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FM 11-23
reradiated to the receiver, the efficiency is very ceive antenna decreases very rapidly as these faclow,
and the signal level at the receiver is there- tors are increased. The most advantageous situafore
low. To compensate for this low efficiency, tion occurs if either or both of the transmit-rehigh-power
transmitters, high-sensitivity re- ceive antennas are located on a site higher than
ceivers, and high-gain antennas are used.
the terrain in the foreground. This permits negative
horizon angle aiming (aiming below a line
B-6. Operating Frequency
tangent with the surface of the earth) of the antennas.
Consequently,
Several
the
factors
scatter angle
determine
and
the
the
frequency range tennas.height of the scatter volume are decreased, promost
suitable for tropo propagation. For frequen- viding maximum scatter volume area and thus an
viding maximum
cies
scatter
below
volume
30 MHz,
area
the
and thus
troposphere
an
appears uniform
and scattering dQes not occur. Also, at these
lower frequencies, it is difficult to construct the b. Circuit Path Distance. Any change that afrequired
high-gain antennas. On the other hand, fects the circuit path distance, such as a terminal
scattering loss and free-space loss increase as the redeployment, will affect the total of refracted
frequency increases. Also, above 10,000 MHz, the energy received. Circuit paths are changed when
wave is greatly affected by atmospheric condi- the horizon angle of either the transmitting or
tions. Thus, the usable frequency range for tro- receiving antenna is changed. Usually, when terpospheric
scatter is between approximately 100 minals are relatively close to each other, the horiand
10,000 MHz. Most tactical troposcatter radio zon angles will be greater and refracted engery
equipments in the army inventry are designed to will be reduced. For example, when two termioperate
between 4,400 and 5,000 MHz.
nals are 80 kilometers apart, an increase of 2 degrees
in the sum of the horizon angle will cause a
B-7. Propagation Losses
10-percent reduction in refracted energy. The effect
of horizon angles decreases as distances be-
The reliability and quality of a circuit depend on tween terminals increase because the fractional
the strength of the signal arriving at the receiv- increase in scatter angle for a given horizon
ing antenna. The total propagation loss for a cir- angle is much less at the greater distances. Howcuit
depends on the distance between antennas, ever, as the distances between terminals increase,
the frequency used, and the scatter angle of the the total path distance must not exceed the maxiintersecting
beams (fig. B-2); this angle, in most mum transmission distance for which the tropo
cases, is determined by intervening terrain fea- equipment was designed.
tures. The relationship between these factors and
the resultant amount of energy refracted from c. Frequency. As the frequency of the transmitthe
common scatter volume is discussed below. ted radio wave is increased, the refractive index
The formula for computing the total loss encoun- of the common scatter volume is decreased. Howtered
between effective transmitted power and ever, army tactical tropo equipment has a frethe
receiver input is contained in appropriate en- quency range of 4,400-5,000 MHz, and the selecgineering
and technical manuals.
tion of any frequency within this range does not
drastically change the total propagation loss ena.
Scatter Angle. The angles at which the countered. Of more importance is the elimination
transmitting and receiving antennas must be of interference. The frequencies selected, when
aimed to clear the horizon are called horizon an- operating in the various modes (space-frequency-,
gles and these horizon angles determine the scat- dual-, or quad-diversity) must therefore adhere
ter angle of a tropo circuit. If there are no ob- to the criteria established for such operations as
structions (such as mountains or hills) in front outlined in the technical manuals for the respecof
the antennas, the antennas are aimed tangen- tive tropo equipments.
tially to the earth's surface and the horizon angle
will be zero. This condition assures a greater
common volume size and provides an increased -8. Fading
amount of refracted energy at the receiving an- Fading has a pronouced effect on circuit reliabiltennas.
When there are obstructions, the anten- ity when the received signal level at the peak of
nas must be aimed at a higher angle to clear the the fade cycle approaches the minimum usable
horizon; this increases the scatter angle, the level of the receiver. There are two types of fadheight
of the scatter volume, and the path dis- ing: short-term and long-term. Personnel intance.
The radiated energy scattered to the re- volved with troposcatter employment should be
B-3