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

							Antenna Handbook ____________________________ 
2-25
Example: 3 half-wavelengths at 7 MHz is—
Length (meters) = 150 (N - 0.05)
Frequency in MHz
 = 150 (3 - .05)
7
 = 150 x 2.95
7
 = 442.50
7
 = 63.2 meters 
						

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MCRP 6-22D
Section IV. Antenna Orientation
The orientation of an antenna is extremely important. Determining
the position of an antenna in relation to the points of the compass
can make the difference between a marginal and good radio circuit.
AZIMUTH
If the azimuth of the radio’s path is not provided, determine it by the
best available means. The accuracy required depends on the radia-
tion pattern of the directional antenna. If the antenna beamwidth is
very wide (e.g., 90° angle between half-power points) an error of90°90°270°270°0°0°180°180°HALF-POWERPOINTS
RELATIVEPOWERRELATIVEFIELD STRENGTHFigure 2-11. Beamwidth Measured on Relative Field Strength 
and Relative Power Patterns.  
						

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2-27
10° is of little consequence. In transportable operation, the rhombic
and vee antennas may have such a narrow beam that great accuracy
is required to determine azimuth. The antenna should be erected for
the correct azimuth. Great accuracy is not required to erect broad-
beam antennas.
Unless a line of known azimuth is available at the site, the direction
of the path is best determined by a magnetic compass. Figure 2-12
on page 2-28 is a map of magnetic declination, showing the varia-
tion of the compass needle from the true north. When the compass
is held so that the needle points to the direction indicated for the
location on the map, all directions indicated by the compass will be
true.
 Improvement of Marginal Communications
It may not always be feasible to orient directional antennas to the
correct azimuth of the desired radio path, and marginal communica-
tions may suffer. To improve marginal communications—
• Check, tighten, and tape cable couplings and connections. 
• Retune all transmitters and receivers in the circuit. 
• Check that the antennas are adjusted for proper operating fre-
quency. 
• Change the heights of antennas. 
• Move the antenna a short distance away and in different loca-
tions from its original location. 
•  Separate transmitters from receiving equipment, if feasible.  
						

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MCRP 6-22D807060504030201001020304050607080
807060504030201001020304050607080
180
180
180
180
160
160
160
160
140
140
140
140
120
120
120
120
100
100
100
100
8080
8080
6060
6060
4040
4040
2020
2020
00
60E
60W
60W55W50W45W40W35W30W25W20W15W10W5W0E5E10E20E25E40E
55W45W50W40W35W30W25W20W
15W
10W
5W0
5E
50E40E35E30E25E20E15E10E10E15E20E25E30E35E40E50E60E
35E
5W
5W5E
10E15E20E
25E35E30E40E
50E60E
0
10W
30E
SMP
NMPFigure 2-12. Magnetic Declination Over the World. 
						

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2-29
Transmission and Reception of Strong Signals
After an adequate site has been selected and the proper antenna ori-
entation obtained, the signal level at the receiver will be propor-
tional to the strength of the transmitted signal.   WARNINGEXCESSIVE SIGNAL STRENGTH MAY RESULT IN ENEMY IN-
TERCEPT AND INTERFERENCE OR IN YOUR INTERFERENCEWITH ADJACENT FREQUENCIES.
If a high-gain antenna is used, a stronger signal can be obtained.
Losses between the antenna and the equipment can be reduced by
using a high quality transmission line, as short as possible, and
properly matched at both ends.  WARNINGBE EXTREMELY CAREFUL WHEN PUTTING UP, TAKING
DOWN, OR MOVING ANTENNAS LOCATED NEAR HIGH VOLT-
AGE OR COMMECIAL POWER LINES. ANTENNA CONTACT
WITH THESE CAN AND MAY RESULT IN ELECTROCUTION
OR SEVERE INJURY TO PERSONNEL HOLDING THE ANTEN-NA OR THE CONNECTING GUY WIRES AND CABLES.
(reverse blank) 
						

							Chapter 3
Transmission Lines
Transmission lines (antenna feed lines) conduct or guide electrical
energy from the transmitter to the receiver. This chapter is oriented
primarily toward transmission lines with field expedient antennas.
For standard issue radios and antennas, use the issued coaxial cable.
As long as radios, cables, and antennas are maintained in working
order, they will operate as designed and won’t require any adjust-
ments or changes based on the information in this chapter.
PROPERTIES
Transmission Line Types
Transmission lines are classified according to construction and
length, and fall into two main categories: balanced line and unbal-
anced line. The terms balanced and unbalanced describe the rela-
tionship between transmission line conductors and the Earth.
Transmission lines may be classified as resonant or nonresonant
lines, each of which may have advantages over the other under a
given set of circumstances.
Balanced Line. A balanced line is composed of two identical con-
ductors, usually circular wires, separated by air or an insulating
material (dielectric). The voltages between each conductor and
ground produced by an RF wave as it moves down a balanced line, 
						

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MCRP 6-22D
are equal and opposite (i.e. at the moment one of the conductors
supports a positive voltage with respect to ground, the other sup-
ports a negative voltage of equal magnitude). Some balanced lines
carry a third conductor in the form of a braided shield, which acts as
ground. Conductor spacings up to several centimeters are com-
monly used. Figure 3-1 shows balanced and unbalanced lines.
Figure 3-1. Balanced and Unbalanced Transmission Lines.PLASTIC COVERINGINSULATORSBRAIDED WIRE SHIELDPLASTIC 
COVERING
SHIELDED LINE OPEN TWIN LINESCONDUCTING GROUND PLANE
OPEN SINGLE WIRE LINEPLASTIC COVERING
BRAID
SHIELDED LINE(COAX)UNBALANCED TRANSMISSION LINESBALANCED TRANSMISSION LINES 
						

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3-3
Unbalanced Line. The unbalanced line is usually open single-wire
line or coaxial cable. It is one-half of a balanced line.
Nonresonant Line. A nonresonant line is a line that has no stand-
ing waves of current and voltage. It is either infinitely long or is ter-
minated in its characteristic impedance. Because there are no
reflections, all of the energy passed along the line is absorbed by the
load (except for the small amount of energy dissipated by the line).
Resonant Line. A resonant transmission line has standing waves of
current and voltage. The line is of finite length and is not terminated
in its characteristic impedance. Reflections are present. A resonant
line, like a tuned circuit, is resonant at some particular frequency.
The resonant line will present to its source of energy a high or a low
resistive impedance at multiples of a quarter-wavelength. Whether
the impedance is high or low at these points depends on whether the
line is short- or open-circuited at the output end. At points that are
not exact multiples of a quarter-wavelength, the line acts as a capac-
itor or an inductor.
MINIMIZING ENERGY LOSS
To communicate with minimal energy loss, elements such as imped-
ance matching and attenuation (line losses) must be considered.
Impedance 
Currents and waves cannot move from place to place without some
dissipation; their flow is impeded. Impedance describes the nature
and size of whatever impedes their flow. Impedance is an important
consideration in selecting the proper transmission line. 
						

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MCRP 6-22D
A radio wave consists of electric and magnetic fields arranged per-
pendicularly to each other and to the direction the wave travels. The
impedance associated with this wave is the ratio of the potential dif-
ference (voltage) to the current (amperage) at a given point along a
transmission line. The following formula illustrates this.
In transmission lines, because of the length-frequency relationship,
the characteristic impedance is more often discussed in terms of
capacitance and inductance. In conventional circuits that contain
inductors and capacitors, the inductance and capacitance are present
in definite lumps. In an RF transmission line, however, these quan-
tities are distributed throughout the entire line and cannot be sepa-
rated from each other.
If a transmitter is connected to a transmission line that is terminated
in a load whose impedance is different from that of the line, only a
portion of the available energy will be accepted by the load antenna,
and the remainder will be reflected back down the line in the direc-
tion of the transmitter. The energy is actually traveling in both
directions along the line.
If a transmitter is connected to a transmission line terminated in a
load whose impedance exactly equals the impedance of the line, the
line will absorb all of the energy except for that lost in the resistive
and dielectric losses of the line. Current flowing through the line
will be uniformly distributed along its length, and the voltage
between the conductors on the line will be equal at all points. When
this condition exists, the line is said to be perfectly matched and
carries only a forward or incident wave. If the impedance of the
transmission line and the load also equal the internal impedance
(output impedance) of the transmitter, a maximum transfer ofVoltage=Impedance
Current 
						

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3-5
energy (lowest system loss) is achieved (i.e., the transmitter or
receiver, transmission line, and antenna are all the same imped-
ance), and the best possible transfer of signal energy will occur.
Optimizing Line Length
When it is necessary to use a transmission line whose impedance is
significantly different from that of the load, it is possible to make
good use of standing waves and the repetitive impedance variations
along the line to match the antenna to the transmitter or the receiver
to the antenna by cutting the line to a specific length.  An example
is when the only available equipment consists of a 300-ohm twin-
lead transmission line; a 50-ohm half-wave dipole antenna; and a
50-ohm internal impedance transceiver. (Note: The internal imped-
ance of most USMC radios is 50 ohms). Ordinarily, this impedance
combination would result in lost energy that could  affect the qual-
ity of communications. However, if a single frequency is used to
communicate, the length between the antenna and the receiver can
be matched. This occurs because the impedance of the receiver is
repeated at intervals of a half-wavelength along the line.
For end-fed, long-wire antennas, a similar impedance match can be
made by feeding the long wire with a quarter-wavelength piece of
wire that is connected to the transmitter on one end and to the end
of the long wire on the other. The quarter-wavelength section
doesn’t need to be a separate piece of wire. For a 2-wavelength,
long-wire antenna, for example, the wire can be cut to 2 1/4 wave-
lengths. The entire quarter-wavelength section then becomes the
transmission line between the radio and the antenna.