Modeling and measurement of radio propagation in rectangular tunnels at low frequencies

Radio propagation in tunnels has been investigated for decades and many methods for modeling tunnel propagation have been proposed. Most of the methods, however, assume that the wavelengths of interest are much less than the tunnel dimensions, which limits their application to relatively high frequency signals. In this paper, we investigate a numerical method that can be applied to analyze power attenuation at low frequencies as wavelengths are comparable to tunnel dimensions. The method is based on numerically solving the complex boundary conditions for frequencies of interest. We validate the method with wideband (spanning from 25 MHz to 1525 MHz) propagation measurements in a rectangular tunnel without an arched ceiling.