FM broadcast 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 (VHF) range, which extends from 30 to 300 MHz.

OIRT bandplan[edit]

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 (OIRT), with the exception of East Germany, which always used the 87.5 to 100 (later 104) 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 modernisation 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 (including Kaliningrad), Belarus,^[6] Moldova,^[7] Ukraine,^[8] and Turkmenistan.^[9]


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 eighties. The frequency plan was created, which was internationally coordinated at Regional Administrative Conference for FM Sound Broadcasting in the VHF band in Geneva, 1984.^[10] 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 eleven 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.^[11] 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 thirty 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 (the "western") 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 (70–70.5 MHz) amateur radio allocation used in many European countries is entirely within the OIRT FM band. Operators on this band and the 6-meter band (50–54 MHz) 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-organisation of TV broadcasting in order to make full use of this band for audio broadcasting.

Historic U.S. bandplan[edit]

In the 1930s investigations were begun into establishing radio stations transmitting on "Very High Frequency" (VHF) assignments above 30 MHz. In October 1937, the Federal Communications Commission (FCC) announced new frequency allocations, which included a band of experimental and educational "Apex" stations, that consisted of 75 channels spanning from 41.02 to 43.98 MHz.^[12] Like the existing AM band these stations employed amplitude modulation, however the 40 kHz spacing between adjacent frequencies was four times as much as the 10 kHz spacing on the standard AM broadcast band, which reduced adjacent-frequency interference, and provided more bandwidth for high-fidelity programming.


Also during the 1930s Edwin Howard Armstrong developed a competing transmission technology, "wide-band frequency modulation", which was promoted as being superior to AM transmissions, in particular due to its high-fidelity and near immunity to static interference. In May 1940, largely as the result of Armstrong's efforts, the FCC decided to eliminate the Apex band, and authorized an FM band effective January 1, 1941, operating on 40 channels spanning 42–50 MHz,^[13] with the first five channels reserved for educational stations. There was significant interest in the new FM band by station owners, however, construction restrictions that went into place during World War II limited the growth of the new service.


Following the end of the war, 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 the Radio Corporation of America (RCA), which was headed by David Sarnoff, was that the FM band needed to be shifted to higher frequencies in order to avoid this potential problem. Armstrong charged that this reassignment had the covert goal of disrupting FM radio development,^[14] however RCA's proposal prevailed, and on June 27, 1945 the FCC announced the reassignment of the FM band to 90 channels from 88–106 MHz,^[15][16] which was soon expanded to 100 channels from 88–108 MHz, with the first 20 channels reserved for educational stations. A period of allowing existing FM stations to broadcast on both the original "low" and new "high" FM bands followed, which ended at midnight on January 8, 1949, at which time all low band transmissions had to end.^[17]


In 1978 one additional frequency reserved for educational stations, 87.9 MHz, was allocated.^[18] In March 2008, the FCC requested public comment on turning the bandwidth currently occupied by analog television channels 5 and 6 (76–88 MHz) over to extending the FM broadcast band when the digital television transition was to be completed in February 2009 (ultimately delayed to June 2009).^[19] This proposed allocation would have effectively assigned frequencies corresponding to the existing Japanese FM radio service (which begins at 76 MHz) for use as an extension to the existing North American FM broadcast band.^[20] Several low-power television stations colloquially known as "Franken-FMs" operated primarily as radio stations on channel 6, using the 87.7 MHz audio carrier of that channel as a radio station receivable on most FM receivers configured to cover the whole of Band II, from 2009 to 2021; since then, a reduced number have received special temporary authority to carry a special audio carrier on their ATSC 3.0 signals to continue the status quo.^[21]