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FM legal on 11 meter CB radio in USA
27MHz CB radio legal modes and bandwidth limits
95.973 limits AM mode to 8 kHz bandwidth, FM mode to 8 kHz bandwidth and SSB mode to 4 kHz bandwidth.
95.971 requires all CB radios to operate in the AM mode (A3E emission). In addition to AM voice modes SSB voice emission types J3E, R3E or H3E are permitted. FM mode is permitted F3E emission.
8K00A3E
8K00F3E
4K00J3E
4K00R3E
4K00H3E
Note that the FCC rules specifically permit full carrier double side band AM DSB-AM as well as suppressed carrier single side band mode or J3E reduced carrier SSB mode or R3E or full carrier with one sideband SSB with carrier H3E. Full carrier with one sideband is often called AME or backwards compatible AM since AM receivers can decode those signals without a BFO.
FCC rules indicate that if a CB transceiver / CB transmitter is capable of SSB operation, it must at least be able to use the upper side band USB. SSB CB radios all have the capability to use either the lower side band LSB or the upper side band USB. As per the rules, all CB rigs are either
AM mode only
AM and SSB CB radios AM USB LSB
AM and FM
AM SSB and FM all-mode AM FM USB LSB CW PA
Part 95 FCC rules 95.971
§ 95.971 CBRS emission types.
Each CBRS transmitter type must be designed such that its capabilities are in compliance with the emission type rules in this section.
(a) Permitted emission types. CBRS transmitter types must transmit AM voice emission type A3E or SSB voice emission types J3E, R3E or H3E, and may also transmit FM voice emission type F3E.
(b) SSB requirements. Each CBRS transmitter type that transmits emission type J3E, R3E, or H3E must be capable of transmitting only the upper sideband with suppressed, reduced or full carrier, respectively, but may additionally be capable of transmitting only the lower sideband, with suppressed, reduced or full carrier, respectively.
AM voice 8 kHz 8K00A3E
FM voice
SSB voice
