U.S. Marine Corps Antenna Mcrp 6 22D Operating Instructions

							Antenna Handbook ______________________________ 
2-5
 . 
Figure 2-2. Radiation Patterns.OMNIDIRECTIONAL
BIDIRECTIONAL
UNIDIRECTIONAL 
						

							2-6 _____________________________________ 
MCRP 6-22D
POLARIZATION 
A radiated wave’s polarization is determined by the direction of the
lines of force making up the electric field. If the lines of electric
force are at right angles to the Earth’s surface, the wave is vertically
polarized (fig. 2-3). If the lines of electric force are parallel to the
Earth’s surface, the wave is horizontally polarized (fig. 2-4). When
a single-wire antenna extracts (receives) energy from a passingEARTHSIGNAL VOLTAGEELECTRIC FIELDDIRECTION OF TRAVEL
Figure 2-3. Vertical Polarization. 
						

							Antenna Handbook ______________________________ 
2-7
radio wave, maximum pickup results if the antenna is oriented in
the same direction as the electric field component. A vertical
antenna receives vertically polarized waves, and a horizontal
antenna receives horizontally polarized waves. If the field rotates as
the waves  travel through space, both horizontal and vertical com-
ponents of the field exist, and the wave is elliptically polarized.
.
Figure 2-4. Horizontal Polarization.EARTHSIGNAL VOLTAGEELECTRIC FIELDDIRECTION OF TRAVEL 
						

							2-8 _____________________________________ 
MCRP 6-22D
Polarization Requirements for Various Frequencies
At medium and low frequencies, ground wave transmission is used
extensively, and it is necessary to use vertical polarization. Vertical
lines of force are perpendicular to the ground, and the radio wave
can travel a considerable distance along the ground surface with a
minimum amount of loss. Because the Earth acts as a relatively
good conductor at low frequencies, horizontal lines of electric force
are shorted out, and the useful range with the horizontal polariza-
tion is limited. 
At high frequencies, with sky wave transmission, it makes little dif-
ference whether horizontal or vertical polarization is used. The sky
wave, after being reflected by the ionosphere, arrives at the receiv-
ing antenna elliptically polarized. Therefore, the transmitting and
receiving antennas can be mounted either horizontally or vertically.
Horizontal antennas are preferred, since they can be made to radiate
effectively at high angles and have inherent directional properties. 
For frequencies in the VHF or UHF range, either horizontal or ver-
tical polarization is satisfactory. Since the radio wave travels
directly from the transmitting antenna to the receiving antenna, the
original polarization produced at the transmitting antenna is main-
tained as the wave travels to the receiving antenna. If a horizontal
antenna is used for transmitting, a horizontal antenna must be used
for receiving. 
Satellites and satellite terminals use circular polarization. Circular
polarization describes a wave whose plane of polarization rotates
through 360° as it progresses forward. The rotation can be clock-
wise or counterclockwise (see fig. 2-5). Circular polarization occurs
when equal magnitudes of vertically and horizontally polarized
waves are combined with a phase difference of 90°. Rotation in one
direction or the other depends on the phase relationship.  
						

							Antenna Handbook ______________________________ 
2-9
Advantages of Vertical Polarization
Simple vertical half-wave and quarter-wave antennas provide
omnidirectional communications. This is desirable in communicat-
ing with a moving vehicle. The disadvantage is that it radiates
equally to the enemy and friendly forces. 
When antenna heights are limited to 3.05 meters (10 feet) or less
over land, as in a vehicular installation, vertical polarization pro-
vides a stronger received signal at frequencies up to about 50 MHz.
From about 50 to 100 MHz, there is only a slight improvement over
horizontal polarization with antennas at the same height. Above 100
MHz, the difference in signal strength between vertical and hori-
zontal polarization is small. However, when antennas are located
near dense forests, horizontally polarized waves suffer lower losses
than vertically polarized waves. 
Vertically polarized radiation is somewhat less affected by reflec-
tions from aircraft flying over the transmission path. With horizon-
tal polarization, such reflections cause variations in received signalDIRECTIONOF TRAVELDIRECTION OF ROTATION,RIGHT-HANDFigure 2-5. Circular Polarization. 
						

							2-10 ____________________________________ 
MCRP 6-22D
strength. An example is the picture flutter in a television set when
an aircraft interferes with the transmission path. This factor is
important in areas where aircraft traffic is heavy. 
When vertical polarization is used, less interference is produced or
picked up from strong VHF and UHF transmissions (TV and FM
broadcasts) because they use horizontal polarization. This factor is
important when an antenna must be located in an urban area that
has TV or FM broadcast stations. 
Advantages of Horizontal Polarization
A simple horizontal half-wave antenna is bidirectional. This charac-
teristic is useful in minimizing interference from certain directions. 
Horizontal antennas are less likely to pick up manmade interfer-
ence, which is ordinarily vertically polarized. When antennas are
located near dense forests, horizontally polarized waves suffer
lower losses than vertically polarized waves, especially above 100
MHz. Small changes in antenna location do not cause large varia-
tions in the field intensity of horizontally polarized waves when an
antenna is located among trees or buildings. When vertical polariza-
tion is used, a change of only a few feet in the antenna location may
have a significant effect on the received signal strength. 
DIRECTIONALITY
Vertical receiving antennas accept radio signals equally from all
horizontal directions, just as vertical transmitting antennas radiate
equally in all horizontal directions. Because of this characteristic,
other stations operating on the same or nearby frequencies may
interfere with the desired signal and make reception difficult or 
						

							Antenna Handbook ____________________________ 
2-11
impossible. However, reception of a desired signal can be improved
by using directional antennas. 
Horizontal half-wave antennas accept radio signals from all direc-
tions. The strongest reception is received in a line perpendicular to
the antenna (i.e., broadside, and the weakest reception is received
from the direction of the ends of the antenna). Interfering signals
can be eliminated or reduced by changing the antenna installation
so that either end of the antenna points directly at the interfering
station. 
Communications over a radio circuit is satisfactory when the
received signal is strong enough to override undesired signals and
noise. The receiver must be within range of the transmitter. Increas-
ing the transmitting power between two radio stations increases
communications effectiveness. Also, changing the types of trans-
mission, changing to a frequency that is not readily absorbed, or
using a directional antenna aids in communications effectiveness. 
Directional transmitting antennas concentrate radiation in a given
direction and minimize radiation in other directions. A directional
antenna may also be used to lessen interception by the enemy and
interference with friendly stations. 
RESONANCE
Antennas can be classified as either resonant or nonresonant,
depending on their design.  In a resonant antenna, almost all of the
radio signal fed to the antenna is radiated. If the antenna is fed with
a frequency other than the one for which it is resonant, much of the
fed signal will be lost and will not be radiated. A resonant antenna
will effectively radiate a radio signal for frequencies close to its
design frequency (usually only 2 percent above or below the design 
						

							2-12 ____________________________________ 
MCRP 6-22D
frequency). If a resonant antenna is used for a radio circuit, a sepa-
rate antenna must be built for each frequency to be used on the
radio circuit. A nonresonant antenna, on the  other hand, will effec-
tively radiate a broad range of frequencies with less efficiency. Res-
onant and nonresonant antennas are commonly used on tactical
circuits. Resonance can be achieved in two ways: physically match-
ing the length of the antenna to the wave and electronically match-
ing the length of the antenna to the wave.
RECEPTION
The radio waves that leave the transmitting antenna will have an
influence on and will be influenced by any electrons in their path.
For example, as an HF wave enters the ionosphere, it is reflected or
refracted back to earth by the action of free electrons in this region
of the atmosphere. When the radio wave encounters the wire or
metallic conductors of the receiving antenna, the radio wave’s elec-
tric field will cause the electrons in the antenna to oscillate back and
forth in step with the wave as it passes. The movement of these
electrons within the antenna is the small alternating electrical cur-
rent which is detected by the radio receiver. 
When radio waves encounter electrons which are free to move
under the influence of the wave’s electric field, the free electrons
oscillate in sympathy with the wave. This generates electric cur-
rents which then create waves of their own. These new waves are
reflected or scattered waves. This process is electromagnetic scat-
tering. All materials that are good electrical conductors reflect or
scatter RF energy. Since a receiving antenna is a good conductor, it
too acts as a scatterer. Only a portion of the energy which comes in
contact with the antenna is converted into received electrical power;
a sizable portion of the total power is re-radiated by the wire. 
						

							Antenna Handbook ____________________________ 
2-13
If an antenna is located within a congested urban environment or
within a building, there are many objects which will scatter or re-
raditate the energy in a manner that can be detrimental to reception.
For example, the electric wiring inside a building can strongly re-
radiate RF energy. If a receiving antenna is in close proximity to
wires, it is possible for the reflected energy to cancel the energy
received directly from the desired signal path. When this condition
exists, the receiving antenna should be moved to another location
within the room where the reflected and direct signals may rein-
force rather than cancel each other.
RECIPROCITY
The various properties of an antenna apply equally, regardless of
whether the antenna is used for transmitting or receiving.  This is
what is meant by reciprocity of antennas.  For example, the more
efficient a certain antenna is for transmitting, the more efficient it
will be for receiving the same frequency.  The directive properties
of a given antenna will be the same whether it is used for transmis-
sion or reception.
For example, figure 2-6 on page 2-14 shows a particular antenna
used with a transmitter radiating a maximum amount of energy at
right angles to the antenna wire.  There is a minimum amount of
radiation along the axis of the antenna.  If this same antenna is used
as a receiving antenna, it receives best in the same directions in
which it produced maximum radiation (i.e., at right angles to the
axis of the antenna).  There is a minimum amount of signal received
from transmitters located in line with the antenna wire. 
						

							2-14 ____________________________________ 
MCRP 6-22D
IMPEDANCE
Impedance is the relationship between voltage and current at any
point in an alternating current circuit. The impedance of an antenna
is equal to the ratio of the voltage to the current at the point on the
antenna where the feed is connected (feed point). If the feed point is
located at a point of maximum current, the antenna impedance is 20
to 100 ohms. If the feed point is moved to a maximum voltage
point, the impedance is as much as 500 to 10,000 ohms. 
The input impedance of an antenna depends on the conductivity or
impedance of the ground. For example, if the ground is a simple
stake driven about a meter into earth of average conductivity, the
impedance of the monopole may be double or even triple the quoted
values. Because this additional resistance occurs at a point on the
antenna circuit where the current is high, a large amount ofMAXIMUM RADIATIONMAXIMUM RECEPTIONTRANSMITTINGANTENNARECEIVINGANTENNAFigure 2-6. Reciprocity.