Propagation of centimeter and millimeter wavelengths through precipitation

Abstract
From theoretical considerations, it is possible to calculate attenuation through rain at several wavelengths. It is shown that attenuation is a linear function of rain rate for a 0.86- cm wavelength. This property is independent of the distribution spectra of drop radius within a ten-percent precision. This later property has been tested experimentally in two different ways. 1) Measurements through rain have been made with a radar at 0.86 cm. Results are reported. They show quite important differences between theoretical and experimental results. 2) Experiments have been conducted in rainfall to measure fall speeds and diameter spectra of drops. The apparatus used for this purpose is briefly described. With the results obtained it is possible to calculate the propagation properties of rainfall at several wavelengths, particularly at 0.86 cm. We hope to be able to compare the results obtained with those observed by radar in the future. The interest in short wavelengths for radiolocation or radio communication is well known, but when using shorter wavelengths we encounter greater attenuation in the atmosphere. We know that for wavelengths between 1 and 10 cm, attenuation by atmospheric gases may be neglected (except near 1.25 cm). At millimetric wavelengths, propagation is only possible in certain bands. At 0.86 cm, attenuation by oxygen and water vapor is not negligible, but can be considered constant. At the wavelengths of 0.4 cm and 0.20 cm, attenuation is dependent upon the concentration of water vapor, that is to says upon meteorological conditions. The effect of atmospheric particles is more important with shorter wavelengths. Practically negligible at 10 cm, attenuation by precipitation begins to be important at 3 cm and 5.5 cm and is strong at 0.86 cm and other milllmetric wavelengths. The purpose of the present work is to study theoretically effects of rainfall at several wavelengths.

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