Science Fair Project Encyclopedia
FM broadcast band
In most of the world, the FM broadcast band, used for broadcasting FM radio stations, goes from 87.5 to 108.0 MHz. In the Americas (ITU region 2) it starts at 87.8, at the top end of TV channel 6. In Japan the FM broadcast band is 76–90 MHz, unlike any other country in the world. The old OIRT band in Eastern Europe was 66–74MHz.
While most countries use frequencies ending in .1, .3, .5, .7, or .9, some use .0, .2, .4, .6, and .8. Still others use .15, .35, .55, .75, .95, or .05, .25, .45, .65, .85 instead. Some, like Italy, allow a station on any 50 kHz boundary where it can be squeezed in. This offset helps to prevent co-channel interference, and take advantage of FM's capture effect and receiver selectivity.
ITU Region II Bandplan and Channel Numbering
In North America, each channel is numbered from 200 (87.9 MHz) to 300 (107.9 MHz) in increments of 1 (0.2 MHz). 87.9 MHz, while technically part of TV channel 6 (82.0–88.0 MHz), is used by two class-D stations in the U.S.. Portable radio tuners often tune down to 87.5, so the same equipment can be marketed worldwide. Automobiles are unlikely to go overseas, but usually tune down to 87.7, so that TV channel 6 audio on 87.75 MHz can be received (although at a somewhat lower volume).
In the United States, the center frequencies of 87.9 though 91.9 are reserved for non-commercial stations only, e.g., religious or educational. The center frequencies 92.1 through 107.9 may contain either commercial or non-commercial stations. (Neither Canada nor Mexico observe this reservation.)
Originally, the FCC devised a bandplan where stations would be assigned at intervals of 4 channels, or 800 kHz separation, for any one geographic area. Thus, in mid-Missouri, stations might be at 88.1, 88.9, 89.7, etc., while in the St. Louis area, stations might be at 88.3, 89.1, 89.9, 90.7, etc. Certain frequencies were designated for Class A only (see FM broadcasting), which had a limit of 3000 watts ERP and an antenna height limit for the center of radiation of 300 feet height above average terrain (HAAT). These frequencies were: 92.1, 92.7, 93.5, 94.3, 95.3, 95.9, 96.7, 97.7, 98.3, 99.3, 100.1, 100.9, 101.7, 102.3, 103.1, 103.9, 104.9, 105.5, 106.3, and 107.1. On other frequencies, stations could be Class B (50000 watts, 500 feet) or Class C (100000 watts, 2000 feet), depending on which Zone they were in.
In the late 1980s (exact year, anyone?), the FCC switched to a "bandplan" based on a distance separation table using currently operating stations, and subdivided the class table to create extra classes and change antenna height limits to meters. Class A power was doubled to 6000 watts, and the frequency restrictions noted above were removed. Basically, as of late 2004, a station can be "squeezed in" anywhere as long as the location and class conforms to the rules in separation table. The rules for second-adjacent-channel spacing do not apply for stations licensed prior to 1964.
The basic formulas determining radio channels for North America as mentioned is as follows:
- (MHz − 47.9) * 5 = ch#
- (ch# / 5) + 47.9 = MHz
Deviation and bandpass
Normally, each channel is 200 kHz (0.2 MHz) wide, and can pass audio and subcarrier frequencies up to 100 kHz. Deviation is up to 200 kHz total, or ±100 kHz, but is typically limited to 150 kHz total (±75 kHz) in order to prevent interference to adjacent channels on the band. Stations in the U.S. may go up to 10% over this if they use stereo or other subcarriers, increasing total modulation by 1% for each 2% used by the subcarriers.
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