Influence of an Inhomogeneous Ground on the Propagation of VLF Radio Waves in the Earth-ionosphere Waveguide

Abstract

Propagation of radio waves in the earth-ionosphere waveguide is considered for the case where the lower boundary is an inhomogeneous smooth surface. An integral equation for the problem is formulated in a direct fashion by utilizing the compensation theorem. After some simplifications, several special cases are considered explicitly. For example, in the case of a two-section path consisting of a long stretch of sea and a short section of land, a relatively simple working formula is obtained. The result shows that the modal excitation factors at VLF for an all sea path are significantly reduced when the foreground is poorly conducting. Another special case considered is when the propagation path is all sea except for a short intermediate land section. In this case, it is found that energy from low-order modes will be transferred to high-order modes with a subsequent reduction of field strength.