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The carrier frequency is 1.25 MHz above the lower edge of the 6 MHz wide channel. This effectively makes the system AM at low modulation frequencies and SSB at high modulation frequencies. The absence of the lower sideband components at high frequencies must be compensated for, and this is done in the IF amplifier.
As originally constituted, what is now CB radio was Class D of the Citizens' Radio Service. Classes A and B were in the UHF radio band and served a similar purpose as Class D while Class C was interspersed among the current CB channels and used for remote control of devices, usually model craft (aircraft, watercraft, or road vehicles).
South Africa has a 23 channel AM / SSB 29 MHz CB allocation (called "29 Megs" or "29 MHz CB") from 29.710 to 29.985 MHz in 12.5 kHz steps. South Africa also permits use of standard CB channels 19–27 (27.185–27.275 MHz) with AM / SSB permitted. Many radios sold in South Africa feature both the 27 MHz and 29 MHz bands.
Brazil allows higher power levels than the US and most of Europe. New Zealand has two 40-channel HF CB bands available, the NZ-specific "NZ CB Band" 26.330–26.770 MHz (40 channels, AM and SSB allowed) and the standardized "mid band" 26.965–27.405 MHz (40 channels, AM and SSB allowed) for a total of 80 HF CB channels.
The power of an AM radio signal plotted against frequency. fc is the carrier frequency, fm is the maximum modulation frequency. In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, that are the result of the modulation process. The sidebands carry the information transmitted by the radio ...
The list of frequency ranges is called a band allocation, which may be set by international agreements, and national regulations. The modes and types of allocations within each frequency band is called a bandplan; it may be determined by regulation, but most typically is set by agreements between amateur radio operators.
The International Telecommunication Union uses an internationally agreed system for classifying radio frequency signals. Each type of radio emission is classified according to its bandwidth, method of modulation, nature of the modulating signal, and type of information transmitted on the carrier signal.
The CB lobby continued to insist that any CB system had to use the 27 MHz band, be AM and a maximum output power of 4 watts (i.e. the US system). Ultimately, the government hinted that they were going to give in to the CB lobby but, as it turned out, only up to a point. CB was eventually legalised on a 27 MHz band but not the band used in the US.