Approximate evaluation of the wave propagation parameters of MF TL communication systems for mine tunnels using image theory

Mine tunnel communications are migrating to wireless; however, medium frequency (MF) communications based on conductor-to-wall transmission line (TL) structures still exist and can be used as emergency or backup systems. This research, addressing MF TL communication systems, is aimed at the finding of simple approximated expressions of the wave propagation parameters characterizing the transmission properties of such systems: attenuation, phase velocity, and characteristic impedance. Bearing in mind the variety of tunnels’ geometries and the lack of rigorous information about the surrounding soil electromagnetic properties, it turns clear that general accurate solutions of the problem are out of reach in a general formulation. In this work, a tunnel model with an equivalent circular cross section and a homogeneous soil with average conductivity is adopted and analyzed. The novelty of the analysis is its intrinsic simplicity due to the employment of image theories, which are applied not only to obtain the capacitance and external inductance of the TL but also to obtain the soil return impedance. Closed-form approximate expressions of the MF TL wave propagation parameters are offered and discussed, their dependence on the frequency and on the position of the signal carrying conductor located inside the tunnel being disclosed.