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alarm amplitude modulation antenna attenuation automatic battery belt BuMines 1C cable cage capacitor carbon monoxide carrier current carrier phone system cellular repeater channel circuit coaxial cable communica communication system conductor connected coupler dedicated wire detect dial dispatcher dispatcher's electrical entry equipment feet foreman frequency haulage haulageway inductor input installed located longwall longwall mining loop loopback loudspeaking maintenance methane miles modulation monitoring systems multipair multiplexing munication noise operation output PABX pager phone paging Paper in Underground personnel phone line portable propagation radio system range receiver relay remote repeater rescue team roof safety seam sensors shaft shown in figure shuttle cars signal single-pair station submains surface switch telephone system tion transceiver transmitted trapped miner trolley carrier phone trolley wire trolley wire-rail twisted pair two-way typical Underground Mine Communications unit vehicles ventilation voice voltage volts