Rain-Induced Cross-Polarization at Centimeter and Millimeter Wavelengths

Abstract

Rain-induced cross-polarization is an important factor in design of dual-polarization microwave radio communication systems. We present current estimates of this effect based upon calculated differential characteristics of canted oblate raindrops and their relationship to experiments. Measured differential attenuation and cross-polarization, mainly at 18 GHz, are used to determine two empirical parameters: an effective average of the absolute value of the canting angle and a measure of the imbalance between positive and negative canting angles. We can then provide estimates for median values of cross-polarization discriminations at other frequencies; these are found to agree fairly well with available measured data. Differential phase shift is the dominant factor in the rain-induced cross-polarization at frequencies below about 10 GHz, and differential attenuation becomes increasingly important at higher frequencies. For a given rain fading, the cross-polarization decreases with increase in frequency and is relatively insensitive to the rain rate, whereas for a given amount of rain the cross-polarization increases with frequency up to about 35 GHz. The cross-polarization discrimination of circularly polarized waves is much poorer than that of linearly polarized waves. When the angle α between the direction of propagation and the axis of symmetry of oblate raindrops is not equal to π/2, as on earth-space paths in satellite communication systems, the differential attenuation and differential phase shift can be approximated by sin <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> α times those for α = π/2, which is the condition for terrestrial paths.