Carrier telephone system for mine railroads

The system, called ZIG, was developed in Germany to operate on a two-wire transmission line to which the locomotive stations are coupled inductively via loop aerials. The system provides communication up to 12.5 miles. Operating frequencies are between 35 and 135 kc/s; frequency modulation is applied because it is less susceptible to large variations of the level. In field trials, the coupling attenuation between the line and mobile stations varied by up to 23 db. Transmission properties of typical lines are discussed in detail, and operation of the system is described for the cases of central-mobile and mobile-mobile stations. A block diagram of the central station is shown. The results of field trials are described.