Hybrid coil

In telecommunication, a hybrid coil is a single transformer that effectively has three windings, and which is designed to be configured as a circuit having four branches, i.e., ports, that are conjugate in pairs.

Note: The primary use of a hybrid coil is to convert between 2-wire and 4-wire operation in concatenated sections of a communications circuit. Such conversion is necessary when repeaters are introduced in a 2-wire circuit. Synonym bridge transformer.

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