Loading Coils

What is a loading coil?

Loading coils were first used in 1911 to extend the reach of POTS (plain old telephone service). The phone company introduce these devices evenly spaced in long loops at 6000 ft intervals and 3000 ft from each end. The function of loading coils is to enhance the frequencies between 300 Hz to 4000 Hz at the expense of higher frequencies. Loading coils have a low pass filter effect making it impossible to coexist with adsl, isdn and maybe dialup. Loading coils must be removed to make adsl work.

TDRs can detect the first loading coil in a line but can't see what is behind it so cleaning up long loops with multiple loading coils can be time consuming.

Example of loading coils
88 mh
Enclosure with multiple
loading coils


As loop distance increases the electrical characteristics of the line change. Capacitance and attenuation increase with distance and frequency. To compensate for capacitance loading coils are introduced.

Inductance units are measured in Henrys (Joseph Henry) since the Henry is a very large unit practical coils are measured in milli-Henrys or micro-Henrys. A typical loading coil is the H88 which is a 88 mH coil suitable for 6000 ft intervals other standard values are 66 mH, 44 mH, 22 mHS etc.


Loading coils and adsl are incompatible. The low pass effect of these devices make higher frequencies impossible to pass. If adsl is installed in a line loaded with coils they must be removed to make adsl work.

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Last Update: October 8, 2014