Notes on duct influences on line-of-sight propagation

Microwave propagation over the sea within the horizon may be treated by ray tracing. Because of the maritime evaporation duct at sea, the receiving field may vanish at a certain antenna height. With increasing duct thickness, this special receiving point is shifted upwards. Vertical polarization puts this point on a lower level than horizontal polarization. Moreover, with increasing duct thickness it is shifted upwards more slowly for vertical polarization than for horizontal polarization. The resultant phase of the receiving field depends on the magnitude of the refractivity on a fixed but arbitrary reference level and on the thickness of the evaporation duct. Between the phase and the refractivity there exists a linear relation. As a function of duct thickness, however, the phase reveals a discontinuity amounting to integer multiples of
2\pi
. In geodesy, where a modified kind of phase measurement is used for telemetry (Tellurometer), the evaporation duct may cause considerable errors.