A simple quasi-static expression for the propagation constant of a two-wire transmission line located at the air-Earth interface is derived. A numerical solution of the mode equation shows that the quasi-static approximation is valid when the wire separation is much less than a free-space wavelength. The quasi-static approximation can be used to determine the complex dielectric constant of the Earth from measurements of either the propagation constant or the characteristic impedance of the transmission line.
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