TC 9-64 _________________________________________________________________________
2-135. This scattering mode of propagation enables VHF and UHF signals to
be transmitted far beyond the normal line-of-sight. To understand how these
signals are transmitted over greater distances, you must first consider the
propagation characteristics of the space wave used in VHF and UHF line-of-
sight communications. When the space wave is transmitted, it undergoes
very little attenuation within the line-of-sight horizon. When it reaches the
horizon, the wave is diffracted and follows the earth's curvature. Beyond the
horizon, the rate of attenuation increases very rapidly and signals soon
become very weak and unusable.
2-136. Tropospheric scattering, on the other hand, provides a usable signal
at distances beyond the point where the diffracted space wave drops to an
unusable level. This is because of the height at which scattering takes place.
The turbulence that causes the scattering can be visualized as a relay station
located above the horizon; it receives the transmitted energy and then
reradiates it in a forward direction to some point beyond the line-of-sight
distance. A high gain-receiving antenna aimed toward this scattered energy
can then capture it.
2-137. The magnitude of the received signal depends on the number of
turbulences causing scatter in the desired direction and the gain of the
receiving antenna. The scatter area used for tropospheric scatter is known as
the scatter volume. The angle at which the receiving antenna must be aimed
to capture the scattered energy is called the scatter angle. The scatter volume
and scatter angles are shown in figure 2-26.
Figure 2-26. Tropospheric Scattering Propagation
2-138. The signal take-off angle (transmitting antenna's angle of radiation)
determines the height of the scatter volume and the size of the scatter angle.
A low signal take-off angle produces a low scatter volume, which in turn
permits a receiving antenna that is aimed at a low angle to the scatter
volume to capture the scattered energy.
2-139. As the signal take-off angle is increased, the height of the scatter
volume is increased. When this occurs, the amount of received energy
decreases. There are two reasons for this: (1) the scatter angle increases as
the height of the scatter volume is increased; (2) the amount of turbulence
2-38
Previous Page
