Transmission line method for propagation characteristic of electromagnetic wave in rectangular tunnel

The paper presents the equivalent transmission line equation which is useful to describe the propagation characteristics of electromagnetic waves in a tunnel. For the transmission line theory, the mathematical structure of the classical telegrapher equation was preserved, while the coefficients (the per-unit-length parameters) were generalized in order to represent the essential characteristic of the wire structure. At frequencies in the range of 200–4000 MHz transmission takes the form of waveguide propagation in a tunnel, since the wavelengths of the UHF waves are smaller than the tunnel dimensions. So, the tunnel is equivalent to the lossy uniform transmission lines under this condition. In the waveguide structure of the tunnel, the equivalent voltage and current must be introduced, which is called the equivalent transmission line theory. In this paper, the calculation of parameters for many propagation modes in a tunnel is ended to the solution of active equivalent transmission line equation. As examples, the coefficients of equivalent circuit and the attenuation rate for the dominant mode of the vertical polarization electromagnetic wave in the rectangular tunnel are computed. Consistent results with the pattern matching theoretical ones are obtained, and the validity of the presented method is thus verified.