V.H.F and u.h.f. field-strength variations corresponding to river beds

The mechanisms of propagation over land and over sea, respectively, are reviewed to detect the causes of the field-strength maximum in the afternoon, in the former case, and of the corresponding minimum in the latter case. The analysis of the received power for two transhorizon links over land along a river bed indicates an effect of the river bed on the afternoon field-strength rise, and a three-part nocturnal trend of field strength. These properties are explained in terms of evaporation and condensation processes of water vapour near the ground, and also by-the superposition of the continental and oceanic types of diurnal variations of field strength. They are in agreement with fading characteristics on line-of-sight links along river beds.