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

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MCRP 6-22D
two conditions represent destructive and constructive combinations
of the reflected and direct waves.
Reflection from the ground at the common midpoint between the
receiving and transmitting antennas may also arrive in a construc-
tive or destructive manner. Generally, in the VHF and UHF range,
the reflected wave is out of phase (destructive) with respect to the
direct wave at vertical angles less than a few degrees above the
horizon. However, since the ground is not a perfect conductor, the
amplitude of the reflected wave seldom approaches that of the
direct wave. Thus, even though the two arrive out of phase, com-
plete cancellation does not occur. Some improvement may result
from using vertical polarization rather than horizontal polarizationTRANSMITTER
RECEIVERFigure 1-6. Reflected Waves. 
						

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over LOS paths because there tends to be less phase difference
between direct and reflected waves. The difference is usually less
than 10 dB, however, in favor of vertical polarization.
Diffraction
Unlike the ship passing beyond the visual horizon, a radio wave
does not fade out completely when it reaches the radio horizon. A
small amount of radio energy travels beyond the radio horizon by a
process called diffraction. Diffraction also occurs when a light
source is held near an opaque object, casting a shadow on a surface
behind it. Near the edge of the shadow a narrow band can be seen
which is neither completely light nor dark. The transition from total
light to total darkness does not occur abruptly, but changes
smoothly as the light is diffracted.
A radio wave passing over either the curved surface of the Earth or
a mountain ridge behaves in much the same fashion as a light wave.
For example, people living in a valley below a high, sharp, moun-
tain ridge can often receive a TV station located many miles below
on the other side. Figure 1-7 illustrates how radio waves from theFigure 1-7. Diffracted Wave. 
						

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MCRP 6-22D
TV station are diffracted by the mountain ridge and bent downward
in the direction of the village. It is emphasized, however, that the
energy decays very rapidly as the angle of propagation departs from
the straight LOS path. Typically, a diffracted signal may undergo a
reduction of 30 to 40 dB by being bent only 5 feet by a mountain
ridge. The actual amount of diffracted signal depends on the shape
of the surface, the frequency, the diffraction angle, and many other
factors. It is sufficient to say that there are times when the use of
diffraction becomes practical as a means for communicating in the
VHF and UHF over long distances.
Tropospheric Refraction, Ducting, and Scattering
Refraction is the bending of a wave as it passes through air layers of
different density (refractive index). In semitropical regions, a layer
of air 5 to 100 meters thick with distinctive characteristics may
form close to the ground, usually the result of a temperature inver-
sion. For example, on an unusually warm day after a rainy spell, the
Sun may heat up the ground and create a layer of warm, moist air.
After sunset, the air a few meters above the ground will cool very
rapidly while the moisture in the air close to the ground serves as a
blanket for the remaining heat. After a few hours, a sizable differ-
ence in temperature may exist between the air near the ground and
the air at a height of 10 to 20 meters, resulting in a marked differ-
ence in air pressure. Thus, the air near the ground is considerably
denser than the air higher up. This condition may exist over an area
of several hundred square kilometers or over a long area of land
near a seacoast. When such an air mass forms, it usually remains
stable until dawn, when the ground begins to cool and the tempera-
ture inversion ends.
When a VHF or UHF radio wave is launched within such air mass,
it may bend or become trapped (forced to follow the inversion
layer). This layer then acts as a duct between the transmitting 
						

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antenna and a distant receiving site. The effects of such ducting can
be seen frequently during the year in certain locations where TV or
VHF FM stations are received over paths of several hundred kilo-
meters. The total path loss within such a duct is usually very low
and may exceed the free space loss by only a few dBs.
It is also possible to communicate over long distances by means of
tropospheric scatter. At altitudes of a few kilometers, the air mass
has varying temperature, pressure, and moisture content. Small
fluctuations in tropospheric characteristics at high altitude create
blobs. Within a blob, the temperature, pressure, and humidity are
different from the surrounding air. If the difference is large enough,
it may modify the refractive index at VHF and UHF. A random dis-
tribution of these blobs exists at various altitudes at all times. If a
high-power transmitter (greater than 1 kW) and high gain antenna
(10 dB or more) are used, sufficient energy may be scattered from
these blobs down to the receiver to make reliable communication
possible over several hundred kilometers. Communication circuits
employing this mode of propagation must use very sensitive receiv-
ers and some form of diversity to reduce the effects of the rapid and
deep fading. Scatter propagation is usually limited to path distances
of less than about 500 km.
NOISE
Noise consists of all undesired radio signals, manmade or natural.
Noise masks and degrades useful information reception. The radio
signal’s strength is of little importance if the signal power is greater
than the received noise power. This is why S/N ratio is the most
important quantity in a receiving system. Increasing receiver ampli-
fication cannot improve the S/N ratio since both signal and noise
will be amplified equally and S/N ratio will remain unchanged.
Normally, receivers have more than enough amplification. 
						

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MCRP 6-22D
Natural Noise
Natural noise has two principle sources: thunderstorms (atmo-
spheric noise) and stars (galactic noise). Both sources generate
sharp pulses of electromagnetic energy over all frequencies. The
pulses propagate according to the same laws as manmade signals,
and receiving systems must accept them along with the desired sig-
nal. Atmospheric noise is dominant from 0 to 5 MHz, and galactic
noise is most important at all higher frequencies. Low frequency
transmitters must generate very strong signals to overcome noise.
Strong signals and strong noise mean that the receiving antenna
does not have to be large to collect a usable signal (a few hundred
microvolts). A 1.5 meter tuned whip will deliver adequately all of
the signals that can be received at frequencies below 1 MHz.
Manmade Noise
Manmade noise is a product of urban civilization that appears wher-
ever electric power is used. It is generated almost anywhere that
there is an electric arc (e.g., automobile ignition systems, power
lines, motors, arc welders, fluorescent lights). Each source is small,
but there are so many that together they can completely hide a weak
signal that would be above the natural noise in rural areas. Man-
made noise is troublesome when the receiving antenna is near the
source, but being near the source gives the noise waves characteris-
tics that can be exploited. Waves near a source tend to be vertically
polarized. A horizontally polarized receiving antenna will generally
receive less noise than a vertically polarized antenna.
Manmade noise currents are induced by any conductors near the
source, including the antenna, transmission line, and equipment
cases. If the antenna and transmission line are balanced with respect
to the ground, then the noise voltages will be balanced and cancel 
						

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with respect to the receiver input terminals (zero voltage across ter-
minals), and this noise will not be received. Near-perfect balance is
difficult to achieve, but any balance helps. 
Other ways to avoid manmade noise are to locate the most trouble-
some sources and turn them off, or move the receiving system away
from them. Moving a kilometer away from a busy street or highway
will significantly reduce noise. Although broadband receiving
antennas are convenient because they do not have to be tuned to
each working frequency, sometimes a narrowband antenna can
make the difference between communicating and not communicat-
ing. The HF band is now so crowded with users that interference
and noise, not signal strength, are the main reasons for poor com-
munications. A narrowband antenna will reject strong interfering
signals near the desired frequency and help maintain good commu-
nications.
(reverse blank) 
						

							Chapter 2
Antenna Fundamentals
All radios, whether transmitting or receiving, require some sort of
antenna. The antenna accepts power from the transmitter and
launches it into space as an electromagnetic or radio wave. At the
receiving end of the circuit, a similar antenna collects energy from
the passing electromagnetic wave and converts it into an alternating
electric current or signal that the receiver can detect. 
How well antennas launch and collect electromagnetic waves
directly influences communications reliability and quality. The
function of an antenna depends on whether it is transmitting or
receiving.
A transmitting antenna transforms the output radio frequency (RF)
energy produced by a radio transmitter (RF output power) into an
electromagnetic field that is radiated through space. The transmit-
ting antenna converts energy from one form to another form. The
receiving antenna reverses this process. It transforms the electro-
magnetic field into RF energy that is delivered to a radio receiver.  
						

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MCRP 6-22D
Section I. Concepts and Terms
To select the right antennas for a radio circuit, certain concepts and
terms must be understood. This section defines several basic terms
and relationships which will help the reader understand antenna
fundamentals. These include: forming a radio wave, radiation fields
and patterns, polarization, directionality, resonance, reception, reci-
procity, impedance, bandwidth, gain, and take-off angle.
FORMING A RADIO WAVE
When an alternating electric current flows through a conductor
(wire), electric and magnetic fields are created around the conduc-
tor. If the length of the conductor is very short compared to a wave-
length, the electric and magnetic fields will generally die out within
a distance of one or two wavelengths.  However, as the conductor is
lengthened, the intensity of the fields enlarge. Thus, an ever-
increasing amount of energy escapes into space. When the length of
the wire approaches one-half of a wavelength at the frequency of
the applied alternating current, most of the energy will escape in the
form of electromagnetic radiation. For effective communications to
occur, the following must exist:  alternating electric energy in the
form of a transmitter, a conductor or a wire, an electric current
flowing through the wire, and the generation of both electric and
magnetic fields in the space surrounding the wire.
RADIATION
Once a wire is connected to a transmitter and properly grounded, it
begins to oscillate electrically, causing the wave to convert nearly
all of the transmitter power into an electromagnetic radio wave. The
electromagnetic energy is created by the alternating flow of elec-
trons impressed on the bottom end of the wire. The electrons travel 
						

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upward on the wire to the top, where they have no place to go and
are bounced back toward the lower end. As the electrons reach the
lower end in phase, i.e., in step with the radio energy then being
applied by the transmitter, the energy of their motion is strongly
reinforced as they bounce back upward along the wire. This regen-
erative process sustains the oscillation. The wire is resonant at the
frequency at which the source of energy is alternating.
The radio power supplied to a simple wire antenna appears nearly
equally distributed throughout its length. The energy stored at any
location along the wire is equal to the product of the voltage and the
current at that point. If the voltage is high at a given point, the cur-
rent must be low. If the current is high, the voltage must be low. The
electric current is maximum near the bottom end of the wire.
Radiation Fields 
When RF power is delivered to an antenna, two fields evolve. One
is an induction field, which is associated with the stored energy; the
other is a radiation field. At the antenna, the intensities of these
fields are large and are proportional to the amount of RF power
delivered to the antenna. At a short distance from the antenna and
beyond, only the radiation field remains. This field is composed of
an electric component and a magnetic component (see fig. 2-1 on
page 2-4). 
The electric and magnetic fields (components) radiated from an
antenna form the electromagnetic field. The electromagnetic field
transmits and receives electromagnetic energy through free space.
A radio wave is a moving electromagnetic field that has velocity in
the direction of travel and components of electric intensity and
magnetic intensity arranged at right angles to each other. 
						

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MCRP 6-22D
 
Figure 2-1. Radiation Fields.
Radiation Patterns
The radio signals radiated by an antenna form an electromagnetic
field with a definite pattern, depending on the type of antenna used.
This radiation pattern shows the antenna’s directional characteris-
tics. A vertical antenna radiates energy equally in all directions
(omnidirectional), a horizontal antenna is mainly bidirectional, and
a unidirectional antenna radiates energy in one direction. However,
the patterns are usually distorted by nearby obstructions or terrain
features. The full- or solid-radiation pattern is represented as a
three-dimensional figure that looks somewhat like a doughnut with
a transmitting antenna in the center (fig 2-2).TRANSMITTING 
     ANTENNA
RECEIVING
ANTENNA
ELECTRIC FIELD
SIGNAL VOLTAGE
MAGNETIC FIELD
DIRECTION OF TRAVEL