Phantom Circuit - Unloaded Phantom

Unloaded phantom is a phantom configuration of loaded lines (a circuit fitted with loading coils). The idea here is not to create additional circuits. Rather, the purpose is to cancel or greatly reduce the effect of the loading coils fitted to a line. The reason for doing this is that loaded lines have a definite cut-off frequency and it may be desired to equalise the line to a frequency which is higher than this, for example to make a circuit suitable for use by a broadcaster. Ideally, the loading would be removed or reduced for a permanent connection, but this is not feasible for temporary arrangements such as a requirement for outside broadcast. Instead, two circuits in a phantom configuration can be used to greatly reduce the inductance being inserted by the loading coils, and hence the loading effect.

It works because the loading coils used on balanced lines have two windings, one for each leg of the circuit. They are both wound on a common core and the windings are so arranged that the magnetic flux induced by both of them is in the same direction. Both windings induce an emf in each other as well as their own self-induction. This effect greatly increases the inductance of the coil and hence its loading effectiveness. By contrast, when the circuit is in the phantom configuration the currents in the two wires of each pair are in the same direction and the magnetic flux is being cancelled. This has precisely the opposite effect and the inductance is greatly reduced.

This configuration is most commonly used on the two pairs of a star-quad cable. It is not so successful with other pairs of wires. The difference in the path of the two pairs can easily destroy the balance and results in crosstalk and interference.

This configuration can also be called "bunched pairs". However, "bunched pairs" can also refer to the straightforward connection of two lines in parallel which is not a phantom circuit and will not reduce the loading.