s some experimental work began on the effect of irregular terrain and of terrain (hills, buildings, etc.) at v.h.f., u.h.f. and s.h.f., especially the diffraction caused by buildings and other obstacles. in relation to the increasing requirements of the mobile services. However, it was only with the development of centimetric radar in the early years of World War II that the limitations of earlier concepts of tropospheric propagation were widely recognised. For 9.8 Survey of propagation modes in the several years attention was concentrated on the role of unusually troposphere strong refraction in the surface layers, especially over water, and the phenomenon of trapped propagation in a duct. It was shown ex- Figure 9.5 illustrates qualitatively the variation of received power perimentally and theoretically that in this mode the rate of attenua- with distance in a homogeneous atmosphere at frequencies above tion beyond the horizon was relatively small. Furthermore, for a about 30MHz. For antenna heights of a wavelength or more, the given height of duct or surface layer having a very large, negative, propagation mode on the free-space range is a space wave consist- vertical gradient of refractive index, there was a critical wavelength ing of a direct and a ground-reflected ray. For small grazing angles above which trapping did not occur; a situation analogous to that in the reflected wave has a phase change of nearly 180° at the Earth's waveguide transmission. surface, but imperfect reflection reduces the amplitude below that Again however it became apparent that further work was required of the direct ray. As the path length increases, the signal strength exhibits successive maxima and minima. The most distant maxi- to explain experimental observations. The increasing use of v.h.f., mum will occur where the path difference is 2/2 where a is the and later u.h.f., for television and radio communication emphasised wavelength. the need for a more comprehensive approach on beyond-the-hori- zon transmission. The importan