(3) The current in the secondary of RC2, however, induces an emf in the primary. A corresponding current
therefore flows down through the primary of RC2 and through the receiver of T2, reproducing the sound introduced at the
transmitter of T1.
(4) Similar analysis can be made for conversations originating in T2, T3, and T4. They will show that the
operation of a phantom group prevents interference between side circuits and the phantom circuit. Of course, perfect
operation requires perfect balance of the lines of the side circuits, and well balanced repeating coils.
73.
Advantages and Limitations of Phantom Circuits.
a. Advantages.
(1) The most important advantage of the phantom circuit over the simplex circuit is the elimination of the ground
return. This gives an additional telephone channel with freedom from the inductive interference to which ground-return
circuits are susceptible. Phantom circuits are used to obtain an additional telegraph channel when good ground
connections--always necessary for its operation in a simplex circuit--are difficult to obtain. Phantom circuits often are
used in teletypewriter communications.
(2) When the side circuits of a phantom group are composed of cables instead of open-wire lines, it is possible to
reduce attenuation and obtain distortionless operation by using loading coils. These coils are arranged so that the currents
in the phantom circuit produce aiding magnetic fluxes in the core of the phantom loading coil, but opposing fluxes in the
cores of the loading coils in the side circuits. In this way, the phantom currents do not affect the side circuits, nor do the
side-circuit currents affect the phantom circuit. Of course, the loading coils must be designed, constructed, and installed
carefully, to maintain a perfect balance and prevent crosstalk between the phantom and side circuits.
b. Limitations. For the circuits of a phantom group to operate without interference, the two metallic
circuits upon which the phantom circuit is superimposed must be well balanced. The method of balancing a line
by use of a rheostat in the low-resistance side of an unbalanced line can be used also in the side circuits of
a phantom group.
Also, for operation without interference, the repeating coils must be identical and
installed properly.
When these conditions are satisfied, a phantom-group operation has advantages that
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