FM broadcasting in the United States

History of FM radio in the U.S.

In the United States FM radio stations broadcast at frequencies of 87.9–107.9 MHz. FM radio was developed in the United States by Edwin Armstrong.

During the 1930s, there were a small number of experimental (known as "Apex") stations attempting to broadcast high fidelity audio using wide-bandwidth AM on VHF frequencies. In 1937 W1XOJ was the first FM radio station, granted a construction permit by the Federal Communications Commission (FCC). On June 17, 1936, FM radio was demonstrated to the FCC for the first time. On January 5, 1940, Edwin H. Armstrong demonstrated FM broadcasting in a long-distance relay network, via five stations in five States. FM radio was assigned the 42 to 50 MHz band of the spectrum in 1940.

After World War II, the FCC moved FM to the frequencies between 88 and 108 MHz on June 27, 1945. The change in frequency was said to be for avoiding possible interference problems between stations in nearby cities and to make "room" for more FM radio channels. However, the FCC was influenced by RCA chairman David Sarnoff, who had the covert goal of disrupting the successful FM network that Edwin Armstrong had established on the old band. The 500,000 receivers built for the original FM radio band could be retrofitted with converters, but many were just replaced. The greater expense was to the radio stations themselves that had to rebuild their stations for the new FM radio band. The move of the FM band, an organized campaign of misinformation by RCA (a company that competed with FM radio by focusing on AM radio and the emerging technology of television), and adverse rulings by the FCC severely set back the development of FM radio. As late as 1947, in Detroit, there were only 3,000 FM receivers in use for the new band, and 21,000 obsolete ones for the old band. On March 1, 1941 W47NV began operations in Nashville, Tennessee, becoming the first modern commercial FM radio station. However, FM radio did not recover from the setback until the upsurge in high fidelity equipment in the late 1950s.

By the end of the 1970s, 50.1% of radio listeners were tuned to FM, ending AM's historical prevalence. By 1982, FM commanded 70% of the global audience and 84% among the 12- to 24-year-old demographic. The shift in popularity of FM radio over AM in United States during the 1970s has been called by record producer Steve Greenberg "a seismic technological shift that had torn apart the very idea of the mass audience upon which pop hits depended". (AM radio would recover by shifting its focus to talk radio, which would be deregulated in the late 1980s after the repeal of the Fairness Doctrine.)

During the 1970s, FM radio experienced a golden age of integrity programming, with disc jockeys playing what they wanted, including album cuts not designated as "singles" and lengthy progressive rock tracks.

FM radio channel assignments in the U.S.

In the United States, frequency-modulated broadcasting stations operate in a frequency band extending from 87.8 MHz to 108.0 MHz, for a total of 20.2 MHz. It is divided into 101 channels, each 0.2 MHz wide, designated "channel 200" through "channel 300". In actual practice, no one except the FCC uses these channel numbers; the frequencies are used instead. (Stations that broadcast on 87.7 MHz are in actuality licensed as television stations.)

To receive a station, an FM receiver is tuned to the center frequency of the station's channel. The lowest and almost-unused channel, channel 200, extends from 87.8 MHz to 88.0 MHz; thus its center frequency is 87.9 MHz. Channel 201 has a center frequency of 88.1 MHz, and so on, up to channel 300, which extends from 107.8 to 108.0 MHz and has a center frequency of 107.9 MHz.

Because each channel is 0.2 MHz wide, the center frequencies of adjacent channels differ by 0.2 MHz. Because the lowest channel is centered on 87.9 MHz, the tenths digit (in MHz) of the center frequency of any FM station in the United States is always an odd number. FM audio for analog television channel 6 is broadcast at a carrier frequency of 87.75 MHz, and many radios can tune this low; full-power stations ceased analog operations in 2009 under FCC orders, but a few low-power stations are still operated solely for their right to use this frequency and broadcast only nominal video programming, if any (these, too, will be forced to shut down in late 2015). For the same reason, assignment restrictions between TV stations on channel 6 and nearby FM stations are stringent: there are only two stations in the United States (KSFH, a 10-watt student station in Mountain View, California; and CSN International translator K200AA in Sun Valley, Nevada) licensed to operate on 87.9 MHz, both because they were forced off of other channels. Therefore, in effect, the FM broadcast band comprises only FM channels 201 (88.1 MHz) through 300 (107.9 MHz).

Originally, FM stations in a market were generally spaced four channels (800 kHz) apart. This spacing was developed in response to problems perceived on the original FM band, mostly due to deficiencies in receiver technology of the time. With modern equipment, this is widely understood to be unnecessary, and in many countries shorter spacings are used. (See FM broadcast band.) Other spacing restrictions relate to mixing products with nearby television, air-traffic control, and two-way radio systems as well as other FM broadcast stations. The most significant such taboo restricts the allocation of stations 10.6 and 10.8 MHz apart, to protect against mixing products which will interfere with an FM receiver's standard 10.7 MHz intermediate frequency stage.

Commercial broadcasting is licensed only on channels 221 through 300 (the upper 80 channels, frequencies between 92.1 and 107.9 MHz), with 200 through 220 (the lower 21 channels, frequencies between 88.1 and 91.9 MHz) reserved for non-commercial educational (NCE) broadcasting. In some "Twin city" markets close to the Canada-United States or Mexico-United States border, such as Detroit, Michigan and Windsor, Ontario, Canada, or San Diego, California and Tijuana, Baja California, Mexico, commercial stations operating from those countries target U.S. audiences on "reserved band" channels, as neither Canada nor Mexico has such a reservation. Because of this necessary sharing, the FCC reserves a few other channels for such NCE stations.

FM stations in the U.S. are now assigned based on a table of separation distance values from currently licensed stations, based on station "class" (power output, antenna height, and geographical location). These regulations (see Docket 80-90) have resulted in approximately double the number of possible stations, and increases in allowable power levels, over the original bandplan scheme described above. All powers are specified as effective radiated power (ERP), which takes into account the magnifying effect (gain) of multiple antenna elements.

The U.S. is divided into Zone I (roughly the northeastern quarter of the U.S. mainland, excluding the far northern areas), Zone I-A (California south of 40 degrees latitude, U.S. Virgin Islands, Puerto Rico), and Zone II (all other locations). The highest-power stations are class C in zone II, and class B in the others. There are no B stations in zone II, nor any C stations in the others. (See the list of broadcast station classes.) Canada is also divided in this manner, based on the most highly populated regions.

High power is useful in penetrating buildings, diffracting around hills, and refracting for some distance beyond the horizon. 100,000-watt FM stations can regularly be heard up to 100 miles (160 km) away, and farther (e.g., 150 miles, 240 km) if there are no competing signals.

A few old "grandfathered" stations do not conform to these power rules. WBCT (93.7) in Grand Rapids, Michigan, runs 320,000 watts ERP, and can increase to 500,000 watts ERP by the terms of its original license. This huge power level does not usually help to increase range as much as one might expect, because VHF frequencies travel in nearly straight lines over the horizon and off into space. Nevertheless, when there were fewer FM stations competing, this station could be heard near Bloomington, Illinois, almost 300 miles (480 km) distant.