A radio scanner is a radio receiver which is capable of scanning through a number of preprogrammed channels or frequencies. The radio scanner can also be programmed to scan or search between two given frequencies in predetermined steps.Depending on the price, these scanning radios offer different features and capabilities. The more expensive scanners cover a wider range of frequencies, have a lot of memory channels and scan through these channels very quickly.Once a radio scanner has a set of different frequencies programmed into some or all of its memory channels and set to scan, it will listen momentarily to each of these channels in turn in ascending order of channel number.These radio scanners are equipped with a squelch which is a device that can be set to suppress the audio output of the receiver if the received signal is not strong enough. This squelch is set to tune out any natural background radio noise.As the radio scans through its preprogrammed frequencies it will stop on a frequency with a signal stronger than the minimum squelch setting. The radio can be set to stay on this frequency until the transmission ends or it can be programmed to stay on this frequency for a predetermined amount of time.Before the scanning radio was developed to achieve the same results as a 100 channel scanner, one would to have 100 radios each tuned to a different frequency. Each of these radios would then be turned on and, if nothing was heard off again, the next radio would then be turned on then off and so on down the line until a signal was heard on one of the radios. All 100 of these radios would also have to be turned on and off again in under two seconds.Many of the modern scanning radio receivers are very wide band receivers. One very popular radio scanner manufacturer offers a wide band scanner which tunes from 150 kHz to 1309.995 MHz.A typical household analog FM receiver usually has a frequency range from about 88MHz to 108MHz and the frequency tuning display is maybe about 2.5 inches long. To match the frequency range of a modern wide band scanner and display the tuned frequency in the same way an analog FM receiver would have to have a tuning display or dial over 25 feet tong.Radio scanners can receive most unencrypted analog FM and AM radio signals and are used mainly to monitor 2 way radio communications. Some which are capable of receiving a wide FM signals can be used also to listen to broadcast stations. A few scanners are even capable of receiving digital radio transmissions.For the most part these at radio scanners are used by hobbyists to listen in the on a two-way radio transmissions from: Fire departments, Ambulances, Police departments, the Coast Guard, Marine and Ship to Shore radio, Aircraft communications from the tower to the aircraft or from one aircraft to another aircraft, amateur radio, CB radio, business radio which might include Taxis, Railroads and towing companies etc, family radio walkie-talkies and even race car drivers to their pit crews.They can also be used to listen in on some of the older wireless or cordless telephones. The newer cordless telephones have adopted spread spectrum technology to prevent telephone conversations from being overheard using radio scanners.Legislation was passed in the United States making it illegal to sell scanners capable of receiving certain frequencies that are used by cell phones. The cell phone companies have also adopted spread Spectrum technology and encryption to prevent eavesdropping.A radio enthusiasts may use their scanner to monitor the radio communications of their local police, Fire and ambulance departments and maybe some of the local amateur radio repeaters.All radio transmitters in the United States are registered with the FCC. All the information about the transmitters, their transmit frequency and their exact location is freely available to the public. In the USA it is a very simple matter to look up all of the frequencies in your town and your surrounding townships and program them into your radio scanner.If you hear a siren and switch on your scanner then usually within a few seconds you will hear the radio communications between the responders and the dispatchers and find out exactly what the emergency is and which departments are responding to it.Scanners are available as base models for home use or handheld units. They range in price from about $75 for a basic entry-level radio to several hundred dollars for the higher end scanners with all the bells and whistles.For the hours of informative entertainment the radio scanner can provide, even a mid range scanner costing a couple of hundred dollars really is it is an extremely good value and a great investment.