Medium-frequency (mf) techniques have been successfully applied to the problem of radio voice communications in both coal and metal and nonmetal mines. Long range is achieved by taking advantage of existing mine conductors that carry and propagate the signal over vast areas. It is very likely that in the near future such systems will become the preferred method for general mine communications. Mf techniques can also be applied to the areas of in-mine control and monitoring. Preliminary work in this area is described. In a western mine, locomotives are being operated under mf remote control in a loading operation. In several eastern mines, mf has been evaluated as a method for monitoring the status of devices in remote areas of the mines. All control and monitor information uses existing mine conductors.
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