Summary

Abstract

KELLER’S [1962] geometrical theory of diffraction (GTD) is used to calculate diffraction of LF radio waves by the earth. Each groundwave mode is represented by a surface-diffracted ray at a complex height that is determined by the groundwave mode propagation constant. Groundwave excitation and shedding coefficients are evaluated by comparing the GTD solution for the field of a vertical Hertzian dipole over a homogeneous, isotropic, spherical earth. The coefficients are then used to calculate the GTD solution for propagation below a homogeneous, isotropic, concentric ionosphere. This solution agrees with the rigorous solution in the shadow region, including the effect that the sky waves decrease slower with distance than the groundwave. The GTD can be combined with ray tracing in complex space [JONES, 1968] to calculate propagation of LF radio waves in the presence of an ionosphere that varies arbitrarily with position.