FM broadcasting in the United States
FM broadcasting in the United States began in the 1930s at engineer and inventor Edwin Howard Armstrong's experimental station, W2XMN. The use of FM radio has been associated with higher sound quality in music radio.
History of FM radio in the U.S.
In the United States, FM radio stations broadcast at frequencies of 87.8-108.0 MHz. FM radio was developed in the United States by Edwin Armstrong.During the 1930s, there were a small number of experimental stations attempting to broadcast high fidelity audio using wide-bandwidth AM on VHF frequencies. In 1937, W1XOJ, Paxton, Massachusetts, was the first FM radio station, granted a construction permit by the Federal Communications Commission. 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. There was interest in the new FM band by station owners. On March 1, 1941 W47NV began operations in Nashville, Tennessee, becoming the first modern commercial FM radio station. construction restrictions that went into place during World War II limited the growth of the new service.
Following the end of World War II, the FCC moved to standardize its frequency allocations. One area of concern was the effects of tropospheric and Sporadic E propagation, which at times reflected station signals over great distances, causing mutual interference. A particularly controversial proposal, spearheaded by RCA and chairman David Sarnoff, was that the FM band needed to be shifted to higher frequencies in order to avoid this potential problem, and to make "room" for more FM radio channels. This reassignment, whose covert goal by RCA was to disrupt the successful FM network that Edwin Armstrong had established on the old band was fiercely opposed as unneeded by Armstrong, but in the end RCA's point of view prevailed. The FCC made its decision final on June 27, 1945. It allocated one hundred FM channels from 88–108 MHz, and assigned the former FM band to 'non government fixed and mobile', and television channel 1, curiously avoiding the subject of the effects of tropospheric and Sporadic E propagation on those services. A period of allowing existing FM stations to broadcast on both low and high bands followed, though as late as 1947, in Detroit, there were only 3,000 FM receivers in use for the new band, and 21,000 for the old band. The dual band transition period ended at midnight on January 8, 1949, at which time any low band transmitters had to be shut down, officially making obsolete 395,000 receivers that had already been purchased by the public for the original band. Although converters allowing low band FM sets to receive high band were manufactured, they ultimately proved to be complicated to install, and often as expensive than buying a new high band set outright. The greater expense was to the radio stations themselves that had to rebuild their stations for the new FM radio band. FM radio as an industry did not recover significantly from the setback until the upsurge in high fidelity equipment in the late 1950s.
In the early 1960s, FM began to benefit from increased investment as broadcasters looked to it to expand their markets; television had been built out by this point, and the shift to music as the dominant format of AM in the wake of television and the rise of rock 'n' roll had led to an AM dial so crowded that the FCC was limiting many stations to daytime-only, a restriction FM was not subject to. The FCC gave FM two boosts in the early 1960s: first by setting a technical standard for stereo broadcasts, and second by adopting the FM Non-Duplication Rule in 1964, prohibiting broadcasters with an AM and FM license in cities of more than 100,000 from transmitting more than 50% of the same programming on both stations.
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".
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 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.| MHz | Chan No. | MHz | Chan No. | MHz | Chan No. | MHz | Chan No. | MHz | Chan No. |
| 87.9 | 200 | 92.9 | 225 | 97.9 | 250 | 102.9 | 275 | 107.9 | 300 |
| 88.1 | 201 | 93.1 | 226 | 98.1 | 251 | 103.1 | 276 | - | - |
| 88.3 | 202 | 93.3 | 227 | 98.3 | 252 | 103.3 | 277 | - | - |
| 88.5 | 203 | 93.5 | 228 | 98.5 | 253 | 103.5 | 278 | - | - |
| 88.7 | 204 | 93.7 | 229 | 98.7 | 254 | 103.7 | 279 | - | - |
| 88.9 | 205 | 93.9 | 230 | 98.9 | 255 | 103.9 | 280 | - | - |
| 89.1 | 206 | 94.1 | 231 | 99.1 | 256 | 104.1 | 281 | - | - |
| 89.3 | 207 | 94.3 | 232 | 99.3 | 257 | 104.3 | 282 | - | - |
| 89.5 | 208 | 94.5 | 233 | 99.5 | 258 | 104.5 | 283 | - | - |
| 89.7 | 209 | 94.7 | 234 | 99.7 | 259 | 104.7 | 284 | - | - |
| 89.9 | 210 | 94.9 | 235 | 99.9 | 260 | 104.9 | 285 | - | - |
| 90.1 | 211 | 95.1 | 236 | 100.1 | 261 | 105.1 | 286 | - | - |
| 90.3 | 212 | 95.3 | 237 | 100.3 | 262 | 105.3 | 287 | - | - |
| 90.5 | 213 | 95.5 | 238 | 100.5 | 263 | 105.5 | 288 | - | - |
| 90.7 | 214 | 95.7 | 239 | 100.7 | 264 | 105.7 | 289 | - | - |
| 90.9 | 215 | 95.9 | 240 | 100.9 | 265 | 105.9 | 290 | - | - |
| 91.1 | 216 | 96.1 | 241 | 101.1 | 266 | 106.1 | 291 | - | - |
| 91.3 | 217 | 96.3 | 242 | 101.3 | 267 | 106.3 | 292 | - | - |
| 91.5 | 218 | 96.5 | 243 | 101.5 | 268 | 106.5 | 293 | - | - |
| 91.7 | 219 | 96.7 | 244 | 101.7 | 269 | 106.7 | 294 | - | - |
| 91.9 | 220 | 96.9 | 245 | 101.9 | 270 | 106.9 | 295 | - | - |
| 92.1 | 221 | 97.1 | 246 | 102.1 | 271 | 107.1 | 296 | - | - |
| 92.3 | 222 | 97.3 | 247 | 102.3 | 272 | 107.3 | 297 | - | - |
| 92.5 | 223 | 97.5 | 248 | 102.5 | 273 | 107.5 | 298 | - | - |
| 92.7 | 224 | 97.7 | 249 | 102.7 | 274 | 107.7 | 299 | - | - |
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 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. 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 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 through 300.
Originally, FM stations in a market were generally spaced four channels 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. 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, with 200 through 220 reserved for non-commercial educational broadcasting. In some "Twin city" markets close to the Canada–United States or Mexico–United States border, such as Detroit, Michigan, and Windsor, Ontario, or San Diego, California, and Tijuana, Baja California, 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". These regulations 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, which takes into account the magnifying effect of multiple antenna elements.
The U.S. is divided into Zone I, Zone I-A, and Zone II. 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. 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 away, and farther if there are no competing signals.
A few old "grandfathered" stations do not conform to these power rules. WBCT 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 distant.