in wire communications, an electric circuit designed to connect some signal source with a receiver by means of circuits that already connect other signal sources and receivers, called side circuits. This is achieved by connecting transformers with a center tap in the secondary winding into the side circuits. The use of phantom circuits is the simplest way to obtain additional signal paths from existing physical circuits. Signals transmitted from the sources through the side circuits and phantom circuits are transmitted only to their intended receivers; no addition of signals occurs. The separation of the phantom circuit from the side circuits is due to the properties of a balanced bridge circuit. Phantom circuits may be unsymmetrical with respect to ground (Figure 1, a) or symmetrical (Figure 1, b).

Figure 1. Diagrams of phantom circuits: (a) unsymmetrical with respect to ground, (b) symmetrical with respect to ground; (S_s) signal source in side circuit, (S_p) signal source in phantom circuit, (R_s) receiver in side circuit, (R_p) receiver in phantom circuit, (T) transformer

Phantom circuits are used in line multiplexing, often as a part of other, more advanced multichannel communication systems, as well as in laying long-distance power supply cables to unmanned repeater stations and in designing service communications circuits that use long-distance trunk lines. Phantom circuits are also used in local telephone and, less frequently, telegraph communications.