Editing Loading coil (section)

===Telephone lines===
{{multiple image
| align = right
| direction = horizontal
| header   =
| image1   = Toroidal telephone loading coil.jpg
| width1   = 170
| image2   = Telephone loading coils 1922.jpg
| width2   = 140
| footer   = ''(left)'' Toroidal 0.175 H loading coil for an AT&T long distance telephone trunkline from New York to Chicago 1922. Each of the 108 twisted pairs in the cable required a coil. The coils were enclosed in an oil-filled steel tank ''(right)'' on the telephone pole. The cable required loading coils every 6000 ft (1.83&nbsp;km).
}}

A common application of loading coils is to improve the [[voice frequency|voice-frequency]] amplitude response characteristics of the [[twisted pair|twisted balanced pairs]] in a telephone cable. Because twisted pair is a [[balanced line|balanced]] format, half the loading coil must be inserted in each leg of the pair to maintain the balance. It is common for both these windings to be formed on the same core. This increases the [[magnetic flux|flux]] linkages, without which the number of turns on the coil would need to be increased. Despite the use of common cores, such loading coils do not comprise [[transformer]]s, as they do not provide [[coupling (electronics)|coupling]] to other circuits.

Loading coils inserted periodically in series with a pair of wires reduce the [[attenuation]] at the higher voice frequencies up to the [[cutoff frequency]] of the [[low-pass filter]] formed by the inductance of the coils (plus the distributed inductance of the wires) and the distributed capacitance between the wires. Above the cutoff frequency, attenuation increases rapidly. The shorter the distance between the coils, the higher the cut-off frequency. The cutoff effect is an artifact of using [[lumped-element model|lumped]] inductors. With loading methods using continuous [[distributed-element model|distributed]] inductance there is no cutoff.

Without loading coils, the line response is dominated by the resistance and capacitance of the line with the attenuation gently increasing with frequency. With loading coils of exactly the right inductance, neither capacitance nor inductance dominate: the response is flat, [[waveform]]s are undistorted and the [[characteristic impedance]] is resistive up to the cutoff frequency. The coincidental formation of an [[audio frequency]] filter is also beneficial in that noise is reduced.

====DSL====
With loading coils, signal attenuation of a circuit remains low for signals within the [[passband]] of the transmission line but increases rapidly for frequencies above the audio cutoff frequency. If the telephone line is subsequently reused to support applications that require higher frequencies, such as in analog or digital [[carrier system]]s or [[digital subscriber line]] (DSL), loading coils must be removed or replaced. Using coils with parallel capacitors forms a filter with the topology of an [[m-derived filter]] and a band of frequencies above the cut-off is also passed. Without removal, for subscribers at an extended distance, e.g., over 4 miles (6.4&nbsp;km) from the central office, DSL cannot be supported.

====Carrier systems====
American early and middle 20th century telephone cables had load coils at intervals of a mile (1.61&nbsp;km), usually in coil cases holding many. The coils had to be removed to pass higher frequencies, but the coil cases provided convenient places for repeaters of digital [[T-carrier]] systems, which could then transmit a 1.5&nbsp;Mbit/s signal that distance. Due to narrower streets and higher cost of copper, European cables had thinner wires and used closer spacing. Intervals of a kilometer allowed European systems to carry 2&nbsp;Mbit/s.