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There are a number of different citizens band (CB) radio frequencies, but in most countries they all occur within what’s known as the 27 megahertz (MHz) band. Radio frequencies travel though the airwaves at various measurable frequencies. The science behind transmission and reception can be somewhat complex, but generally speaking the sender and the receiver need to be tuned into the same wavelength, which can be measured by its energy output. CB radios usually operate within the 27MHz band, but the number of available frequencies is usually around 40, which is the number of CB channels that are available in most places. The numbers don't always align exactly, though. Some frequencies reserved, or private, such as for law enforcement; most, though, are publicly available, and users simply have to wait until a particularly frequency is available to begin broadcasting over it.
There are a couple of different ways to communicate using radio technology. CB, FM, and AM frequencies are usually the most common, though each has its own specifications. Radio frequencies can range as high as 300 gigahertz for Extremely High Frequencies to below 300 hertz for Extremely Low Frequencies. CB radio frequencies almost always fall into what’s known as the High Frequency range, which is from 3 to 30 megahertz.
That said, CB radio usually only covers from about 26.965 to 27.405 megahertz. It’s important to note here that the frequency is different from the channel. Radio operators usually have to have one of each, and at least in the United States and Canada, the number of channels is fixed at 40.
Most of the available frequencies in the CB band are shared, which means that they are publicly accessible and can be used by anyone with the right equipment. Only one transmitter and one receiver can be using any given frequency at the same time, though. As such, people who want to use a particular frequency that’s already in use either need to wait until it becomes available again or else switch to another set of numerical coordinates.
Sometimes people will commit to communicate only on a certain frequency, but others prefer to shift around. Most of what’s said can also be overheard by non-participants. This can be both a pro and a con. In the pre-Internet years, CB radio was often a form of entertainment, and people could and often did get to know each other and carve out unique identities over the airwaves. The medium isn’t usually very good for personal conversations or the sharing of sensitive information, though.
In most cases anyone with a radio and the necessary know-how can use CB radio frequencies, either to communicate over or just to listen in on. In most places they’re most common with long-distance truckers and police forces, though. These two groups often need a simple and dependable way to communicate with each other and broadcast information to anyone in the vicinity who is listening. People who chase storms and other major weather events sometimes also transmit over these frequencies.
At least in the United States, certain channels have become more or less dedicated to certain uses, even if informally. Truck drivers typically talk on channel 19, for instance, with a frequency of 27.185 megahertz. These transmissions are usually openly available for anyone to hear, but most of the time truckers talk in a form of code that only they understand.
Some common CB radio codes remain in everyday use. For instance, 10-4 means “message received and understood,” 10-17 means “en route,” and 10-22 means “cancel last message.” Most citizen band radio models have a list of the basic codes. Local police departments typically put out their own handbook of codes, too, which can allow people to listen to chatter more easily.
In most cases CB radio frequencies only allow for a maximum range of 4 miles (6.4 km). Due to the overwhelming amount of interference and geographical obstacles, however, the range of a particular radio unit could be much less. Depending on the local laws, not all frequencies may be available for use, either. Police broadcasts are almost always able to be heard, but only authorized users can actually broadcast transmissions. Many countries set limits on the allowable frequency band and may penalize users who attempt to broadcast higher or lower than the 27MHz zone. This is usually because of possible overlap with other types of radio broadcasting.
@irontoenail - Citizen Band Radio has been, in some ways, the predecessor of the internet, at least in terms of communication and social organization.
For example, truckers once used it in order to organize rallies and protests, and to let each other know where the police had set up speed traps, or where the fuel was cheapest. This is much like people use Facebook and other social media to communicate today, letting each other know about gatherings or cheap deals or whatever.
I believe more people use the Family Radio service frequencies now than Citizen Band, since it's a bit more clear and easy to use.
I always wondered why it seemed so easy for people to listen in on police radios, but I guess it makes sense if they are essentially using a public station in order to communicate. It's probably not even illegal to use it.
I always thought of it as some kind of special advanced technology that only criminals and people in movies use, but I was catching a ride with a friend of a friend the other day and she had a police frequency detector in her car! She was just a high school student who had bought it because she thought it was interesting. Or at least, that's what she told me.