2-37
TROPOSPHERIC SCATTERING
When a radio wave passing through the troposphere meets a turbulence, it makes an abrupt change in
velocity. This causes a small amount of the energy to be scattered in a forward direction and returned to
Earth at distances beyond the horizon. This phenomenon is repeated as the radio wave meets other
turbulences in its path. The total received signal is an accumulation of the energy received from each of
the turbulences.
This scattering mode of propagation enables vhf and uhf signals to be transmitted far beyond the
normal line-of-sight. To better 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.
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.
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 angle are shown in figure 2-26.
Figure 2-26.—Tropospheric scattering propagation.
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.
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) scatter angle
