Editing Radio broadcasting (section)

===AM===
{{main|AM broadcasting}}
AM stations were the earliest broadcasting stations to be developed. AM refers to [[amplitude modulation]], a mode of broadcasting radio waves by varying the amplitude of the carrier signal in response to the amplitude of the signal to be transmitted. The medium-wave band is used worldwide for AM broadcasting. Europe also uses the [[long wave]] band. In response to the growing popularity of [[FM broadcasting#Stereo FM|FM stereo]] radio stations in the late 1980s and early 1990s, some [[North America]]n stations began broadcasting in [[AM stereo]], though this never gained popularity and very few receivers were ever sold.

The signal is subject to interference from electrical storms ([[lightning]]) and other [[electromagnetic interference]] (EMI).<ref>Based on the "interference" entry of ''The Concise Oxford English Dictionary'', 11th edition, online</ref> One advantage of AM radio signal is that it can be detected (turned into sound) with simple equipment. If a signal is strong enough, not even a power source is needed; building an unpowered [[crystal radio|crystal radio receiver]] was a common childhood project in the early decades of AM broadcasting.

AM broadcasts occur on [[North America]]n airwaves in the [[medium wave]] frequency range of 525 to 1,705 [[Hertz|kHz]] (known as the "standard broadcast band"). The band was expanded in the 1990s by adding nine [[Communication channel|channels]] from 1,605 to 1,705&nbsp;kHz. Channels are spaced every 10&nbsp;kHz in the [[Americas]], and generally every 9 kHz everywhere else.

AM transmissions cannot be ionospheric [[radio propagation|propagated]] during the day due to strong absorption in the [[D-layer]] of the ionosphere. In a crowded channel environment, this means that the power of regional channels which share a frequency must be reduced at night or directionally beamed in order to avoid interference, which reduces the potential nighttime audience. Some stations have frequencies unshared with other stations in North America; these are called [[clear-channel station]]s. Many of them can be heard across much of the country at night. During the night, absorption largely disappears and permits signals to travel to much more distant locations via ionospheric reflections. However, fading of the signal can be severe at night.

AM radio transmitters can transmit audio frequencies up to 15&nbsp;kHz (now limited to 10&nbsp;kHz in the US due to FCC rules designed to reduce interference), but most receivers are only capable of reproducing frequencies up to 5&nbsp;kHz or less. At the time that AM broadcasting began in the 1920s, this provided adequate fidelity for existing microphones, 78 rpm recordings, and loudspeakers. The fidelity of sound equipment subsequently improved considerably, but the receivers did not. Reducing the bandwidth of the receivers reduces the cost of manufacturing and makes them less prone to interference. AM stations are never assigned adjacent channels in the same service area. This prevents the sideband power generated by two stations from interfering with each other.<ref>{{cite web | title=Types of Technology, FM vs AM | website=kwarner.bravehost.com | date=July 13, 2012 | url=http://kwarner.bravehost.com/tech.htm | archive-url=https://web.archive.org/web/20120713084431/http://kwarner.bravehost.com/tech.htm | archive-date=July 13, 2012 | url-status=dead | access-date=August 10, 2019}}</ref> [[Bob Carver]] created an [[AM stereo]] tuner employing [[notch filter]]ing that demonstrated that an AM broadcast can meet or exceed the 15&nbsp;kHz baseband bandwidth allotted to [[FM broadcasting|FM]] stations without objectionable interference. After several years, the tuner was discontinued. Bob Carver had left the company and the Carver Corporation later cut the number of models produced before discontinuing production completely.<ref>{{Cite web |title=Landing Page |url=https://www.bobcarvercorp.com/ |access-date=2025-03-25 |website=BobCarverCorp.com |language=en}}</ref>

As well as on the medium wave bands, amplitude modulation (AM) is also used on the [[shortwave]] and [[long wave]] bands.  Shortwave is used largely for national broadcasters, international propaganda, or [[religious broadcasting]] organizations. Shortwave transmissions can have international or inter-continental range depending on atmospheric conditions.<ref>{{Cite book|url=https://books.google.com/books?id=HudwCgAAQBAJ|title=Beginning Shortwave Radio Listening|last=Grodkowski|first=Paul|date=August 24, 2015|publisher=Booktango|isbn=9781468964240|language=en}}</ref> Long-wave AM broadcasting occurs in Europe, Asia, and Africa. The [[ground wave]] propagation at these frequencies is little affected by daily changes in the ionosphere, so broadcasters need not reduce power at night to avoid interference with other transmitters.