Page 153 - Satellite Communications, Fourth Edition

Polarization 133

local vertical is a complicated function of position, but the following
general points can be observed. When the electric field is parallel to the
ground (horizontal), t 0, the second term on the right-hand side of the
equation for U contributes a 15-dB amount to the XPD, whereas with
circular polarization the contribution is only about 0.13 dB. With the
electric field vector in the reference plane containing the direction of
propagation and the local vertical, t 90° q (all angles in degrees),
and the cos4t term becomes cos4q.

5.7 Ice Depolarization
As shown in Fig. 4.3 an ice layer is present at the top of a rain region,
and as noted in Table 4.1, the ice crystals can result in depolarization.
The experimental evidence suggests that the chief mechanism pro-
ducing depolarization in ice is differential phase shift, with little dif-
ferential attenuation present. This is so because ice is a good dielectric,
unlike water, which has considerable losses. Ice crystals tend to be
needle-shaped or platelike and, if randomly oriented, have little effect,
but depolarization occurs when they become aligned. Sudden increases
in XPD that coincide with lightning flashes are thought to be a result
of the lightning producing alignment. An International Radio
Consultative Committee (CCIR) recommendation for taking ice depo-
larization into account is to add a fixed decibel value to the XPD value
calculated for rain. Values of 2 dB are suggested for North America and
4 to 5 dB for maritime regions, and it is further suggested that the
effects of ice can be ignored for time percentages less than 0.1 percent
(Ippolito, 1986).

5.8 Problems and Exercises
5.1. Explain what is meant by a plane TEM wave.

5.2. Two electric fields, in time phase and each of unity amplitude, act at right
angles to one another in space. On a set of x-y axes draw the path traced by the
tip of the resultant electric field vector. Given that the total power developed
across a 50 Ω load is 5 W, find the peak voltage corresponding to the unity
amplitude.
5.3. Two electric fields with an amplitude ratio of 3:1 and in time phase, act
at right angles to one another in space. On a set of x-y axes draw the path
traced by the tip of the resultant. Given that the total power developed across
a 50 Ω load is 10 W, find the peak voltage corresponding to the unity
amplitude.