Loading coil

In telecommunication, a loading coil is a coil that does not provide coupling to any other circuit, but is inserted in a circuit to increase its inductance.

Note 1: Loading coils inserted periodically in a pair of wires reduce the attenuation at the higher voice frequencies up to the cutoff frequency of the low-pass filter formed by (a) the inductance of the coils and distributed inductance of the wires, and (b) the distributed capacitance between the wires. Above the cutoff frequency, attenuation increases rapidly.

Note 2: A common application of loading coils is to improve the voice-frequency[?] amplitude response characteristics of twisted cable pairs. When connected across a twisted pair at regular intervals, loading coils, in concert with the distributed resistance and capacitance of the pair, form an audio-frequency[?] filter that improves the high-frequency audio response of the pair.

Note 3: When loading coils are in place, signal attenuation increases rapidly for frequencies above the audio cutoff frequency. Thus, when a pair is used to support applications that require higher frequencies, such as carrier systems, loading coils must be absent.

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