Roger A. Dalke

Abstract: Circa 1960, the National Bureau of Standards intensively studied over-the-horizon radio propagation due to tropospheric (aka forward) scatter. The results of that effort, published in the form of graphs and/or empirical mathematical functions based on curve fitting, led to the development of important radio propagation models. Unfortunately, there is scant documentation describing exactly how the published data is related to the underlying theoretical basis for scattering theory. In this report, we describe the electromagnetic theory that results in the forward scatter Common Volume Integral. This is followed by a description of analytical methods used to obtain solutions. The results are then compared to propagation model predictions. In general, fairly good agreement between theory and models was obtained for the Irregular Terrain Model and IF-73 air/ground propagation model. Good agreement with the IF-77 Electromagnetic Wave Propagation Model was obtained when frequency gain corrections are negligible. Otherwise, the IF-77 and theoretical results differed significantly. The reason for this was not determined.

Keywords: troposcatter; Irregular Terrain Model (ITM); forward scatter; tropospheric scatter; IF-77 Electromagnetic Wave Propagation Model

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