Dubanton's and Wait's Approximations of the Earth Return Impedance Used for Evaluating Transmission-Line Wave Propagation Parameters

In this letter, a new theoretical support of the well-known Dubanton's approximation widely used in the evaluation of the earth return impedance of transmission lines is provided. For that purpose, the electromagnetic-wave theory of guided cylindrical waves along an axial conductor inside a circular tunnel, developed by J. Wait, is utilized, considering that the tunnel radius is very large and that only the fundamental quasi-transverse electric-magnetic mode propagates.