Control of mode conversions on bifilar line in tunnels

The influence of frequency and of the position of the line on mode conversions is studied and previous observations of other researchers are explained. The means whereby exchanges between the two propagation modes may be provoked at the level desired, to meet the needs of propagation and coupling between the transmitter and the balanced mode are investigated. These investigations lead to the definition and realization of an ideal mode converter. Such a mode converter reduces the coupling loss, whatever the frequency, and allows a good control of the field level in the tunnel. It consists in introducing a four-port network or a transformer on the line, where a mode conversion is needed. Theoretical curves, checked experimentally, are given, which make it possible to determine the nature of the four-port network as a function of the conversion coefficient desired.