Specification of polarization parameters for optimal-performance inrain of dual circularly polarized radio links

Rain depolarization of nominally circularly polarized waves alters the polarization state of the wave to produce a polarization ellipse in which the major axis is tilted with respect to the horizontal. Tilt angles for right-hand and left-hand polarization are opposite in sign. It is shown that these tilt angles are almost independent of rain rate and only slightly dependent on frequency. At 62 GHz the magnitude of tilt is close to 60°C. The suggestion is made that the antenna system be deliberately designed to match, as closely as possible, the polarization state of the incoming wave when the expected rain attenuation is at the design maximum (rain margin) for a given communications link. This strategy assures that the crosstalk between polarizations will be minimized when the signal-to-thermal noise ratio (SNR) is at its lowest value. In clear weather, the SNR will be larger, and a greater degree of polarization crosstalk can be tolerated. Finding an optimal match involves selection of the correct rotational orientation of at least one of the antennas as well as specification of its axial ratio to lie between minimum and maximum limits