foreground terrain effects on overland UHF transmissions

This paper describes an experimental study of the influence of the shape of foreground terrain profiles near terminals of UHF links on the received field. A gently rounded shape of the foreground profile causes a marked diffraction pattern to be superimposed on the normal variation of field strength with height. The diffraction geometry shows similarity to knife-edge geometry and the rounded terrain feature appears to act geometrically as an equivalent knife edge. The amplitude of the spatial variations in signal are, however, much greater than knife-edge theory predicts. A sizeable foreground diffraction enhancement of received field can be realized by locating the antenna at the height of the first diffraction maximum. Changing refraction due to meteorological variations can change both the position in height and intensity of the diffraction pattern.