Repeating coil

In telecommunication, a repeating coil is a voice-frequency[?] transformer characterized by a closed core, a pair of identical balanced primary (line) windings, a pair of identical but not necessarily balanced secondary (drop) windings, and low transmission loss[?] at voice frequencies.

Note: It permits transfer of voice currents from one winding to another by magnetic induction, matches line and drop impedances, and prevents direct conduction between the line and the drop.

Source: from Federal Standard 1037C and from MIL-STD-188