The filling in of an antenna null by off-path scattering on a tropospheric scatter circuit

The concept and characteristics of Antenna-to-Medium Coupling loss have already been discussed in the literature in some detail. These earlier analyses show that the coupling loss is not significant unless the beamwidth of the antenna becomes equal to or less than the angular width of the scatter volume as seen from the antenna. An inverse phenomenon occurs if one tries to design an antenna null to protect a given location beyond the horizon; namely, if the width of the null is narrower than the angular width of the scatter volume, the null can be filled in by off-path scattering and the desired protection might not be achieved. This paper outlines the analysis used in evaluating the "filling in" of the antenna null by off-path scattering and presents curves which should prove useful in obtaining quantitative estimates. An examination of the interrelation between the scattering process and the free-space antenna pattern reveals that the scattering process averages the free-space antenna pattern, the averaging interval being of the order of several degrees. Thus a qualitative conclusion can be drawn that if it is desired to protect an installation with a 40 db null, this null must be maintained over at least several degrees in order to be effective. A cosine squared pattern is not satisfactory.