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

							Antenna Handbook ____________________________ 
2-15
transmitter power will dissipate as heat into the ground rather than
radiated as intended. Therefore, it is essential to provide as good a
ground or artificial ground (counterpoise) connection as possible
when using a vertical whip or monopole.
The amount of power an antenna radiates depends on the amount of
current which flows in it. Maximum power is radiated when there is
maximum current flowing. Maximum current flows when the
impedance is minimized—when the antenna is resonated so that its
impedance is pure resistance. (When capacitive reactance is made
equal to inductive reactance, they cancel each other, and impedance
equals pure resistance.)
BANDWIDTH
The bandwidth of an antenna is that frequency range over which it
will perform within certain specified limits. These limits are with
respect to impedance match, gain, and/or radiation pattern charac-
teristics.  Typical specification limits are—
• An impedance mismatch of less than 2:1 relative to some stan-
dard impedance such as 50 ohms.
• A loss in gain or efficiency of no more than 3 dB.
• A directivity pattern whose main beam is 13 dB greater than
any of the side lobes, and a back lobe at least 15 dB below the
main beam.
•  Bandwidth is measured by changing the frequency of a con-
stant-strength test signal above and below center frequency and
measuring power output. The high and low frequencies, where
power is one-half (-3 dB) of what it was at center, define the
bandwidth.  It is expressed as frequency (high minus low) or in
percentage (high-low/center x 100%). 
						

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MCRP 6-22D
In the radio communication process, intelligence changes from
speech or writing into a low frequency signal that is used to modu-
late, or cause change, in a much higher frequency radio signal.
When transmitted by an antenna, these radio signals carry the intel-
ligence to the receiving antenna, where it is picked up and recon-
verted into the original speech or writing. There are natural laws
which limit the amount of intelligence or signal that can be trans-
mitted and received at a given time. The more words per minute,
the higher the rate and the modulation frequency, so a wider or
greater bandwidth is needed. To transmit and receive all the intelli-
gence necessary, the antenna bandwidth must be as wide or wider
than the signal bandwidth, otherwise it will limit the signal frequen-
cies, causing voices and writing to be unintelligible. Too wide a
bandwidth is also bad, since it accepts extra voices and will degrade
the S/N ratio. Figure 2-7 shows how signal bandwidth is defined
and gives some examples of bandwidth required to transmit ordi-
nary types of intelligence.
GAIN
The antenna’s gain depends on its design. Transmitting antennas are
designed for high efficiency in radiating energy, and receiving
antennas are designed for high efficiency in picking up (gaining)
energy. On many radio circuits, transmission is required between a
transmitter and only one receiving station. Energy is radiated in one
direction because it is useful only in that direction. Directional
receiving antennas increase the energy gain in the favored direction
and reduce the reception of unwanted noise and signals from other
directions. Transmitting and receiving antennas should have small
energy losses and should be efficient as radiators and receptors.  
						

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2-17
Figure 2-7. Bandwidth.BANDWIDTH=1.34 MHz
FREQUENCY, MHz 2.02.22.42.6
2.83.0
3.23.43.6
3.84.0 0 50 100
TUNED CENTER FREQUENCY
-15
-6
-9 -30
POWER OUTPUT
PERCENT OF POWER AT 3.0 MHz
DECIBELSINTELLIGENCEBANDWIDTH
Voice, AM6.0 KHz
Voice, FM46.0 KHz
One microsecond pulses10,000.0 KHz
Bandwidths necessary to transmit and receive 
some ordinary kinds of intelligence 
						

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MCRP 6-22D
TAKE-OFF ANGLE
The antenna’s take-off angle is the angle above the horizon that an
antenna radiates the largest amount of energy (see fig. 2-8). VHF
communications antennas are designed so that the energy is radi-
ated parallel to the Earth (do not confuse take-off angle and polar-
ization). The take-off angle of an HF communications antenna can
determine whether a circuit is successful or not. HF sky wave
antennas are designed for specific take-off angles, depending on the
circuit distance. High take-off angles are used for short-range com-
munications, and low take-off angles are used for long-range com-
munications.
Figure 2-8. Take-Off Angle.ANTENNAMAIN ENERGYFROM ANTENNATAKE-OFF
ANGLE 
						

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Section II. Ground Effects 
Since most tactical antennas are erected over the Earth and not out
in free space, except for those on satellites, the ground will alter the
free space radiation patterns of antennas. The ground will also
affect some of the electrical characteristics of an antenna. It has the
greatest effect on those antennas that must be mounted relatively
close to the ground in terms of wavelength. For example, medium-
and high-frequency antennas, elevated above the ground by only a
fraction of a wavelength, will have radiation patterns that are quite
different from the free-space patterns. 
GROUNDED ANTENNA THEORY
The ground is a good conductor for medium and low frequencies
and acts as a large mirror for the radiated energy. The ground
reflects a large amount of energy that is radiated downward from an
antenna mounted over it. Using this characteristic of the ground, an
antenna only a quarter-wavelength long can be made into the equiv-
alent of a half-wave antenna. A quarter-wave antenna erected verti-
cally, with its lower end connected electrically to the ground (fig.
2-9 on page 2-20), behaves like a half-wave antenna. The ground
takes the place of the missing quarter-wavelength, and the reflec-
tions supply that part of the radiated energy that normally would be
supplied by the lower half of an ungrounded half-wave antenna. 
						

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MCRP 6-22D
 
Figure 2-9.  Quarter-Wave Antenna
Connected to Ground.
TYPES OF GROUNDS
When grounded antennas are used, it is especially important that the
ground has as high a conductivity as possible. This reduces ground
losses and provides the best possible reflecting surface for the
down-going radiated energy from the antenna. At low and medium
frequencies, the ground acts as a good conductor. The ground con-
nection must be made in such a way as to introduce the least possi-
ble amount of resistance to ground. At higher frequencies, artificial
grounds constructed of large metal surfaces are common. 1/4QUARTER-WAVEVERTICAL ANTENNAIMAGE ANTENNAEARTH 
						

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2-21
The ground connections take many forms, depending on the type of
installation and the loss that can be tolerated. In many simple field
installations, the ground connection is made by one or more metal
rods driven into the soil. Where more satisfactory arrangements
cannot be made, ground leads can be connected to existing devices
which are grounded. Metal structures or underground pipe systems
are commonly used as ground connections. In an emergency, a
ground connection can be made by forcing one or more bayonets
into the soil. 
When an antenna must be erected over soil with low conductivity,
treat the soil to reduce resistance. Treat the soil with substances that
are highly conductive when in solution. Some of these substances,
listed in order of preference, are sodium chloride (common salt),
calcium chloride, copper sulfate (blue vitriol), magnesium sulfate
(Epsom salt), and potassium nitrate (saltpeter). The amount
required depends on the type of soil and its moisture content.   WARNINGWHEN THESE SUBSTANCES ARE USED, IT IS IMPORTANT
THAT THEY DO NOT GET INTO NEARBY DRINKING WATERSUPPLIES.
For simple installations in the field, a single ground rod can be fab-
ricated from pipe or conduit. It is important that a low resistance
connection be made between the ground wire and the ground rod.
The rod should be cleaned thoroughly by scraping and sandpaper-
ing at the point where the connection is to be made, and a clean
ground clamp should be installed. A ground wire can then be sol-
dered or joined to the clamp. This joint should be covered with tape
to prevent an increase in resistance because of oxidation.  
						

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MCRP 6-22D
Counterpoise
When an actual ground connection cannot be used because of the
high resistance of the soil or because a large buried ground system
is not practical, a counterpoise may be used to replace the usual
direct ground connection. The counterpoise (fig. 2-10) consists of a
device made of wire, which is erected a short distance above the
ground and insulated from it. The size of the counterpoise should be
at least equal to or larger than the size of the antenna. 
When the antenna is mounted vertically, the counterpoise should be
made into a simple geometric pattern. Perfect symmetry is not
required. The counterpoise appears to the antenna as an artificial
ground that helps to produce the required radiation pattern. 
Figure 2-10. Wire Counterpoise. ANTENNACOUNTERPOISESUPPORT 
						

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In some VHF antenna installations on vehicles, the metal roof of the
vehicle (or shelter) is used as a counterpoise for the antenna. Small
counterpoises of metal mesh are sometimes used with special VHF
antennas that must be located a considerable distance above the
ground. 
Ground Screen
A ground screen consists of a fairly large area of metal mesh or
screen that is laid on the surface of the ground under the antenna.
There are two specific advantages to using ground screens. First,
the ground screen reduces ground absorption losses that occur when
an antenna is erected over ground with poor conductivity. Second,
the height of the antenna can be set accurately, and the radiation
resistance of the antenna can be determined more accurately. 
						

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MCRP 6-22D
Section III. Calculating Antenna Length
An antenna’s length must be considered in two ways: physical and
electrical. The two are never the same. The reduced velocity of the
wave on the antenna and a capacitive effect (end effect) make the
antenna seem longer electrically than physically. The contributing
factors are the ratio of the diameter of the antenna to its length and
the capacitive effect of terminal equipment (e.g., insulators or
clamps) used to support the antenna. 
To calculate the antenna’s physical length, use a correction of 0.95
for frequencies between 3 and 50 MHz. The figures given are for a
half-wave antenna. 
The length of a long-wire antenna (one wavelength or longer) for
harmonic operation is calculated by using the following formula,
where N = number of half-wavelengths in the total length of the
antenna.Length (meters) = 150 x 0.95=  142.50
Frequency in MHzFrequency in MHz
Length (feet) = 492 x 0.95=  468
Frequency in MHzFrequency in MHz
Length (meters) = 150 (N - 0.05)
Frequency in MHz
Length (feet) = 492 (N - 0.05)
Frequency in MHz