Analysis of Radio Frequency Transmission along a Trolley Wire in a Mine Tunnel

An idealized model of a trolley wire communication system in a tunnel is considered. To facilitate the analysis the tunnel cross section is taken to be semicircular and the surrounding rock medium is homogeneous with a finite conductivity. The metallic rails or other conductors on the flat floor of the tunnel are represented by a plane metallic reflecting surface of infinite extent. The trolley wire and associated feeder line are assumed to be equivalent to a single cylindrical conductor that is parallel to the axis of the tunnel but located anywhere within the cross section. The appropriate modal equation is then solved for the propagation constant of the dominant mode. It is shown that the attenuation rate increases significantly as the trolley wire approaches the tunnel wall. However, even when the trolley wire is within 20 cm (8 in) of the wall, the attenuation rate at 200 kHz is still less than 2 dB per kilometer.