Theoretical and experimental analysis of propagation in tunnels

Telecommunications in tunnels is being investigated in order to ensure a radio link between a mobile and a ground station. The problem of providing data communications between a ground station and vehicles becomes important with the diffusion of fully automated driverless urban transportation systems. To achieve high transmission rates, it is necessary to use at least centimetric microwave links. It is well known that high-frequency waves can propagate inside a tunnel which behaves as an oversized waveguide. The influence of changes in the cross section of the tunnel (such as broadening or a narrowing) on the electric field variation is analyzed. The excitation of the tunnel from a wave impinging the entrance plane or the radiation of the tunnel towards free space (when a transmitter is situated inside the tunnel) are considered both experimentally and theoretically through the geometrical theory of diffraction.>