FM

From HFUnderground

Revision as of 15:30, 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 Above 150 MHz

In the United States, 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). The Part 95 license free Family Radio Service (FRS) also uses narrow for all 22 channels in simplex mode. For 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.

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.