interaction
Transmitted
frequency
Why are UHF and VHF antennas
on board ship installed as
high as possible?
3.
1.
2.
3.
4.
kw
kw
kw
kw
Most Whip antennas require
some kind of a tuning system
to improve bandwidth and power
handling capabilities.
1.
2.
2-33.
2-27.
20
30
40
50
What determines the gain and
directivity of a Rhombic
antenna?
1.
2.
3.
4.
In general, log-periodic
antennas have which of the
following
characteristics?
Rotate 360 degrees
Rotate from horizontal to
vertical and back
Ability to handle high
transmitter power
Ability to produce high
antenna gain
What is the average power
handling capability of an
Inverted Cone antenna?
1.
2.
3.
4.
limiting
the Yagi
of
SCOTCHCOAT
RTV
Laminated fiberglass
Rubber shield
2-35.
1.
2.
3.
4.
2-36.
1.
2.
3.
4.
2-39.
2-42.
Vertical
Horizontal
Right–hand
circular
Left-hand circular
2-43.
2-44.
LPI
Low power interference
Low probability of
intercept
Low phase intercept
Last pass intercept
2-45.
1.
2.
3.
4.
2-46.
2-40.
Antenna tuning is accomplished
using what piece or pieces of
equipment?
1.
2.
3.
4.
3.
4.
9
provided
4
5
6
7
A surface search
An air search
A navigation
A height-finder
Of the following methods,
which is NOT used to feed a
cylindrical
paraboloid
reflector?
1.
2.
Couplers
Tuners
Multicouplers
All the above
Mhz
Mhz
Mhz
Mhz
What type of radar would use a
truncated paraboloid reflector
that has been rotated 90
degrees?
1.
2.
3.
4.
Conical
Peripheral
Vertical
Horizontal
2– 6
4-12
10-30
40–60
How many channels are
with the AN/SRA-12
multicoupler?
1.
2.
3.
4.
The reflectors for the AN/WSC6 (V) are mounted on threeaxis pedestals and provide
auto tracking using what
scanning technique?
AN/WSC-3
AN/URT–23
AN/URC-80
AN/FRT-84
The AN/SRA-57 coupler group
operates in which of the
following
frequency
ranges?
1.
2.
3.
4.
acronym
1
2
3
4
The AN/URA-38 antenna coupler
is an automatic tuning system
primarily used with which
radio transmitter?
1.
2.
3.
4.
1
2
3
4
What does the
stand for?
Antenna multicouplers are used
to match more than one
transmitter or receiver to
what number of antennas?
1.
2.
3.
4.
counterpoise angle
input impedance
radiation angle
feed point
The AN/WSC-5 (V) shore station
antenna consists of what
number of OE-82A/WSC-1 (V)
assemblies?
1.
2.
3.
4.
2-38.
The
The
The
The
The
OE-82B/WSC-I(V)
antenna
group uses what type of
polarization?
1.
2.
3.
4.
2-37.
2-41.
The adjustable stub on the
AS-390/SRC uhf antenna is used
to adjust what antenna
characteristic?
A linear array of dipoles
A slit in the side of a
waveguide
A thin waveguide radiator
A quarter–wave stub
2-47.
1.
2.
3.
4.
2-48.
2-50.
Which of the following is NOT
a mode of operation for the
AN/SPN-35A radar set?
1.
2.
3.
4.
One–eighth
One-quarter
One–half
Three-quarter
What is the advantage, if any,
to offsetting a feedhorn
radiator for a parabolic dish?
1.
2.
1.
2.
2-53.
A broader beam angle
The elimination of
shadows
A narrower beam angle
No advantage
T
F
What is the range in nautical
miles of the AN/GPN-27 radar?
When a person is standing in
an rf field, power in excess
of what level will cause a
noticeable rise in body
temperature?
1.
2.
3.
4.
1.
2.
3.
4.
2-54.
55
75
105
155
What is the purpose of the
jackscrew on the
AS–3263/SPS–49(V)
antenna?
1.
2.
3.
4.
2-55.
1.
To adjust the beam width
To vary the antenna feed
horn focal distance
To adjust the beam
elevation
angle
To lockdown the antenna
for PM
2.
The
OE-172/SPS–55
antenna
normally operates in the
linearly polarized mode, for
what reason would you use the
circular polarized mode?
1.
2.
3.
4.
4.
To compensate for the
ships pitch and roll
To prevent jamming
To reduce return echoes
from precipitation
To achieve over the
horizon coverage
10
5
10
15
20
milliwatts
milliwatts
milliwatts
milliwatts
When working aloft, what
safety precaution(s) must
followed?
3.
2-51.
Final
Dual
Surveillance
Simultaneous
The two primary safety
concerns associated with rf
fields are rf burns and
injuries caused by dielectric
heating.
3.
4.
2-49.
2-52.
The elements of a broadside
array are spaced one-half
wavelength apart and are
spaced how many wavelengths
away from the reflector?
be
Tag out the antenna at
the switchboard to
prevent it from becoming
operational
Secure motor safety
switches for rotating
antennas
Wear the proper oxygen
breathing apparatus when
working near a stack
All the above
ASSIGNMENT
Textbook
3-1.
2.
3.
4.
2.
3.
4.
Waveguides,”
3-5.
3-6.
3-7.
2.
3.
4.
3.
4.
Leakage current in a two–wire
transmission line is the
current that flows through what
component?
1.
2.
3.
4.
The
The
The
The
resistor
inductor
insulator
conductor
11
3-1
reciprocal
property?
Inductance
Resistance
Capacitance
Reciprocity
Electric
only
Magnetic
only
Both electric and
Capacitive
magnetic
input–gain
rate
voltage–gain ratio
output impedance
input
impedance
The
characteristic
impedance
(2.) of a transmission line is
calculated by using which of
the following ratios?
1.
2.
Between ground and any
single point on the line
Along the length of the
line
According to the thickness
of the line
According to the crosssectional area of the line
pages
A measurement of the voltage to
current ratio (Ein / Iout ) at the
input end of a transmission
line is called the
1.
2.
3.
4.
3-8.
3,
A transmission line that has
current flowing through it has
which of the following fields
about it?
1.
2.
3.
4.
Expected value of current
flow through the insulation
Expected value of voltage
supplied by the transmitter
Value of the lump and
distributed constants of
the line divided by
impedance
Value of the lumped
constants of the line as
seen by the source and the
load
chapter
Conductance is the
of what electrical
1.
2.
3.
4.
Disperse energy in all
directions
Detune a transmitter to
match the load
Guide electrical energy
from point to point
Replace the antenna in a
communications
system
Distributed constants in a
transmission line are
distributed in which of the
following
ways?
1.
3-4.
and
The conductance value of a
transmission
line
represents
which of the following values?
1.
3-3.
“Transmission Lines
through 3-32.
A transmission line is designed
to perform which of the
following
functions?
1.
3-2.
Assignment:
3
Rsource to Rload of the line
Imax to Im i n at every point
along the line
E to I at every point along
the line
E in to Eo u t of the line
3-9.
Maximum transfer of energy from
the source to the transmission
line takes place when what
impedance
relationship
exists
between the source and the
transmission line?
1.
2.
3.
4.
3-10.
3.
4.
3-11.
3-15.
of the following sets of
represents a type of loss
transmission line?
I 2R and induction only
Induction and dielectric
only
Dielectric and radiation
only
and
I2R , induction,
dielectric
3-16.
3-17.
3-12.
1.
2.
3.
4.
3-13.
3-18.
Copper
Radiation
Induction
Dielectric
1.
2.
3.
4.
3-19.
12
is
Load line
Coaxial line
Two-wire open line
Twisted-pair line
Electrical power lines are most
often made of which of the
following types of transmission
lines?
1.
2.
3.
4.
They are in phase with each
other
They are equal to ZO of the
line
They are out of phase with
each other
They are evenly distributed
along the line
Vswr
Pswr
Iswr
Rswr
Which of the following lines
NOT a transmission medium?
1.
2.
3.
4.
When a dc voltage is applied to
a transmission line and the
load absorbs all the energy,
what is the resulting
relationship
between
current
and voltage?
rswr
pswr
vswr
iswr
Which of the following ratios
samples the magnetic field
along a line?
1.
2.
3.
4.
What transmission-line loss is
caused by magnetic lines of
force not returning to the
conductor?
Incident
Refracted
Reflected
Diffracted
The ratio of maximum voltage to
minimum voltage on a
transmission line is referred
to as the
1.
2.
3.
4.
Copper
Voltage
Induction
Dielectric
Incident
Refracted
Reflected
Diffracted
Waves that travel from the
output end to the input end of
a transmission line are
referred to as what type of
waves?
1.
2.
3.
4.
Skin effect is classified as
which of the following types of
loss?
1.
2.
3.
4.
The initial waves that travel
from the generator to the load
of a transmission line are
referred to as what type of
waves?
1.
2.
3.
4.
When the load impedance
equals the source impedance
When the load impedance is
twice the source impedance
When the load impedance is
half the source impedance
When the load impedance is
one-fourth the source
impedance
Which
terms
in a
1.
2.
3-14.
Twin-lead line
Shielded-pair line
Two–wire open line
Two–wire ribbon line
3-20.
1.
2.
3.
4.
3-21.
3-24.
line
3-27.
Low radiation losses
Inexpensive
construction
Low high–frequency losses
Easy maintenance
3-28.
Waveguides
Twin–lead flat lines
Single-conductor
lines
Coaxial
transmission
lines
3.
4.
3-29.
1.
2.
3.
4.
Skin effect
Copper loss
Conductor density
Waveguide material
used
3.
4.
3–30.
being
Which of the following
dielectrics is used in
waveguides?
2.
3.
1.
2.
3.
4.
Air
Mica
Insulating
Insulating
4.
oil
foam
13
by
The widest (height/width)
The narrowest (height/
width)
The shortest (length)
The longest (length)
The cutoff frequency for a
waveguide is controlled by the
physical dimensions of the
waveguide and is defined as the
frequency at which two quarter
wavelengths are
1.
3-25.
An open half-wave section
An open quarter–wave
section
A shorted half–wave section
A shorted quarter-wave
section
The range of operating
frequencies is determined
which of the following
waveguide
dimensions?
1.
2.
In a coaxial line, the currentcarrying area of the inner
conductor is restricted to a
small surface layer because of
which of the following
properties?
An inductor
A resistor
A capacitor
A transformer
At high frequencies, which of
the following devices works
best as an insulator?
1.
2.
Small surface area
Large surface area
Shape of the waveguides
Waveguide material being
used
I 2R loss
Physical size
Wall
thickness
Dielectric
loss
At very high frequencies,
ordinary insulators in a twowire transmission line display
the characteristics of what
electrical
component?
1.
2.
3.
4.
Copper I2R losses are reduced
by what physical property of
waveguides?
1.
2.
3.
4.
Which of the following
characteristics of a waveguide
cause its lower–frequency
limitation?
1.
2.
3.
4.
The most efficient transfer of
electromagnetic energy can be
provided by which of the
following mediums?
1.
2.
3.
4.
3-23.
Coaxial line
Twisted pair
Shielded pair
Two-wire open
What is the primary advantage
of a rigid coaxial line?
1.
2.
3.
4.
3-22.
3-26.
Uniform capacitance throughout
the length of the line is an
advantage of which of the
following
transmission
lines?
shorter than the “a”
dimension
shorter than the “b”
dimension
longer than the “a”
dimension
longer than the “b”
dimension
3-31.
1.
2.
3.
4.
3-32.
wavelength
wavelength
wavelength
wavelength
3–38.
E field only
H field only
E and H fields
Stationary fields
2.
3.
4.
Electric
only
Magnetic
only
Electromagnetic
3-39.
H lines have which of the
following
distinctive
characteristics?
2.
3.
4.
3-35.
1.
continuous
lines
generated by
3.
4.
closed loops
only in the
3–40.
For an electric field to exist
at the surface of a conductor,
the field must have what
angular relationship to the
conductor?
1.
2.
3.
4.
3-36.
They are
straight
They are
voltage
They form
They form
waveguide
3-41.
1.
2.
3.
4.
Cutoff
Incidence
Refraction
Penetration
14
Decreasing the frequency of
the input energy
Increasing the frequency of
the input energy
Increasing the power of the
input energy
Decreasing the power of the
input energy
wavefronts
modes of operation
fields of operation
fields of distribution
The most efficient transfer of
energy occurs in a waveguide in
what mode?
1.
2.
3.
4.
If the wall of a waveguide is
perfectly flat, the angle of
reflection is equal to which of
angles?
the following
be
The various field configura–
tions that can exist in a
waveguide are referred to as
1.
2.
3.
4.
0°
30°
45°
90°
Group velocity is somewhat
faster
Group velocity is somewhat
slower
Group velocity is twice
that of free velocity
Free velocity is twice that
of group velocity
The group velocity of a
wavefront in a waveguide may
increased by which of the
following actions?
2.
1.
10°
30°
45°
90°
How does the group velocity of
an electromagnetic field in a
waveguide compare to the
velocity of a wavefront through
free space?
1.
A difference in potential
across a dielectric causes
which of the following fields
to develop?
1.
2.
3.
3-34.
0.1
0.2
0.5
0.7
The cutoff frequency in a
waveguide occurs at exactly
what angle of reflection?
1.
2.
3.
4.
Which of the following fields
is/are present in waveguides?
1.
2.
3.
4.
3-33.
3-37.
In practical applications,
which of the following
dimensions describes the wide
dimension of the waveguide at
the operating frequency?
Sine
Dominant
Transverse
Time–phase
3-42.
1.
2.
3.
4.
3-43.
3.
4.
2.
3.
4.
3-48.
Half-sine and dominant
Transverse electric and
transverse
magnetic
Transverse electric and
dominant
Transverse magnetic and
half-sine
3-49.
One
Two
Three
Four
One
Two
Three
Four
A slot
A loop
A probe
A horn
Sine waves
Dominant
waves
Standing waves
Transverse waves
As an inductive reactance
As a shunt resistance
As a capacitive reactance
As a shorted 1/4 wave stub
A horn can be used as a
waveguide termination device
because it provides which of
the following electrical
functions?
1.
2.
3.
4.
15
to
A waveguide iris that covers
part of both the electric and
magnetic planes acts as what
type of equivalent circuit at
the resonant frequency?
1.
2.
3.
4.
3-51.
Efficiency
Bandwidth coverage
Power–handling
capability
Each of the above
A waveguide that is not
perfectly impedance matched
its load is not efficient.
Which of the following
conditions in a waveguide
causes this inefficiency?
1.
2.
3.
4.
3-50.
By doubling the size of the
probe
By increasing the length of
the probe
By decreasing the length of
the probe
By placing the probe
directly in the center of
the energy field
Increasing the size of the loop
wire increases which of the
following
loop
capabilities?
1.
2.
3.
4.
Which of the following devices
CANNOT be used to inject or
remove energy from a waveguide?
1.
2.
3.
4.
Loose coupling is a method used
to reduce the amount of energy
being transferred from a
How is loose
waveguide.
coupling achieved when using a
probe?
1.
With the mode description,
TE 1,1 , what maximum number of
half-wave patterns exist across
the diameter of a circular
waveguide?
1.
2.
3.
4.
3-46.
times the radius of
waveguide
times the diameter of
waveguide
times the diameter of
waveguide
times the radius of
waveguide
With a mode description of
TE 1,0 , what maximum number of
half-wave patterns exist across
the “a” dimension of a
waveguide?
1.
2.
3.
4.
3–45.
1.17
the
1.17
the
1.71
the
1.71
the
The field configuration in
waveguides is divided into what
two
categories?
1.
2.
3-44.
3-47.
How is the cutoff wavelength
for a circular waveguide
computed?
A reflective load
An absorptive load
An abrupt change in
impedance
A gradual change in
impedance
3-52.
For a waveguide to be
terminated with a resistive
load, that load must be matched
to which of the following
properties of the waveguide?
1.
2.
3.
4.
3-57.
The bandwidth
The
frequency
The inductance
The
characteristic
impedance
A flexible waveguide is used in
short sections because of the
power-loss
disadvantages.
What
is the cause of this power
loss?
1.
2.
3.
4.
3-53.
A resistive device with the
sole purpose of absorbing all
the energy in a waveguide
without causing reflections is
a/an
1.
2.
3.
4.
3-58.
The choke joint is used for
what purpose in a waveguide?
1.
2.
iris
horn
antenna
dummy load
3.
4.
3-54.
A resistive load most often
dissipates energy in which of
the following forms?
3-59.
1.
2.
3.
4.
3-55.
Reflections will be caused by
an abrupt change in which of
the following waveguide’s
physical
characteristics?
1.
2.
3.
4.
3-56.
Heat
Light
Magnetic
Electrical
3-60.
1.
2.
3.
4.
3–61.
cracking
reflections
energy gaps
electrolysis
Corrosion
Damaged
surfaces
Improperly sealed
Each of the above
joints
What type of corrosion occurs
when dissimilar metals are in
contact with each other?
1.
2.
3.
4.
16
Oscillation
Large power loss
Decrease in bandwidth
Field–pattern
distortion
In your waveguide inspection,
you should be alert for which
of the following problems?
1.
2.
3.
4.
A waveguide bend that in the E
and H plane must be greater
than two wavelengths to prevent
To reduce standing waves
To restrict the volume of
electron
flow
To prevent the field from
rotating
To provide a joint that can
be disassembled during
maintenance
A circular waveguide is
normally used in a rotating
joint because rotating a
rectangular
waveguide
would
cause which of the following
unwanted conditions?
1.
2.
3.
4.
Size and shape only
Size and dielectric
material
only
Dielectric material and
shape only
Size, shape, and dielectric
material
Walls are not smooth
E and H fields are not
perpendicular
Cannot be terminated in its
characteristic
impedance
Wall size cannot be kept
consistent
Contact
Metallic
Electrical
Electrolytic
3–62.
1.
2.
3.
4.
3-63.
2.
3.
4.
3-65.
2.
3.
4.
1/8
1/4
1/2
3/4
wavelength
wavelength
wavelength
wavelength
3-71.
3-72.
What factor(s) determine(s) the
primary frequency of a resonant
cavity?
1.
2.
3.
4.
Size only
Shape only
Size and shape
Q of the cavity
17
magic
T
E–type T
H-type T
H-type T junction
H–type junction
Magic T
Rat race
Duplexer
Hybrid ring
The hybrid ring is usually used
as what type of device in radar
systems?
1.
2.
3.
4.
Low Q
High Q
Inductive reactance
Capacitive
reactance
H and T
H and E
Hybrid Ring and
Q and magic T
Low power handling capabilities
and internal power losses are
the primary disadvantages of
which of the following
junctions?
1.
2.
3.
4.
Be enclosed by conducting
walls
Possess
resonant
properties
Contain
oscillating
electromagnetic
fields
Be round or elliptical in
shape
The Q
The power
The cutoff frequency
The resonant frequency
A waveguide junction in which
the arm area extends from the
main waveguide in the same
direction as the electric field
is an example of what type
junction?
1.
2.
3.
4.
What property gives a resonant
cavity a narrow bandpass and
allows very accurate tuning?
1.
2.
3.
4.
3-67.
3-70.
a
What are the two basic types of
waveguide T junctions?
1.
2.
3.
4.
To sample the energy in a
waveguide
To change the phase of the
energy in the waveguide
To change the direction of
energy travel in the
waveguide
To allow energy in the
waveguide to travel in one
direction only
Of the following
characteristics, which is NOT
required for a device to be
considered a resonant cavity?
1.
3-66.
3-69.
of
What is the electrical distance
between the two holes in a
simple
directional
coupler?
1.
2.
3.
4.
Tuning is the process of
changing what property of
resonant
cavity?
1.
2.
3.
4.
Change in mode
Electrolysis at a joint
Moisture in the waveguide
Gradual change in frequency
What is the primary purpose
a directional coupler?
1.
3-64.
3-68.
Internal arcing in a waveguide
is usually a symptom of which
of the following conditions?
Mixer
Detector
Duplexer
Impedance
matcher
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