Effects of Elevation and Ground Conductivity on Horizontal Dipole Excitation of the Earth‐Ionosphere Waveguide

Horizontal electric dipole excitation of vertical E at the ground for 19.8‐kHz radiation beneath a hypothetical daytime ionosphere and beneath a highly anisotropic ionosphere has been numerically investigated as a function of ground conductivity and source elevation. East to west propagation is considered with a magnetic dip angle of 50°, and full allowance is made for earth curvature and ionospheric anisotropy. In horizontal dipole excitation between a highly anisotropic ionosphere and a good conducting ground, broadside launching, except for sources very close to the ground, is to be preferred to end‐on launching. Since TE waves are launched broadside, this is a direct consequence of mode coupling in the ionosphere. For source elevations comparable to or greater than a wavelength, the horizontal and vertical electric dipole excitations can become comparable. In excitation by a horizontal electric dipole beneath a daytime ionosphere, end‐on launching is to be preferred to broadside launching for a ground‐based horizontal dipole exciter. However, for an elevated horizontal dipole source, the preferred direction of launch depends upon the ground conductivity as well as transmitter‐receiver distance. Thus, even with weak ionospheric anisotropy, mode coupling can be significant. Up to elevations of 40 km (the highest considered in this paper), the elevated vertical dipole is a more efficient source of vertical E at the ground than is the elevated horizontal dipole.