FM

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Revision as of 15:39, 3 July 2025

No static at all.

Frequency Modulation

With FM, the carrier frequency is modulated by the audio signal, with the transmitter power remaining constant. FM typically produces better quality signals than AM when the received signal is strong, but worse quality under poor reception conditions.

It is rarely used on HF frequencies, with some exceptions - land mobile systems operating in the 25-30 MHz region and the 27 MHz 11 meter CB operators in many countries - often in conjunction with AM and SSB, amateur radio operators using the FM in the 10 meter band, land mobile radio users operating on a small number of channelized frequencies in the 25 to 30 MHz range and some specialized maritime mobile (marine radio) systems used by fishing fleets, fishery radio networks, etc. In military contexts, "FM" is used to refer to the VHF-FM combat net radio / tactical radio systems operating in the (roughly) 30-88 MHz range, e.g. SINCGARS.

FM (in the form of narrow FM, also known as FMN, NFM or NBFM for narrow band FM) is heavily used on the VHF/UHF bands. Different services have different regulations regarding deviation limits, bandwidth, etc. for FM voice use worldwide.

FM mode is also known by its emission designation F3 or F3E. Phase modulation (PM) produces the same end result and uses the emission designator G3 / G3E for phase modulated voice. Some radio services, such as VHF marine, use the G3E emission for FM voice - 16K0G3E instead of the F3E designation.

WFM or WBFM is used worldwide on the FM broadcast band, which varies from country to country. The 88-108 MHz band (87.7 MHz to 107.9 MHz) is the de facto standard. Other standards, like the Japanese band or the OIRT band 65 MHz - 74 MHz used in former Soviet bloc countries (former USSR, the CIS, etc.) WFM usually has a 75 kHz bandwidth and multiple sub-carriers. The Japanese FM band used to be allocated the 76 MHz to 90 MHz spectrum, it is now 76 MHz - 95 MHz.

Narrow Band FM - Land Mobile Radio Above 150 MHz

In the United States, the FCC requires Part 90 land mobile (business radio, public safety radio systems) operating above 150 MHz must use narrow band FM deviation (2.5 kHz deviation, 11.25 kHz bandwidth limit). This requirement went into effect in 2013 and is commonly known as the FCC narrowbanding mandate. Land mobile radios (Motorola, etc.) should be set to "12.5 kHz channel spacing" or "12.5 kHz channel" mode for narrow mode.

Other radios call this "12K" mode or simply "NARR" or "NARROW". Computer programming settings generally require switching the radio to NFM mode for narrowband FM (vs. FM mode for regular 4/5 kHz bandwidth). Land mobile licenses must reflect the use of the narrowband emission designator - 11K3F3E, 11K2F3E, 11K0F3E are all considered identical in the eyes of the FCC. Another example would be Alinco radios, which use 8.5 kHz bandwidth narrow band FM for narrow mode - emission: 8K50F3E and 16 kHz bandwidth for regular wide mode - emission: 16K0F3E. Baofeng, etc. radios such as the UV-5R, UV-82, UV-25 and so on have a "WIDE" and "NARR" setting available, NARROW being narrow band FM, 9K20F3E or 9K10F3E emission depending on the source.

The Part 95 license free Family Radio Service (FRS) also uses narrow for all 22 channels in simplex mode. FRS shares all 22 of its frequencies with the GMRS service.

GMRS, regular "wide" deviation (4 kHz - 5 kHz deviation) is allowed on channels 15-22 (in simplex mode and as part of GMRS repeaters). Narrow mode is required on 3 of the 5 license free MURS channels on the VHF band. 4 kHz deviation is generally identified as "20 kHz bandwidth" or "20 kHz channel" with emission designator 16K0F3E or 16K0G3E on VHF marine, 5 kHz is called "25 kHz bandwidth" with emission designator 20K0F3E. In practice, 4 kHz and 5 kHz deviation are interchangeable with each other, however the wider deviation signal will sound louder on most receivers, and may cause distortion if the receiver is configured in the narrowband mode.

Amateur radio and the VHF marine band are exempt from this requirement, as are services operating below 150 MHz (for land mobile, that means the 25 MHz - 50 MHz VHF low band and 72-76 MHz VHF mid band). Outside the U.S., use of narrow deviation is commonplace even on VHF low band.