Editing Loading coil (section)

=== 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.