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

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MCRP 6-22D
Another problem could be misaligned directional antennas. If direc-
tional antennas are not correctly pointed at each other, communica-
tion is degraded. The directional antennas’ electrical characteristics
can change over several field deployments, especially if the antenna
is subjected to harsh use. These electrical characteristic changes can
cause the radiation pattern to change. Then, when the antenna is
physically pointed at the distant station, the main radiation may be
aimed in another direction. 
To correct these electrical characteristic changes, have the distant
station transmit. Slowly turn the receiving antenna while listening
to the received signal. When the received signal is strongest, the
antenna is properly aligned for the circuit. Secure the antenna in this
position and have the distant station align its antenna in the same
way. When both antennas are properly adjusted, the maximum radi-
ation from each antenna is directed at the other antenna.
SITING VHF ANTENNAS
Antenna sites should be as high as possible and clear of obstruc-
tions such as hills, dense woods, and buildings. If it is necessary to
site the antennas on or around hills, choose a site that allows LOS to
the distant station or stations. If possible, place the antenna on the
military crest of a hill, not on the ridge line. Antennas located on the
ridge line provide an aiming stake for enemy observation and fire
(fig. 8-1). 
Place high ground between the antenna and the enemy to block the
enemy’s observation and the antenna’s radiation, reducing the
enemy’s intercept capability (fig. 8-2).    
						

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8-7
      
In a dense forest, get the antenna tip above the treetops. This height
allows the radio signal to propagate in the clear space above the
trees. If it is impossible to raise the antenna above the trees, a hori-
zontally polarized antenna provides better communications throughFigure 8-1. Ridge Line Antenna Farm.DESIRED 
COMMUNICATIONSENEMYFigure 8-2. Antenna Sited on a Military Crest. 
						

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MCRP 6-22D
trees than a vertically polarized antenna. Figure 8-3 shows good,
fair, and poor antenna siting in dense trees.
A clearing in a forest improves propagation if the antenna is placed
so that the clearing is between the antenna and the distant station
(for a directional antenna). Place an omnidirectional antenna in the
center of a clearing, with the antenna as high as possible (fig. 8-4).
A communicator may have little choice in selecting a transmitter or
receiver site location. Often the site is determined by the opera-
tional requirements of a superior command. However, when a
choice is available, determine the HF antenna site by wave-path
geometry.GOOD
FAIRPOORDISTANT STATIONFigure 8-3. Antennas Sited in Dense Trees. 
						

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8-9
Figure 8-4. Directional Antennas Sited in a Clearing.
Transmitting Antenna Site
Any site that has a horizon whose obstructions subtend vertical
angles of less than 2° from level in any of the directions of trans-
mission can be considered immediately as a satisfactory site from
the standpoint of radiation. As a simple rule, a satisfactory horizon
clearance exists when any obstruction subtends a vertical angle that
does not exceed one-half of the desired beam angle in the vertical
plane in that direction. If the vertical beam angle for a given circuit
is low for the lowest order hop, then the horizon in that direction
can be as much as 5° above level as seen from the antenna location. 
In hilly or mountainous country, choosing a site for long-distance
transmission, requiring very low beam angles, can be difficult.
When the only possible site presents horizon obstructions in the pre-
ferred wave path, it may be necessary to design an antenna that uses
a higher order of hop, and to direct the beam at a corresponding
higher angle to obtain the desired 2-to-1 horizon clearance angle. DISTANT STATION
GOODFAIRPOOR 
						

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MCRP 6-22D
For example, if the computed vertical beam angle for a one-hop cir-
cuit is 6° at an azimuth of 332°, and the horizon in this direction is a
range of mountains with a height of 8° as seen from the antenna
site, the performance of the circuit would be greatly compromised
by the obstruction of the mountains. It might be better to work this
circuit with two hops—a vertical beam of 20° could be used
instead, with adequate horizon clearance for the wave path. If the
circuit required 6° for a two-hop circuit 5,400 kilometers long with
the same obstruction sited, the circuit could be changed to three
hops, which, for the same layer heights, would permit using a beam
at 14°. The latter solution lacks the full 2-to-1 horizon-clearance
angle, but it may be an acceptable compromise and perhaps prefera-
ble to using four hops.
Short-range, sky wave circuits using one-hop high-angle radiation
give a great latitude in the choice of sites. For F layer transmission
to distances of 500 miles and less, the vertical beam or angles are
always greater than 30°. Satisfactory sites for such transmission can
often be located in rather deep valleys without any compromise on
the circuit performance.
Forests on or near the site require some consideration. Because the
theoretical radiation pattern is calculated on perfect reflectivity
from ground, some precautions are necessary to obtain actual per-
formance that substantially agrees with theoretical performance.
Choose a site that provides conditions as nearly perfect as possible
with respect to wave-reflecting surfaces around the antenna. There
should be few or no trees and buildings out to the necessary dis-
tance from the antenna. The point of wave reflection should be flat
and cleared. An excellent choice is a site that borders the sea or a
lake. Water is a wave-reflecting surface. 
						

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Receiving Antenna Sites
Choosing a receiving antenna site is similar to choosing a transmit-
ting antenna site. The dominant angles of arrival of the incoming
waves at the site are determined mainly by the characteristics of the
transmitting antenna. Best results are obtained with complementary
transmitting and receiving antennas. If a horizon obstruction exists
at the optimum angle of wave arrival, a compromise, noncomple-
mentary antenna may be necessary. When possible, move the trans-
mitting antenna to align with the receiving antenna.
The receiving site must be as free as possible from electrical noise.
The tolerable amount of manmade noise at a particular receiving
station site depends on the prevailing natural atmospheric noise lev-
els. At a well selected site, reception should always be limited only
by natural atmospheric noise. Any manmade noise at the site should
always be substantially less than the atmospheric noise received
during the low-noise periods.
Aside from broadcast systems, most communications systems re-
quire that antennas be positioned so that their main lobes of radia-
tion are aligned with each other. This requires knowledge of the
great circle bearing to the other antenna and the local magnetic vari-
ation from true north. 
The great circle bearing between two locations is calculated by
methods that are beyond the scope of this publication. A way to find
the great circle bearings is to request a frequencies of optimum
transmission chart from the Electromagnetic Compatibility Analy-
sis Center. 
						

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MCRP 6-22D
ANTENNA FARM INTERNAL ARRANGEMENT
Frequency Band
The higher the frequency, the shorter the wavelength. The shorter
the wavelength, the more nearly LOS. The more nearly LOS, the
more critical is a clear LOS path for the signal.
Antenna Selection and Placement
Selection. The key to antenna selection rests with the answers to
the following three questions:
• To whom will you be transmitting?  Where will they be?
• What is the path between you and them?
•  What kind of net?  Point-to-point or multistation?
Placement. Antenna placement within the antenna farm should
take into account the following three factors:
Cosite Interference. Evaluating interference can be difficult be-
cause of the nature of the systems involved and the complexity of
the signals. The mechanisms are varied. In the simpler cases they
may be direct interference into the radio receiver. In other cases,
they may be spurious products or combinations of products which
arrive at the receiver input and produce a net resultant interference
into the receiver intermediate frequency section. The latter may be
frequency translations resulting from sum and difference products
within the same system. In still other cases, the receiver may see an
identical signal to the regular signal.
Interference produces beats or noise in a radio receiver which have
detrimental effects depending on the frequency, deviation, channel
separation and linearity of the transmission medium, as well as the 
						

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nature of the interfering signal. Sometimes the interfering signals
combine with other frequencies in the system, including carrier-
sum and difference frequencies, to produce interference in a third
radio channel. The products may hold up automatic gain control
during critical fading periods, with serious effect on system noise.
Usually, noise in the base band channels is an end product.
Radio system interference may be introduced through antennas,
wave guides, cabling, or by spurious products produced in the radio
equipment itself. Interference introduced into the cabling or in the
equipment can be prevented by good installation practices, includ-
ing proper separation of high- and low-level cabling, proper ground-
ing practices, shielding where necessary, and good equipment
design and assembly. Interference introduced by coupling between
wave guides in the same station is usually produced by radiation
from wave guide and filter flanges which are not properly tightened,
or which are damaged and cannot be mated properly.
Antenna Coupling. Antenna coupling is a frequency-independent
problem that may occur whenever other antennas (whether trans-
mitting or not) or metallic objects are located within one wave of the
transmitting antenna. Antenna coupling may be either beneficial or
detrimental. Yagis, log periodic arrays, and half-square antennas,
for example, derive their gain and directivity from antenna cou-
pling. Unintended antenna coupling, on the other hand, may signifi-
cantly reduce the signal strength in the desired direction and either
degrade or stop communications.
Coupling is based on two principles. One, that current flowing
through a wire creates a magnetic field around it; and, two, that sig-
nals in phase reinforce each other whereas signals out of phase can-
cel each other. Receiving antennas have current flowing in them
(the received signal). Because there is a flowing current that creates
a magnetic field, a receiving antenna will simultaneously receive
and reradiate the same signal. Receiving antennas, in fact, tend to 
						

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MCRP 6-22D
reradiate about one-half of the power they receive. If the receiving
antenna is within one wavelength of a transmitting antenna (it
makes absolutely no difference whether or not the receiving
antenna’s radio is tuned to the same frequency as the transmitting
radio), the receiving antenna will reradiate a portion of the signal
which may be out of phase with the original signal, altering the
transmitting antenna’s radiation pattern.
Direction of Desired Transmission. Separate antennas according
to the direction of desired transmission. For example: If antenna A
is used to communicate to the east, and antenna B is used to com-
municate to the north, then locate antenna A south and east of
antenna B. Do not make the signal from one antenna pass through,
or around, another antenna on the way to its intended receiver.
Accomplish this by the physical location of the antennas, by mask-
ing the antennas, or by placing the antennas at different elevations.
Requirements
Separate antennas based on the frequencies at which they will oper-
ate and the power they will transmit to avoid cosite interference.
For a 10 percent separation—
PowerDistance (Meters)
1 kilowatt500
400 watts315
150 watts200
100 watts150
40 watts100
20 watts70
10 watts50
2 watts22
1 watt15 
						

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8-15
Multiply separations by 10 for each halving of frequency separation
(i.e., 10 for 5 percent; 100 for 2.5 percent).
For  a 5 percent separation—
For a 2.5 percent separation—
Separate antennas by a minimum of wavelength at the lowest fre-
quency at which they will operate to alleviate antenna coupling.
Separate antennas according to the desired direction of transmission
(i.e., don’t send the propagated wave through other antennas).
Polarization
The preferable polarization with respect to vegetation depends on
the forest and the amount of foliage. Use a polarization with an
inherent advantage when heavy vegetation cannot be avoided. AnyPower (Watts)Distance (Meters)
10500
2220
1150
Power (Watts)Distance (Meters)
105000
22200
11500
BandLowest Frequency
(MHz)Minimum Separation
(Feet)
HF2492
VHF3032.8
UHF2254.37