The design and implementation of a UHF radio system using distributed antennas, passive reflectors and two-way signal boosters in a room and pillar limestone mine

The Bureau of Mines (BOM) has established a project to demonstrate means for improving underground communications in metal/nonmetal room and pillar mines. The Black River Mine, a large limestone mine near Butler, Kentucky, was selected for conducting propagation tests on 466 and 812 MHz and to develop a practical operating system. Tests of communications between handheld 2 watt ultra high frequency (UHF) transceivers in the mine proved satisfactory for approximately 2,000 feet (610 m) through straight and level haulageways but the range of communication into intersecting crosscuts was quite limited. It was evident that radiation from the transceivers was not being efficiently reflected by limestone pillars into the intersecting crosscuts. Two passive aluminum reflectors, each four feet (1.22 m) square, were installed near the roof and positioned 45° with respect to the axis at an intersection of two haulageways and the range of communications down the intersecting haulageways was significantly extended. Two distributed antenna and base station systems, each feeding four antennas located at principal intersections of haulage and travel roads, were installed and signal or fade margins above 20 dB quieting were measured to determine where reflectors were needed. The reflectors were very effective when they were within direct line of sight and less than 1,000 feet (305 m) from an antenna. The two antenna systems provided satisfactory coverage along the principal travel and haulageways in approximately 70 percent of the central portion of the mine but coverage was unreliable in the active mining areas around the perimeter of the mine. The alternatives of either installing four additional base station and distributed antenna systems or extending the original distributed antenna systems by adding two-way multichannel signal boosters to compensate for cable attenuation were evaluated. The signal booster concept was selected because it is less expensive, simpler to control, avoids hetrodyne interference and communications can be readily extended as the mined area is expanded.