The propagation of metre radio waves beyond the normal horizon. Part 1: Some theoretical considerations, with particular reference to propagation over land

The effect on radio wave propagation of departures from the normal variation with height of the refractive index of the lower atmosphere is considered with special reference to frequencies of 90 and 45 Mc/s. The relative importance of changes in refraction near the surface of the earth and of reflection from high-level inversion layers is investigated, and some illustrative examples are worked out. The number of parameters concerned in the problem prevents an all-embracing survey of the many possibilities within the scope of the present work, but a few general conclusions are reached. It is unlikely that radio ducts of sufficient magnitude to be greatly significant at the frequencies in question can be maintained over long distances in broken country. Of the two mechanisms¿refraction near the surface of the earth and reflection at an elevated inversion layer¿the latter is the one which is likely more often to give abnormally high field strengths at ranges of a few hundred kilometres, especially for relatively low terminal points. Consideration is also given to the effects of scattering of radio energy by turbulent eddies in the atmosphere, and an approximate calculation is made of the magnitude of this scattering for horizontally polarized waves on a frequency of 90 Mc/s, and at distances up to 500 km. It appears likely that such scattering is of less importance in the creation of unusually high field strengths up to distances of the order of 250 km than the mechanisms of abnormal refraction, including reflection at elevated inversion layers.