A comparison of rain attenuation and drop size distributionsmeasured in Chilbolton and Singapore

Abstract

Attenuation of radio waves caused by precipitation, especially in the form of rain, is considered to be the limiting factor for new communication systems that will exploit the radio wave spectrum at frequencies higher than about 30 GHz. Over the last 40 years, much effort has gone into theoretical studies characterizing rain in terms of statistical drop size distributions (DSDs), the shapes and velocity dependence of raindrops, and the calculation of raindrop extinction cross sections. This paper focuses on specific data sets and data processing and different ways of viewing DSDs that may help in quantifying some of the important parameters in radio wave propagation from experimental data. Values for the coefficients k and α in the relationship for specific rain attenuation γ = kRα are presented, together with rain-rate-dependent parameters for fits of DSDs to standard statistical distributions. These are based on data from Chilbolton, England, and from Singapore. The distributions measured at Chilbolton and Singapore are very different, which strongly suggests that drop size distributions differ under different climatic conditions. Comparisons are also presented of attenuations calculated with values of k and α determined from the DSDs and values found from logarithmic regression between simultaneous rain rate and attenuation measurements at 57, 97, 135, and 210 GHz at Chilbolton. This paper gives a strong indication that the International Telecommunication Union Radiocommunication Sector model for specific rain attenuation is inadequate at frequencies higher than about 70 GHz.