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

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MCRP 6-22D
ranges are assigned to many activities, including citizens (public)
communications (e.g., mobile, police, weather, taxis, and general
purpose).
POLARIZATION
In many countries, FM and TV broadcasting in the VHF range use
horizontal polarization. One reason is because it reduces ignition
interference, which is mainly vertically polarized. Mobile commu-
nications often use vertical polarization for two reasons. First, the
vehicle antenna installation has physical limitations, and second, so
that reception or transmission will not be interrupted as the vehicle
changes its heading to achieve omnidirectionality. 
Using directional antennas and horizontal polarization (when possi-
ble) will reduce manmade noise interference in urban locations.
Horizontal polarization, however, should be chosen only where an
antenna height of many wavelengths is possible. Ground reflections
tend to cancel horizontally polarized waves at low angles. Use only
vertically polarized antennas when the antenna must be located at a
height of less than about 10 meters above ground, or where omnidi-
rectional radiation or reception is desired.
GAIN AND DIRECTIVITY
VHF and UHF (above 30 MHz) antenna gain and directivity are
extremely important for several reasons. Assuming the same
antenna gain and propagation path, the received signal strength
drops as frequency is increased. At VHF and UHF, more of the
received signal is lost in the transmission line than is lost at HF. A
10 to 20 dB loss is not uncommon in a 30 meter length of coaxial
line at 450 MHz.  
						

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5-3
At frequencies below 30 MHz, system sensitivity is almost always
limited by receiver noise rather than by noise external to the
antenna (e.g., atmospheric and manmade interference). Generally,
wider modulation or signal bandwidths are employed in VHF and
UHF transmissions than at HF. Since system noise power is directly
proportional to bandwidth, additional antenna gain is necessary to
preserve a usable S/N ratio.
VHF and UHF antenna directivity (gain) aids security by restricting
the amount of power radiated in unwanted directions. Receiver sen-
sitivity is generally poorer at VHF and UHF (with the exception of
high quality state-of-the-art receivers). Obstructions (e.g., build-
ings, trees, hills) may seriously decrease the signal strength avail-
able to the receiving antenna because VHF and UHF signals travel
a straight LOS path. 
Gain
Obtaining communications reliability over difficult VHF and UHF
propagation paths requires considerable attention to the design of
high-gain, directive antenna arrays at least at one end of the com-
munications link. Unlike HF communications, the shorter VHF and
UHF wavelengths support walkie-talkie transceivers and simple
mobile transmission units. Communicating or receiving with such
devices over distances beyond 1 or 2 km requires maximum
antenna gain at the base station site or fixed end of the link.
Directivity
Because VHF and UHF wavelengths are so short, reliable predic-
tion of diffraction, refraction, and reflection effects are not practi-
cal. One must depend entirely on LOS paths. For best results,
attempt to establish VHF and UHF communications paths that are
as free of obstacles as possible. The VHF and UHF wavelengths are 
						

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MCRP 6-22D
short enough that it is possible to construct resonant antenna arrays.
An array provides directivity (the ability to concentrate radiated
energy into a beam that can be aimed at the intended receiver).
Arrays of resonant elements, (e.g., half-wave dipoles), can be con-
structed of rigid metal rods or tubing or of aluminum or copper foil
laid out or pasted on a flat nonconducting surface. Directing power
helps to increase the range of the communications path and tends to
decrease the likelihood of interception or jamming from hostile
radio stations. However, such highly directive antennas place an
added burden on the operator to ensure that the antenna is pointed
properly.
TRANSMISSION LINES
Choosing transmission lines at VHF and UHF depends on many
factors. Generally, twin-lead has much lower loss than small diame-
ter coaxial cable. Twin-lead is preferred over coaxial when trans-
mission line lengths exceed 10 meters. Twin-lead is much more
susceptible to picking up objectionable manmade noise than is well
shielded coaxial cable. Also, most modern VHF and UHF equip-
ment employs unbalanced input and output circuitry with a 50-ohm
impedance. Such equipment requires either using coaxial cable or a
balun to feed a twin-lead or two-wire balanced transmission line.
Noise pickup by twin-lead transmission lines may be considerably
reduced by twisting the line along its length. 
When using twin-lead, the spacing between the wires of the line
should not exceed 0.05 l. If the spacing is an appreciable part of a
wavelength, the line will radiate and receive energy like the
antenna. This effect will alter the intended antenna radiation pat-
tern. To further reduce local noise pickup, keeping twin-lead clear
of metal objects (e.g., gutters and window frames). Twice the wire
spacing in the twin-lead is sufficient clearance. 
						

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5-5
RADIATORS
A radiator is the antenna component that transmits RF energy.
Vertical Radiator
A vertical radiator for general coverage use at UHF should be one-
quarter wavelength long. Longer vertical antennas do not have their
maximum radiation at right angles to the line of the radiator. They
are not practical for use where the greatest possible radiation paral-
lel to the Earth is desired. It is important that the antenna be decou-
pled from the coaxial transmission line. This will prevent unwanted
radiation currents from flowing along the outside of the cable,
which will distort the antenna pattern. Use a sleeve, ground plane,
or counterpoise to perform decoupling.
Cross Section Radiator
Aluminum tubing is commonly used for dipoles and radiation ele-
ments. They are so short that the expense of larger diameter con-
ductors is relatively low. With such conductors, the antenna will
tune much more broadly. This is very desirable, particularly when
an antenna or array is used over an entire frequency band.
Large cross section radiators have a shorter resonant length than a
radiator or mode of small diameter wire. A tubing radiator mode is
seldom longer than 90 percent of a half-wavelength for a dipole at
frequencies above 100 MHz. 
INSULATION
Insulation or dielectric material quality is more important at VHF
and UHF than at frequencies below 30 MHz. Many insulators that
perform well in the HF range are poor or unusable for fabricating 
						

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MCRP 6-22D
antennas operating at frequencies above 100 MHz. Ordinary wood
is a good example. In minimal rainfall climates, using very dry red-
wood, maple, or fir boiled in paraffin wax for several hours is fairly
successful up to frequencies as high as 560 MHz. The neck of a
glass soft drink bottle or other similar items work reasonably well
up to frequencies as high as 1 GHz. Several modern plastics, used
throughout the world, also make excellent insulators (e.g., fiber-
glass, polystyrine, polyethylene, and Styrofoam). Pieces of these
plastics in usable shapes can be found almost everywhere, and, with
a little ingenuity, can be used as insulation in the design of many
VHF and UHF antennas. Avoiding insulation entirely is possible by
choosing an antenna design with elements supported at lower volt-
age (high current) points (e.g., the Yagi antenna).
INTERFERENCE
Obtaining optimum coupling between the antenna and transmission
line and between the transmission line and the receiver or transmit-
ter circuits is a major concern.
Noise
While atmospheric and manmade noise usually limit the ultimate
sensitivity of an HF receiving system, a VHF or UHF receiving sys-
tem is almost always limited by receiver noise. External (atmo-
spheric) noise is virtually nonexistent at frequencies higher than
100 MHz. Automobile ignition and other forms of manmade static
affect frequencies well into the UHF band.
Multipath Interference
VHF and UHF radio waves are highly attenuated when they travel
through most materials. Select a location which is as free as possi-
ble of obstacles in the direction of desired propagation. It is possible 
						

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to use relay stations or carefully placed reflectors when obstacles
interfere with the direct path.
When operating from areas where the transmission path is bounded
rather closely by reflective objects (e.g., buildings or metal towers)
the possibility of multipath exists. Whenever conditions are such
that radio signals travel two or more separate paths from the trans-
mitting to the receiving antenna, a phenomenon known as an inter-
ference pattern is created around the receiving antenna. There will
be zones where the incoming signal is received very strongly (con-
structive interference) with areas of weak signal between (destruc-
tive interference). If each of the two signal has the same strength,
complete cancellation will occur in the destructive interference
zones, and no signal will be received. At VHF and UHF the created
interference patterns are small enough to permit moving out of a
destructive zone and into a constructive zone within the space of
only a meter or so. 
It is difficult to predict the location of an interference pattern. Opti-
mizing antenna location is a must for good results. Sometimes a
secondary path is created as the result of reflection from a moving
object (e.g., an automobile or airplane). The resulting interference
pattern will not be stationary, but will move past the antenna so that
the received signal appears to flutter between good and poor recep-
tion. Multipath problems can be particularly severe when either the
transmitting or receiving antenna is moving. Diversity techniques
such as two separated antennas or circular polarization should help
to alleviate the effects of multipath interference. High gain (highly
directive) antennas, both on the transmitter and the receiver, can
reduce signal loss from multipath interference. 
						

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MCRP 6-22D
Vegetated Areas
VHF and UHF communications through a dense forest over dis-
tances of more than a few kilometers can often be very difficult. In
many tropical regions, trees and underbrush absorb VHF and UHF
radio energy. In addition to the ordinary free space loss between
transmitting and receiving antennas, a radio wave passing through a
forest undergoes an additional loss that is measured in dBs per km.
This extra loss increases rapidly as the transmission frequency
increases.
Near the ground (i.e., antenna heights of less than 3 meters) vertical
polarization is preferred. However, if it is possible to elevate both
receiving and transmitting antennas as much as 10 to 20 meters,
horizontal polarization is preferable to vertical polarization. Con-
siderable reduction in total path loss results if either or both the
transmitting and receiving antennas can be placed above the tree
level through which communications must be made.
Increasing antenna gain may provide an improved signal strength
that exceeds the added antenna gain by reducing the number of
multipath reflections from trees along the propagation path. The
higher gain antenna exhibits a much narrower radiation pattern
which includes fewer trees in its beam. Generally, this effect is most
noticeable with antenna gains higher than 15 dB or azimuthal half-
power beam of less than 35°.
Communications through heavily forested areas over distances
greater than 10 kms may require a transmitter power of at least 10
watts and antenna gains of 10 dB or more, depending on antenna
height, terrain features, type of trees, moisture content, and numer-
ous other factors. If communication is required over distances 
						

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5-9
exceeding 30 kms, it may be necessary to use high-angle iono-
spheric propagation in the 3 to 10 MHz frequency range (i.e., HF)
to obtain a reliable circuit.
ANTENNA TYPES
The vertical whip is the most commonly used antenna. The OE-254
is a broadband, omnidirectional, biconical antenna. Antennas
located in places which are enclosed mostly in a metal shell or con-
tainer (e.g., an automobile) cannot be expected to perform as well
as if located outside the enclosure. Most of the antenna types usable
in the HF range are also usable in the VHF and UHF bands. In the
VHF and UHF ranges, use the same antenna for transmitting and
receiving.
Vertical Whip
It is easy to use and part of every radio set. In mobile situations, it is
the only antenna that can be used. In stationary operations, the ver-
tical whip is not a good choice. It cannot be elevated for good omni-
directional VHF LOS communications, and it radiates in useless
directions if communications are point-to-point.
If the tactical situation prevents using an antenna other than the ver-
tical whip, steps can be taken to improve its performance. Ensure
that the antenna is vertical. This can be a problem when using the
manpack short whip or tape in the prone position. Use the flexible
base on the tape to ensure that the antenna is in a vertical position.
Place a reflector behind the whip to direct radiation in a general
direction. A reflector is a vertical wire or another whip placed one-
quarter wavelength behind the radiating whip. Place the reflector at
the same height as the whip, and insulate it from the ground. The 
						

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MCRP 6-22D
reflector reflects some of the radio energy back towards the whip
and provides a broad beam of energy towards the distant station.
Characteristics are—
Frequency range:30 to 88 MHz
Polarization:Vertical
Power capability:Matched to specific radio
Radiation pattern
Azimuthal (bearing):
Vertical (take-off angle):
OE-254
The OE-254 (fig. 5-1) is scheduled to replace the RC-292. Unlike
the RC-292, the OE-254 does not require tuning for specific bands
and can cover the 30 to 87.975 MHz VHF band without adjust-
ments. Three upward and three downward radial elements simulate
two cones which provide omnidirectional VHF LOS radiation. The
antenna is usually mounted on a 33-foot 8-inch mast for an overall
height of 41 feet 9 inches. The antenna may be installed at lower
heights; however, care should be taken to ensure that the lower and
upper mast adapter assemblies are always used. An 80-foot coaxial
cable comes with the antenna for direct connection to a radio.
Frequency range:30 to 88 MHz
Polarization:Vertical
Power capability:350 watts
Radiation pattern
Azimuthal (bearing):Omnidirectional
Vertical (take-off angle): 
						

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5-11Figure 5-1. Installed OE-254 Antenna.