Abstract :
Simultaneous field strength measurements on u.h.f. transmissions at 560 Mc/s and 774 Mc/s were carried out over the North Sea, on transmission paths ranging from 123 miles (198 km) to 591 miles (950 km). The results show that with the same meteorological conditions the long-term received field strengths tend to be higher, although not to a significant extent, for the 774-Mc/s transmission than for the 560-Mc/s transmission. When meteorological conditions are favourable during anticyclonic weather, the signals received are, at times, very much in excess of the free-space value. These high signals are associated with horizontally stratified boundary layers at heights up to 500 metres resulting from subsidence in the lower atmosphere. Boundary layers at greater heights are thought to be partly responsible for the signals received at the greater distances during abnormal propagation conditions. The field strength exceeded in each month for certain time-percentages, the types of signal received and the fading rate are also discussed.
