Voice frequency electromagnetic trapped miner location devices were tested for reliability and effectiveness in 93 coal mines. Tests included uplink pulsed carrier waves, downlink pulsed carrier waves, and downlink voice evaluations. Frequencies were 630, 1,050, 1,950, and 3,030 hertz. Overburden depths ranged from 70 to 1,550 feet. Successful communications were established at 91 of the 93 mines. Data indicate the transmitters and receivers will operate in practically all coal mine environments. Reliability and accuracy as location devices appear to depend on noise, depth, and presence of metallic objects that distort the magnetic field. In addition, theoretical models appear to define best case situations in terms of noise and conductivity. Minor hardware problems were encountered, mostly involving battery contacts, switches, and antenna cables on the receivers. No hardware problems occurred with the transmitters.