Citizens band radio (CB) is a short-range personal radio service, commonly associated with frequencies near 27 MHz. The band is often called 11 metres, after its approximate wavelength. CB and the amateur service have separate operating rules and should not be treated as the same radio allocation.
A widely used 40-channel plan has channel centres from 26.965 to 27.405 MHz. The sequence includes gaps and is not simply forty consecutive frequencies in numeric channel order. Some countries also provide other channel plans.
AM, FM and SSB are used where permitted. The European CEPT harmonisation decision defines a 26.960–27.410 MHz band with specified excluded channel centres and power limits of 4 W for angle modulation, 4 W for double-sideband AM and 12 W PEP for SSB. National implementation determines the applicable channels, equipment and operating conditions.
At 27 MHz, a first free-space estimate gives:
Wavelength ≈ 300 / 27 = 11.1 m Quarter wavelength ≈ 2.78 m Half wavelength ≈ 5.56 m
These are electrical starting points, not finished construction dimensions. Loading coils can shorten a mobile whip, with consequences for loss and bandwidth. The vehicle body or another suitable counterpoise is part of many mobile antenna systems.
Local communication depends on terrain, antenna height, polarisation and noise. Ionospheric propagation can also produce distant signals through sporadic E or F-region paths. These conditions vary with season, time and solar activity; they can bring both long-distance contacts and interference.
Computer-assisted experiments include weak-signal software such as WSJT-CB. The availability of a mode or frequency in an application does not establish that it is permitted for CB transmission in a particular country. A group-issued CB identifier is also different from an amateur radio licence or authorisation.
Accurate documentation should state the actual RF frequency, modulation, occupied bandwidth, equipment and power measurement used.