Modulation įrequency modulation or FM is a form of modulation which conveys information by varying the frequency of a carrier wave the older amplitude modulation or AM varies the amplitude of the carrier, with its frequency remaining constant. It radiates circularly polarized radio waves. Only broadcast stations with large enough geographic separations between their coverage areas can operate on close or the same frequencies.Ĭrossed dipole antenna of station KENZ's 94.9 MHz, 48 kW transmitter on Lake Mountain, Utah. The ITU publishes Protection Ratio graphs which give the minimum spacing between frequencies based on their relative strengths. To minimise inter-channel interference, stations operating from the same or geographically close transmitter sites tend to keep to at least a 500 kHz frequency separation even when closer frequency spacing is technically permitted. There are other unusual and obsolete FM broadcasting standards in some countries, with non-standard spacings of 1, 10, 30, 74, 500, and 300 kHz.

In most countries the maximum permitted frequency error of the unmodulated carrier is specified, which typically should be within 2 kHz of the assigned frequency. In some parts of Europe, Greenland and Africa, only even multiples are used. Some other countries follow this plan because of the import of vehicles, principally from the United States, with radios that can only tune to these frequencies. In most of South Korea, the Americas, the Philippines and the Caribbean, only odd multiples are used. The frequency of an FM broadcast station (more strictly its assigned nominal center frequency) is usually a multiple of 100 kHz. Where the OIRT band is used, the 87.5–108.0 MHz band is referred to as the CCIR band. This band, sometimes referred to as the OIRT band, is slowly being phased out.

Assigned frequencies are at intervals of 30 kHz.

In the former Soviet republics, and some former Eastern Bloc countries, the older 65.8–74 MHz band is also used.

Usually 87.5 to 108.0 MHz is used, or some portion thereof, with few exceptions: Throughout the world, the FM broadcast band falls within the VHF part of the radio spectrum.

Fm radiotuner to fmstations