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Radio Wave Propagation in Tunnels
POLYTECHNIC UNIV BROOKLYN NY
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This report examines the radio propagation model for narrow and long tunnels. Modal analysis is used to model the path gain in 2-D and 3-D rectangular tunnels and the coupling loss of L, T and cross tunnels. Modal attenuation is determined by the wavelength lamda and given tunnel dimension through the factor square lamda cubic w where w is the width of tunnel, by the mode number and dielectric constant of the walls. In oversized tunnels, many nodes ma propagate, however higher modes have high attenuation constant so that a few lower modes are dominant at long distances from the transmitter. The antenna polarization giving the least path loss in 3-D tunnels, is found by comparing attenuation constants of both vertical and horizontal polarization. For the propagation into branches off the main tunnel, mode coupling at the discontinuities is obtained by hybrid ray-mode conversion. By accounting for the mode diffraction at each comer, we compute the coupling loss into cross junctions, the T-junction and L-bends. Because the cross tunnel has four edges and they generate more comer diffracted modal fields than other structures, and have the strongest coupling. The coupling loss is found to increase slightly with tunnel width.
APPROVED FOR PUBLIC RELEASE