Editing Loading coil (section)

{{Short description|Inductor in a transmission line}}
{{Use dmy dates|date=January 2025}}

[[File:Pupin coils in PTT Museum in Belgrade 04.jpg|300px|thumb|[[Mihajlo Idvorski Pupin|Pupin]] coils in [[PTT Museum in Belgrade]] (Serbia)]]

A '''loading coil''' or '''load coil''' is an [[inductor]] that is inserted into an [[electronic circuit]] to increase its [[inductance]]. The term originated in the 19th century for inductors used to prevent signal distortion in long-distance telegraph transmission cables. The term is also used for inductors in [[antenna (radio)|radio antenna]]s, or between the antenna and its [[feedline]], to make an [[Electrical length#Electrical lengthening and shortening|electrically short]] antenna [[resonance|resonant]] at its operating frequency.

The concept of loading coils was discovered by [[Oliver Heaviside]] in studying the problem of slow signalling speed of the first [[transatlantic telegraph cable]] in the 1860s. He concluded additional inductance was required to prevent amplitude and time delay distortion of the transmitted signal. The mathematical condition for distortion-free transmission is known as the [[Heaviside condition]]. Previous telegraph lines were overland or shorter and hence had less delay, and the need for extra inductance was not as great. [[Submarine communications cable]]s are particularly subject to the problem, but early 20th century installations using [[balanced pair]]s were often continuously loaded with iron wire or tape rather than discretely with loading coils, which avoided the sealing problem.

Loading coils are historically also known as '''Pupin coils''' after [[Mihajlo Idvorski Pupin|Mihajlo Pupin]], especially when used for the Heaviside condition and the process of inserting them is sometimes called ''pupinization''.