Depolarization is an effect wherein a satellite link's wave polarization is altered after being launched by the antenna. Some texts refer to depolarization as cross-polarization. For the case of a linearly polarized wave passing through the medium, components of the opposite polarization will be developed. For the case of circular polarization, there will be a tendency to develop into an elliptical wave. This is particularly important for the case of frequency reuse systems, where depolarization effectively reduces the polarization isolation and can tend to increase crosstalk on the signal.
Depolarization on a satellite can be caused by rain, ice, snow, multipath, effects, and refractive effects. It can also be caused by Faraday rotation.
Faraday Rotation. Analysis of the propagation of a linearly polarized high-frequency wave in the ionosphere shows that it experiences rotation of the plane of polarization such that a wave launched with vertical polarization does not remain vertical. Depending on the frequency, length of the path in the ionosphere, and orientation with respect to the earth's magnetic field, the amount of Faraday rotation may vary from negligible to 360° or many rotations.
Of most importance for satellite links is the fact that the rotation varies inversely as the square of the frequency. Typically, at 4 GHz, the rotation is only several degrees.
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