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A loading coil is a device placed along a circuit in a telephone line that compensates for the losses in signal over long distances. Two wires are coiled into the shape of a flat doughnut, often referred to as a torrid form. The structure induces an electrical current via magnetic fields to compensate for lost signal power. About an inch-and-a-half wide, the loading coil is located along the circuit instead of serving as a coupling to it. Loading coils are designed to moderate the loss in signals, or attenuation, or minimize it at high voice frequencies in telephone lines.
Loading coils are typically placed every 6,000 feet (about 1.8 kilometers) along a telephone line. They lower the electrical losses at high frequencies, up to frequency strengths that a circuit’s filter or amplifier would start to reduce the level. Signals lose strength significantly when this cutoff frequency is reached, so loading coils are not used in twisted cable pairs where the frequencies are inadequately high.
The coils are sometimes based on the concept of pupin coils, named after inventor Michael Pupin who patented a design for a loading coil in the late 1800s. Capacitors, which store energy when an electric field is present, were used in this design. In the early 21st century, capacitance is still defined by cable conductor length and the space between conductors.
The length of the cable has an effect on how much energy is lost along the way. Inductance, the storage of energy in a magnetic field, helps to equalize losses along a voice cable, which is the function of a loading coil. Even if the loss in intensity is not typically noticeable on a voice signal, it can be increased to the point that it does over a long distance. Loading coils are even used on digital subscriber lines (DSL) of modern telephone systems. In the past, inductor coils were placed inside load pots, steel structures on telephone lines that were up to three feet in height.
Frequencies can be further modified using an unloaded phantom configuration. If the loading coil reduces the frequency power too much, then this design can change the effect to accommodate broadcasting applications. The inductance can be adjusted to a specified level as well. Another consideration is the type of loading coil used, because of different requirements for telephone lines and the varying distances that signals need to travel over a complex line.