Multiple-Path Fading on Line-of-Sight Microwave Radio Systems as a Function of Path Length and Frequency

Transmission over line-of-sight microwave radio paths is analyzed with the aid of a piece-wise linear approximation of the atmospheric index of refraction. The simple model is adequate; it predicts published experimental results. A short path is defined on which no deep fading can occur and the maximum length of such a path is estimated from measured data for New Jersey. Expressions are presented for the worst-case amplitude-frequency response and for the maximum echo delay for short paths. It is shown that if the normal Fresnel-zone clearance is maintained on short paths, no fading will occur due to substandard conditions of propagation. W. T. Barnett's result is also predicted from this model: the distribution of attenuation on long paths is a function of L3/λ where L is the path length and λ is the free space wavelength. The distribution of deep fades on long paths is predicted by this model to have the same slope as the Rayleigh distribution, the slope normally found in measurements of attenuation distributions on long paths.