FM broadcast band
The FM broadcast band is a range of radio frequencies used for FM broadcasting by radio stations. The range of frequencies used differs between different parts of the world. In Europe and Africa and in Australia and New Zealand, it spans from 87.5 to 108 megahertz - also known as VHF Band II - while in the Americas it ranges from 88 to 108 MHz. The FM broadcast band in Japan uses 76 to 95 MHz, and in Brazil, 76 to 108 MHz. The International Radio and Television Organisation band in Eastern Europe is from 65.9 to 74.0 MHz, although these countries now primarily use the 87.5 to 108 MHz band, as in the case of Russia. Some other countries have already discontinued the OIRT band and have changed to the 87.5 to 108 MHz band.
Narrow band frequency modulation was developed and demonstrated by Hanso Idzerda in 1919.
Wide band frequency modulation radio originated in the United States during the 1930s; the system was developed by the American electrical engineer Edwin Howard Armstrong. However, FM broadcasting did not become widespread, even in North America, until the 1960s.
Frequency-modulated radio waves can be generated at any frequency. All the bands mentioned in this article are in the very high frequency range, which extends from 30 to 300 MHz.
CCIR bandplan
Center frequencies
While all countries use FM channel center frequencies ending in 0.1, 0.3, 0.5, 0.7, and 0.9 MHz, some countries also use center frequencies ending in 0.0, 0.2, 0.4, 0.6, and 0.8 MHz. A few others also use 0.05, 0.15, 0.25, 0.35, 0.45, 0.55, 0.65, 0.75, 0.85, and 0.95 MHz.An ITU conference in Geneva, Switzerland, on December 7, 1984, resolved to discontinue the use of 50 kHz channel spacings throughout Europe.
- Most countries have used 100 kHz or 200 kHz channel spacings for FM broadcasting since this ITU conference in 1984.
- Some digitally-tuned FM radios are unable to tune using 50 kHz or even 100 kHz increments. Therefore, when traveling abroad or importing receivers, stations that broadcast on certain frequencies using such increments may not be heard clearly. This problem will not affect reception on an analog-tuned radio.
- A few countries, such as Italy, which have heavily congested FM bands, still allow a station on any multiple of 50 kHz wherever one can be squeezed in.
- The 50 kHz channel spacings help prevent co-channel interference, and these take advantage of FM's capture effect and receiver selectivity.
ITU Region 2 bandplan and channel numbering
The center frequencies of the FM channels are spaced in increments of 200 kHz. The frequency of 87.9 MHz, while technically part of TV channel 6, is used by just two FM class-D stations in the United States. Portable radio tuners often tune down to 87.5 MHz, so that the same radios can be made and sold worldwide. Automobiles usually have FM radios that can tune down to 87.7 MHz, so that TV channel 6's audio at 87.75 MHz could be received while driving. This is largely no longer possible due to the 2009 digital television transition, though in 2023, the FCC authorized 14 low-powered Channel 6 television stations to continue to operate radio services indefinitely.
In the United States, the 21 channels with center frequencies of 87.9–91.9 MHz constitute the reserved band, exclusively for non-commercial educational stations. The other channels may be used by both commercial and non-commercial stations.
Originally, the American Federal Communications Commission devised a bandplan in which FM radio stations would be assigned at intervals of four channels for any one geographic area. Thus, in one area, stations might be at 88.1, 88.9, 89.7, etc., while in an adjacent area, stations might be at 88.3, 89.1, 89.9, 90.7 etc. Certain frequencies were designated for Class A only, which had a limit of 3 kilowatts of effective radiated power and an antenna height limit for the center of radiation of 300 feet height above average terrain. 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, a station could be Class B or Class C, depending on which zone it was in.
In the late 1980s, 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 six kilowatts, and the frequency restrictions noted above were removed. As of late 2004, a station can be "squeezed in" anywhere as long as the location and class conform to the rules in the FCC separation table. The rules for second-adjacent-channel spacing do not apply for stations licensed before 1964.
In 2017, Brazil laid the groundwork to reclaim channels 5 and 6 for sound broadcasting use and required new radio receivers to be able to tune into the new extended band. Five transmitters of public broadcaster Brazil Communication Company were the first extended-band stations to begin broadcasts on May 7, 2021.
In 2023, Chile announced the expansion of the FM band to 76-108 MHz as part of the analog TV shutdown, scheduled for April 2024.
Deviation and bandpass
Normally each channel is 200 kHz wide, and can pass audio and subcarrier frequencies up to 100 kHz. Deviation is typically limited to 150 kHz total in order to prevent adjacent-channel interference on the band. Stations in the U.S. may go up to 10% over this limit if they use non-stereo subcarriers, increasing total modulation by 0.5% for each 1% used by the subcarriers. Some stations may be limited to deviation in order to reduce transmitted bandwidth so that additional stations can be squeezed in.OIRT bandplan
The OIRT FM broadcast band covers 65.8 to 74 MHz. It was used in the Soviet Union and most of the other Warsaw Pact member countries of the International Radio and Television Organisation in Eastern Europe, with the exception of East Germany, which always used the 87.5 to 100 MHz broadcast band—in line with Western Europe.The lower portion of the VHF band behaves a bit like shortwave radio in that it has a longer reach than the upper portion of the VHF band. It was ideally suited for reaching vast and remote areas that would otherwise lack FM radio reception. In a way, FM suited this band because the capture effect of FM could mitigate interference from skywaves.
Transition to the 87.5 to 108 MHz band started as early as the 1980s in some East European countries. Following the collapse of the communist governments, that transition was remarkably accelerated as private stations have been established. This was also prompted by the lack of equipment for the OIRT band and the modernization of existing transmission networks.
Many countries have completely ceased broadcasting on the OIRT FM band, although use continues in others, mainly the former republics of the USSR. The future of broadcasting on the OIRT FM band is limited, due to the lack of new consumer receivers for this band outside of Russia.
Countries which still use the OIRT band are Russia, Belarus, Moldova, Ukraine, and Turkmenistan.
In Czechoslovakia, the decision to use the 87.5 to 108 MHz band instead of 65.9 to 74 MHz band was made in the beginning of the 80s. The frequency plan was created, which was internationally coordinated at the 1984 Regional Administrative Conference for FM Sound Broadcasting in the VHF band in Geneva. Allocated frequencies are still valid and are used in the Czech Republic and Slovakia. The first transmitter was put into operation on 102.5 MHz near Prague in November 1984. Three years later, there were 11 transmitters in service across the country, including three in the Prague neighborhood of Žižkov. In 1988, the plan was to set up 270 transmitters in 45 locations eventually. The transition was finished in 1993.
In Poland all OIRT broadcast transmitters were closed down at the end of 1999.
Hungary closed down its remaining broadcast transmitters in 2007, and for 30 days in July of that year, several Hungarian amateur radio operators received a temporary experimental permit to perform propagation and interference experiments in the 70–70.5 MHz band.
In Belarus, only government-run public radio stations are still active on OIRT. All stations on OIRT in Belarus are a mirror of normal FM broadcasts. The main purpose of those stations is compatibility with older equipment.
In 2014, Russia began replacing OIRT-banded transmitter with CCIR-banded FM transmitters. The main reason for the change to CCIR FM is to reach more listeners.
Unlike Western practice, OIRT FM frequencies are based on 30 kHz rather than 50, 100, or 200 kHz multiples. This may have been to reduce co-channel interference caused by Sporadic E propagation and other atmospheric effects, which occur more often at these frequencies. However, multipath distortion effects are less annoying than on the CCIR band.
Stereo is generally achieved by sending the stereo difference signal, using a process called polar modulation. Polar modulation uses a reduced subcarrier on 31.25 kHz with the audio on both side-bands. This gives the following signal structure: L + R --> 31.25 kHz reduced subcarrier L - R.
The 4-meter band amateur radio allocation used in many European countries is entirely within the OIRT FM band. Operators on this band and the 6-meter band use the presence of broadcast stations as an indication that there is an "opening" into Eastern Europe or Russia. This can be a mixed blessing because the 4 meter amateur allocation is only 0.5 MHz or less, and a single broadcast station causes considerable interference to a large part of the band.
The System D television channels R4 and R5 lie wholly or partly within the 87.5–108 MHz FM audio broadcast band. Countries which still use System D therefore have to consider the re-organization of TV broadcasting in order to make full use of this band for audio broadcasting.