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

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MCRP 6-22D
Antenna Within Vehicle Interior
Antennas located inside vehicles will lose some radiation through
the window openings. It is difficult to predict how much radiation
will escape or how much the original antenna radiation pattern will
be affected by the enclosure. The pattern modification depends on
the vehicle size and its openings and on the location of the antenna
inside. Select antennas which operate above the cutoff frequency of
the window openings. An opening in a metal container which is less
than 0.5 l in the dimension perpendicular to the plane of polariza-
tion will be severely attenuated or cutoff as it traverses the opening.
Lower frequency waves will suffer even greater attenuation. 
HF Antenna Types Usable at VHF and UHF 
Simple vertical half-wave dipole and quarter-wave monopole
antennas are very popular for omnidirectional transmission and
reception over short-range distances. For longer distances, rhombic
antennas made of wire and somewhat similar in design to HF ver-
sions may be used to good advantage at frequencies as high as 1
GHz. Another HF antenna, the Yagi, is equally popular in the VHF
and UHF ranges. However, while Yagis with more than three or
four elements are seldom used at HF, Yagi designs with as many as
15 elements, or more, are quite common at VHF and UHF.
Dual-Function Antennas
Because there is no sure method of accurately pointing a transmit-
ting antenna, accomplish this task by first using the antenna in
reception and orienting it for the best received signal. Positioning
the antenna in this manner assures that, according to the reciprocity
theorem, the antenna is optimally oriented for transmitting. If two-
way communications are desired (i.e., transmitting and receiving
alternately) a switch to toggle quickly back and forth between trans-
mitter and receiver is required. Design the switch so that it will not 
						

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5-13
upset the impedance on the transmission line, causing unwanted
received or transmitted power loss. When using coaxial cable as
transmission line, use suitable, commercially available, low-loss
coaxial antenna switching relays if possible.
In the 225 to 400 MHz and 450 to 470 MHz frequency ranges, most
antennas are quite small. Mount two identical antennas, using one
for transmitting and one for receiving. Separate transmission lines
may then be used, eliminating the need for an antenna relay.
(reverse blank) 
						

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6-1
Chapter 6
Field Repair and Expedients
Antennas sometimes break or sustain damage, causing poor com-
munications or communications failure. If a spare is available, use
it to replace the damaged antenna. When there is no spare, construct
an expedient antenna. The following paragraphs discuss antenna
and support repair and constructing and adjusting expedient anten-
nas. 
REPAIR TECHNIQUES 
Whip Antennas 
A broken antenna (whip) can be repaired temporarily. If the whip is
broken into two sections, rejoin the sections. Remove the paint and
clean the sections where they will rejoin to ensure a good electrical
connection. Place the sections together, secure them with a pole or
branch, and lash them with bare wire or tape above and below the
break (see fig. 6-1 (A) on page 6-2).
If the whip is badly damaged, use a length of field wire (WD1/TT)
the same length as the original antenna. Remove the insulation from
the lower end of the field wire antenna, twist the conductors to-
gether, stick them in the antenna base connector, and secure with a
wooden block. Use either a pole or a tree to support the antenna
wire (see fig. 6-1 (B) on page 6-2). 
						

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MCRP 6-22D
Wire Antennas 
Expedient wire antenna repair may involve repairing or replacing
the antenna or transmission line wire or repairing or replacing the
assembly that supports the antenna. When one or more antenna
wires are broken, repair the antenna by reconnecting the broken
wires. Lower the antenna to the ground, clean the ends of the wires,
and twist the wires together. Solder the connection if possible. If the
antenna is damaged beyond repair, construct a new one. Make sure
that the substitute wire is the same length as the original. 
Antenna supports may also require repair or replacement. Use a
substitute item in place of a damaged support. If properly insulated,
any material of adequate strength can be used. If the radiating ele-
ment is not properly insulated, field antennas may be shorted to
ground and rendered ineffective. Many commonly found items canFigure 6-1. Emergency Repair of Broken Whip. 
						

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6-3
be used as field expedient insulators (fig. 6-2). The best are plastic
or glass, including plastic spoons, buttons, bottle necks, and plastic
bags. Less effective than plastic or glass, but better than no insula-
tors at all, are wood and rope, or both, in that order. The radiating
element—the actual antenna wire—should touch only the antenna
terminal and be physically separated from all other objects, other
than the supporting insulator.PLASTIC SPOON
NYLON ROPEBUTTON PLASTIC BAG
BOTTLE NECK
WOOD(DRY)
RUBBER OR CLOTH STRIP(DRY)NYLON ROPE
BEST
GOOD
FAIRFigure 6-2. Improvised Insulators. 
						

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MCRP 6-22D
Guys 
Lines used to stabilize antenna supports are called guys. These lines
are usually wire, manila rope, or nylon rope. If a rope breaks, repair
it by tying the two broken ends together. If the rope is too short after
tying, lengthen it by adding another piece of rope or a piece of dry
wood or cloth. If a guy wire breaks, replace it with another piece of
wire. Figure 6-3 shows how to repair a guy line with a spoon. 
Masts 
Some antennas are supported by masts. If a mast breaks, replace it
with another mast the same length. If long poles are not available,
overlap short poles and lash them with rope or wire to provide a
pole of the required length. Figure 6-3 shows how to make an expe-
dient mast repair. 
TIPS ON CONSTRUCTION AND ADJUSTMENT 
Constructing the Antenna
The best kinds of wire for antennas are copper and aluminum. In an
emergency, use any type that is available. The exact length of most
antennas is critical. An expedient antenna should be the same length
as the antenna it replaces. 
Antennas supported by trees can usually survive heavy wind storms
if the trunk of a tree or a strong branch is used as a support. To keep
the antenna taut and to prevent it from breaking or stretching as the
trees sway, attach a spring or old inner tube to one end of the
antenna. Another technique is to pass a rope through a pulley or
eyehook, attach the rope to the end of the antenna, and load the rope
with a heavy weight to keep the antenna tightly drawn.  
						

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6-5
Guys used to hold antenna supports are made of rope or wire. To
ensure that wire guys will not affect antenna operation, cut the wire
into several short lengths and connect the pieces with insulators. Figure 6-3. Repaired Guy Line and Mast. 
						

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MCRP 6-22D
Adjusting the Antenna 
An improvised antenna may change a radio set’s performance. Use
the following methods to determine if the antenna is operating prop-
erly. 
Use a distant station to test the antenna. If the signal received from
this station is strong, the antenna is operating satisfactorily. If the
signal is weak, adjust the antenna and transmission line height and
length to receive the strongest signal at a given receiver volume
control setting. This is the best method of tuning an antenna when
transmission is dangerous or forbidden. 
Most Marine Corps cadets use the transmitter to adjust the antenna.
Set the transmitter controls in position for normal operation. Then,
tune the system by adjusting the antenna height and length and the
transmission line length to obtain the best transmission output.   WARNINGSERIOUS INJURY OR DEATH CAN RESULT FROM CONTACT
WITH THE RADIATING ANTENNA OR MEDIUM- OR HIGH-
POWER TRANSMITTER. TURN THE TRANSMITTER OFFWHILE MAKING ADJUSTMENTS TO THE ANTENNA. 
Impedance-matching a load to its source is an important consider-
ation in transmissions’ systems. If the load and source are mis-
matched, part of the power is reflected back along the transmission
line towards the source. This reflection prevents maximum power
transfer, causes erroneous measurements of other parameters, or
causes circuit damage in high-power applications.  
						

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6-7
The power reflected from the load interferes with the incident (for-
ward) power, creating standing waves of voltages and current along
the line. The ratio of standing-wave maximal to minimal is directly
related to the impedance mismatch of the load. The standing-wave
ratio (SWR) provides the means to determine impedance and mis-
match. 
FIELD EXPEDIENT ANTENNAS
VHF Considerations
SINCGARS VHF radios provide the primary means of communica-
tions means for Marine Corps forces around the world. SINCGARS
radios operate in both single-channel and frequency hopping modes.
It is important for CIS personnel to remember that when using SIN-
CGARS radios in the frequency hopping mode, field expedient VHF
antennas should not be used. CIS personnel should only use the
whip antenna or the OE-254 antenna when operating in the fre-
quency hopping mode.
HF Considerations
Vertical antennas are omnidirectional. They transmit and receive
equally well in all directions. Most manpack portable radios use a
vertical whip antenna. Improvise a by using a metal pipe or rod of
the correct length, held erect by guys. Insulate the lower end of the
antenna from the ground by placing it on a large block of wood or
other insulating material. Support a vertical wire antenna with a tree
or a wooden pole (fig. 6-4 on page 6-8). For short, vertical anten-
nas, use the pole without guys (if properly supported at the base). If
the vertical mast is too short to support the wire upright, modify the
connection at the top of the antenna.  
						

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MCRP 6-22D
End-Fed Half-Wave Antenna 
Construct an expedient, end-fed half-wave antenna from available
materials (e.g., field wire, rope, and wooden insulators). This
antenna’s electrical length is measured from the antenna terminal
on the radio set to the far end of the antenna (fig. 6-5). Figure 6-4. Field Substitutes for Support of
Vertical Wire Atennas.