This paper analyzes radio wave propagation characteristics in a tunnel with a rectangular cross section and multi-layer-structure walls. These walls consist of a surface moisture layer, concrete reinforcing mesh, concrete, and bedrock. The wavelengths are assumed to be small compared with the tunnel's cross section. In the - analysis, the geometrical optic approximation is used to determine the radio wave propagation routes in this tunnel. Also, the input impedance of the material that forms each layer of the multi-layer walls is given by an equivalent transmission matrix and applied to the calculation of the reflection loss for radio waves against tunnel walls. In addition, we give an example of the calculation of radio wave propagation characteristics in a tunnel using the tunnel cross-section dimensions, dielectric constant of rock, reinforced concrete, and other parameters.
