Experimental data for groundwave propagation over cylindrical surfaces

Abstract

Experimental data for the fields of EM groundwaves propagating over cylindrical homogeneous paths and two-section mixed paths were obtained by microwave (4.765 GHz) modeling. The cylindrical surfaces, which have a radius of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20 \lambda_{0}</tex> , closely approximate spherical surfaces insofar as groundwave propagation is concerned. The model is a curved tank which was constructed as a stratified combination of Plexiglas over distilled water, giving a predictable highly inductive surface impedance. Aluminum foil laid on the Plexiglas produced a nearly perfectly conducting surface wherever needed for the mixed path cases. For both homogeneous and mixed paths the residue series theory agrees well with the experimental data in the shadow and penumbra regions, but the agreement becomes poorer as the observation point moves into the illuminated region where an excess number of residue terms is needed. The results show that a groundwave which crosses an abrupt boundary rapidly acquires the characteristic properties of the second medium at small heights above the surface, and that the equivalent integral solution for mixed paths is therefore adequate close to the surface. This is especially useful near the boundary where the residue series converges poorly. It is concluded that if the constitutive electrical parameters of the earth are precisely known and constant, the theory can be reliably applied to LF and VLF groundwave propagation over the earth where the constraints are even less severe.