Abstract
Radio propagation in mines and most tunnels by natural means is not adequate for general use, ranges so achieved usually being limited to a-few hundred metres at the most. In this work the transmission-line principle for artificially propagating very-high-frequency (VHF) radio signals through such environments has been studied experimentally and theoretically, and has been developed to allow practical communication over any desired distance in coal mines and other tunnels. The external coupling mechanisms of the two main types of transmission line through their normal imperfections have in particular been investigated, considering especially the extent to which these imperfections themselves may be usefully controlled. Guiding principles have been established for optimizing the choices of line and frequency for any given application. Methods of extending the basic range by judicious branching and networking are then examined, together with the use of active devices - line repeaters and multiple base stations - to increase the coverage further to any desired extent; these techniques themselves introduce problems which in turn are treated. Finally, a description is given of an engineered system based on the principles developed. This has been in operational use for over two years, and a production version is being installed in a number of British mines. Although the principles described are applicable to all kinds of tunnel, and to some extent surface applications also, the main emphasis has been on coal mines; the question of intrinsic safety of equipment is here important, and has influenced both the conduct of the experimental work and the design of the final equipment.