A radio scanner is a device that allows the user to listen in on someone else's radio conversation. Radio scanners are especially useful for those who have a curiosity to monitor emergency communications channels or air traffic control channels. A radio scanner is often capable of receiving signals from a variety of applications and can be programmed to only pick up specific frequencies.
Since human beings first picked up the ability to audibly communicate, other human beings have had the desire to listen in on what is being said. While eavesdropping is socially unacceptable, listening to a radio scanner is acceptable. This is because the public airwaves are just that -- public. There is no expectation of privacy like there may be in other forms of communication.
The main purpose of a radio scanner is to scan a width of frequencies to look for conversations. Once it finds a conversation, it may remain on that frequency until there is a sufficient lull in the conversation. Scanners may remain on the frequency for a fraction of a second after a transmission stops, or may wait a significantly longer period of time. This is usually determined by the user.
The radio scanner is simply nothing more than another type of radio receiver. It receives a signal and amplifies them so that they can be heard audibly. These radio receivers are often not meant for high-quality audio transmissions and therefore may not have the same clarity of sound many are used to hearing from a radio.
While most people may listen to a radio scanner out of a harmless fascination for what is taking place, police and others say they can be quite dangerous. If criminals keep a scanner handy, they may be able to realize a raid is going to take place and therefore take steps in order to leave the area or conceal evidence. This is one reason why law enforcement agencies across the nation have started using cellular frequencies not capable of being intercepted by radio scanners.
Scanners can also be used at some sporting events to enhance a fan's experience while in the stands. This is especially popular at NASCAR events. In fact, NASCAR actually will inform fans, both on its website and at the track, as to what the frequencies are that teams will be using for the day.
To hear local emergency traffic and air traffic control frequencies, they will need to be programmed into the radio scanner. In most cases, the instructions that come with the unit will include popular frequencies for an area and how to program them into the device. In other cases, they may refer you to a website with information that is updated frequently with this information.