AM_broadcasting - Wikiparency

# AM broadcasting **AM broadcasting** is [[Radio|radio broadcasting]] that carries its programme material by [[Amplitude_modulation|amplitude modulation]] of a carrier wave, the method used for the first audio broadcasts ever made and still in everyday use on the medium-wave, longwave and shortwave bands. A three.js microsim built for this article lays out a single 10 kHz medium-wave channel and the sidebands a modulated carrier fills it with, so the reader can see directly why AM channels are spaced the way they are. The [[Sideband|sidebands]] either side of the carrier are what actually carry the sound; early listeners often heard them through nothing more than a [[Crystal_radio|crystal set]], a detector with no amplifier and no power supply of its own, which is part of why full carrier power was kept in the transmitted signal even though doing so wastes most of the transmitter's output. AM's simplicity carried it from the first licensed stations of 1920 through the mass-audience "Golden Age" of network radio, and it has spent the seven decades since in a long, still continuing, competition with FM, digital and internet alternatives for listeners' attention. ## History Broadcasting — sending one signal to be heard by an unlimited, unaddressed audience — is a different problem from the point-to-point wireless telegraphy that came before it, which sent coded messages to one intended receiver at a time. A usable broadcast service needed both a way to carry a continuous audio waveform rather than on-off code, and a receiver simple and cheap enough for an ordinary household to own. ### Early broadcasting development Wireless telegraphy, refined for point-to-point message traffic by [[Guglielmo_Marconi|Marconi]] and others from the 1890s, used a spark to produce a broadband, damped burst of radio-frequency energy well suited to on-off keying and poorly suited to carrying a smooth audio waveform. Broadcasting an intelligible voice or piece of music required a transmitter that could hold a single, steady carrier frequency and vary its amplitude continuously, a genuinely different engineering problem from making a spark loud enough to be heard at a distance. ### Early amplitude modulation (AM) transmitter technologies Three distinct technologies were tried before the vacuum tube settled the question. #### Alternator transmitter Reginald Fessenden's National Electric Signaling Company had General Electric build a high-frequency alternator, designed largely by Ernst Alexanderson, that generated a continuous radio-frequency wave mechanically, by spinning a specially built generator fast enough to produce tens of thousands of cycles per second directly. A carbon microphone in series with the antenna could then vary that continuous wave's amplitude directly, without any of the electronic amplification later transmitters would need. Fessenden used such an alternator at Brant Rock, Massachusetts, to demonstrate voice and music transmission to invited engineers on December 21, 1906; the same event is often reported, on far thinner and later documentation, as a Christmas Eve broadcast heard by ships at sea, a claim modern historians have been unable to confirm in any contemporary source.[^fessenden1906] #### Arc transmitters Valdemar Poulsen's arc converter, developed in Denmark in the early 1900s, generated continuous waves from a direct-current arc burning in a hydrogen-rich atmosphere between rotating electrodes, and proved easier to scale to high power than an alternator.[^poulsenarc] In the United States, the Federal Telegraph Company and its engineer Leonard Fuller built arc stations for point-to-point traffic and, from around 1912, for experimental radiotelephone transmission, before the vacuum tube made arc transmitters obsolete for anything but the highest-power, lowest-frequency work. #### Vacuum tube transmitters Lee De Forest's Audion, patented in 1906, added a third electrode, a grid, to John Ambrose Fleming's two-electrode vacuum-tube diode, giving the new tube the ability to amplify a weak signal rather than merely rectify it.[^deforest1906] The Audion alone made a poor oscillator, but Edwin Armstrong's regenerative circuit of 1912 fed part of a tube's amplified output back into its own input, turning modest amplification into both a much stronger receiver and a practical, self-sustaining transmitter oscillator.[^armstrong1912] Vacuum-tube transmitters were lighter, cheaper to run, and far more easily tuned than either an alternator or an arc, and had displaced both for broadcasting purposes by the early 1920s. ### Receivers The cheapest possible AM [[Radio_receiver|receiver]] is a crystal set: an antenna, a tuned circuit, a crystal [[Diode|diode]] detector across which the tiny received [[Voltage|voltage]] appears, and an earphone, needing no battery or mains power because the received radio energy itself drives the earphone. Vacuum-tube receivers, and later the [[Superheterodyne_receiver|superheterodyne receiver]], added amplification, sharper tuning, and eventually [[Automatic_gain_control|automatic gain control]] to hold a steady volume as a distant signal faded in and out, at the cost of needing their own power supply. ### Early experimental broadcasts Beyond Fessenden's 1906 demonstration, a scattering of amateur and university experimenters transmitted voice and music into the years around 1910–1920 with no audience beyond whoever happened to be listening with a receiver of their own; Frank Conrad's amateur station in Pittsburgh, later relicensed as 8XK and then as KDKA, was one of several such stations broadcasting recorded music and talk on an irregular schedule before any station did so as an organized, advertised service. ### Organized broadcasting KDKA, the station that grew out of Conrad's experiments, received the first US broadcasting license granted for that purpose on October 27, 1920, and made its first organized broadcast five days later, reporting the Harding–Cox presidential election returns on the night of November 2, 1920.[^kdka1920] The idea of a station broadcasting on a fixed schedule to a general audience spread explosively: hundreds of stations went on the air across the United States within two years of KDKA's debut. ### Radio networks Individual stations soon linked by wire to share programming from a single production centre, but the two largest English-speaking markets built that link on opposite commercial models. #### United States The Radio Corporation of America incorporated the National Broadcasting Company in 1926 to feed a common schedule of sponsored, advertising-supported programming to affiliated stations across the country, and the Columbia Broadcasting System followed as a rival network the next year, fixing advertiser-supported network radio as the American pattern.[^nbc1926] #### United Kingdom A group of radio-set manufacturers formed the British Broadcasting Company in 1922 to provide a service that would, in turn, sell more receivers; Parliament reconstituted it in 1927 as the British Broadcasting Corporation, a public body funded by a licence fee on receiving sets rather than by advertising, a structural difference from the American networks that has shaped British broadcasting ever since.[^bbc1927] ### "Golden Age of Radio" With a national audience and network schedules in place, the two decades or so before television became widespread are commonly called radio's "Golden Age": serialized drama, comedy, variety and big-band music, and, increasingly, news, filled the evening schedule of a typical American household gathered around a single living-room receiver. ### Decline in popularity Television took over much of that evening audience through the 1950s, and FM, whose far wider [[Frequency_response|bandwidth]] gives music noticeably better fidelity and, from the 1960s, stereo sound, took over much of the music audience over the following decades. AM kept its advantage in talk, news and sports, formats where fidelity matters less than reach, but lost still more listening to satellite radio, internet streaming and podcasting from the 1990s onward, a decline this article's own revitalization section describes the industry's response to. ## AM band revitalization efforts in the United States From the 1980s on, as competition for listeners grew, American regulators and broadcasters tried a series of largely separate fixes aimed at keeping AM commercially viable rather than at changing the underlying modulation itself. ### Fairness Doctrine repeal The Federal Communications Commission repealed the Fairness Doctrine, which had required stations to devote time to contrasting views on controversial public issues, in 1987; freed of that obligation, opinionated, single-viewpoint talk formats grew rapidly and came to dominate much of the surviving AM schedule.[^fairness1987] ### AM stereo and AMAX standards Several incompatible AM stereo systems were proposed in the early 1980s, and rather than choose one, the FCC in 1982 let station owners and receiver manufacturers pick a winner in the marketplace; Motorola's C-QUAM system emerged as the de facto standard over the following years, adopted piecemeal as [[Transistor|transistorized]] car and portable receivers already on the market added stereo decoders.[^cquam1982] AMAX was a separate, voluntary certification mark for receivers meeting minimum fidelity and interference-rejection standards, an attempt to counter the perception that AM sets, as much as AM transmissions, were to blame for the format's declining reputation for sound quality. ### Expanded band Ten new channels, from 1610 to 1700 kHz, were added above the traditional top of the American AM band to relieve crowding on the existing channels, with the FCC assigning the first stations to the new frequencies in 1997.[^expandedband] ### HD radio The FCC authorized iBiquity's in-band, on-channel digital system, marketed as HD Radio, for AM and FM stations in 2002, letting a station transmit a digital signal in the same channel as its existing analogue one so that suitably equipped receivers could decode a clearer, though still short-range, digital layer alongside the ordinary analogue broadcast.[^hdradio2002] ### FM translator stations Because skywave propagation at night (described below, under Technical information) forces many AM stations to reduce power or leave the air after dark, the FCC's 2015 AM revitalization order let AM licensees apply for low-power FM translator stations to rebroadcast their signal, giving listeners a steadier, static-free way to hear the same station regardless of the AM signal's own night-time behaviour.[^amrevital2015] ### Additional activities In 2020 the FCC further authorized AM stations to convert entirely to the HD Radio digital signal, dropping analogue compatibility altogether for stations that choose to do so, on the reasoning that a fully digital signal delivers better coverage and audio quality within the same channel than the earlier hybrid analogue-plus-digital arrangement.[^alldigital2020] ### Electric vehicles Several automakers began dropping AM receivers from electric vehicles in the early 2020s, arguing that an EV's motors and power electronics generate electrical noise that interferes with AM reception far more than it does with FM. Members of the US Congress responded with the AM Radio for Every Vehicle Act, which passed the House of Representatives in 2024 but was not enacted before that Congress ended and was reintroduced in the next one; whether it has since become law is not settled here.[^amact] ## Other digital standards Outside the United States, the digital standard most often paired with AM broadcasting is Digital Radio Mondiale (DRM), developed by an international broadcaster-and-manufacturer consortium and described in the technical literature from around 2001 as a replacement for, rather than an overlay on, the analogue AM signal.[^drm2001] Unlike the American HD Radio approach, DRM was designed from the outset to work across the longwave, medium-wave and shortwave bands alike, including a fully digital mode that occupies the same channel width as an analogue AM signal without an analogue component at all, alongside modes intended to widen a station's channel for higher audio quality. Adoption has been concentrated in a handful of national and international broadcasters, particularly for shortwave, rather than becoming the near-universal standard HD Radio is within the United States; the two approaches reflect the same underlying goal, fitting a digital signal into spectrum planned around AM's older channel widths, solved in two incompatible ways on either side of the Atlantic. ## Technical information An AM broadcast channel's width, and how far its signal reaches, both follow directly from where in the [[Radio_spectrum|radio spectrum]] the channel sits. ### Broadcast band frequencies #### Longwave broadcasting A longwave broadcast band, roughly 148.5 to 283.5 kHz, is allocated in parts of Europe, Africa and Asia, spaced in 9 kHz channels; no longwave broadcast band exists in the Americas, where the medium-wave band alone carries AM broadcasting. #### Medium-wave broadcasting The medium-wave band is AM broadcasting's main home: roughly 520 to 1700 kHz worldwide, divided into 10 kHz channels in the Americas and 9 kHz channels in most of the rest of the world. A channel of twice a station's audio bandwidth is exactly what the underlying modulation demands: a 5 kHz baseband, typical of a talk or music service, needs a 10 kHz channel by the same relation that halves in single-sideband transmission, so the 10 kHz American channel spacing is not an arbitrary regulatory round number but close to the minimum full-carrier AM itself allows for that audio bandwidth.[^johnson124b] #### Shortwave broadcasting Several internationally coordinated shortwave bands, scattered between about 2.3 and 26.1 MHz, carry AM broadcasting intended for reception far beyond a single country's borders. Shortwave's usefulness for that purpose rests on [[Ionosphere|ionospheric]] [[Skywave|skywave]] propagation, which can return a signal to earth thousands of kilometres from the transmitter, in contrast to the shorter, mostly [[Ground_wave|ground-wave]] range of a medium-wave station in daylight; the same skywave path is also less stable than a ground wave, so shortwave listening includes a fading, [[Multipath_propagation|multipath]] character that medium-wave listeners rarely hear. #### VHF AM broadcasting Amplitude modulation has never displaced FM for VHF broadcast services, but AM remains the deliberate choice for civil aviation voice communication in the VHF band, because when two stations transmit on the same AM frequency at once both remain at least partially audible, letting a pilot hear that a channel is occupied, where FM's tendency to suppress the weaker of two simultaneous signals could hide a second, safety-critical transmission entirely. ### Other distribution methods Some AM programming reaches listeners by means other than a full-power broadcast transmitter: low-power "carrier current" transmitters feed a signal directly into building or campus wiring for reception only nearby, travellers' information stations broadcast recorded advisories on a dedicated frequency near a highway or airport, and most surviving stations now also stream their programme over the internet using ordinary [[Data_compression|data compression]], reaching listeners who never tune an AM set at all. ### Microbroadcasting At the opposite end of the scale from a full-power station, unlicensed or minimally licensed microbroadcasters run AM transmitters at power levels low enough, and antennas short enough, to stay within a country's rules for operation without an individual station license, trading range of a few hundred metres to a few kilometres for freedom from the cost and process of a full broadcast license. ## Microsims A three.js companion built for the Wikitube framework, rather than carried over from an existing sketch, lays a single 10 kHz medium-wave channel alongside a modulated carrier's sidebands, showing how much of that channel a given audio bandwidth actually fills; it is described here only in outline, without its own controls or readouts, since those belong on its own page. *Try:* in the [[Doppler_effect]] sketch, watch the two observer markers report two different frequencies from a single moving source at the same instant; two AM carriers a few hundred hertz apart produce the same kind of pair of close frequencies, which a receiver tuned across both renders audible as a slow heterodyne whistle rather than as two separately identifiable tones. *Try:* in the [[Radar]] sketch, widen the pulse and watch the close pair of targets in the zoomed inset merge into one hump once the pulse is no longer short compared with their separation; an AM receiver's own tuned circuit sets an equivalent limit, in frequency rather than in range, on how close two stations' channels can sit before one receiver can no longer separate them. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/AM_broadcasting) : [Wikitube](https://en.wikitube.io/wiki/AM_broadcasting) Skeleton mirrored at revision 1370821293. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Amplitude_modulation]] - [[Sideband]] - [[Single-sideband_modulation]] - [[Crystal_radio]] - [[Frequency_modulation]] - [[Guglielmo_Marconi]] - [[History_of_radio]] - [[Shortwave_radio]] ## References [^fessenden1906]: Radio World / Broadcasters' Desktop Reference historiography of Reginald Fessenden's career: a demonstration of voice and music transmission from Brant Rock, Massachusetts, to invited engineers on December 21, 1906, is documented in a contemporary account; the popular "Christmas Eve" ships-at-sea broadcast story is not corroborated by any source earlier than the late 1920s and has been challenged by historians including James E. O'Neal. Citation needed: a primary 1906 document (rather than a later recollection) fixing the December 21 event's content and audience precisely. https://www.thebdr.net/what-do-we-really-know-about-reginald-fessenden/ [^poulsenarc]: Engineering and Technology History Wiki. "Milestones: Poulsen-Arc Radio Transmitter, 1902." https://ethw.org/Milestones:Poulsen-Arc_Radio_Transmitter,_1902 [^deforest1906]: Mix (trade publication). "1906 Lee De Forest Triode Vacuum Tube." on De Forest's Audion, a three-electrode development of Fleming's two-electrode vacuum-tube valve. https://www.mixonline.com/technology/1906-lee-de-forest-triode-vacuum-tube-383581 [^armstrong1912]: Britannica. "Regenerative circuit." On Edwin Armstrong's 1912 feedback circuit and its use as both a sensitive receiver and a self-sustaining oscillator. https://www.britannica.com/technology/regenerative-circuit [^kdka1920]: Engineering and Technology History Wiki. "KDKA, First Commercial Radio Station": license granted October 27, 1920; first broadcast, of the Harding–Cox election returns, November 2, 1920. https://ethw.org/KDKA,_First_Commercial_Radio_Station [^nbc1926]: Historical broadcasting record of RCA's incorporation of the National Broadcasting Company, September 1926. Citation needed: a primary RCA or NBC corporate record fixing the exact incorporation date. https://eyesofageneration.com/september-9-1926-nbc-was-incorporated-by-rcathe-incorporation-process-was-t/ [^bbc1927]: The British Broadcasting Company, formed by radio manufacturers in 1922, was dissolved and re-established by royal charter as the license-fee-funded British Broadcasting Corporation on January 1, 1927. Citation needed: the 1926 royal charter and 1927 Wireless Telegraphy licence documents themselves. [^fairness1987]: Britannica, "Fairness doctrine": the FCC abolished the doctrine in 1987, a decision that stood after Congress failed to override it by statute. https://www.britannica.com/topic/Fairness-Doctrine [^cquam1982]: Federal Communications Commission, "AM Stereo Broadcasting": the Commission in 1982 declined to mandate a single AM stereo system and instead let the marketplace choose; Motorola's C-QUAM became the system in general use. https://www.fcc.gov/media/radio/am-stereo-broadcasting [^expandedband]: Citation needed: the FCC's own 1997 public notice or order assigning the first stations to the 1610-1700 kHz expanded AM band, rather than a secondary compilation of the assignments. [^hdradio2002]: HD Radio (iBiquity/Xperi), "US Regulatory": FCC authorization of in-band, on-channel digital AM/FM broadcasting, 2002. https://hdradio.com/broadcasters/us-regulatory/ [^amrevital2015]: Federal Communications Commission, "AM Revitalization": the Commission's October 2015 order allowing AM licensees to apply for FM translator stations, among other measures. https://www.fcc.gov/media/radio/am-revitalization [^alldigital2020]: Federal Communications Commission fact sheet, October 2020, authorizing AM stations to convert voluntarily to an all-digital signal; published in final form, Federal Register, December 3, 2020. https://docs.fcc.gov/public/attachments/DOC-367361A1.pdf [^amact]: Citation needed: the AM Radio for Every Vehicle Act's final legislative status (it passed the U.S. House as H.R. 8449 in the 118th Congress in 2024 without being enacted, and a further version, S.315, was introduced in the 119th Congress in 2025); no record confirming enactment is asserted here. [^drm2001]: Stott, J. "Digital Radio Mondiale (DRM)." EBU Technical Review, March 2001. https://tech.ebu.ch/docs/techreview/trev_286-stott.pdf [^johnson124b]: Johnson, D. *Fundamentals of Electrical Engineering I*. 2014, p. 124 (PDF page): AM bandwidth is twice the baseband width for a baseband much narrower than the carrier frequency. Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/fundamentals-of-electrical-engineering-1 . CC BY.