An Experimental Look at Rf Propagation in Narrow Tunnels

A measurement campaign was undertaken at the Camp Lejeune North Carolina (CLNC) Military Operations on Urban Terrain (MOUT) site to characterize subterranean RF propagation. Previous related studies for subterranean wireless communications purposes have focused on railroad, auto, subway and mine tunnels having wide cross-section dimensions exceeding 3 meters. An overlooked infrastructure element with significant tactical communications value is the municipal sewer tunnel network. As long as the sewer line is accessible and navigable by the warfighter, it will have utility and so interior diameters down to 1-meter are not out of the question. The CLNC MOUT subterranean tunnel complex has such a narrow diameter, as well as representative L-bends and T-junctions. A series of path loss measurements were conducted for various spatial separations of transmit and receive antennas in the ultrahigh frequency (UHF) band covering 374 MHz as well as the 900 MHz and 2.4 GHz Industrial, Scientific, and Medical (ISM) bands. In contrast to the wider, predominately vehicular tunnels, these transmission wavelengths are critically close to the sewer tunnel dimensions. The test methodology is explained and the results are compared with predictions based on modeling the sewer tunnel as an oversized, lossy circular waveguide. The unique findings presented herein are applicable to other tightly confined spaces such as natural cave passageways