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

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MCRP 6-22D
Attenuation
Transmission lines do not transfer all of the energy applied at one
end of the line to the opposite end. Attenuation is energy that is lost
when converted into heart, partially due to conductor (wire) resis-
tance. More energy is lost due to the insulation material used to
space the conductors (dielectric loss). Some insulating materials
(e.g., Teflon) have extremely low loss while others (e.g., rubber or
wood) have relatively high loss, especially at frequencies above
about 30 MHz. Old, dry wood (especially redwood) may be boiled
in paraffin or bee’s wax to make a fairly good insulator at frequen-
cies up to about 200 MHz. Polyethylene, a common insulation
material used in coaxial cables, has an average loss of about twice
that of Teflon in the 100-MHz range for cables having a diameter of
less than about one centimeter. Dry air is a better insulator than
most solid, liquid, or flexible materials. Some inert gases (e.g.,
nitrogen, helium, and argon) are superior to air and are often used
under pressure to fill coaxial cables used with high-powered trans-
mitters.
Since attenuation results from conductor resistance and dielectric
loss, transmission lines using large diameter conductors lose less
energy than cables having small diameter conductors. Also, trans-
mission lines having a large spacing between conductors (high
impedance) will lose less energy than those with a smaller spacing
(lower impedance) since they carry smaller currents and there is
less energy lost in conductor resistance. Thus, 300-ohm twin-lead
has less loss than coaxial cable at most frequencies. Among coaxial
cables, the larger the diameter, the lower the loss, assuming the
same insulator is used. It is also true that coaxial cable, which has
an impedance of 75 ohm, has slightly lower loss than 50-ohm cable,
when both cables have about the same diameter. When there is a
choice, it is best to use the largest available transmission line which
matches the impedance of both the antenna and transmitter. 
						

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3-7
MAKING THE BEST USE OF AVAILABLE 
TRANSMISSION LINES
It is often necessary to feed a balanced antenna (e.g., horizontal
dipole) with coaxial cable. While this is not considered good prac-
tice, it will perform satisfactorily under most conditions. When
coaxial cable is used for this purpose, it should run perpendicular to
the dipole wires for a distance greater than one-half of the length of
the dipole. This will help to prevent unwanted RF power from being
induced on the outside shield of the cable. It is also advisable to
make sure that the total length of the coaxial cable and one side of
the antenna is not equal to a half-wavelength or any multiple
thereof. This will prevent the outside conductor from becoming res-
onant and acting as a radiating part of the antenna. The same pre-
caution should be taken with twin-lead transmission line.
Occasionally, it may also be necessary to feed an unbalanced
antenna (e.g., a whip with twin-lead or balanced line). Again this is
not considered good practice, but the bad effects can be minimized
if care is taken. If the transmitter has a balanced output circuit, little
difficulty will be experienced. However if the output is unbalanced,
the hot terminal or coaxial center at the transmitter output must be
connected to the same wire of the twin-lead as is the vertical whip
at the other end of the twin-lead. This ensures that the ground side
of the transmitter output is connected to the side of the twin-lead
that goes to the ground side of the unbalanced antenna. 
If the twin-lead is reversed and the antenna ground terminal is con-
nected to the hot terminal of the transmitter, a large portion of the
transmitter output may be wasted, making communications either
difficult or impossible. Twin-lead of the type commonly used with
television sets usually has one tinned and one bare copper conduc-
tor. This color coding readily permits correct connection of the
transmitter to the antenna. It is also advisable to make the length of 
						

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MCRP 6-22D
the twin-lead equal to a half-wavelength or any multiple of a half-
wavelength. When possible, the twin-lead should be twisted so that
it forms a long helix with about one twist every thirty centimeters,
or so. Twisting helps to prevent transmission line radiation and
reduces noise pickup when receiving.
Twin-Lead Limitations
It is generally best not to use twin-lead or balanced line at frequen-
cies higher than about 200 MHz for three reasons.
First, the spacing between the two wires becomes sufficiently large
in terms of a wavelength that radiation from the line occurs. When
lengths over 30 meters are employed, this radiation may represent a
significant loss of energy. 
Second, if the twin-lead or balanced lines must come in close con-
tact (less than 2 or 3 cm) with metal, masonry, or wood surfaces,
additional losses will be encountered due to the substantial imped-
ance change which takes place along the section of the line next to
the surface. This mismatch loss becomes apparent at frequencies
above 200 MHz because the length of the section affected becomes
a substantial portion of a wavelength long. At lower frequencies,
the section of line involved is too short to be seriously affected. 
Third, twin-lead picks up more locally generated interference than
coaxial cable since the outer conductor of the coaxial cable acts as a
shield for the center conductor. Radiation and noise pickup by twin-
lead can be partially prevented by twisting it once every 20 or 30
centimeters.
When using common, TV twin-lead (300 ohm), preference should
be given to the deep brown rather than the light, colorless variety.
The darker colored twin-lead withstands the effects of sunlight and 
						

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3-9
moisture after prolonged outdoor exposure much better than the
clear type. The clear, colorless, twin-lead tends to crack after a few
months exposure to the Sun. It also begins to absorb moisture which
greatly increases energy loss.
Directly Connecting the Transceiver and Antenna
In many instances the transmitter or receiver may be connected
directly to the antenna wire without using a transmission line. This
is particularly true with indoor antennas in the HF range and with
many VHF whip antennas designed for use with manpack trans-
ceivers.
When a direct connection is made between a transmitter and the
antenna at frequencies below 30 MHz or where the length of the
antenna wire is much shorter than 0.25 l, the output circuit of the
transmitter usually contains a matching device which may be used
to tune the antenna efficiently to resonance. This tuning actually
matches the impedance of the antenna to the output impedance of
the transmitter. 
When a VHF transceiver is designed to connect directly to a short
whip or self-contained, collapsible rod, the output circuit is usually
designed to accommodate the range of impedances likely to be
encountered at the base of the whip or rod. 
The efficiency of these devices is usually low since the ground
return circuit for the antenna may range from nothing more than the
case of the transmitter to the hand and body of an individual hold-
ing the device. The impedance of the antenna may vary with fre-
quency over a range of 5 to 1 or greater. Thus, antenna efficiencies
of from 25 to 50 percent are not uncommon with such devices. 
						

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MCRP 6-22D
BALUNS
There are times when a balanced antenna must be used with a trans-
mitter or receiver which has an unbalanced output or input circuit.
While it is possible to make a direct connection between balanced
and unbalanced devices, it is certainly not good practice. A balun
can be used to transform energy from balanced to unbalanced
devices and vice versa. 
The word balun comes from balanced to unbalanced transformer.
Many balun types are easily constructed in the field. Using them
can often make the difference between marginal communications
and completely solid contact. This may be especially true in the
receiving case where a balun can result in a substantial reduction in
the amount of manmade noise and interference received by a poorly
balanced antenna system. The balun is usually placed at the antenna
terminals so that a coaxial transmission line can be used. However,
it is possible to feed a balanced antenna with twin-lead or any kind
of balanced line, and the balun is placed near the transmitter or
receiver terminals (see figs. 3-2 and 3-3).
Figure 3-2. A Balun Placed at the Antenna.BALANCED ANTENNABALUNCOAX TRANSMITTER OR
RECEIVER 
						

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3-11
Figure 3-3. Balun Placed at the Transmitter or Receiver.
Cable Connectors
Cable connector fittings are available for all standard transmission
lines. Although it takes some time to prepare the cable ends and sol-
der the fittings on, it may be well worth it later if rapid assembly or
disassembly of a communications system is necessary.
Balanced Antenna
It is highly desirable to use a receiving antenna which is balanced
with respect to ground. This insures the antenna’s insensitivity to
locally generated noise. Balancing only the receiving antenna is not
enough. The entire receiving system must be balanced to success-
fully reject noise. The antenna should be connected to its receiver
so as not to disrupt the antenna’s balance. Receivers are supplied
with either balanced or unbalanced antenna terminals, and some-
times both.ANTENNACOAXTRANSMITTER OR
RECEIVERBALUNTWIN LEAD(reverse blank) 
						

							Chapter 4
 HF Antenna Selection
The HF portion of the radio spectrum is very important to commu-
nications. Radio waves in the 3 to 30 MHz frequency range are the
only ones that are capable of being reflected or returned to Earth by
the ionosphere with predictable regularity. To optimize the proba-
bility of a successful sky wave communications link, select the fre-
quency and take-off angle that is most appropriate for the time of
day transmission is to take place.
Merely selecting an antenna that radiates at a high elevation angle is
not enough to ensure optimum communications. Various large con-
ducting objects, in particular the Earth’s surface, will modify an
antenna’s radiation pattern. Sometimes, nearby scattering objects
may modify the antenna’s pattern favorably by concentrating more
power toward the receiving antenna. Often, the pattern alteration
results in less signal transmitted toward the receiver. 
When selecting an antenna site, the operator should avoid as many
scattering objects as possible. How the Earth’s surface affects the
radiation pattern depends on the antenna’s height. The optimum
height above electrical ground is about 0.4 l at the transmission fre-
quency. However, the exact height is not critical.
Although NVIS is the chief mode of short-haul HF propagation, the
ground wave and direction (LOS) modes are also useful over short
paths. How far a ground wave is useful depends on the electrical
conductivity of the terrain or body of water over which it travels.
The direct wave is useful only to the radio horizon, which extends
slightly beyond the visual horizon. 
						

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MCRP 6-22D
ANTENNA SELECTION PROCEDURE
Selecting the right antenna for an HF radio circuit is very important.
When selecting an HF antenna, first consider the type of propaga-
tion. Ground wave propagation requires low take-off angle and ver-
tically polarized antennas. The whip antenna included with all radio
sets provides good omnidirectional ground wave radiation. 
If a directional antenna is needed, select one with good, low-angle
vertical radiation. An example is an AN/MRC-138 with its compo-
nent 32-foot whip set up on a 200-mile circuit. With the radiation
characteristics of the whip antenna, the radiated power of the trans-
mitter or whip could be 300 watts for the take-off angle required for
a 200-mile circuit. 
If a 35-foot half-wave horizontal dipole is used instead of the whip,
the radiated power would be 5,000 watts. By using the dipole
instead of the whip, the radiated power is increased more than 16
times. A circuit with 5,000 watts of radiated power produces a bet-
ter signal than a 300-watt circuit using the same frequency.
Selecting an antenna for sky wave propagation is more complex.
First, find the circuit (range) distance so that the required take-off
angle can be determined. Table 4-1 gives approximate take-off
angles for daytime and nighttime sky wave propagation. A circuit
distance of 966 kilometers (600 miles) requires a take-off angle of
approximately 25° during the day and 40° at night. Select a high-
gain antenna (25° to 40°). If propagation predictions are available,
skip this step, since the predictions will probably give the take-off
angles required.
Next, determine the required coverage. A radio circuit with mobile
(vehicular) stations or several stations at different directions from the
transmitter requires an omnidirectional antenna. A point-to-point 
						

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4-3
circuit uses either a bidirectional or a directional antenna. Normally,
the receiving station locations dictate this choice (see table 4-1).
Before selecting a specific antenna, examine the available construc-
tion materials. At least two supports are needed to erect a horizontal
dipole, with a third support in the middle for frequencies of 5 MHz
or less. If these supports or other items to use as supports are
unavailable, the dipole cannot be constructed, and another antenna
should be selected. Examine the proposed antenna site to determine
if the antenna will fit. If not, select a different antenna. Table 4-1. Take-Off Angle vs. Distance.
Take-off Angle
(Degrees)Distance
F2 Region DaytimeF2 Region Nighttime
kilometersmileskilometersmiles
03220200045082800
52415150037032300
101932120028981800
15145090022541400
20112770017711100
2596660016101000
307254501328825
356444001127700
40564350966600
45443275805500
50403250685425
60258160443275
7015395290180
80805014590
900000 
						

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MCRP 6-22D
The site is another consideration. Usually, the tactical situation
determines the position of the communications antennas. The ideal
setting would be a clear, flat area (i.e., no trees, buildings, fences,
power lines, or mountains). Unfortunately, an ideal location is sel-
dom available. Choose the clearest, flatest area possible. If the pro-
posed site is obstructed, try to maintain the horizontal distance
listed in table 4-2. Often, an antenna must be constructed on irregu-
lar sites. This does not mean that the antenna will not work. It
means that the site will affect the antenna’s pattern and function. 
Table 4-2. Assuming a 30-Foot Antenna and
75-Foot Trees
Take-Off Angle
(Degrees)Required Horizontal Distance
from Trees
018kilometers
51932meters
10966meters
15644meters
20483meters
25370meters
30298meters
35241meters
40201meters
45169meters
50145meters
60105meters
7064meters
8032meters
900meters