CB radio in the United States

The CB Radio Service spectrum is divided into 40 numbered radio frequency channels from 26.965 to 27.405 MHz. Channel spacing is 10 kHz between channel centers with exceptions where CBRS channels are adjacent to Radio Control Radio Service. The initial channel allocations had a gap equal to two channel spaces between channels 22 and 23. Those channels were assigned to the Business Radio Service. Beginning in 1977, those two channels (and 15 others above CB Channel 23) were reallocated to CB use. Channel 23 was left as it was so that users of pre-1977 equipment could use that equipment with minimal confusion.

Channel 19 is the most commonly-used channel by truck drivers on highways, to the point that some radios even have a dedicated button to bring up channel 19 instantly. In many areas of the US, other channels have been used in the past for similar purposes including 10, 17, and 21.[9] Channel 13 has been used in some areas for marine use[8] and for recreational vehicles.[9]

Channel 9 is reserved by regulation[3] for emergency use and to provide traveler assistance. In decades past, the channel was monitored by volunteers who could relay messages to the authorities, and often monitored directly by the authorities themselves. With the popularity of cellular phones since the 1990s, support for Channel 9 as an emergency channel has diminished, though volunteer organizations such as Radio Emergency Associated Communication Teams (REACT), and private individuals still monitor Channel 9 in some areas.

Regional mode conventions include Channels 30 or 35 through 40 for SSB operation, often with 38 known as a calling channel. There is no consensus yet for FM operation, legalized in 2021.

Among several other services that share the CB frequencies is the former Class C Citizens Band service, renamed to the Radio Control Radio Service (RCRS) in 1976, outlined in Subpart C of the Part 95 rules for radio-controlled ("R/C") devices. No voice transmissions are permitted. It has six channels in the 27 MHz band. Five are unused 10 kHz CB assignments between channels 3–4, 7–8, 11–12, 15–16 and 19–20, and the sixth is shared with Channel 23. R/C transmitters may use up to 4 watts on the first five channels and 25 watts on the last, 27.255 MHz. Some in-house paging systems, and car alarms with a paging feature, also use these frequencies, especially 27.255 MHz, where the higher power is permitted.

The 27 MHz RCRS channels are not officially numbered by the FCC. R/C enthusiasts usually designate them by color, and fly different-colored flags from the antenna to show who is on which channel.[11]

Because of interference from CB radios, legal or otherwise, the noise level, and the limited number of channels, most "serious" hobby radio-controlled models operate on other bands.[11]

The RCRS service has 50 channels just for model aircraft in the 72–73 MHz range, and 30 more channels for surface models such as cars and boats in the 75.4–76 MHz range. 0.75 watts is allowed on these numbered channels. Licensed amateur radio operators can use any amateur frequency for R/C, but those enthusiasts tend to use frequencies in their 6-meter band.[11]

Many toy R/C cars and wireless keyboards and mice operate on the 27 MHz R/C channels, especially 27.145 MHz. But most of these devices run far less than 4 watts and do not operate under the RCRS service. Instead, they operate under the FCC's Part 15 rules, which allow a wide variety of low powered devices to use the frequencies from 26.96 to 27.28 MHz, which covers CB Channels 1 through 27.

Some other of the R/C toys operate on the 49 MHz Part 15 channels, and often a pair of R/C cars will be sold with one on 27.145 and one on 49.860 to avoid interference. This allows less selective, and therefore less expensive, receivers to be used than if they were using channels in the same band.

In the days when CB required a license, some low-powered or toy walkie-talkies were exempt because they operated within Part 15. However, in 1976, the FCC phased in a shift of these 100 mW[12] devices to the 49 MHz band, with operation on the CB frequencies to cease in 1983.[13] More recently in the 1990s, low-powered handhelds using FM voice on the 27 MHz radio-control channels were also sold to operate legally under Part 15.

Broadband over Power Lines (BPL) technology uses a wide range of HF frequencies to transmit data (3.5 through 30 MHz), which includes the CB frequencies. There is great potential for interference, as power lines were never specifically designed to shield radio frequencies. RF leakage from BPL is regulated under Part 15 and is a big problem for amateur radio operators across all frequencies that the BPL uses.

Another class of devices operating in the 27 MHz band are ISM (Industrial, Scientific and Medical) devices regulated by the FCC's Part 18 rules. Induction welding of plastics, and some types of diathermy machines commonly operate in this range. These devices centered on 27.12 MHz with a tolerance of ±163 kHz, that is, 26.957 to 27.283 MHz.[14]