Oblique radio transmission in the ionosphere, and the lorentz polarisation term

The application of the Sellmeyer and Lorentz dispersion formulae to the problem of oblique reflection of radio waves from a parabolic type of layer is discussed. The work of Ratcliffe on the problem is extended and shown to be consistent with that of N. Smith. This theoretical analysis has been applied to a large number of experimental measurements of the maximum usable frequency at oblique incidence. The results of two sets of measurements of the maximum usable frequency over distances of 1000 km. and 700 km. are in close agreement with values calculated from the Sellmeyer type of formula. In the case of the 1000 km. observations, the divergence between calculated and observed values is less than 0.2 of what it should be if the Lorentz formula applied. In the case of more accurate measurements over a distance of 700 km., the mean divergence between calculated and observed values is only about 0.03 of that which should occur on the basis of the Lorentz formula. These experimental results are thus in agreement with the theoretical conclusion of Darwin, that the Sellmeyer type of formula is applicable to the case of refraction of radio waves in the ionosphere.