b. Operation. The simplexed leg of the simplexed-phantom circuit contains a ground-return circuits which
permits transmission of telegraph signals between telegraph instruments T7 and T8. The connections of these instruments
are similar to those of the simplex circuit. One terminal of set T7 is connected to the center terminal of the secondary of
repeating-coil RC5, and one terminal of set T8 to the center terminal of secondary RC6. The other terminal of each set is
grounded as effectively as possible, to furnish the return path. By using an analysis similar to that given in the discussion
of simplex and phantom circuits, the currents of the simplex leg, the phantom circuit, and the side circuit can be traced.
Such an analysis will show that a signal originating in T1 is reproduced only in T2, one originating in T3 is reproduced
only in T4, one originating in T5 is reproduced only in T6, and a telegraph impulse originating in T7 is reproduced only in
T8. In the usual operation of a simplexed-phantom circuit, the side circuits and the phantom circuit are used to obtain
three telephone channels, since these circuits involve full metallic-line operation. The simplex leg provides a telegraph
channel.
c. Limitations.
The same general limitations discussed in connection with simplex and phantom
circuits, and concerned primarily with the maintenance of perfect balance in the metallic lines and repeating
coils, apply to simplexed-phantom circuits. The existence of four channels in a simplexed-phantom circuit
makes perfect balance even more critical than in the other types. For this reason, and because they are more
difficult to maintain in the field, simplexed-phantom circuits often are not used in forward areas of operation.
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