DEPOLARIZATION OF CIRCULARLY POLARIZED MILLIMETRE WAVES BY PRECIPITATION IN NIGERIA

Abstract

Rain induced depolarization due to distorted rain drops may be a serious problem to satellite communication at short wavelengths when dual - polarized signals are employed. This study investigates the effects of rain induced depolarization on circularly polarized millimeter and centimeter wave propagation in a tropical location (Nigeria). Propagation along the Earth-space path is considered. Cross-polarization discrimination (XPD) and copolar attenuation (CPA) are calculated for an effective raindrop canting angle of 10° with standard deviation of 19° and for varying elevation angles. Three tropical rain types classified as widespread, shower and thunderstorm are assumed. The frequency range of the study covers the band 4-100GHz. The study presents results for the variation of XPD with frequency, path length to the rain region, XPD as a function of fade depths (CPA) and so on. The study found that for convective rain types (shower and thunderstorm) cross polarization discrimination becomes very poor as elevation angle decreases and frequency and rain rate increases. For instance, as elevation angle becomes low (e.g. 23°), because the path length to the rain region becomes longer, XPD falls rapidly with increasing rain rate. Copolar attenuation on the other hand increases with increasing rain rate and frequency. The study also found that depolarization of circularly polarized waves is more severe when signal transmission is through convective rain type.