Rain-Induced Cross-Polarization at Centimeter and Millimeter Wavelengths

Understanding depolarization properties of the transmission medium is of crucial importance in planning frequency reuse by employing orthogonal polarizations in a radio communication system. The raininduced depolarization, which concurs with heavy rain attenuation, 1557 Fig. 1--Canted oblate spheroidal raindrop. has attracted considerable theoretical and experimental efforts. 1 - 9 A mathematical model of canted oblate spheroidal raindrops as shown in Fig. 1 has been assumed in most theoretical investigations. Crosscoupling between vertical and horizontal polarizations occurs as a result of the differential attenuation and differential phase shift between two polarizations parallel and perpendicular to a major axis of the oblate raindrops. Recently, Morrison et al.10,11 have given the calculated differential characteristics for various rain rates throughout the microwave range from 4 to 100 GHz. These results are based upon numerical solutions 12 of the scattering of a plane electromagnetic wave by oblate spheroidal raindrops using a point-matching procedure or perturbation about an equivolumic sphere. The modified perturbation scheme 11 offers a substantial improvement over Oguchi's previous perturbation calculations. 1 Very recently, Oguchi13'14 also used a pointmatching procedure to make similar calculations. The purpose of this paper is to assess our current understanding of the rain-induced microwave depolarization in view of these new calculations and their relationship to the measured data.