Propagation characteristics of the dominant mode in tunnels with elliptical cross section

Abstract

AbstractThis paper seeks to obtain basic knowledge of the wave propagation characteristics for radio communication in tunnels or subways. The problem of a hollow cylinder surrounded by a lossy medium is modeled and the propagation characteristics of the dominant mode, which is most important in practice, are rigorously obtained. For a medium of elliptical cross section with high conductivity, the propagation characteristics can be calculated by a perturbation method, whereas for those media with low conductivity, such as concrete and rock, this method fails. Herein the governing characteristic equation for the propagation constant is derived and the constant is calculated numerically.Since the elliptical cross section has two symmetric axes, there exist odd and even dominant modes. It is shown numerically that in the hollow elliptical cylinder in an actual tunnel, the two attenuation constants for the two modes decrease with frequency and that the odd mode attenuation is less than that of the even mode. The attenuation is also investigated with respect to the medium constants and is found to be dependent more on the dielectric constant than conductivity. Also, measurement of attenuation according to the modeled experiment is found to be in good agreement with the analytical results.