Editing Loading coil (section)

==Applications==
{{Unreferenced section|date=January 2008}}
[[File:Loaded line schematic.svg|thumb|600px|center|Schematic of a balanced loaded telephone line. The capacitors are not discrete components but represent the distributed capacitance between the closely spaced wire conductors of the line, this is indicated by the dotted lines. The loading coils prevent the audio (voice) signal from being distorted by the line capacitance. The windings of the loading coil are wound such that the magnetic flux induced in the core is in the same direction for both windings.]]

===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.

=== Radio antenna <span id="antenna_loading_anchor" class="anchor"></span> ===
[[File:CB antenna.jpg|thumb|upright=1.3|A typical mobile antenna with a center-placed loading coil]]
[[File:Large antenna loading coil.jpg|thumb|upright=1.3|An enormous antenna loading coil used in a powerful [[longwave]] [[radiotelegraphy|radiotelegraph]] station in New Jersey in 1912]]
Another type of loading coil is used in radio [[antenna (radio)|antenna]]s. [[Monopole antenna|Monopole]] and [[dipole antenna|dipole]] radio antennas are designed to act as [[resonator]]s for radio waves; the power from the transmitter, applied to the antenna through the antenna's [[transmission line]], excites [[standing wave]]s of voltage and current in the antenna element. To be "naturally" resonant, the antenna must have a physical length of one quarter of the [[wavelength]] of the radio waves used (or a multiple of that length, with odd multiples usually preferred). At resonance, the antenna acts electrically as a pure [[electrical resistance|resistance]], absorbing all the power applied to it from the transmitter.

In many cases, for practical reasons, it is necessary to make the antenna shorter than the resonant length, this is called an [[electrically short]] antenna. An antenna shorter than a quarter wavelength presents [[capacitive reactance]] to the transmission line.<ref>{{Cite web |title=Introduction to Radio Equipment - Chapter 20 |url=https://maritime.org/doc/radio/chap20.php |access-date=2025-03-17 |website=maritime.org}}</ref> Some of the applied power is reflected back into the transmission line and travels back toward the transmitter {{Citation needed|date=April 2021}}. The two currents at the same frequency running in opposite directions causes [[standing wave]]s on the transmission line {{Citation needed|date=April 2021}}, measured as a [[standing wave ratio]] (SWR) greater than one. The elevated currents waste energy by heating the wire, and can even overheat the transmitter.

To make an [[electrical length|electrically short]] antenna resonant, a loading coil is inserted in series with the antenna. The coil is built to have an [[inductive reactance]] equal and opposite to the capacitive reactance of the short antenna, so the combination of reactances cancels. When so loaded the antenna presents a pure resistance to the transmission line, preventing energy from being reflected. The loading coil is often placed at the base of the antenna, between it and the transmission line (''base loading''), but for more efficient radiation, it is sometimes inserted near the midpoint of the antenna element (''center loading'').{{citation needed|date=April 2021}}

Loading coils for powerful transmitters can have challenging design requirements, especially at low frequencies. The [[radiation resistance]] of short antennas can be very low, as low a few ohms in the [[low frequency|LF]] or [[very low frequency|VLF]] bands, where antennas are commonly short and inductive loading is most needed. Because resistance in the coil winding is comparable to, or exceeds the radiation resistance, loading coils for extremely electrically short antennas must have extremely low AC [[electrical resistance|resistance]] at the operating frequency. To reduce [[skin effect]] losses, the coil is often made of tubing or [[Litz wire]], with single layer windings, with turns spaced apart to reduce [[Proximity effect (electromagnetism)|proximity effect]] resistance. They must often handle high voltages. To reduce power lost in [[dielectric loss]]es, the coil is often suspended in air supported on thin ceramic strips. The capacitively loaded antennas used at low frequencies have extremely narrow bandwidths, and therefore if the frequency is changed the loading coil must be adjustable to tune the antenna to resonance with the new transmitter frequency. [[Inductor#Variable inductor|Variometers]] are often used.

===Bulk power transmission===
To reduce losses due to high capacitance on long-distance [[Electric power transmission#Bulk transmission|bulk power transmission lines]], inductance can be introduced to the circuit with a [[flexible AC transmission system]] (FACTS), a [[static VAR compensator]], or a [[static synchronous series compensator]]. Series compensation can be thought of as an inductor connected to the circuit in series if it is supplying inductance to the circuit.