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The Phantom Circuit

Reference is made in Table IV to the phantom circuit. This circuit is obtained, as indicated in Fig. 19, from two pairs of line wires (or from two cable pairs). By the use of the phantom circuit, three telephone conversations can be carried on simultaneously over two pairs of wires.

Figure 19. A phantom telephone circuit is obtained over four wires as shown. The letter T signifies a telephone set.

If the transformers (or repeating coils, as they are commonly called in telephony) are tapped at exactly their electrical center, and if the impedance of each line wire is the same, the currents from the phantom circuit will divide equally. Since these currents will flow in opposite directions through the transformer windings, their magnetic effects will neutralize, and no magnetic flux will be produced to induce speech currents in the side circuits. Instantaneous directions of current flow for a phantom circuit are shown; side-circuit currents will flow as if the phantom circuit were not present.


Home Transmission Lines The Phantom Circuit

Last Update: 2011-05-30