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Abstract-At the ultra-high frequencies (UHF) common to portable radios, the mine tunnel acts as a dielectric waveguide, directing and absorbing energy as a radio signal propagates. Understanding radio propagation behavior in a dielectric waveguide is critical for designing reliable, optimized communication systems in an underground mine. One of the major parameters used to predict the power attenuation in lossy waveguides is the attenuation constant. In this paper, we theoretically and experimentally investigate the attenuation constants for a rectangular waveguide with dielectric walls. We provide a new derivation of the attenuation constant based on the classic Fresnel reflection coefficients. The new derivation takes advantage of ray representation of plane waves and provides more insight into understanding radio attenuation in tunnels. We also investigate the impact of different parameters on the attenuation constant, including the tunnel transverse dimensions, permittivity, conductivity, frequency, and polarization, with an aim to find their theoretical optimal values that result in the minimum power loss. Additionally, measurements of the attenuation constants of the dominant mode at different frequencies (455, 915, 2450, and 5800 MHz) for a straight concrete tunnel are presented and compared to theoretical predictions. It is shown that the analytical results match the measured results very well at all four frequencies.

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