Surface Boundary conditions for lossy dielectrics to model electromagnetic wave propagation in tunnels

This paper presents, an original technique for modeling the propagation in arbitrary-shaped tunnels, it is based on the Transmission Line Matrix method (TLM) for the analysis of guided structures and the concept of Surface Impedance Boundary Condition (SIBC) for modeling the tunnel walls. Surface impedance is a frequency concept; by using a pade-rational approximation and bilinear transform, the implementation in time domain is obtained. The SIBC implementation proposed is validated for the case of a parallel-plate waveguide. Wave propagation in a circular tunnel with lossy dielectric walls is then studied, showing that this algorithm is suitable for radio-wave propagation in tunnels at high frequencies. Useful information for on-board antenna design in railway systems can thus be obtained.