Page 151 - Satellite Communications, Fourth Edition

Polarization 131

Maximum values quoted by Miya (1981) for Faraday rotation are 9°
at 4 GHz and 4° at 6 GHz. In order to counter the depolarizing effects
of Faraday rotation, circular polarization may be used. With circular
polarization, a Faraday shift simply adds to the overall rotation and does
not affect the copolar or cross-polar components of electric field.
Alternatively, if linear polarization is to be used, polarization tracking
equipment may be installed at the antenna.

5.6 Rain Depolarization

The ideal shape of a raindrop is spherical, since this minimizes the
energy (the surface tension) required to hold the raindrop together. The
shape of small raindrops is close to spherical, but larger drops are better
modeled as oblate spheroids with some flattening underneath, as a
result of the air resistance. These are sketched in Fig. 5.12a and b. For
vertically falling rain, the axis of symmetry of the raindrops will be
parallel to the local vertical as shown in Fig. 5.12b, but more realisti-
cally, aerodynamic forces will cause some canting, or tilting, of the drops.
Thus there will be a certain randomness in the angle of tilt as sketched
in Fig. 5.12c.
As shown earlier, a linearly polarized wave can be resolved into two com-
ponent waves, one vertically polarized and the other horizontally polar-
ized. Consider a wave with its electric vector at some angle t relative to
the major axis of a raindrop, which for clarity is shown horizontal in
Fig. 5.13. The vertical component of the electric field lies parallel to the
minor axis of the raindrop and therefore encounters less water than the hor-
izontal component. There will be a difference therefore in the attenuation
and phase shift experienced by each of the electric field components. These
differences are termed as the differential attenuation and differential phase
shift, and they result in depolarization of the wave. For the situation shown
in Fig. 5.13, the angle of polarization of the wave emerging from the rain
is altered relative to that of the wave entering the rain. Experience has

(a) (b) (c)
Figure 5.12 Raindrops: (a) small spherical, (b) flattening resulting from air resistance,
and (c) angle of tilt randomized through aerodynamic force.