Effective use of natural modes in VHF and UHF tunnel propagation

An experimental study of the electromagnetic wave propagation in various tunnels, at frequencies between 1 MHz and 1,000 MHz is presented. The results obtained provide a better understanding of natural propagation mechanisms in underground galleries containing no transmission line. The analysis of these results reveals the existence of a cutoff frequency and a strong correlation between the propagation characteristics and the transverse dimensions of the tunnel. Further experiments are made in tunnels where a transmission line is used as a waveguide; they show under what conditions the natural modes can be useful in this case. These modes can advantageously replace the so-called monofilar mode which is generally used for coupling between the transmission line and the mobile transceivers. Experimental checking shows that when using a well adapted transmission line, exploitation of natural propagation can greatly reduce the cost of the line installation without reducing its performance.