Radio propagation over a discontinuity in the Earth´s electrical properties. Part II: Coastal refraction

The paper presents merely one aspect of the general problem of propagation over a coastline, but special consideration is given to it since it differs from other effects in being present only at oblique incidence. By adopting a model for the coastal region the analysis given in Part I of the paper (see page 43) may be used to provide expressions for the angle of refraction appropriate to various positions of the transmitter and receiver relative to the coast. It is shown that the onset of the refraction occurs abruptly at the coastline, the theory suggesting that the direction of propagation is initially turned through an angle ¿/2. Within a wavelength of the coast, however, the refraction depends critically upon the way in which it is defined, and expressions which are equivalent at larger distances here lead to widely differing values. Nevertheless, if the transmitter is far away, it is certain that the refraction is largely confined to within a few wavelengths of the discontinuity and, moreover, decreases as the receiver recedes from the coastline; ultimately the original direction of propagation is restored. But, if the transmitter is itself within the coastal region, there is a permanent distortion of the field and a directional error which is virtually independent of the position of the receiver.