The effect of non-uniform illumination on site shielding from buildings at microwave frequency

Predicting the diffracted signal strength behind an obstacle is valuable in estimating the obtainable site shielding factor (SSF), where the SSF is defined as the difference between unobstructed and diffracted signal levels. Presently, the majority of models by which diffracted field strength may be predicted assume that the diffracting obstacle is clearly illuminated by the transmitter. For the case where a building is being relied on to provide protection from microwave frequency interference it is more common for the interference path to be trans-horizon. In this situation a threat is posed on the receiver only under anomalous propagation conditions. In this paper a flat roofed building is treated as a double knife edge system and the exact, surface integral method of determining diffracted field strength is extended so that it may be applied to conditions of non-uniform illumination. Additionally, results from an experiment involving the long term monitoring of site shielding on a 50 km trans-horizon path at 11.2 GHz are presented. >