Accurate Electromagnetic Analysis of MF TL Backup Communications in Cylindrical Tunnels

Medium frequency transmission line (MF TL) communications in circular cylindrical underground tunnels critically depend on the soil return impedance. The soil impedance can be approximately calculated using a very simple closed-form expression based on an image method-the so-called Dubanton approach. In this paper, we present an analytical exact method, based on electromagnetic field analysis, using Bessel and Hankel functions to correctly evaluate the soil impedance and the wave propagation parameters (attenuation, phase velocity, and characteristic impedance) that characterize the fundamental quasi-TEM mode propagating along circular tunnels. By comparing the computation results of both methods we have been able to quantify the errors involved in the Dubanton approach. The approximation developed by Wait is also used for verification purposes.