71.
Phantom Telephone Circuit.
a. A phantom circuit provides an additional telephone channel on two 2-wire metallic circuits. Side circuit Nos.
1 and 2 in figure 70 are the two 2-wire metallic circuits of such an arrangement. These two circuits, together with the
phantom circuits, constitute a phantom group.
b. The phantom group contains six repeating coils located at the central office. These coils are similar to those
used in simplex circuits. Repeating coils RC1 and RC2 are connected in circuit No. 1 at the line terminals of the
switchboard. Telephone sets T1 and T2 are connected at the switchboard to the switchboard terminals of RC1 and RC2,
respectively. Similarly, repeating coils RC3 and RC4 are connected inside circuit No. 2, and telephone sets T3 and T4 are
connected at the switchboard to the respective switchboard terminals of these repeating coils. The phantom circuit uses
two additional repeating coils, RC5 and RC6. One line terminal of RC5 is connected to the center terminal of RC1, and
the other terminal is connected to the center terminal of RC3. Similarly, the two line terminals of RC6 are connected,
respectively, to the center terminals of RC2 and RC4. The center terminals of RC5 and RC6 are not connected to any
other point. Telephone set T5 is connected at the switchboard to the switchboard terminals of RC6. T5 and T6 also may
represent telegraph sets instead of telephones, so that phantom operation provides either additional telephone or telegraph
channels. It is also possible to construct a phantom group without the use of repeating coils RC5 and RC6 by connecting
set T5 directly to the center terminals of RC1 and RC3, and connecting set T6 to the center terminals of RC2 and RC4.
This arrangement affords economics in initial cost and maintenance, since it eliminates two repeating coils, but it is more
susceptible to inductive interference..
72.
Analysis of Phantom Circuit.
a. Path of phantom-circuit current.
(1) As in the simplex circuit, the center-tapped iron-core repeating coil used in a phantom group
prevents the voice currents of the phantom circuit from interfering with those of the side circuits. Assume
that at a given instant the phantom circuit in figure 70 is operating from left to right; that is, a person at telephone
set T5 is talking to someone at telephone T6. If the voice current in the primary winding of RC5 is assumed to
flow down at this instant, the emf induced in the secondary will cause a corresponding current to flow up
147
Previous Page
