Editing Radio broadcasting (section)

===FM===
{{main|FM broadcasting}}
FM refers to [[frequency modulation]], and occurs on [[VHF]] airwaves in the frequency range of 88 to 108 [[Megahertz|MHz]] everywhere except [[Japan]] and [[Russia]]. Russia, like the former [[Soviet Union]], uses 65.9 to 74&nbsp;MHz frequencies in addition to the world standard. Japan uses the 76 to 90&nbsp;MHz frequency band.

[[Edwin Howard Armstrong]] invented wide-band FM radio in the early 1930s to overcome the problem of [[Electromagnetic interference|radio-frequency interference]] (RFI), which plagued AM radio reception. At the same time, greater fidelity was made possible by spacing stations further apart in the [[radio frequency]] spectrum. Instead of 10&nbsp;kHz apart, as on the AM band in the US, FM channels are 200&nbsp;kHz (0.2&nbsp;MHz) apart. In other countries, greater spacing is sometimes mandatory, such as in New Zealand, which uses 700&nbsp;kHz spacing (previously 800&nbsp;kHz). The improved fidelity made available was far in advance of the audio equipment of the 1940s, but wide interchannel spacing was chosen to take advantage of the noise-suppressing feature of wideband FM.

Bandwidth of 200 [[kHz]] is not needed to accommodate an audio signal &mdash; 20&nbsp;kHz to 30&nbsp;kHz is all that is necessary for a narrowband FM signal. The 200&nbsp;kHz bandwidth allowed room for ±75&nbsp;kHz signal deviation from the assigned frequency, plus guard bands to reduce or eliminate adjacent channel interference. The larger bandwidth allows for broadcasting a 15&nbsp;kHz bandwidth audio signal plus a 38&nbsp;kHz [[FM broadcasting#Stereo FM|stereo "subcarrier"]]—a piggyback signal that rides on the main signal. Additional unused capacity is used by some broadcasters to transmit utility functions such as background [[music]] for public areas, [[Global Positioning System|GPS]] auxiliary signals, or financial market data.

The AM radio problem of interference at night was addressed in a different way. At the time FM was set up, the available frequencies were far higher in the spectrum than those used for AM radio - by a factor of approximately 100. Using these frequencies meant that even at far higher power, the range of a given FM signal was much shorter; thus its market was more local than for AM radio. The reception range at night is the same as in the daytime. All FM broadcast transmissions are line-of-sight, and ionospheric bounce is not viable. The much larger bandwidths, compared to AM and SSB, are more susceptible to phase dispersion. Propagation speeds are fastest in the ionosphere at the lowest sideband frequency. The celerity difference between the highest and lowest sidebands is quite apparent to the listener. Such distortion occurs up to frequencies of approximately 50&nbsp;MHz. Higher frequencies do not reflect from the ionosphere, nor from storm clouds. Moon reflections have been used in some experiments, but require impractical power levels.

The original FM radio service in the U.S. was the [[Yankee Network]], located in [[New England]].<ref>Halper, Donna L. [http://www.bostonradio.org/essays/shepard-fm.html "John Shepard's FM Stations—America's first FM network."] {{Webarchive|url=https://web.archive.org/web/20060212104948/http://www.bostonradio.org/essays/shepard-fm.html |date=February 12, 2006 }} Boston Radio Archives (BostonRadio.org).</ref><ref>{{cite web | title=The Yankee Network in 1936 | website=The Archives @ BostonRadio.org | url=http://www.bostonradio.org/yankee-36.html | access-date=August 10, 2019}}</ref><ref>{{cite web | title=FM Broadcasting Chronology | website=Jeff Miller Pages | date=June 23, 2017 | url=http://jeff560.tripod.com/chronofm.html | access-date=August 10, 2019}}</ref> Regular FM broadcasting began in 1939 but did not pose a significant threat to the AM broadcasting industry. It required purchase of a special receiver. The frequencies used, 42 to 50&nbsp;MHz, were not those used today. The change to the current frequencies, 88 to 108&nbsp;MHz, began after the end of [[World War II]] and was to some extent imposed by AM broadcasters as an attempt to cripple what was by now realized to be a potentially serious threat.

FM radio on the new band had to begin from the ground floor. As a commercial venture, it remained a little-used audio enthusiasts' medium until the 1960s. The more prosperous AM stations, or their owners, acquired FM licenses and often broadcast the same programming on the FM station as on the AM station ("[[simulcasting]]"). The FCC limited this practice in the 1960s. By the 1980s, since almost all new radios included both AM and FM tuners, FM became the dominant medium, especially in cities. Because of its greater range, AM remained more common in rural environments.