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FM 11-23
familiar with the characteristics of fading and cies, using different antenna polarization, or travthe
means available to reduce it.
eling slightly different distances will arrive at a
receiving location with different strengths. Divera.
Long-Term Fading. Long-term
Lo
fading
en a ang. she
is the
nte sity reception consists of combining two or more
result of seasonal changes in the atmosphere.
of these different strength signals at the receiv-
These changes affect the refractive index of the
scatter As the volume. circuit distance
ing location and then causing the strongest signal
screate, v.olu-term fasing thecircu s dTe ise ,to n- be selected for use, thus increasing the probacreased,
long-term fihang decreasest Tne preiom- bility that a usable signal is continuously availainant
reason is that as circuit distance is increased,
the height of the scatter volume is in- blefor communcations.
creased. Most of the atmospheric changes which b. There are several types of diversity operacause
long-term fading occur in the lower part of tions available to troop radio systems. The terms
the troposphere. As the circuit distance is in- dual-diversity and quadruple-diversity indicate
creased, the scatter volume is elevated to a less the number of independent received signals.
changeable region. A method of increasing scat- Other techniques that provide two or more reter
volume height is by increasing the horizon ceived signals are spaced antennas (space diverangle
of the antenna. However, because long-term sity) and dual transmission frequencies (frefading
effects are considered during the engineer- quency diversity). Combinations of these divering
phase of tropo system design, the system is sity methods are used to provide greater circuit
installed in such a way that the horizon angle of reliability than would be available if only one dithe
antenna is determined by the near physical versity system were used. The techniques of inobstacle.
stallation and operation of these various systems
are technical in nature and are found in approb.
Short-Term Fading. Short-term fading, priate engineering and technical manuals; howwhich
is the most predominant type, usually ocever,
a brief description of some
curs at a few
of
cycles
the
per
more
second.
fre-
It is caused by
quently used
rapid
methods
changes
is
in
given below.
the scatter
The
volume
deci-
because the
turbulences which cause the scattering effect are availability of equipment, the
availability
degree of
constantly changing positions.
of
These
equipment,
changes in system/circuit
the degree
reliability
of
desired, and the tactical
position are due to variations in temperature, hu- situation.
midity, and density. The received signal is a composite
of many small signals, all of which have (1) Space diversity. This involves two or
traveled slightly different path lengths in arriv- more receivers and antennas operating on the
ing at the receiving antenna. The amplitude of same frequency.
each of these small signals is constantly chang- (2) Frequency diversity. This involves two
ing. The signals combine in random-phase rela- or more receivers, transmitters, and antennas option;
sometimes in an overall reinforcement, and erating on different frequencies.
other times in an overall cancellation. While an
(3) Quadruple space-frequency
increase in
diversity.
transmitter output power or receiver
This method requires four receivers, two transgain
may alleviate short-term fading, diversity
mitters,
operation (para B-9) is the most
and two
commonly used
send-receive
terminal. The two send-receive
antennas
antennas
for each
also
method
method to
to
counteract
counteract
short-term
short-term
fading.
fading.
How-
How-
terminal. The two send-receive antennas also
ever, diversity operation rarely improves circuit have dual polarization (vertical and horizontal).
conditions attributable to long-term fading. B-0. Reliability
Tropo propagation provides a communications
B-9. Diversity Operation
system with extremely high reliability. Current
a. Short-term fading (para B-8) produces a data indicates that tactical tropo systems have a
signal of varying strength at the receiver and capability for 98-percent reliability when operattherefore
has an adverse effect on the quality of ing in the nondiversity phase over a given discommunications.
To counteract this effect, diver- tance. Dual-diversity operations increase reliabilsity
reception techniques are employed. These ity to approximately 99.2 percent, while quadrutechniques
are based on the fact that two or more ple-diversity operations reflect a reliability factor
transmitted signals, on different carrier frequen- of 99.5 percent.
B-4