For many of us, it's hard to imagine a time before cell phones. Until very recently, most of the people who carried mobile communication devices were people who needed them specifically to perform their jobs -- people like freight truck drivers and police. Before cell phones, many people used radiotelephones for mobile communications. These were installed in cars, so that people could communicate from construction sites or get much-needed help during emergency situations. Decades ago, a single tower with perhaps two dozen frequencies could control the radiotelephone communications for a whole city.
Citizens' band (CB) radios were often used by long-haul truckers and became popular with the general public in the 1970s. Essentially, a CB radio is a low-power, short-distance transmitter and receiver that is limited to a narrow range of radio frequencies. While most governments regulate which radio frequencies can be used for which purposes, average citizens are allowed to broadcast on the "citizens' band," sending their messages out to all CB users within range of the transmission who are listening to the channel.
Ham radio, or amateur radio, is a way of communicating with people around the globe with radio waves. A ham radio can be used for entertainment or public service, though many users are simply hobbyists and technology enthusiasts. With a well-built ham radio and a skilled relay system, operators can talk to friends locally or around the world. They can even communicate with astronauts in space. Unlike CB operators, ham radio operators do not broadcast to the general public, and there is a set of rules that governs their behavior. Ham radio operators must be licensed, and they communicate to other individuals or in small round-table groups. While it has long been possible to use a ham radio as a mobile communication device, it is sometimes more difficult than using a radiotelephone or a CB radio, since many ham radio antennas are very large and must be mounted onto a vehicle or other stable structure to make transmission possible.
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