Source text. Published from the recorded source PDF for NEETS Module 10: Wave Propagation, Transmission Lines, and Antennas.
NEETS MODULE 10-Wave Propagation, Transmission Lines,
and Antennas
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4 ANTENNAS
LEARNING OBJECTIVES
After you finish this chapter, you should be able to do the following:
1. State the basic principles of antenna radiation and list the parts of an antenna.
2. Explain current and voltage distribution on an antenna.
3. Describe how electromagnetic energy is radiated from an antenna.
4. Explain polarization, gain, and radiation resistance characteristics of an antenna.
5. Describe the theory of operation of half- wave and quarter-wave antennas.
6. List the various array antennas.
7. Describe the directional array antennas presented and explain the basic operation
of each.
8. Identify various special antennas presented, such as long-wire, V, rhombic,
turnstile, ground-plane, and corner-reflector; describe the operation of each.
9. List safety precautions when working aloft and around antennas.
4.1 INTRODUCTION
If you had been around in the early days of electronics, you would have considered an
ANTENNA (AERIAL) to be little more than a piece of wire strung between two trees or
upright poles. In those days, technicians assumed that longer antennas automatically
provided better reception than shorter antennas. They also believed that a mysterious
MEDIUM filled all space, and that an antenna used this medium to send and receive its
energy. These two assumptions have since been discarded. Modern antennas have
evolved to the point that highly directional, specially designed antennas are used to relay
worldwide communications in space through the use of satellites and Earth station
antennas (fig. 4-1). Present transmission theories are based on the assumption that space
itself is the only medium necessary to propagate (transmit) radio energy.
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A tremendous amount of knowledge and information has been gained about the design of
antennas and radio-wave propagation. Still, many old-time technicians will tell you that
when it comes to designing the length of an antenna, the best procedure is to perform all
calculations and try out the antenna. If it doesn't work right, use a cut-and-try method
until it does. Fortunately, enough information has been collected over the last few
decades that it is now possible to predict the behavior of antennas. This chapter will
discuss and explain the basic design and operation of antennas.
Figure 4-1 Satellite/earth station communications system
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4.2 PRINCIPLES OF ANTENNA RADIATION
After an rf signal has been generated in a transmitter, some means must be used to radiate
this signal through space to a receiver. The device that does this job is the antenna. The
transmitter signal energy is sent into space by a TRANSMITTING ANTENNA; the rf
signal is then picked up from space by a RECEIVING ANTENNA.
The rf energy is transmitted into space in the form of an electromagnetic field. As the
traveling electromagnetic field arrives at the receiving antenna, a voltage is induced into
the antenna (a conductor). The rf voltages induced into the receiving antenna are then
passed into the receiver and converted back into the transmitted rf information.
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. The dimensions are
determined by the transmitting frequencies. The dimensions of the receiving antenna are
not critical for relatively low radio frequencies. However, as the frequency of the signal
being received increases, the design and installation of the receiving antenna become
more critical. An example of this is a television receiving antenna. If you raise it a few
more inches from the ground or give a slight turn in direction, you can change a snowy
blur into a clear picture.
The conventional antenna is a conductor, or system of conductors, that radiates or
intercepts electromagnetic wave energy. An ideal antenna has a definite length and a
uniform diameter, and is completely isolated in space. However, this ideal antenna is not
realistic. Many factors make the design of an antenna for a communications system a
more complex problem than you would expect. These factors include the height of the
radiator above the earth, the conductivity of the earth below it, and the shape and
dimensions of the antenna. All of these factors affect the radiated-field pattern of the
antenna in space. Another problem in antenna design is that the radiation pattern of the
antenna must be directed between certain angles in a horizontal or vertical plane, or both.
Most practical transmitting antennas are divided into two basic classifications, HERTZ
(half-wave) ANTENNAS and MARCONI (quarter-wave) ANTENNAS. Hertz antennas
are generally installed some distance above the ground and are positioned to radiate
either vertically or horizontally. Marconi antennas operate with one end grounded and are
mounted perpendicular to the Earth or to a surface acting as a ground. Hertz antennas are
generally used for frequencies above 2 megahertz. Marconi antennas are used for
frequencies below 2 megahertz and may be used at higher frequencies in certain
applications.
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A complete antenna system consists of three parts: (1) The COUPLING DEVICE, (2) the
FEEDER, and (3) the ANTENNA, as shown in figure 4-2. The coupling device (coupling
coil) connects the transmitter to the feeder. The feeder is a transmission line that carries
energy to the antenna. The antenna radiates this energy into space.
The factors that determine the type, size, and shape of the antenna are (1) the frequency
of operation of the transmitter, (2) the amount of power to be radiated, and (3) the general
direction of the receiving set. Typical antennas are shown in figure 4-3.
Figure 4-2 Typical antenna system.
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Figure 4-3 Typical antennas
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4.3 CURRENT AND VOLTAGE DISTRIBUTION ON AN ANTENNA
A current flowing in a wire whose length is properly related to the rf produces an electro-
magnetic field. This field is radiated from the wire and is set free in space. We will
discuss how these waves are set free later in this chapter. Remember, the principles of
radiation of electromagnetic energy are based on two laws:
1. A MOVING ELECTRIC FIELD CREATES A MAGNETIC (H) FIELD.
2. A MOVING MAGNETIC FIELD CREATES AN ELECTRIC (E) FIELD.
In space, these two fields will be in phase and perpendicular to each other at any given
time. Although a conductor is usually considered present when a moving electric or
magnetic field is mentioned, the laws that govern these fields say nothing about a
conductor. Therefore, these laws hold true whether a conductor is present or not.
Figure 4-4 shows the current and voltage distribution on a half-wave (Hertz) antenna. In
view A, a piece of wire is cut in half and attached to the terminals of a high-frequency ac
generator. The frequency of the generator is set so that each half of the wire is 1/4
wavelength of the output. The result is a common type of antenna known as a DIPOLE.
Figure 4-4 Current and voltage distribution on
an antenna
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At a given time the right side of the generator is positive and the left side negative.
Remember that like charges repel. Because of this, electrons will flow away from the
negative terminal as far as possible, but will be attracted to the positive terminal. View B
shows the direction and distribution of electron flow. The distribution curve shows that
most current flows in the center and none flows at the ends. The current distribution over
the antenna will always be the same no matter how much or how little current is flowing.
However, current at any given point on the antenna will vary directly with the amount of
voltage developed by the generator.
One-quarter cycle after electrons have begun to flow, the generator will develop its
maximum voltage and the current will decrease to 0. At that time the condition shown in
view C will exist. No current will be flowing, but a maximum number of electrons will be
at the left end of the line and a minimum number at the right end. The charge distribution
view C along the wire will vary as the voltage of the generator varies. Therefore, you
may draw the following conclusions:
1. A current flows in the antenna with an amplitude that varies with the generator voltage.
2. A sinusoidal distribution of charge exists on the antenna. Every 1/2 cycle, the charges
reverse polarity.
3. The sinusoidal variation in charge magnitude lags the sinusoidal variation in current by
1/4 cycle.
Q1. What are the two basic classifications of antennas?
Q2. What are the three parts of a complete antenna system?
Q3. What three factors determine the type, size, and shape of an antenna?
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4.4 RADIATION OF ELECTROMAGNETIC ENERGY
The electromagnetic radiation from an antenna is made up of two components, the E field
and the H field. We discussed these fields in chapters 1 and 2. The two fields occur 90
degrees out of phase with each other. These fields add and produce a single
electromagnetic 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 and, at any given point, decreases as the distance
increases.
Various factors in the antenna circuit affect the radiation of these waves. In figure 4-5, for
example, if an alternating current is applied at the A end of the length of wire from A to
B, 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 farther. 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. The energy of the wave
would be gradually dissipated by the resistance of the wire of this back-and-forth motion
(oscillation); however, each time it reaches the starting point, the wave is reinforced by
an amount sufficient to replace the energy lost. This results in continuous oscillations of
energy along the wire and a high voltage at the A end of the wire. These oscillations are
applied to the antenna at a rate equal to the frequency of the rf voltage.
Figure 4-5 Antenna and rf source
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These impulses must be properly timed to sustain oscillations in the antenna. The rate at
which the waves travel 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. Remember, the
distance a wave travels during the period of 1 cycle is known as the wavelength and is
found by dividing the rate of travel by the frequency.
Look at the current and voltage (charge) distribution on the antenna in figure 4-6. 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. 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 current nodes. View
B shows voltage loops and a voltage node. View C shows 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 the electromagnetic waves are transmitted into space at
maximum radiation. When the antenna is not at resonance, the waves tend to cancel each
other and lose energy in the form of heat.
Figure 4-6 Standing waves of voltage and
current on an antenna
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Q4. If a wave travels exactly the length of an antenna from one end to the other and back
during the period of 1 cycle, what is the length of the antenna?
Q5. What is the term used to identify the points of high current and high voltage on an
antenna?
Q6. What is the term used to identify the points of minimum current and minimum voltage
on an antenna?
4.5 ANTENNA CHARACTERISTICS
You can define an antenna 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 is fed into the receiver.
Fortunately, separate antennas seldom are required for both transmitting and receiving rf
energy. Any antenna can transfer energy from space to its input receiver with the same
efficiency that it transfers energy from the transmitter into space. Of course, this is
assuming that the same frequency is used in both cases. This property of
interchangeability of the same antenna for transmitting and receiving is known as antenna
RECIPROCITY. Antenna reciprocity is possible because antenna characteristics are
essentially the same for sending and receiving electromagnetic energy.
4.5.1 Reciprocity of Antennas
In general, the various properties of an antenna apply equally, regardless of whether you
use the antenna for transmitting or receiving. The more efficient a certain antenna is for
transmitting, the more efficient it will be for receiving on the same frequency. Likewise,
the directive properties of a given antenna also will be the same whether it is used for
transmitting or receiving.
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Assume, for example, that a certain antenna used with a transmitter radiates a maximum
amount of energy at right angles to the axis of the antenna, as shown in figure 4-7, view
A. Note the minimum amount of radiation along the axis of the antenna. Now, if this
same antenna were used as a receiving antenna, as shown in view B, it would receive best
in the same directions in which it produced maximum radiation; that is, at right angles to
the axis of the antenna.
Figure 4-7 Reciprocity of antennas
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4.5.2 Antenna Gain
Another characteristic of a given antenna that remains the same whether the antenna is
used for transmitting or receiving is GAIN. Some antennas are highly directional that is,
more energy is propagated in certain directions than in others. The ratio between the
amount of energy propagated in these directions compared to the energy that would be
propagated if the antenna were not directional is known as its gain. When a transmitting
antenna with a certain gain is used as a receiving antenna, it will also have the same gain
for receiving.
4.5.3 Polarization
Let's review polarization briefly. In chapter 2 you learned that the radiation field is
composed of electric and magnetic lines of force. These lines of force are always at right
angles to each other. Their intensities rise and fall together, reaching their maximums 90
degrees apart. The electric field determines the direction of polarization of the wave. In a
vertically polarized wave, the electric lines of force lie in a vertical direction. In a
horizontally polarized wave, the electric lines of force lie in a horizontal direction.
Circular polarization has the electric lines of force rotating through 360 degrees with
every cycle of rf energy.
The electric field was chosen as the reference field because the intensity of the wave is
usually measured in terms of the electric field intensity (volts, millivolts, or microvolts
per meter). When a single-wire antenna is used to extract energy from a passing radio
wave, maximum pickup will result when the antenna is oriented in the same direction as
the electric field. Thus a vertical antenna is used for the efficient reception of vertically
polarized waves, and a horizontal antenna is used for the reception of horizontally
polarized waves. In some cases the orientation of the electric field does not remain
constant.
Instead, the field rotates as the wave travels through space. Under these conditions both
horizontal and vertical components of the field exist and the wave is said to have an
elliptical polarization.
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Q7. The various properties of a transmitting antenna can apply equally to the same
antenna when it is used as a receiving antenna. What term is used for this property?
Q8. The direction of what field is used to designate the polarization of a wave?
Q9. If a wave's electric lines of force rotate through 360 degrees with every cycle of rf
energy, what is the polarization of this wave?
4.5.3.1 Polarization Requirements for Various Frequencies
Ground-wave transmission is widely used at medium and low frequencies. Horizontal
polarization cannot be used at these frequencies because the electric lines of force are
parallel to and touch the earth. Since the earth acts as a fairly good conductor at low
frequencies, it would short out the horizontal electric lines of force and prevent the radio
wave from traveling very far. Vertical electric lines of force, on the other hand, are
bothered very little by the earth. Therefore vertical polarization is used for ground-wave
transmission, allowing the radio wave to travel a considerable distance along the ground
surface with minimum attenuation.
Sky-wave transmission is used at high frequencies. Either horizontal or vertical
polarization can be used with sky-wave transmission because the sky wave arrives at the
receiving antenna elliptically polarized. This is the result of the wave traveling obliquely
through the Earth's magnetic field and striking the ionosphere. The radio wave is given a
twisting motion as it strikes the ionosphere. Its orientation continues to change because of
the unstable nature of the ionosphere. The relative amplitudes and phase differences
between the horizontal and vertical components of the received wave also change.
Therefore, the transmitting and receiving antennas can be mounted either horizontally or
vertically.
Although either horizontally or vertically polarized antennas can be used for high
frequencies, horizontally polarized antennas have certain advantages and are therefore
preferred. One advantage is that vertically polarized interference signals, such as those
produced by automobile ignition systems and electrical appliances, are minimized by
horizontal polarization. Also, less absorption of radiated energy by buildings or wiring
occurs when these antennas are used. Another advantage is that support structures for
these antennas are of more convenient size than those for vertically polarized antennas.
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For frequencies in the vhf or uhf range, either horizontal or vertical polarization is
satisfactory. These radio waves travel directly from the transmitting antenna to the
receiving antenna without entering the ionosphere. The original polarization produced at
the transmitting antenna is maintained throughout the entire travel of the wave to the
receiver. Therefore, if a horizontally polarized antenna is used for transmitting, a
horizontally polarized antenna must be used for receiving. The requirements would be the
same for a vertical transmitting and receiving antenna system.
For satellite communications, parallel frequencies can be used without interference by
using polarized radiation. The system setup is shown in figure 4-8. One pair of satellite
antennas is vertically polarized and another pair is horizontally polarized. Either
vertically or horizontally polarized transmissions are received by the respective antenna
and retransmitted in the same polarization. For example, transmissions may be made in
the 3.7 to 3.74 GHz range on the vertical polarization path and in the 3.72 to 3.76 GHz
range on the horizontal polarization path without adjacent frequency (co-channel)
interference.
Figure 4-8 Satellite transmissions using polarized radiation
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4.5.3.2 Advantages of Vertical Polarization
Simple vertical antennas can be used to provide OMNIDIRECTIONAL (all directions)
communication. This is an advantage when communications must take place from a
moving vehicle.
In some overland communications, such as in vehicular installations, antenna heights are
limited to 3 meters (10 feet) or less. In such instances vertical polarization results in a
stronger receiver signal than does horizontal polarization at frequencies up to about 50
megahertz. From approximately 50 to 100 megahertz, vertical polarization results in a
slightly stronger signal than does horizontal polarization with antennas at the same
height. Above 100 megahertz, the difference in signal strength is negligible.
For transmission over bodies of water, vertical polarization is much better than horizontal
polarization for antennas at the lower heights. As the frequency increases, the minimum
antenna height decreases. At 30 megahertz, vertical polarization is better for antenna
heights below about 91 meters (300 feet); at 85 megahertz, antenna heights below 15
meters (50 feet); and still lower heights at the high frequencies. Therefore, at ordinary
antenna mast heights of 12 meters (40 feet), vertical polarization is advantageous for
frequencies less than about 100 megahertz.
Radiation is somewhat less affected by reflections from aircraft flying over the
transmission path when vertical polarization is used instead of horizontal polarization.
With horizontal polarization, such reflections cause variations in received signal strength.
This factor is important in locations where aircraft traffic is heavy.
When vertical polarization is used, less interference is produced or picked up because of
strong vhf and uhf broadcast transmissions (television and fm). This is because vhf and
uhf transmissions use horizontal polarization. This factor is important when an antenna
must be located in an urban area having several television and fm broadcast stations.
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4.5.3.3 Advantages of Horizontal Polarization
A simple horizontal antenna is bi-directional. This characteristic is useful when you
desire to minimize interference from certain directions. Horizontal antennas are less
likely to pick up man-made interference, which ordinarily is vertically polarized.
When antennas are located near dense forests or among buildings, horizontally polarized
waves suffer lower losses than vertically polarized waves, especially above 100
megahertz. Small changes in antenna locations do not cause large variations in the field
intensity of horizontally polarized waves. When vertical polarization is used, a change of
only a few meters in the antenna location may have a considerable effect on the received
signal strength. This is the result of interference patterns that produce standing waves in
space when spurious reflections from trees or buildings occur.
When simple antennas are used, the transmission line, which is usually vertical, is less
affected by a horizontally mounted antenna. When the antenna is mounted at right angles
to the transmission line and horizontal polarization is used, the line is kept out of the
direct field of the antenna. As a result, the radiation pattern and electrical characteristics
of the antenna are practically unaffected by the presence of the vertical transmission line.
Q10. What type of polarization should be used at medium and low frequencies?
Q11. What is an advantage of using horizontal polarization at high frequencies?
Q12. What type of polarization should be used if an antenna is mounted on a moving
vehicle at frequencies below 50 megahertz?
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4.6 RADIATION RESISTANCE
Radiated energy is the useful part of the transmitter's signal. However, it represents as
much of a loss to the antenna as the energy lost in heating the antenna wire. In either
case, the dissipated power is equal to I2R. In the case of heat losses, the R is real
resistance. In the case of radiation, R is an assumed resistance; if this resistance were
actually present, it would dissipate the same amount of power that the antenna takes to
radiate the energy. This assumed resistance is referred to as the RADIATION
RESISTANCE.
Radiation resistance varies at different points on the antenna. This resistance is always
measured at a current loop. For the antenna in free space, that is, entirely removed from
any objects that might affect its operation, the radiation resistance is 73 ohms. A practical
antenna located over a ground plane may have any value of radiation resistance from 0 to
approximately 100 ohms. The exact value of radiation resistance depends on the height of
the antenna above the ground. For most half-wave wire antennas, the radiation resistance
is about 65 ohms. It will usually vary between 55 and 600 ohms for antennas constructed
of rod or tubing. The actual value of radiation resistance, so long as it is 50 ohms or
more, has little effect on the radiation efficiency of the antenna. This is because the
ohmic resistance is about 1 ohm for conductors of large diameter. The ohmic resistance
does not become important until the radiation resistance drops to a value less than 10
ohms. This may be the case when several antennas are coupled together.
4.6.1 Radiation Types and Patterns
The energy radiated from an antenna forms a field having a definite RADIATION
PATTERN. A radiation pattern is a plot of the radiated energy from an antenna. This
energy is measured at various angles at a constant distance from the antenna. The shape
of this pattern depends on the type of antenna used. In this section, we will introduce the
basic types of radiation (isotropic and anisotropic) and their radiation patterns.
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4.6.1.1 Isotropic Radiation
Some antenna sources radiate energy equally in all directions. Radiation of this type is
known as ISOTROPIC RADIATION. We all know the Sun radiates energy in all
directions. The energy radiated from the Sun measured at any fixed distance and from
any angle will be approximately the same. Assume that a measuring device is moved
around the Sun and stopped at the points indicated in figure 4-9 to make a measurement
of the amount of radiation. At any point around the circle, the distance from the
measuring device to the Sun is the same. The measured radiation will also be the same.
The Sun is therefore considered an isotropic radiator.
To plot this pattern, we will assume that the radiation is measured on a scale of 0 to 10
units and that the measured amount of radiation is 7 units at all points. We will then plot
our measurements on two different types of graphs, rectangular- and polar-coordinate
graphs. The RECTANGULARCOORDINATE GRAPH of the measured radiation,
shown in view A of figure 4-10, is a straight line plotted against positions along the
circle. View B shows the POLAR-COORDINATE GRAPH for the same isotropic
source.
Figure 4-9 Isotropic radiator
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In the rectangular-coordinate graph, points are located by projection from a pair of
stationary, perpendicular axes. In the polar-coordinate graph, points are located by
projection along a rotating axis (radius) to an intersection with one of several concentric,
equally-spaced circles. The horizontal axis on the rectangular-coordinate graph
corresponds to the circles on the polar-coordinate graph. The vertical axis on the
rectangular-coordinate graph corresponds to the rotating axis (radius) on the polar-
coordinate graph.
4.6.1.2 Rectangular-Coordinate Pattern
Look at view A of figure 4-10. The numbered positions around the circle are laid out on
the HORIZONTAL AXIS of the graph from 0 to 7 units. The measured radiation is laid
out on the VERTICAL AXIS of the graph from 0 to 10 units. The units on both axes are
chosen so the pattern occupies a convenient part of the graph.
The horizontal and vertical axes are at a right angle to each other. The point where the
axes cross each other is known as the ORIGIN. In this case, the origin is 0 on both axes.
Now, assume that a radiation value of 7 units view B is measured at position 2. From
position 2 on the horizontal axis, a dotted line is projected upwards that runs parallel to
the vertical axis. From position 7 on the vertical axis, a line is projected to the right that
runs parallel to the horizontal axis. The point where the two lines cross (INTERCEPT)
represents a value of 7 radiation units at position 2. This is the only point on the graph
that can represent this value.
Figure 4-10 Comparison of rectangular- and polar-coordinate graphs for an isotropic source
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As you can see from the figure, the lines used to plot the point form a rectangle. For this
reason, this type of plot is called a rectangular-coordinate graph. A new rectangle is
formed for e ach different point plotted. In this example, the points plotted lie in a straight
line extending from 7 units on t he vertical scale to the projection of position 7 on the
horizontal scale. This is the characteristic pattern in rectangular coordinates of an
isotropic source of radiation.
4.6.1.3 Polar-Coordinate Pattern
The polar-coordinate graph has proved to be of great use in studying radiation patterns.
Compare views A and B of figure 4-10. Note the great difference in the shape of the
radiation pattern when it is transferred from the rectangular-coordinate graph in view A
to the polar-coordinate graph in view B. The scale of radiation values used in both graphs
is identical, and the measurements taken are both the same. However, the shape of the
pattern is drastically different.
Look at view B of figure 4-10 and assume that the center of the concentric circles is the
Sun. Assume that a radius is drawn from the Sun (center of the circle) to position 0 of the
circle. When you move to position 1, the radius moves to position 1; when you move to
position 2, the radius also moves to position 2, and so on.
The positions where a measurement was taken are marked as 0 through 7 on the graph.
Note how the position of the radius indicates the actual direction from the source at
which the measurement was taken. This is a distinct advantage over the rectangular-
coordinate graph in which the position is indicated along a straight-line axis and has no
physical relation to the actual direction of measurement. Now that we have a way to
indicate the direction of measurement, we must devise a way to indicate the magnitude
of the radiation.
Notice that the rotating axis is always drawn from the center of the graph to some
position on the edge of the graph. As the axis moves toward the edge of the graph, it
passes through a set of equally-spaced, concentric circles. In this example view B, they
are numbered successively from 1 to 10 from the center out. These circles are used to
indicate the magnitude of the radiation.
The advantages of the polar-coordinate graph are immediately evident. The source, which
is at the center of the observation circles, is also at the center of the graph. By looking at
a polar-coordinate plot of a radiation pattern, you can immediately see the direction and
strength of radiation put out by the source. Therefore, the polar-coordinate graph is more
useful than the rectangular-coordinate graph in plotting radiation patterns.
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4.6.1.4 Anisotropic Radiation
Most radiators emit (radiate) stronger radiation in one direction than in another. A
radiator such as this is referred to as ANISOTROPIC. An example of an anisotropic
radiator is an ordinary flashlight. The beam of the flashlight lights only a portion of the
space surrounding it. If a circle is drawn with the flashlight as the center, as shown in
view B of figure 4-11, the radiated light can be measured at different positions around the
circle. Again, as with the isotropic radiator, all positions are the same distance from the
center, but at different angles. However, in this illustration the radiated light is measured
at 16 different positions on the circle.
Directly behind the flashlight (position 0) the radiation measured is minimum.
Accordingly, a 0 value is assigned to this position in the rectangular-coordinate graph
(fig. 4-11, view A). This radiation remains at minimum until position 4 is reached.
Between positions 4 and 6, the measuring device enters the flashlight beam. You can see
this transition from darkness to brightness easily in view B. Radiation is fairly constant
between positions 6 and 10. Maximum brightness occurs at position 8, which is directly
in the path of the flashlight beam. From positions 10 to 12, the measuring device leaves
the flashlight beam and the radiation measurement falls off sharply. At position 13 the
radiation is again at 0 and stays at this value back to position 0.
Figure 4-11 Anisotropic radiator
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Radiation from a light source and radiation from an antenna are both forms of
electromagnetic waves. Therefore, the measurement of radiation of an antenna follows
the same basic procedure as that just described for the Sun and the flashlight. Before
proceeding further with the study of antenna patterns, you should be sure you understand
the methods used to graph the measured radiation (magnitude of the radiation). Study the
rectangular- and polar-coordinate systems of plotting presented in the following section.
Q13. What is the radiation resistance of a half-wave antenna in free space?
Q14. A radiating source that radiates energy stronger in one direction than another is
known as what type of radiator?
Q15. A radiating source that radiates energy equally in all directions is known as what
type of radiator?
Q16. A flashlight is an example of what type of radiator?
In figure 4-11, view A, the radiation pattern of the flashlight is graphed in rectangular
coordinates. The illustration of the flashlight beam in view B clearly indicates the shape
of the flashlight beam. This is not evident in the radiation pattern plotted on the
rectangular-coordinate graph. Now look at figure 4-12. The radiation pattern shown in
this figure looks very much like the actual flashlight beam. The pattern in figure 4-12 is
plotted using the same values as those of figure 4-11, view A, but is drawn using polar
coordinates.
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The positions marked off on the two polar-coordinate graphs in figures 4-10 and 4-12
were selected and numbered arbitrarily. However, a standard method allows the positions
around a source to be marked off so that one radiation pattern can easily be compared
with another. This method is based on the fact that a circle has a radius of 360 degrees.
The radius extending vertically from the center (position 0 in figure 4-10) is designated 0
degrees. At position 4 the radius is at a right angle to the 0-degree radius. Accordingly,
the radius at position 4 is marked 90 degrees, position 8 is 180 degrees, position 12 is 270
degrees, and position 16 is 360 degrees. The various radii drawn on the graph are all
marked according to the angle each radius makes with the reference radius at 0 degrees.
The radiation pattern in figure 4-12 is obtained by using the same procedure that was
used for (figure 4-10, view B). The radiation measured at positions 1, 2, 3, and 4 is 0.
Position 5 measures approximately 1 unit. This is marked on the graph and the rotating
radius moves to position 6. At this position a reading of 5.5 units is taken. As before, this
point is marked on the graph. The procedure is repeated around the circle and a reading is
obtained from positions 6 through 11. At position 12 no radiation is indicated, and this
continues on to position 16.
Figure 4-12 Polar-coordinate graph for anisotropic radiator
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The polar-coordinate graph now shows a definite area enclosed by the radiation pattern.
This pattern indicates the general direction of radiation from the source. The enclosed
area is called a LOBE. Outside of this area, minimum radiation is emitted in any
direction. For example, at position 2 the radiation is 0. Such a point is called a NULL. In
real situations, some radiation is usually transmitted in all directions. Therefore, a null is
used to indicate directions of minimum radiation. The pattern of figure 4-12 shows one
lobe and one continuous null.
4.7 ANTENNA LOADING
You will sometimes want to use one antenna system for transmitting and receiving on
several different frequencies. Since the antenna must always be in resonance with the
applied frequency, you may need to either physically or electrically lengthen or shorten
the antenna.
Except for trailing-wire antennas used in aircraft installations (which may be lengthened
or shortened), physically lengthening the antenna is not very practical. But you can
achieve the same result by changing the electrical length of the antenna. To change the
electrical length, you can insert either an inductor or a capacitor in series with the
antenna. This is shown in figure 4-13, views A and B. Changing the electrical length by
this method is known as LUMPED-IMPEDANCE TUNING, or LOADING. The
electrical length of any antenna wire can be increased or decreased by loading. If the
antenna is too short for the wavelength being used, it is resonant at a higher frequency
than that at which it is being excited. 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 antenna is too long for
the transmitting frequency, it offers an inductive reactance. Inductive reactance can be
compensated for by introducing a lumped-capacitive reactance, as shown in view B. An
antenna without loading is represented in view C.
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4.8 BASIC ANTENNAS
Before you look at the various types of antennas, consider the relationship between the
wavelength at which the antenna is being operated and the actual length of the antenna.
An antenna does not necessarily radiate or receive more energy when it is made longer.
Specific dimensions must be used for efficient antenna operation.
Nearly all antennas have been developed from two basic types, the Hertz and the
Marconi. The basic Hertz antenna is 1/2 wavelength long at the operating frequency and
is insulated from ground. It is often called a DIPOLE or a DOUBLET. The basic Marconi
antenna is 1/4 wavelength long and is either grounded at one end or connected to a
network of wires called a COUNTERPOISE. The ground or counterpoise provides the
equivalent of an additional 1/4 wavelength, which is required for the antenna to resonate.
Figure 4-13 Electrically equal antenna
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4.8.1 Half-Wave Antennas
A half-wave antenna (referred to as a dipole, Hertz, or doublet) consists of two lengths of
wire rod, or tubing, each 1/4 wavelength long at a certain frequency. It is the basic unit
from which many complex antennas are constructed. The half-wave antenna operates
independently of ground; therefore, it may be installed far above the surface of the Earth
or other absorbing bodies. For a dipole, the current is maximum at the center and
minimum at the ends. Voltage is minimum at the center and maximum at the ends, as was
shown in figure 4-6.
4.8.1.1 Radiation Patterns
In the following discussion, the term DIPOLE is used to mean the basic half-wave
antenna. The term DOUBLET is used to indicate an antenna that is very short compared
with the wavelength of the operating frequency. Physically, it has the same shape as the
dipole.
4.8.2 Radiation Pattern of a Doublet
The doublet is the simplest form of a practical antenna. Its radiation pattern can be plotted
like the radiation pattern of the flashlight (fig. 4-12). Figure 4-14 shows the development
of vertical and horizontal patterns for a doublet. This in NOT a picture of the radiation,
but three-dimensional views of the pattern itself. In three views the pattern resembles a
doughnut. From the dimensions in these views, two types of polar-coordinate patterns can
be drawn, horizontal and vertical. The HORIZONTAL PATTERN view A is derived
from the solid pattern view C by slicing it horizontally. This produces view B, which is
converted to the polar coordinates seen in view A. The horizontal pattern illustrates that
the radiation is constant in any direction along the horizontal plane.
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A VERTICAL PATTERN view E is obtained from the drawing of the vertical plane view
D of the radiation pattern view C. The radiation pattern view C is sliced in half along a
vertical plane through the antenna. This produces the vertical plane pattern in view D.
Note how the vertical plane in view D of the radiation pattern differs from the horizontal
plane in view B. The vertical pattern view E exhibits two lobes and two nulls. The
difference between the two patterns is caused by two facts: (1) no radiation is emitted
from the ends of the doublet; and (2) maximum radiation comes from the doublet in a
direction perpendicular to the antenna axis. This type of radiation pattern is both
NONDIRECTIONAL (in a horizontal plane) and DIRECTIONAL (in a vertical plane).
Figure 4-14 Development of vertical and horizontal patterns
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From a practical viewpoint, the doublet antenna can be mounted either vertically or
horizontally. The doublet shown in figure 4-14 is mounted vertically, and the radiated
energy spreads out about the antenna in every direction in the horizontal plane. Since
ordinarily the horizontal plane is the useful plane, this arrangement is termed
NONDIRECTIONAL. The directional characteristics of the antenna in other planes is
ignored. If the doublet were mounted horizontally, it would have the effect of turning the
pattern on edge, reversing the patterns given in figure 4-14. The antenna would then be
directional in the horizontal plane. The terms "directional" and "nondirectional" are used
for convenience in describing specific radiation patterns. A complete description always
involves a figure in three dimensions, as in the radiation pattern of figure 4-14.
Q17. What terms are often used to describe basic half-wave antennas?
Q18. If a basic half-wave antenna is mounted vertically, what type of radiation pattern
will be produced?
Q19. In which plane will the half-wave antenna be operating if it is mounted
horizontally?
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4.8.3 Radiation Pattern of a Dipole
The radiation pattern of a dipole (fig. 4-15) is similar to that of the doublet (fig. 4-14).
Increasing the length of the doublet to 1/2 wavelength has the effect of flattening out the
radiation pattern. The radiation pattern in the horizontal plane of a dipole is a larger circle
than that of the doublet. The vertical-radiation pattern lobes are no longer circular. They
are flattened out and the radiation intensity is greater.
4.8.3.1 Methods of Feeding Energy to an Antenna
Voltage and current distribution for the half-wave antenna (shown in figure 4-16) is the
same as that for the antenna discussed earlier in this chapter. A point closely related to
the voltage and current distribution on an antenna is the method of feeding the transmitter
output to the antenna. The simplest method of feeding energy to the half-wave antenna is
to connect one end through a capacitor to the final output stage of the transmitter. This
method is often called the END-FEED or VOLTAGE-FEED method. In this method the
antenna is fed at a point of high voltage (the end).
Figure 4-15 Radiation pattern of a dipole
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Energy may also be fed to the half-wave antenna by dividing the antenna at its center and
connecting the transmission line from the final transmitter output stage to the two center
ends of the halved antenna. Since the antenna is now being fed at the center (a point of
low voltage and high current), this type of feed is known as the CENTER-FEED or
CURRENT-FEED method. The point of feed is important in determining the type of
transmission line to be used.
4.8.4 Quarter-Wave Antennas
As you have studied in the previous sections, a 1/2 wavelength antenna is the shortest
antenna that can be used in free space. If we cut a half-wave antenna in half and then
ground one end, we will have a grounded quarter-wave antenna. This antenna will
resonate at the same frequency as the ungrounded half-wave antenna. Such an antenna is
referred to as a QUARTER-WAVE or Marconi antenna. Quarter-wave antennas are
widely used in the military. Most mobile transmitting and receiving antennas (fig. 4-17)
are quarter-wave antennas.
Figure 4-16 Standing waves of current and voltage
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As stated above, a grounded quarter-wave antenna will resonate at the same frequency as
an ungrounded half-wave antenna. This is because ground has high conductivity and acts
as an electrical mirror image. This characteristic provides the missing half of the antenna,
as shown in the bottom part of figure 4-18. In other words, the grounded quarter-wave
antenna acts as if another quarter-wave were actually down in the earth.
Figure 4-17 Mobile antennas
Figure 4-18 Grounded quarter-wave antenna image
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4.8.4.1 Characteristics of Quarter-Wave Antennas
The grounded end of the quarter-wave antenna has a low input impedance and has low
voltage and high current at the input end, as shown in figure 4-18. The ungrounded end
has a high impedance, which causes high voltage and low current. The directional
characteristics of a grounded quarter-wave antenna are the same as those of a half-wave
antenna in free space.
As explained earlier, ground losses affect radiation patterns and cause high signal losses
for some frequencies. Such losses may be greatly reduced if a perfectly conducting
ground is provided in the vicinity of the antenna. This is the purpose of a GROUND
SCREEN (figure 4-19, view A) and COUNTERPOISE view B.
Figure 4-19 Groundscreen and counterpoise
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The ground screen in view A is composed of a series of conductors buried 1 or 2 feet (0.3
to 0.6 meter) below the surface of the earth and arranged in a radial pattern. These
conductors reduce losses in the ground in the immediate vicinity of the antenna. Such a
radial system of conductors is usually ½ wavelength in diameter.
A counterpoise view B is used when easy access to the base of the antenna is necessary.
It is also used when the earth is not a good conducting surface, such as ground that is
sandy or solid rock. The counterpoise serves the same purpose as the ground screen but it
is usually elevated above the earth. No specific dimensions are necessary in the
construction of a counterpoise nor is the number of wires particularly critical. A practical
counterpoise may be assembled from a large screen of chicken wire or some similar
material. This screen may be placed on the ground, but better results are obtained if it is
placed a few feet above the ground.
Q20. Since the radiation pattern of a dipole is similar to that of a doublet, what will
happen to the pattern if the length of the doublet is increased?
Q21. What is the simplest method of feeding power to the half-wave antenna?
Q22. What is the radiation pattern of a quarter-wave antenna?
Q23. Describe the physical arrangement of a ground screen.
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4.8.5 Folded Dipole
The use of parasitic elements and various stacking arrangements causes a reduction in the
radiation resistance of a center-fed, half-wave antenna. Under these conditions obtaining
a proper impedance match between the radiator and the transmission line is often
difficult. A convenient method of overcoming these difficulties is to use a FOLDED
DIPOLE in place of the center-fed radiator. (See views A and B of figure 4-20).
Figure 4-20 Folded-dipole antennas
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A FOLDED DIPOLE is an ordinary half-wave antenna that has one or more additional
conductors connected across its ends. Additional conductors are mounted parallel to the
dipole elements at a distance equal to a very small fraction of a wavelength. Spacing of
several inches is common.
The feed-point impedance can be further increased by using three or four properly spaced
parallel conductors. Standard feed-line SPREADERS are used to maintain this spacing
when required. In any folded dipole, the increase of impedance is the square of the
number of conductors used in the radiator. Thus, a three-wire dipole has nine times (32)
the feed-point impedance of a simple center-fed dipole. A second method of stepping up
the impedance of a folded dipole is to use two conductors with different radii, as shown
in view B.
The directional characteristics of a folded dipole are the same as those of a simple dipole.
However, the reactance of a folded dipole varies much more slowly as the frequency is
varied from resonance. Because of this the folded dipole can be used over a much wider
frequency range than is possible with a simple dipole.
Q24. What is the difference in the amount of impedance between a three-wire dipole and
a simple center-fed dipole?
Q25. Which has a wider frequency range, a simple dipole or a folded dipole?
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4.9 ARRAY ANTENNAS
An array antenna is a special arrangement of basic antenna components involving new
factors and concepts. Before you begin studying about arrays, you need to study some
new terminology.
4.9.1 Definition of Terms
An array antenna is made up of more than one ELEMENT, but the basic element is
generally the dipole. Sometimes the basic element is made longer or shorter than a half-
wave, but the deviation usually is not great.
A DRIVEN element is similar to the dipole you have been studying and is connected
directly to the transmission line. It obtains its power directly from the transmitter or, as a
receiving antenna, it delivers the received energy directly to the receiver. A PARASITIC
ELEMENT is located near the driven element from which it gets its power. It is placed
close enough to the driven element to permit coupling.
A parasitic element is sometimes placed so it will produce maximum radiation (during
transmission) from its associated driver. When it operates to reinforce energy coming
from the driver toward itself, the parasitic element is referred to as a DIRECTOR. If a
parasitic element is placed so it causes maximum energy radiation in a direction away
from itself and toward the driven element, that parasitic element is called a
REFLECTOR.
If all of the elements in an array are driven, the array is referred to as a DRIVEN
ARRAY (sometimes as a CONNECTED ARRAY). If one or more elements are parasitic,
the entire system usually is considered to be a PARASITIC ARRAY.
MULTIELEMENT ARRAYS frequently are classified according to their directivity. A
BIDIRECTIONAL ARRAY radiates in opposite directions along the line of maximum
radiation. A UNIDIRECTIONAL ARRAY radiates in only one general direction.
Arrays can be described with respect to their radiation patterns and the types of elements
of which they are made. However, you will find it useful to identify them by the physical
placement of the elements and the direction of radiation with respect to these elements.
Generally speaking, the term BROADSIDE ARRAY designates an array in which the
direction of maximum radiation is perpendicular to the plane containing these elements.
In actual practice, this term is confined to those arrays in which the elements themselves
are also broadside, or parallel, with respect to each other.
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A COLLINEAR ARRAY is one in which all the elements lie in a straight line with no
radiation at the ends of the array. The direction of maximum radiation is perpendicular to
the axis of the elements.
An END-FIRE ARRAY is one in which the principal direction of radiation is along the
plane of the array and perpendicular to the elements. Radiation is from the end of the
array, which is the reason this arrangement is referred to as an end-fire array.
Sometimes a system uses the characteristics of more than one of the three types
mentioned. For instance, some of the elements may be collinear while others may be
parallel. Such an arrangement is often referred to as a COMBINATION ARRAY or an
ARRAY OF ARRAYS. Since maximum radiation occurs at right angles to the plane of
the array, the term broadside array is also used.
The FRONT-TO-BACK RATIO is the ratio of the energy radiated in the principal
direction compared to the energy radiated in the opposite direction for a given antenna.
4.9.2 Phasing
Various reflected and refracted components of the propagated wave create effects of
reinforcement and cancellation. At certain distant points from the transmitter, some of the
wave components meet in space. Reception at these points is either impaired or
improved. If the different components arrive at a given point in the same phase, they add,
making a stronger signal available. If they arrive out of phase, they cancel, reducing the
signal strength.
4.9.3 Radiation Pattern
Effects similar to those described in the preceding paragraph can be produced at the
transmitting point itself. Consider the antennas shown in figure 4-21, views A and B.
View A shows an unobstructed view of the radiation pattern of a single dipole. In view B
two dipoles, shown as points 1 and 2, are perpendicular to the plane of the page. They are
spaced 1/4 wavelength apart at the operating frequency. The radiation pattern from either
antenna 1 or 2, operating alone, would be uniform in all directions in this plane, as shown
in view A. Suppose that current is being fed to both antennas from the same transmitter in
such a way that the current fed to antenna 2 lags the current in antenna 1 by 90 degrees.
Energy radiating from antenna 1 toward receiving location X will reach antenna 2 after
1/4 cycle of operation. The energy from both antennas will add, and propagation toward
X will be strong.
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Radiation from antenna 2 toward receiving location Y will reach antenna 1 after 1/4
cycle. The energy in antenna 1 was 1/4 cycle behind that of antenna 2 to begin with;
therefore, the radiation from antenna 1 toward receiving point Y will be exactly 180
degrees out of phase with that of antenna 2. As a result, the radiation fields will cancel
and there will be no radiation toward Y.
At receiving points away from the line of radiation, phase differences occur between 0
and 180 degrees, producing varying amounts of energy in that direction. The overall
effect is shown by the radiation pattern shown in view B. The physical phase relationship
caused by the 1/4-wavelength spacing between the two elements, as well as the phase of
the currents in the elements, has acted to change the radiation pattern of the individual
antennas.
4.9.4 Stub Phasing
In the case just discussed, the currents fed to the two antennas from the same transmitter
were 90 degrees out of phase. Sections of transmission line, called STUBS, are frequently
used for this purpose. These stubs can be adjusted to produce any desired phase
relationship between connected elements.
Figure 4-21 Phasing of antenna in free space
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When two collinear half-wave elements are connected directly so their currents are in the
same phase, the effect is the same as that of a full-wave antenna, as shown in figure 4-22,
view A. The current in the first 1/2 wavelength is exactly 180 degrees out of phase with
that in the second 1/2 wavelength. This is the opposite of the desired condition. In the
illustration, arrows are used to indicate the direction of current flow in the antenna.
(Using arrows is a convenient means of determining the phase on more complicated
arrays.)
Figure 4-22 Phasing of connected elements
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When the two elements are connected by a shorted 1/4-wavelength stub, as shown in
view B, current travels down one side of the stub and up the other. It travels a distance of
a 1/2 wavelength in the stub itself. As a result, the current moves through 1/2 cycle of
change. When the current reaches the second element, it is in the desired phase. Since the
current on one side of the stub is equal and opposite to the current on the other side, the
fields produced here cancel and no radiation is transmitted from the stub itself.
4.10 DIRECTIVITY
The DIRECTIVITY of an antenna or an array can be determined 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 altered in such
a way that they change the pattern and distribute it more uniformly in all directions. The
elements can be considered as a group of antennas fed from a common source and facing
different directions. On the other hand, the elements could be arranged so that the
radiation would be focused in a single direction. With no increase in power from the
transmitter, the amount of radiation in a given direction would be greater. Since the input
power has no increase, this increased directivity is achieved at the expense of gain in
other directions.
4.10.1 Directivity and Interference
In many applications, sharp directivity is desirable although no need exists for added
gain. Examine the physical disposition of the units shown in figure 4-23. Transmitters 1
and 2 are sending information to receivers 1 and 2, respectively, along the paths shown
by the solid arrows. The distance between transmitter 1 and receiver 1 or between
transmitter 2 and receiver 2 is short and does not require high-power transmission. The
antennas of the transmitters propagate well in all directions. However, receiver 1 picks up
some of the signals from transmitter 2, and receiver 2 picks up some of the signals from
transmitter 1, as shown by the broken arrows. This effect is emphasized if the receiving
antennas intercept energy equally well in all directions.
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The use of highly directional arrays as radiators from the transmitters tends to solve the
problem. The signals are beamed along the paths of the solid arrows and provide very
low radiation along the paths of the broken arrows. Further improvement along these
lines is obtained by the use of narrowly directed arrays as receiving antennas. The effect
of this arrangement is to select the desired signal while discriminating against all other
signals. This same approach can be used to overcome other types of radiated interference.
In such cases, preventing radiation in certain directions is more important than producing
greater gain in other directions.
Look at the differences between the field patterns of the single-element antenna and the
array, as illustrated in figure 4-24. View A shows the relative field-strength pattern for a
horizontally polarized single antenna. View B shows the horizontal-radiation pattern for
an array. The antenna in view A radiates fairly efficiently in the desired direction toward
receiving point X. It radiates equally as efficiently toward Y, although no radiation is
desired in this direction. The antenna in view B radiates strongly to point X, but very
little in the direction of point Y, which results in more satisfactory operation.
Figure 4-23 Directivity and interference
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4.10.2 Major and Minor Lobes
The pattern shown in figure 4-24, 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 arrays frequently contain several lobes of varying
intensity, you should learn to use appropriate terminology. In general, major lobes are
those in which the greatest amount of radiation occurs. Minor lobes are those in which
the radiation intensity is least.
Q26. What is the purpose of antenna stubs?
Q27. What is the primary difference between the major and minor lobes of a radiation
pattern?
Figure 4-24 Single antenna versus array
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4.11 DIRECTIONAL ARRAYS
You have already learned about radiation patterns and directivity of radiation. These
topics are important to you because using an antenna with an improper radiation pattern
or with the wrong directivity will decrease the overall performance of the system. In the
following paragraphs, we discuss in more detail the various types of directional antenna
arrays mentioned briefly in the "definition of terms" paragraph above.
4.11.1 Collinear Array
The pattern radiated by the collinear array is similar to that produced by a single dipole.
The addition of the second radiator, however, tends to intensify the pattern. Compare the
radiation pattern of the dipole (view A of figure 4-25) and the two-element antenna in
view B. You will see that each pattern consists of two major lobes in opposite directions
along the same axis, QQ1. There is little or no radiation along the PP1 axis. QQ1
represents the line of maximum propagation. You can see that radiation is stronger with
an added element. The pattern in view B is sharper, or more directive, than that in view
A. This means that the gain along the line of maximum energy propagation is increased
and the beam width is decreased. As more elements are added, the effect is heightened, as
shown in view C. Unimportant minor lobes are generated as more elements are added.
Figure 4-25 Single half-wave antenna versus two half-wave antennas in phase
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More than four elements are seldom used because accumulated losses cause the elements
farther from the point of feeding to have less current than the nearer ones. This introduces
an unbalanced condition in the system and impairs its efficiency. Space limitations often
are another reason for restricting the number of elements. Since this type of array is in a
single line, rather than in a vertically stacked arrangement, the use of too many elements
results in an antenna several wavelengths long.
4.11.1.1 Radiation Pattern
The characteristic radiation pattern of a given array is obtained at the frequency or band
of frequencies at which the system is resonant. The gain and directivity characteristics are
lost when the antenna is not used at or near this frequency and the array tunes too sharply.
A collinear antenna is more effective than an end-fire array when used off its tuned
frequency. This feature is considered when transmission or reception is to be over a wide
frequency band. When more than two elements are used, this advantage largely
disappears.
4.11.1.2 Length and Phasing
Although the 1/2 wavelength is the basis for the collinear element, you will find that
greater lengths are often used. Effective arrays of this type have been constructed in
which the elements are 0.7 and even 0.8 wavelength long. This type of array provides
efficient operation at more than one frequency or over a wider frequency range. Whatever
length is decided upon, all of the elements in a particular array should closely adhere to
that length. If elements of different lengths are combined, current phasing and
distribution are changed, throwing the system out of balance and seriously affecting the
radiation pattern.
Q28. What is the maximum number of elements ordinarily used in a collinear array?
Q29. Why is the number of elements used in a collinear array limited?
Q30. How can the frequency range of a collinear array be increased?
Q31. How is directivity of a collinear array affected when the number of elements is
increased?
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4.11.1.3 Spacing
The lower relative efficiency of collinear arrays of many elements, compared with other
multi-element arrays, relates directly to spacing and mutual impedance effects. Mutual
impedance is an important factor to be considered when any two elements are parallel
and are spaced so that considerable coupling is between them. There is very little mutual
impedance between collinear sections. Where impedance does exist, it is caused by the
coupling between the ends of adjacent elements. Placing the ends of elements close
together is frequently necessary because of construction problems, especially where long
lengths of wire are involved.
The effects of spacing and the advantages of proper spacing can be demonstrated by
some practical examples. A collinear array consisting of two half-wave elements with
1/4-wavelength spacing between centers has a gain of 1.8 dB. If the ends of these same
dipoles are separated so that the distance from center to center is 3/4 wavelengths and
they are driven from the same source, the gain increases to approximately 2.9 dB.
A three-dipole array with negligible spacing between elements gives a gain of 3.3 dB. In
other words, when two elements are used with wider spacing, the gain obtained is
approximately equal to the gain obtainable from three elements with close spacing. The
spacing of this array permits simpler construction, since only two dipoles are used. It also
allows the antenna to occupy less space. Construction problems usually dictate small-
array spacing.
4.11.2 Broadside Arrays
A broadside array is shown in figure 4-26, view A. Physically, it looks somewhat like a
ladder. When the array and the elements in it are polarized horizontally, it looks like an
upright ladder. When the array is polarized vertically, it looks like a ladder lying on one
side (view B). View C is an illustration of the radiation pattern of a broadside array.
Horizontally polarized arrays using more than two elements are not common. This is
because the requirement that the bottom of the array be a significant distance above the
earth presents construction problems. Compared with collinear arrays, broadside arrays
tune sharply, but lose efficiency rapidly when not operated on the frequencies for which
they are designed.
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4.11.2.1 Radiation Pattern
Figure 4-27 shows an end view of two parallel half-wave antennas (A and B) operating in
the same phase and located 1/2 wavelength apart. At a point (P) far removed from the
antennas, the antennas appear as a single point. Energy radiating toward P from antenna
A starts out in phase with the energy radiating from antenna B in the same direction.
Propagation from each antenna travels over the same distance to point P, arriving there in
phase. The antennas reinforce each other in this direction, making a strong signal
available at P. Field strength measured at P is greater than it would be if the total power
supplied to both antennas had been fed to a single dipole. Radiation toward point P1 is
built up in the same manner.
Figure 4-26 Typical broadside array
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Next consider a wavefront traveling toward point Q from antenna B. By the time it
reaches antenna A, 1/2 wavelength away, 1/2 cycle has elapsed. Therefore energy from
antenna B meets the energy from antenna A 180 degrees out of phase. As a result, the
energy moving toward point Q from the two sources cancels. In a like manner, radiation
from antenna A traveling toward point Q1 meets and cancels the radiation in the same
direction from antenna B. As a result, little propagation takes place in either direction
along the QQ1 axis. Most of the energy is concentrated in both directions along the PP1
axis. When both antenna elements are fed from the same source, the result is the basic
broadside array.
When more than two elements are used in a broadside arrangement, they are all parallel
and in the same plane, as shown in figure 4-26, view B. Current phase, indicated by the
arrows, must be the same for all elements. The radiation pattern shown in figure 4-26,
view C, is always bi-directional. This pattern is sharper than the one shown in figure 4-27
because of the additional two elements. Directivity and gain depend on the number of
elements and the spacing between them.
Figure 4-27 Parallel elements in phase
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4.11.2.2 Gain and Directivity
The physical disposition of dipoles operated broadside to each other allows for much
greater coupling between them than can occur between collinear elements. Moving the
parallel antenna elements closer together or farther apart affects the actual impedance of
the entire array and the overall radiation resistance as well. As the spacing between
broadside elements increases, the effect on the radiation pattern is a sharpening of the
major lobes. When the array consists of only two dipoles spaced exactly 1/2 wavelength
apart, no minor lobes are generated at all. Increasing the distance between the elements
beyond that point, however, tends to throw off the phase relationship between the original
current in one element and the current induced in it by the other element. The result is
that, although the major lobes are sharpened, minor lobes are introduced, even with two
elements. These, however, are not large enough to be of concern.
If you add the same number of elements to both a broadside array and a collinear array,
the gain of the broadside array will be greater. Reduced radiation resistance resulting
from the efficient coupling between dipoles accounts for most of this gain. However,
certain practical factors limit the number of elements that may be used. The construction
problem increases with the number of elements, especially when they are polarized
horizontally.
Q32. What is the primary cause of broadside arrays losing efficiency when not operating
at their designed frequency?
Q33. When more than two elements are used in a broadside array, how are the elements
arranged?
Q34. As the spacing between elements in a broadside array increases, what is the effect
on the major lobes?
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4.11.3 End-Fire Arrays
An end-fire array looks similar to a broadside array. The ladder-like appearance is
characteristic of both (fig. 4-28, view A). The currents in the elements of the end-fire
array, however, are usually 180 degrees out of phase with each other as indicated by the
arrows. The construction of the end-fire array is like that of a ladder lying on its side
(elements horizontal). The dipoles in an end-fire array are closer together (1/8-
wavelength to 1/4 -wavelength spacing) than they are for a broadside array.
Closer spacing between elements permits compactness of construction. For this reason an
end-fire array is preferred to other arrays when high gain or sharp directivity is desired in
a confined space. However, the close coupling creates certain disadvantages. Radiation
resistance is extremely low, sometimes as low as 10 ohms, making antenna losses greater.
The end-fire array is confined to a single frequency. With changes in climatic or
atmospheric conditions, the danger of detuning exists.
Figure 4-28 Typical end-fire array
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4.11.3.1 Radiation Pattern
The radiation pattern for a pair of parallel half-wave elements fed 180 degrees out of
phase is shown in figure 4-29, view A. The elements shown are spaced 1/2 wavelength
apart. In practice, smaller spacings are used. Radiation from elements L and M traveling
toward point P begins 180 degrees out of phase. Moving the same distance over
approximately parallel paths, the respective wavefronts from these elements remain 180
degrees out of phase. In other words, maximum cancellation takes place in the direction
of P. The same condition is true for the opposite direction (toward P1). The P to P1 axis
is the line of least radiation for the end-fire array.
Consider what happens along the QQ1 axis. Energy radiating from element M toward Q
reaches element L in about 1/2 cycle (180 degrees) after it leaves its source. Since
element L was fed 180 degrees out of phase with element M, the wavefronts are now in
the same phase and are both moving toward Q reinforcing each other. Similar
reinforcement occurs along the same axis toward Q1. This simultaneous movement
towards Q and Q1 develops a bi-directional pattern. This is not always true in end-fire
operation. Another application of the end-fire principle is one in which the elements are
spaced ¼ wavelength apart and phased 90 degrees from each other to produce a
unidirectional pattern.
Figure 4-29 Parallel elements 180 degrees out of phase
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In figure 4-29, view A, elements A and B are perpendicular to the plane represented by
the page; therefore, only the ends of the antennas appear. In view B the antennas are
rotated a quarter of a circle in space around the QQ1 axis so that they are seen in the
plane of the elements themselves. Therefore, the PP1 axis, now perpendicular to the page,
is not seen as a line. The RR1 axis, now seen as a line, is perpendicular to the PP1 axis as
well as to the QQ1 axis. The end-fire array is directional in this plane also, although not
quite as sharply. The reason for the greater broadness of the lobes can be seen by
following the path of energy radiating from the midpoint of element B toward point S in
view B. This energy passes the A element at one end after traveling slightly more than
the perpendicular distance between the dipoles. Energy, therefore, does not combine in
exact phase toward point S. Although maximum radiation cannot take place in this
direction, energy from the two sources combines closely enough in phase to produce
considerable reinforcement. A similar situation exists for wavefronts traveling toward T.
However, the wider angle from Q to T produces a greater phase difference and results in
a decrease in the strength of the combined wave.
Directivity occurs from either one or both ends of the end-fire array, along the axis of the
array, as shown by the broken arrows in figure 4-28, view A; hence, the term end-fire is
used.
The major lobe or lobes occur along the axis of the array. The pattern is sharper in the
plane that is at right angles to the plane containing the elements (figure 4-29, view A). If
the elements are not exact half-wave dipoles, operation is not significantly affected.
However, because of the required balance of phase relationships and critical feeding, the
array must be symmetrical. Folded dipoles, such as the one shown in figure 4-20, view A,
are used frequently because the impedance at their terminals is higher. This is an
effective way of avoiding excessive antenna losses. Another expedient to reduce losses is
the use of tubular elements of wide diameter.
4.11.3.2 Gain and Directivity
In end-fire arrays, directivity increases with the addition of more elements and with
spacings approaching the optimum. The directive pattern for a two-element, bi-
directional system is illustrated in figure 4-29. View A shows radiation along the array
axis in a plane perpendicular to the dipoles, and view B shows radiation along the array
axis in the plane of the elements. These patterns were developed with a 180-degree phase
difference between the elements. Additional elements introduce small, minor lobes.
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With a 90-degree phase difference in the energy fed to a pair of end-fire elements spaced
approximately 1/4 wavelength apart, unidirectional radiation can be obtained. The pattern
perpendicular to the plane of the two elements is shown in figure 4-30, view A. The
pattern shown in view B, taken in the same plane, is for a six-element array with 90-
degree phasing between adjacent elements. Since both patterns show relative gain only,
the increase in gain produced by the six-element array is not evident. End-fire arrays are
the only unidirectional arrays wholly made up of driven elements.
Q35. What are some disadvantages of the end-fire array?
Q36. Where does the major lobe in the end-fire array occur?
Q37. To maintain the required balance of phase relationships and critical feeding, how
must the end-fire array be constructed?
Figure 4-30 Unidirectional end-fire arrays
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4.11.4 Parasitic Arrays
If a small light bulb were placed in the center of a large room, the illumination would be
very poor. However, if a reflector were placed behind the bulb, the space in front of the
reflector would be brighter and the space behind the reflector would be dimmer. The light
rays would be concentrated. Also, if a lens were placed in front of the bulb, the light
would be even more concentrated and a very bright spot would appear on the wall in
front of the lens. A flashlight is a practical combination of the small bulb, the reflector,
and the lens. The energy from an antenna can be reflected and concentrated in a similar
manner.
Although we do not usually discuss the gain of a flashlight, we can continue the
comparison of an antenna and a flashlight to explain the meaning of antenna gain.
Suppose the spot on the wall in front of the flashlight becomes 10 times brighter than it
was when only the open bulb was used. The lens and reflector have then produced a 10-
fold gain in light. For antennas, the simple half-wave antenna corresponds to the open
bulb in the flashlight. Suppose an antenna system concentrates the radio waves so that at
a particular point the field strength is 10 times more than it would be at the same distance
from a half-wave antenna. The antenna system is then said to have a gain of 10.
Parasitic arrays represent another method of achieving high antenna gains. A parasitic
array consists of one or more parasitic elements placed in parallel with each other and, in
most cases, at the same line-of-sight level. The parasitic element is fed inductively by
radiated energy coming from the driven element connected to the transmitter. It is in NO
way connected directly to the driven element.
When the parasitic element is placed so that it radiates away from the driven element, the
element is a director. When the parasitic element is placed so that it radiates toward the
driven element, the parasitic element is a reflector.
The directivity pattern resulting from the action of parasitic elements depends on two
factors. These are (1) the tuning, determined by the length of the parasitic element; and
(2) the spacing between the parasitic and driven elements. To a lesser degree, it also
depends on the diameter of the parasitic element, since diameter has an effect on tuning.
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4.11.4.1 Operation
When a parasitic element is placed a fraction of a wavelength away from the driven
element and is of approximately resonant length, it will re-radiate the energy it intercepts.
The parasitic element is effectively a tuned circuit coupled to the driven element, much as
the two windings of a transformer are coupled together. The radiated energy from the
driven element causes a voltage to be developed in the parasitic element, which, in turn,
sets up a magnetic field. This magnetic field extends over to the driven element, which
then has a voltage induced in it. The magnitude and phase of the induced voltage depend
on the length of the parasitic element and the spacing between the elements. In actual
practice the length and spacing are arranged so that the phase and magnitude of the
induced voltage cause a unidirectional, horizontal-radiation pattern and an increase in
gain.
In the parasitic array in figure 4-31, view A, the parasitic and driven elements are spaced
¼ wavelength apart. The radiated signal coming from the driven element strikes the
parasitic element after 1/4 cycle. The voltage developed in the parasitic element is 180
degrees out of phase with that of the driven element. This is because of the distance
traveled (90 degrees) and because the induced current lags the inducing flux by 90
degrees (90 + 90 = 180 degrees). The magnetic field set up by the parasitic element
induces a voltage in the driven element 1/4 cycle later because the spacing between the
elements is 1/4 wavelength. This induced voltage is in phase with that in the driven
element and causes an increase in radiation in the direction indicated in figure 4-31, view
A. Since the direction of the radiated energy is stronger in the direction away from the
parasitic element (toward the driven element), the parasitic element is called a reflector.
The radiation pattern as it would appear if you were looking down on the antenna is
shown in view B. The pattern as it would look if viewed from the ends of the elements is
shown in view C.
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Figure 4-31 Patterns obtained using a
reflector with proper spacing
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Because the voltage induced in the reflector is 180 degrees out of phase with the signal
produced at the driven element, a reduction in signal strength exists behind the reflector.
Since the magnitude of an induced voltage never quite equals that of the inducing
voltage, even in very closely coupled circuits, the energy behind the reflector (minor
lobe) is not reduced to 0.
The spacing between the reflector and the driven element can be reduced to about 15
percent of a wavelength. The parasitic element must be made electrically inductive before
it will act as a reflector. If this element is made about 5 percent longer than 1/2
wavelength, it will act as a reflector when the spacing is 15 percent of a wavelength.
Changing the spacing and length can change the radiation pattern so that maximum
radiation is on the same side of the driven element as the parasitic element. In this
instance the parasitic element is called a director.
Combining a reflector and a director with the driven element causes a decrease in back
radiation and an increase in directivity. This combination results in the two main
advantages of a parasitic array unidirectivity and increased gain. If the parasitic array is
rotated, it can pick up or transmit in different directions because of the reduction of
transmitted energy in all but the desired direction. An antenna of this type is called a
ROTARY ARRAY. Size for size, both the gain and directivity of parasitic arrays are
greater than those of driven arrays. The disadvantage of parasitic arrays is that their
adjustment is critical and they do not operate over a wide frequency range.
4.11.4.2 Gain and Directivity
Changing the spacing between either the director or the reflector and the driven element
results in a change in the radiation pattern. More gain and directivity are obtained by
changing the length of the parasitic elements.
The FRONT-TO-BACK RATIO of an array is the proportion of energy radiated in the
principal direction of radiation to the energy radiated in the opposite direction. A high
front-to-back ratio is desirable because this means that a minimum amount of energy is
radiated in the undesired direction. Since completely suppressing all such radiation is
impossible, an infinite ratio cannot be achieved. In actual practice, however, rather high
values can be attained. Usually the length and spacing of the parasitic elements are
adjusted so that a maximum front-to-back ratio is obtained, rather than maximum gain in
the desired direction.
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Q38. What two factors determine the directivity pattern of the parasitic array?
Q39. What two main advantages of a parasitic array can be obtained by combining a
reflector and a director with the driven element?
Q40. The parasitic array can be rotated to receive or transmit in different directions.
What is the name given to such an antenna?
Q41. What are the disadvantages of the parasitic array?
4.11.5 Multielement Parasitic Array
A MULTIELEMENT PARASITIC array is one that contains two or more parasitic
elements with the driven element. If the array contains two parasitic elements (a reflector
and a director) in addition to the driven element, it is usually known as a THREE-
ELEMENT ARRAY. If three parasitic elements are used, the array is known as a FOUR-
ELEMENT ARRAY, and so on. Generally speaking, if more parasitic elements are added
to a three-element array, each added element is a director. The field behind a reflector is
so small that additional reflectors would have little effect on the overall radiation pattern.
In radar, from one to five directors are used.
4.11.5.1 Construction
The parasitic elements of a multi-element parasitic array usually are positioned as shown
in figure 4-32, views A and B. Proper spacings and lengths are determined
experimentally. A folded dipole (view B) is often used as the driven element to obtain
greater values of radiation resistance.
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4.11.5.2 Yagi Antennas
An example of a multielement parasitic array is the YAGI ANTENNA (figure 4-32,
views A and B). The spacings between the elements are not uniform. The radiation from
the different elements arrives in phase in the forward direction, but out of phase by
various amounts in the other directions.
The director and the reflector in the Yagi antenna are usually welded to a conducting rod
or tube at their centers. This support does not interfere with the operation of the antenna.
Since the driven element is center-fed, it is not welded to the supporting rod. The center
impedance can be increased by using a folded dipole as the driven element.
The Yagi antenna shown in figure 4-32, view A, has three directors. In general, the
greater number of parasitic elements used, the greater the gain. However, a greater
number of such elements causes the array to have a narrower frequency response as well
as a narrower beamwidth. Therefore, proper adjustment of the antenna is critical. The
gain does not increase directly with the number of elements used. For example, a three-
element Yagi array has a relative power gain of 5 dB. Adding another director results in a
2 dB increase. Additional directors have less and less effect.
Figure 4-32 Yagi antenna
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A typical Yagi array used for receiving and transmitting energy is shown with a support
frame in figure 4-33. This antenna is used by the military services. It operates at
frequencies of from 12 to 50 megahertz and consists of two separate arrays (one high-
frequency and one low-frequency antenna array) mounted on one frame. The various
elements are indicated in the figure. The high-frequency (hf) array consists of one
reflector, one driven element, and two directors; the low-frequency (lf) array has the
same arrangement with one less director. The lengths of the elements in the high-
frequency array are shorter than those in the low-frequency array. The physical lengths of
the elements in the individual arrays are equal, but the electrical lengths can be varied by
means of the tuning stubs at the center of the elements. The array can be rotated in any
desired direction by a remotely controlled, electrically driven, antenna rotator.
Figure 4-33 A typical parasitic array used for transmitting and receiving
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Q42. What is the advantage of adding parasitic elements to a Yagi array?
Q43. The Yagi antenna is an example of what type of array?
4.12 SPECIAL ANTENNAS
In this section we will cover some special communications and radar antennas. Some of
these antennas we touch on briefly since they are covered thoroughly in other courses.
Previously discussed antennas operate with standing waves of current and voltage along
the wires. This section deals principally with antenna systems in which the current is
practically uniform in all parts of the antenna. In its basic form, such an antenna consists
of a single wire grounded at the far end through a resistor. The resistor has a value equal
to the characteristic impedance of the antenna. This termination, just as in the case of an
ordinary transmission line, eliminates standing waves. The current, therefore, decreases
uniformly along the wire as the terminated end is approached. This decrease is caused by
the loss of energy through radiation. The energy remaining at the end of the antenna is
dissipated in the terminating resistor. For such an antenna to be a good radiator, its length
must be fairly long. Also, the wire must not be too close to the ground. The return path
through the ground will cause cancellation of the radiation. If the wire is sufficiently
long, it will be practically nonresonant over a wide range of operating frequencies.
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4.12.1 Long-Wire Antenna
A LONG-WIRE ANTENNA is an antenna that is a wavelength or longer at the operating
frequency. In general, the gain achieved with long-wire antennas is not as great as the
gain obtained from the multi-element arrays studied in the previous section. But the long-
wire antenna has advantages of its own. The construction of long-wire antennas is simple,
both electrically and mechanically, with no particularly critical dimensions or
adjustments. The long-wire antenna will work well and give satisfactory gain and
directivity over a frequency range up to twice the value for which it was cut. In addition,
it will accept power and radiate it efficiently on any frequency for which its overall
length is not less than approximately 1/2 wavelength. Another factor is that long-wire
antennas have directional patterns that are sharp in both the horizontal and vertical
planes. Also, they tend to concentrate the radiation at the low vertical angles. Another
type of long-wire antenna is the BEVERAGE ANTENNA, also called a WAVE
ANTENNA. It is a horizontal, long-wire antenna designed especially for the reception
and transmission of low-frequency, vertically polarized ground waves. It consists of a
single wire, two or more wavelengths long, supported 3 to 6 meters above the ground,
and terminated in its characteristic impedance, as shown in figure 4-34.
Q44. To radiate power efficiently, a long-wire antenna must have what minimum overall
length?
Q45. What is another name for the Beverage antenna?
Figure 4-34 Beverage antenna
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4.12.2 V Antenna
A V ANTENNA is a bi-directional antenna used widely in military and commercial
communications. It consists of two conductors arranged to form a V. Each conductor is
fed with currents of opposite polarity.
The V is formed at such an angle that the main lobes reinforce along the line bisecting the
V and make a very effective directional antenna (see figure 4-35). Connecting the two-
wire feed line to the apex of the V and exciting the two sides of the V 180 degrees out of
phase cause the lobes to add along the line of the bisector and to cancel in other
directions, as shown in figure 4-36. The lobes are designated 1, 2, 3, and 4 on leg AA',
and 5, 6, 7, and 8 on leg BB'. When the proper angle between AA' and BB' is chosen,
lobes 1 and 4 have the same direction and combine with lobes 7 and 6, respectively. This
combination of two major lobes from each leg results in the formation of two stronger
lobes, which lie along an imaginary line bisecting the enclosed angle. Lobes 2, 3, 5, and 8
tend to cancel each other, as do the smaller lobes, which are approximately at right angles
to the wire legs of the V. The resultant waveform pattern is shown at the right of the V
antenna in figure 4-36.
Figure 4-35 Basic V antenna
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Q46. What is the polarity of the currents that feed the V antenna?
Figure 4-36 Formation of directional radiation pattern from a resonant V antenna
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4.12.3 Rhombic Antenna
The highest development of the long-wire antenna is the RHOMBIC ANTENNA (see
figure 4-37). It consists of four conductors joined to form a rhombus, or diamond shape.
The antenna is placed end to end and terminated by a noninductive resistor to produce a
uni-directional pattern. A rhombic antenna can be made of two obtuse-angle V antennas
that are placed side by side, erected in a horizontal plane, and terminated so the antenna is
nonresonant and unidirectional.
Figure 4-37 Basic rhombic antenna
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The rhombic antenna is WIDELY used for long-distance, high-frequency transmission
and reception. It is one of the most popular fixed-station antennas because it is very
useful in point-to-point communications.
4.12.3.1 Advantages
The rhombic antenna is useful over a wide frequency range. Although some changes in
gain, directivity, and characteristic impedance do occur with a change in operating
frequency, these changes are small enough to be neglected.
The rhombic antenna is much easier to construct and maintain than other antennas of
comparable gain and directivity. Only four supporting poles of common heights from 15
to 20 meters are needed for the antenna.
The rhombic antenna also has the advantage of being noncritical as far as operation and
adjustment are concerned. This is because of the broad frequency characteristics of the
antenna.
Still another advantage is that the voltages present on the antenna are much lower than
those produced by the same input power on a resonant antenna. This is particularly
important when high transmitter powers are used or when high-altitude operation is
required.
4.12.3.2 Disadvantages
The rhombic antenna is not without its disadvantages. The principal one is that a fairly
large antenna site is required for its erection. Each leg is made at least 1 or 2 wavelengths
long at the lowest operating frequency. When increased gain and directivity are required,
legs of from 8 to 12 wavelengths are used. These requirements mean that high-frequency
rhombic antennas have wires of several hundred feet in length. Therefore, they are used
only when a large plot of land is available.
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Another disadvantage is that the horizontal and vertical patterns depend on each other. If
a rhombic antenna is made to have a narrow horizontal beam, the beam is also lower in
the vertical direction. Therefore, obtaining high vertical-angle radiation is impossible
except with a very broad horizontal pattern and low gain. Rhombic antennas are used,
however, for long-distance sky wave coverage at the high frequencies. Under these
conditions low vertical angles of radiation (less than 20 degrees) are desirable. With the
rhombic antenna, a considerable amount of the input power is dissipated uselessly in the
terminating resistor. However, this resistor is necessary to make the antenna
unidirectional. The great gain of the antenna more than makes up for this loss.
4.12.3.3 Radiation Patterns
Figure 4-38 shows the individual radiation patterns produced by the four legs of the
rhombic antenna and the resultant radiation pattern. The principle of operation is the
same as for the V and the half-rhombic antennas.
Figure 4-38 Formation of a rhombic antenna beam
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4.12.3.4 Terminating Resistor
The terminating resistor plays an important part in the operation of the rhombic antenna.
Upon it depend the unidirectivity of the antenna and the lack of resonance effects. An
antenna should be properly terminated so it will have a constant impedance at its input.
Terminating the antenna properly will also allow it to be operated over a wide frequency
range without the necessity for changing the coupling adjustments at the transmitter.
Discrimination against signals coming from the rear is of great importance for reception.
The reduction of back radiation is perhaps of lesser importance for transmission. When
an antenna is terminated with resistance, the energy that would be radiated backward is
absorbed in the resistor.
Q47. What is the main disadvantage of the rhombic antenna?
4.12.4 Turnstile Antenna
The TURNSTILE ANTENNA is one of the many types that has been developed
primarily for omni-directional vhf communications. The basic turnstile consists of two
horizontal half-wave antennas mounted at right angles to each other in the same
horizontal plane. When these two antennas are excited with equal currents 90 degrees out
of phase, the typical figure-eight patterns of the two antennas merge to produce the nearly
circular pattern shown in figure 4-39, view A. Pairs of such antennas are frequently
stacked, as shown in figure 4-40. Each pair is called a BAY. In figure 4-40 two bays are
used and are spaced 1/2 wavelength apart, and the corresponding elements are excited in
phase. These conditions cause a part of the vertical radiation from each bay to cancel that
of the other bay. This results in a decrease in energy radiated at high vertical angles and
increases the energy radiated in the horizontal plane. Stacking a number of bays can alter
the vertical radiation pattern, causing a substantial gain in a horizontal direction without
altering the overall horizontal directivity pattern. Figure 4-39, view B, compares the
circular vertical radiation pattern of a single-bay turnstile with the sharp pattern of a four-
bay turnstile array. A three-dimensional radiation pattern of a four-bay turnstile antenna
is shown in figure 4-39, view C.
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Figure 4-39 Turnstile antenna radiation pattern
Figure 4-40 Stacked turnstile antennas
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4.12.5 Ground-Plane Antenna
A vertical quarter-wave antenna several wavelengths above ground produces a high angle
of radiation that is very undesirable at vhf and uhf frequencies. The most common means
of producing a low angle of radiation from such an antenna is to work the radiator against
a simulated ground called a GROUND PLANE. A simulated ground may be made from a
large metal sheet or several wires or rods radiating from the base of the radiator. An
antenna so constructed is known as a GROUND-PLANE ANTENNA. Two ground-plane
antennas are shown in figure 4-41, views A and B.
Figure 4-41 Ground-plane antennas
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4.12.6 Corner Reflector
When a unidirectional radiation pattern is desired, it can be obtained by the use of a
corner reflector with a half-wave dipole. A CORNER-REFLECTOR ANTENNA is a
half-wave radiator with a reflector. The reflector consists of two flat metal surfaces
meeting at an angle immediately behind the radiator. In other words, the radiator is set in
the plane of a line bisecting the corner angle formed by the reflector sheets. The
construction of a corner reflector is shown in figure 4-42. Corner-reflector antennas are
mounted with the radiator and the reflector in the horizontal position when horizontal
polarization is desired. In such cases the radiation pattern is very narrow in the vertical
plane, with maximum signal being radiated in line with the bisector of the corner angle.
The directivity in the horizontal plane is approximately the same as for any half-wave
radiator having a single-rod type reflector behind it. If the antenna is mounted with the
radiator and the corner reflector in the vertical position, as shown in view A, maximum
radiation is produced in a very narrow horizontal beam. Radiation in a vertical plane will
be the same as for a similar radiator with a single-rod type reflector behind it.
Q48. What is the primary reason for the development of the turnstile antenna?
Figure 4-42 Corner-reflector antennas
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4.13 RF SAFETY PRECAUTIONS
Although electromagnetic radiation from transmission lines and antennas is usually of
insufficient strength to electrocute personnel, it can lead to other accidents and compound
injuries. Voltages may be induced in ungrounded metal objects, such as wire guys, wire
cable (hawser), hand rails, or ladders. If you come in contact with these objects, you
could receive a shock or rf burn. This shock can cause you to jump or fall into nearby
mechanical equipment or, when working aloft, to fall from an elevated work area.
Take care to ensure that all transmission lines or antennas are deenergized before
working near or on them.
Either check or have someone check all guys, cables, rails, and ladders around your work
area for rf shock dangers. Use working aloft "chits" and safety harnesses for your own
safety. Signing a "working aloft chit" signifies that all equipment is in a non-radiating
status. The person who signs the chit should ensure that no rf danger exists in areas where
you or other personnel will be working.
Nearby ships or parked aircraft are another source of rf energy that you must consider
when you check a work area for safety. Combustible materials can be ignited and cause
severe fires from arcs or heat generated by rf energy. Also, rf radiation can detonate
ordnance devices by inducing currents in the internal wiring of the devices or in the
external test equipment or leads connected to them.
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 4-
43.
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Figure 4-43 Examples of rf radiation warning signs
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4.13.1 Rf Burns
Close or direct contact with rf transmission lines or antennas may result in rf burns. These
are usually deep, penetrating, third-degree burns. To heal properly, these burns must heal
from the inside to the skin's surface. To prevent infection, you must give proper attention
to all rf burns, including the small "pinhole" burns. Petrolatum gauze can be used to
cover these burns temporarily, before the injured person reports to medical facilities for
further treatment.
4.13.2 Dielectric Heating
DIELECTRIC HEATING is the heating of an insulating material by placing it in a high-
frequency electric field. The heat results from internal losses during the rapid reversal of
polarization of molecules in the dielectric material.
In the case of a human in an rf field, the body acts as a dielectric. If the power in the rf
field exceeds 10 milliwatts per centimeter, a person in that field will have a noticeable
rise in body temperature. The eyes are highly susceptible to dielectric heating. For this
reason, you should not look directly into devices radiating rf energy. The vital organs of
the body also are susceptible to dielectric heating. For your own safety, you must NOT
stand directly in the path of rf radiating devices.
4.13.3 Precautions When Working Aloft
When radio or radar antennas are energized by transmitters, you must not go aloft unless
advance tests show that little or no danger exists. A casualty can occur from even a small
spark drawn from a charged piece of metal or rigging. Although the spark itself may be
harmless, the "surprise" may cause you to let go of the antenna involuntarily and you may
fall. There is also a shock hazard if nearby antennas are energized.
Rotating antennas also might cause you to fall when you are working aloft. Motor safety
switches controlling the motion of rotating antennas must be tagged and locked open
before you go aloft near such antennas.
When working near a stack, you should 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.
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4.14 SUMMARY
This chapter has presented information on the various types of antennas. The information
that follows summarizes the important points of this chapter.
An ANTENNA is a conductor, or system of conductors, that radiates or receives energy
in the form of electromagnetic waves.
HERTZ (half-wave) and MARCONI (quarter-wave) are the two basic
classifications of antennas.
RECIPROCITY of antennas means that the various properties of the antenna apply
equally to transmitting and receiving.
RADIATION RESISTANCE is the amount of resistance which, if inserted in place of
the antenna, would consume the same amount of power that is actually radiated by the
antenna.
RADIATION PATTERNS can be plotted on a rectangular- or polar-coordinate graph.
These patterns are a measurement of the energy leaving an antenna.
An ISOTROPIC RADIATOR radiates energy equally in all directions.
An ANISOTROPIC RADIATOR radiates energy directionally.
A LOBE is the area of a radiation pattern that is covered by radiation.
A NULL is the area of a radiation pattern that has minimum radiation.
ANTENNA LOADING is the method used to change the electrical length of an antenna.
This keeps the antenna in resonance with the applied frequency. It is accomplished by
inserting a variable inductor or capacitor in series with the antenna.
A HALF-WAVE ANTENNA (Hertz) consists of two lengths of rod or tubing, each a
quarter-wave long at a certain frequency, which radiates a doughnut pattern.
A QUARTER-WAVE ANTENNA (Marconi) is a half-wave antenna cut in half with
one end grounded. The ground furnishes the missing half of the antenna.
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The GROUND SCREEN and the COUNTERPOISE are used to reduce losses caused
by the ground in the immediate vicinity of the antenna. The ground screen is buried
below the surface of the earth. The counterpoise is installed above the ground.
The FOLDED DIPOLE consists of a dipole radiator, which is connected in parallel at its
ends to a half-wave radiator.
AN ARRAY is a combination of half-wave elements operating together as a single
antenna. It provides more gain and greater directivity than single element antennas.
A DRIVEN ARRAY derives its power directly from the source.
A PARASITIC ARRAY derives its power by coupling the energy from other elements
of the antenna.
The BIDIRECTIONAL ARRAY radiates energy equally in two opposing directions.
The UNIDIRECTIONAL ARRAY radiates energy efficiently in a single direction.
The COLLINEAR ARRAY has elements in a straight line. Maximum radiation occurs at
right angles to this line.
The BROADSIDE ARRAY has elements parallel and in the same plane. Maximum
radiation develops in the plane at right angles to the plane of the elements.
The END-FIRE ARRAY has elements parallel to each other and in the same plane.
Maximum radiation occurs along the axis of the array.
MATCHING STUBS are used between elements to maintain current in the
proper phase.
The GAIN OF A COLLINEAR ANTENNA is greatest when the elements are spaced
from 0.4 to 0.5 wavelength apart or when the number of elements is increased.
The OPTIMUM GAIN OF A BROADSIDE ARRAY is obtained when the elements are
spaced 0.65 wavelength apart.
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A PARASITIC ARRAY consists of one or more parasitic elements with a driven
element. The amount of power gain and directivity depends on the lengths of the parasitic
elements and the spacing between them.
MULTIELEMENT ARRAYS, such as the YAGI, have a narrow frequency response as
well as a narrow beamwidth.
A LONG-WIRE ANTENNA is an antenna that is a wavelength or more long at the
operating frequency. These antennas have directive patterns that are sharp in both the
horizontal and vertical planes.
BEVERAGE ANTENNAS consist of a single wire that is two or more
wavelengths long.
A V ANTENNA is a bi-directional antenna consisting of two horizontal, long wires
arranged to form a V.
The RHOMBIC ANTENNA uses four conductors joined to form a rhombus shape. This
antenna has a wide frequency range, is easy to construct and maintain, and is noncritical
as far as operation and adjustment are concerned.
The TURNSTILE ANTENNA consists of two horizontal, half-wire antennas mounted at
right angles to each other.
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ANSWERS TO QUESTIONS Q1. THROUGH Q48.
A1. Half-wave (Hertz) and quarter-wave (Marconi).
A2. Coupling device, feeder, and antenna.
A3. Frequency of operation of the transmitter, amount of power to be radiated, and
general direction of the receiving set.
A4. One-half the wavelength.
A5. Current and voltage loops.
A6. Current and voltage nodes.
A7. Reciprocity of antennas.
A8. Electric (E) field.
A9. Circular polarization.
A10. Vertical polarization.
A11. Less interference is experienced by man-made noise sources.
A12. Vertical polarization.
A13. 73 ohms.
A14. Anisotropic radiator.
A15. Isotropic radiator.
A16. Anisotropic radiator.
A17. Dipole, doublet and Hertz.
A18. Nondirectional.
A19. Vertical plane.
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A20. The pattern would flatten.
A21. To connect one end through a capacitor to the final output stage of the
transmitter.
A22. A circular radiation pattern in the horizontal plane, or same as a half wave.
A23. It is composed of a series of conductors arranged in a radial pattern and buried
1 to 2 feet below the ground.
A24. Nine times the feed-point impedance.
A25. Folded dipole.
A26. To produce desired phase relationship between connected elements.
A27. Major lobes have the greatest amount of radiation.
A28. Four.
A29. As more elements are added, an unbalanced condition in the system occurs which
impairs efficiency.
A30. By increasing the lengths of the elements of the array.
A31. Directivity increases.
A32. Lower radiation resistance.
A33. Parallel and in the same plane.
A34. They sharpen.
A35. Extremely low radiation resistance, confined to one frequency, and affected by
atmospheric conditions.
A36. Along the major axis
A37. Symmetrically.
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A38. Length of the parasitic element (tuning) and spacing between the parasitic and
driven elements.
A39. Increased gain and directivity.
A40. Rotary array.
A41. Their adjustment is critical and they do not operate over a wide frequency range.
A42. Increased gain.
A43. Multielement parasitic array.
A44. One-half wavelength.
A45. Wave antenna.
A46. Opposite.
A47. It requires a large antenna site.
A48. For omni-directional vhf communications.