Citizens band radio
Citizens band radio, used in many countries, is a land mobile radio system, a system allowing short-distance person-to-many persons bidirectional voice communication among individuals, using two way radios operating on 40 channels near 27 MHz (11 m) in the high frequency band. Citizens band is distinct from other personal radio service allocations such as FRS, GMRS, MURS, UHF CB and the Amateur Radio Service. In many countries, CB operation does not require a license, and it may be used for business or personal communications. Like many other land mobile radio services, multiple radios in a local area share a single frequency channel, but only one can transmit at a time. The radio is normally in receive mode to receive transmissions of other radios on the channel; when users want to talk they press a "push to talk" button on their radio, which turns on their transmitter. Users on a channel must take turns talking. Transmitter power is limited to 4 watts in the US and the EU. CB radios have a range of about 3 miles (4.8 km) to 20 miles (32 km) depending on terrain, for line of sight communication; however, various radio propagation conditions may intermittently allow communication over much greater distances.