A charging coil compensates for signal losses in telephone lines by inducing an electric current via magnetic fields. They are placed every 6,000 feet and reduce electrical losses at high frequencies. Coils are based on pupin coils and capacitance is defined by the length of the cable conductor. Inductance helps equalize losses along a vocal cord. Frequencies can be further changed using an unloaded phantom setup. Different types of loading coils are used for different telephone lines and distances.
A charging coil is a device placed along a circuit in a telephone line that compensates for signal losses over long distances. Two strands are rolled into a flat donut shape, often referred to as a torrid shape. The structure induces an electric current via magnetic fields to compensate for lost signal strength. About an inch and a half wide, the loading coil sits alongside the circuit instead of acting as a coupling to it. Loading coils are designed to moderate or minimize signal loss, or attenuation, at high voice frequencies in telephone lines.
Charging coils are typically placed every 6,000 feet (about 1.8 kilometers) along a telephone line. They reduce electrical losses at high frequencies, down to frequencies that a circuit’s filter or amplifier would begin to reduce. The signals lose strength significantly when this cut-off frequency is reached, so load coils are not used in twisted pairs of wire where the frequencies are not high enough.
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 were used in this design, which store energy when an electric field is present . In the early 21st century, capacitance is still defined by the length of the cable conductor and the space between the conductors.
The length of the cable affects the amount of energy lost along the way. Inductance, the storage of energy in a magnetic field, helps equalize losses along a vocal cord, which is the function of a loading coil. While the loss in intensity is not typically noticeable on a speech signal, it can be increased to the point that it does over a long distance. Upload reels are also used on the digital subscriber lines (DSL) of modern telephone systems. In the past, induction coils were placed inside cargo tanks, steel structures on telephone lines up to three feet high.
Frequencies can be further changed using an unloaded phantom setup. If the load coil reduces frequency power too much, this design can modify the effect to suit broadcast applications. Inductance can also be adjusted to a specified level. Another consideration is the type of loading coil used, due to the different requirements for telephone lines and the different distances signals must travel on a complex line.
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