__________________________________________________________ Radio Wave Propagation
decreases with height. As the distance between the transmitting and
receiving antennas is increased, the height of the scatter volume must also be
increased. The received signal level, therefore, decreases as circuit distance is
increased.
2-140. The tropospheric region that contributes most strongly to
tropospheric scatter propagation lies near the midpoint between the
transmitting and receiving antennas and just above the radio horizon of the
antennas.
2-141. Because tropospheric scatter depends on turbulence in the
atmosphere, changes in atmospheric conditions have an effect on the strength
of the received signal. Both daily and seasonal variations in signal strength
occur as a result of changes in the atmosphere. These variations are called
long-term fading.
2-142. In addition to long-term fading, the tropospheric scatter signal often
is characterized by very rapid fading because of multipath propagation.
Because the turbulent condition is constantly changing, the path lengths and
individual signal levels are also changing, resulting in a rapidly changing
signal. Although the signal level of the received signal is constantly changing,
the average signal level is stable; therefore, no complete fade-out occurs.
2-143. Another characteristic of a tropospheric scatter signal is its relatively
low power level. Because very little of the scattered energy is reradiated
toward the receiver, the efficiency is very low and the signal level at the final
receiver point is low. Initial input power must be high to compensate for the
low efficiency in the scatter volume. This is accomplished by using high-
power transmitters and high-gain antennas, which concentrate the
transmitted power into a beam, thus increasing the intensity of energy of
each turbulence in the volume. The receiver must also be very sensitive to
detect the low-level signals.
Application of Tropospheric Scattering
2-144. Tropospheric scatter propagation is used for point-to-point
communications. A correctly designed tropospheric scatter circuit will provide
highly reliable service for distances ranging from 50 miles to 500 miles.
Tropospheric scatter systems may be particularly useful for communications
to locations in rugged terrain that are difficult to reach with other methods of
propagation. One reason for this is that the tropospheric scatter circuit is not
affected by ionospheric and auroral disturbances.
SUMMARY
Now that you have completed this chapter, let us review some of the new
terms, concepts, and ideas that you have learned. You should have a
thorough understanding of these principles before moving on to chapter 3.
The induction field contains an E field and an H field and is localized near
phase with each other.
The radiation field contains E and H fields that are propagated from the
antenna into space in the form of electromagnetic waves. The E and H fields
of the radiation field are in phase with each other.
2-39
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