Theory of EM wave propagation through tunnels


  • James R. Wait


A rather ingenious communication technique is being developed for use in Belgian coal mines. The idea is to use a two-wire transmission line or equivalent loosely braided coaxial cable that can be suspended from the upper wall. A transmitter placed in the vicinity of the line excites a strongly unbalanced mode that normally would propagate like a coaxial or TEM mode with relatively high attenuation. The key step in the Belgian system, and the closely related French system, is to convert this unbalanced mode to a balanced mode that is much less attenuated because the return current flows mostly in the second wire rather than through the surrounding rock. The physical basis of this system has many similarities with the leaky feeder device described by Martin and used in British coal mines. We summarize the analysis of the propagation characteristics for an idealized mine tunnel of circular cross section. The extension to a rectangular tunnel is also considered. Some comparison is made with relevant experimental data. Actually, we deal only with the twin open wire pair that is the basic ingredient of the so-called INIEX/Deryck system. However, as suggested above, the leaky braided cable system and the slotted cylindrical shielded cables have common features. The important point is that propagation modes which depend on current flow in the tunnel walls will suffer excessive attenuation while modes which utilize the second conductor as a return current path will have less attenuation.