Ray tracing analysis of propagation characteristics in tunnels including transmitting antenna

The radio wave propagation characteristics of a communication system inside a tunnel are reported. A leaky coaxial cable communication system has a large transmission loss in the high frequency range, so a novel system using optical fibers and a flat antenna is proposed. This system is free from the transmission loss due to the optical fiber, however, the cost of electro-optical (E/O) and opto-electronic (O/E) units are expensive. To overcome this difficulty, it is required that the coverage area of the antenna should be maximized. It is then necessary to investigate the optimal radiation pattern of the source antenna installed inside the tunnel. The tunnels may be regarded as oversized lossy waveguides and the field distributions inside them are calculated by using waveguide theory, however, it is difficult to include the radiation pattern when investigating the source near region. We use the ray-tracing method based on the theory of geometrical optics to include the location of the source and its radiation pattern. We demonstrate the validity of the ray tracing method by comparison with measurements using a scaled model and a hollow tunnel. We also discuss the optimal radiation pattern of the source antenna inside the tunnel.