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

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MCRP 6-22D
Ground Reflected Wave. The ground reflected wave reaches the
receiving antenna after being reflected from the Earth’s surface.
Cancellation of the radio signal can occur when the ground
reflected component and the direct wave component arrive at the
receiving antenna at the same time and are 180° out of phase with
each other.
Surface Wave. The surface wave follows the Earth’s curvature. It
is affected by the Earth’s conductivity and dielectric constant. 
Frequency Characteristics Of Ground Waves. Various frequen-
cies determine which wave component will prevail along any given
signal path. For example, when the Earth’s conductivity is high and
the frequency of a radiated signal is low, the surface wave is the
predominant component. For frequencies below 10 MHz, the sur-
face wave is sometimes the predominant component. However,
above 10 MHz, the losses that are sustained by the surface wave
component are so great that the other components (direct wave and
sky wave) become predominant.
At frequencies of 30 to 300 kHz, ground losses are very small, so
the surface wave component follows the Earth’s curvature. It can be
used for long-distance communications provided the radio operator
has enough power from the transmitter. The frequencies 300 kHz to
3 MHz are used for long-distance communications over sea water
and for medium-distance communications over land.
At high frequencies, 3 to 30 MHz, the ground’s conductivity is
extremely important, especially above 10 MHz where the dielectric
constant or conductivity of the Earth’s surface determines how
much signal absorption occurs. In general, the signal is strongest at
the lower frequencies when the surface over which it travels has a
high dielectric constant and conductivity. 
						

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1-11
Earth’s Surface Conductivity. The dielectric constant or Earth’s
surface conductivity determines how much of the surface wave
signal energy will be absorbed or lost. Although the Earth’s surface
conductivity as a whole is generally poor, the conductivity of vary-
ing surface conditions, when compared one with an other, would be
as stated in table 1-3. 
Sky Wave Propagation. Radio communications that use sky wave
propagation depend on the ionosphere to provide the signal path
between the transmitting and receiving antennas. 
Ionospheric Structure. The ionosphere has four distinct layers. In
the order of increasing heights and decreasing molecular densities,
these layers are D, E, F1, and F2. During the day, when the rays of
the Sun are directed toward that portion of the atmosphere, all four
layers may be present. At night, the F1 and F2 layers seem to merge
into a single F layer, and the D and E layers fade out. The actual
number of layers, their height above the Earth, and their relative
intensity of ionization vary constantly.Table 1-3. Surface Conductivity.
Surface TypeRelative Conductivity
Large body fresh waterVery good
Ocean or sea waterGood
Flat or hilly loamy soilFair
Rocky terrainPoor
DesertPoor
JungleVery poor 
						

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MCRP 6-22D
The D layer exists only during the day and has little effect in bend-
ing the paths of HF radio waves. The main effect of the D layer is to
attenuate HF waves when the transmission path is in sunlit regions. 
The E layer is used during the day for HF radio transmission over
intermediate distances (less than 2,400 km/1,500 miles [mi]). At
night the intensity of the E layer decreases, and it becomes useless
for radio transmission.
The F layer exists at heights up to 380 km/240 mi above the Earth
and is ionized all the time. It has two well-defined layers (F1 and
F2) during the day, and one layer (F) at night. At night the F layer
remains at a height of about 260 km/170 mi and is useful for long-
range radio communications (over 2,400 km/1,500 mi). The F2
layer is the most useful for long-range radio communications, even
though its degree of ionization varies appreciably from day to day
(fig. 1-5).
The Earth’s rotation around the Sun and changes in the Sun’s activ-
ity contribute to ionospheric variations. There are two main classes
of these variations: regular (predictable) and irregular, occuring
from abnormal behavior of the Sun.
Regular Ionospheric Variations. The four regular variations are—
•Daily: caused by the rotation of the Earth.
•Seasonal: caused by the north and south progression of the Sun.
•27-day: caused by the rotation of the Sun on its axis.
•1-year: caused by the sunspot activity cycle going from maxi-
mum through minimum back to maximum levels of intensity.  
						

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1-13F1 & F2
F1 E 
D 
F2 COMBINE
F2 250-500 km (250-420 km at night)
F1 200-250 km
E     90-130 km
D       75-90 kmSUN
AT NIGHTF2F1E
DDAYLIGHT POSITIONSFigure 1-5. Ionospheric Structure. 
						

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MCRP 6-22D
Irregular Ionospheric Variations. In planning a communications
system, the current status of the four regular variations must be
anticipated. There are also unpredictable irregular variations that
must be considered. They have a degrading effect (at times blank-
ing out communications) which cannot be controlled or compen-
sated for at the present time. Some irregular variations are— 
•Sporadic E. When excessively ionized, the E layer often blanks
out the reflections from the higher layers. It can also cause
unexpected propagation of signals hundreds of miles beyond
the normal range. This effect can occur at any time.
•Sudden ionospheric disturbance (SID). A sudden ionospheric
disturbance coincides with a bright solar eruption and causes
abnormal ionization of the D layer. This effect causes total
absorption of all frequencies above approximately 1 MHz. It
can occur without warning during daylight hours and can last
from a few minutes to several hours. When it occurs, receivers
seem to go dead.
•Ionospheric storms. During these storms, sky wave reception
above approximately 1.5 MHz shows low intensity and is
subject to a type of rapid blasting and fading called flutter fad-
ing. These storms may last from several hours to days and usu-
ally extend over the entire Earth. 
Sunspots. Sunspots generate bursts of radiation that cause high
levels of ionization. The more sunspots, the greater the ionization.
During periods of low sunspot activity, frequencies above 20 MHz
tend to be unusable because the E and F layers are too weakly ion-
ized to reflect signals back to Earth. At the peak of the sunspot
cycle, however, it is not unusual to have worldwide propagation on
frequencies above 30 MHz. 
						

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Frequency Characteristics in the Ionosphere. The range of long-
distance radio transmission is determined primarily by the ioniza-
tion density of each layer. The higher the frequency, the greater the
ionization density required to reflect radio waves back to Earth. The
upper (E and F) layers reflect the higher frequencies because they
are the most highly ionized. The D layer, which is the least ionized,
does not reflect frequencies above approximately 500 kHz. Thus, at
any given time and for each ionized layer, there is an upper fre-
quency limit at which radio waves sent vertically upward are
reflected back to Earth. This limit is called the critical frequency.
Radio waves directed vertically at frequencies higher than the criti-
cal frequency pass through the ionized layer out into space. All radio
waves directed vertically into the ionosphere at frequencies lower
than the critical frequency are reflected back to Earth. Radio waves
used in communications are generally directed towards the iono-
sphere at some oblique angle, called the angle of incidence. Radio
waves at frequencies above the critical frequency will be reflected
back to Earth if transmitted at angles of incidence smaller than a cer-
tain angle, called the critical angle. At the critical angle, and at all
angles larger than the critical angle, the radio waves pass through the
ionosphere if the frequency is higher than the critical frequency. As
the angle of transmission decreases, an angle is reached at which the
radio waves are reflected back to Earth. 
Transmission Paths. Sky wave propagation refers to those types
of radio transmissions that depend on the ionosphere to provide sig-
nal paths between transmitters and receivers.
The distance from the transmitting antenna to the place where the
sky waves first return to Earth is the skip distance. The skip distance
depends on the angle of incidence, the operating frequency, and the 
						

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MCRP 6-22D
ionosphere’s height and density. The antenna’s height, in relation to
the operating frequency, affects the angle that transmitted radio
waves strike and penetrate the ionosphere and then return to Earth.
This angle of incidence can be controlled to obtain the desired cov-
erage area. Lowering the antenna height increases the angle of trans-
mission and provides broad and even signal patterns in a large area. 
Using near-vertical transmission paths is known as near-vertical
incidence sky wave (NVIS). Raising the antenna height lowers the
angle of incidence. Lowering the angle of incidence produces a skip
zone in which no usable signal is received. This area is bounded by
the outer edge of usable ground wave propagation and the point
nearest the antenna at which the sky wave returns to Earth. In short-
range communications situations, the skip zone is an undesirable
condition. However, low angles of incidence make long-distance
communications possible. 
When a transmitted wave is reflected back to the Earth’s surface, the
Earth absorbs part of the energy. The remaining energy is reflected
back into the ionosphere to be reflected back again. This means of
transmission—alternately reflecting the radio wave between the
ionosphere and the Earth—is called hops. Hops enable radio waves
to be received at great distances from the point of origin. 
Fading. Fading is the periodic increase and decrease of received
signal strength. Fading occurs when a radio signal is received over a
long-distance path in the high frequency range. The precise origin
of this fading is seldom understood. There is little common knowl-
edge of what precautions to take to reduce or eliminate fading’s
troublesome effects. Fading associated with sky wave paths is the
greatest detriment to reliable communications. Too often, those
responsible for communication circuits rely on raising the transmit-
ter power or increasing antenna gain to overcome fading. Unfortu-
nately, such actions often do not work and seldom improve 
						

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1-17
reliability. Only when the signal level fades down below the back-
ground noise level for an appreciable fraction of time will increased
transmitter power or antenna gain yield an overall circuit improve-
ment. Choosing the correct frequency and using transmitting and
receiving equipment intelligently ensure a strong and reliable
receiving signal, even when low power is used.
Maximum Usable and Lowest Usable Frequencies. Using a
given ionized layer and a transmitting antenna with a fixed angle of
radiation, there is a maximum frequency at which a radio wave will
return to Earth at a given distance. This frequency is called the max-
imum usable frequency (MUF). It is the monthly median of the
daily highest frequency that is predicted for sky wave transmission
over a particular path at a particular hour of the day. The MUF is
always higher than the critical frequency because the angle of inci-
dence is less than 90°. If the distance between the transmitter and
the receiver is increased, the MUF will also increase. Radio waves
lose some of their energy through absorption by the D layer and a
portion of the E layer at certain transmission frequencies.
The total absorption is less and communications more satisfactory
as higher frequencies are used—up to the level of the MUF. The
absorption rate is greatest for frequencies ranging from approxi-
mately 500 kHz to 2 MHz during the day. At night the absorption
rate decreases for all frequencies. As the frequency of transmission
over any sky wave path decreases from high to low frequencies, a
frequency will be reached at which the received signal overrides the
level of atmospheric and other radio noise interference. This is
called the lowest useful frequency (LUF) because frequencies
lower than the LUF are too weak for useful communications. The
LUF depends on the transmitter power output as well as the trans-
mission distance. When the LUF is greater than the MUF, no sky
wave transmission is possible. 
						

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MCRP 6-22D
Propagation Prediction. Although a detailed discussion of propa-
gation prediction methods is beyond the scope of this publication, it
should be noted that propagation predictions can be obtained from a
system planning, engineering, and evaluation device (SPEED).
Other Factors Affecting Propagation
In the VHF and UHF ranges, extending from 30 to 300 MHz and
beyond, the presence of objects (e.g., buildings or towers) may pro-
duce strong reflections that arrive at the receiving antenna in such a
way that they cancel the signal from the desired propagation path
and render communications impossible. Most Marines are familiar
with distant TV station reception interference caused by high-flying
aircraft. The signal bouncing off of the aircraft alternately cancels
and reinforces the direct signal from the TV station as the aircraft
changes position relative to the transmitting and receiving antennas.
This same interference can adversely affect the ordinary voice com-
munications circuit at VHF and UHF, rendering the received signal
unintelligible for brief periods of time. Receiver locations that
avoid the proximity of an airfield should be chosen if possible.
Avoid locating transmitters and receivers where an airfield is at or
near midpoint of the propagation path of frequencies above 20
MHz.
Many other things may affect the propagation of a radio wave.
Hills, mountains, buildings, water towers, tall fences, aircraft, and
even other antennas can have a marked affect on the condition and
reliability of a given propagation path. Conductivity of the local
ground or body of water can greatly alter the strength of the trans-
mitted or received signal. Energy radiation from the Sun’s surface
also greatly affects conditions within the ionosphere and alters the
characteristics of long-distance propagation at 2 to 30 MHz.  
						

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1-19
Path Loss
Radio waves become weaker as they spread from the transmitter.
The ratio of received power to transmitted power is called path loss.
LOS paths at VHF and UHF require relatively little power since the
total path loss at the radio horizon is only about 25 decibels (dB)
greater than the path loss over the same distance in free space
(absence of ground). This additional loss results from some energy
being reflected from the ground, canceling part of the direct wave
energy. This is unavoidable in almost every practical case. The total
path loss for an LOS path above average terrain varies with the fol-
lowing factors: total path loss between transmitting and receiving
antenna terminals, frequency, distance, transmitting antenna gain,
and receiving antenna gain.
Reflected Waves
Often, it is possible to communicate beyond the normal LOS dis-
tance by exploiting the reflection from a tall building, nearby moun-
tain, or water tower (fig. 1-6 on page 1-20). If the top portion of a
structure or hill can be seen readily by both transmitting and receiv-
ing antennas, it may be possible to achieve practical communica-
tions by directing both antennas toward the point of maximum
reflection. If the reflecting object is very large in terms of a wave-
length, the path loss, including the reflection, can be very low.
If a structure or hill exists adjacent to an LOS path, reflected energy
may either add to or subtract from the energy arriving from the
direct path. If the reflected energy arrives at the receiving antenna
with the same amplitude (strength) as the direct signal but has the
opposite phase, both signals will cancel and communication will be
impossible. However, if the same condition exists but both signals
arrive in phase, they will add and double the signal strength. These