ABSTRACT
A communications channel is an integral part of a pilot relaying system that is used to protect a transmission line. It is needed to deliver information from one terminal of the line to the other, so that a fault location can be determined. Fiber optics is similar to pilot wire costs, in increasing linearly, but the cost per mile is considerably higher. Audio tones fall within the frequency band that is audible to the human ear. These low-frequency channels are used independently on a pair of pilot wires, on a voice channel on power line carrier and on a voice channel on microwave. Single-side-band carrier accommodates multiple channels in a manner comparable with that in microwave, but has a substantially higher terminal cost than a dedicated power line carrier. If hybrids are used to permit multiple channels to cooperatively use the same coupling devices for various functions, their losses must be evaluated.
