Radio and television stations are licensed to broadcast their signals over specific radio frequencies controlled by the Federal Communication Commission (FCC). Consumers routinely buy receivers (radios and televisions) which can translate those signals into sound and/or visual images again. This same technology, however, extends into many other types of radiowave broadcasts. Emergency services, airlines and even baby monitors and cordless phones all have assigned frequencies. Devices called radio scanners can be used to receive these signals, allowing users to eavesdrop on those communications.
Radio scanners first became popular in the 1970s, around the same time as Citizens' Band radios. The FCC allowed public access to 40 frequencies above the FM radio band, which encouraged truck drivers and hobbyists to purchase special transmitters and receivers. Interest in receiving other frequencies also arose, leading to the creation of the first radio scanners capable of intercepting police, fire and airline channels. These original radio scanners were limited by their need for special 'crystal' tuners for each desired frequency. These tuners could prove to be very expensive or difficult to locate.
As technology progressed, the crystal tuning system was replaced with electronic devices capable of scanning the entire radio spectrum. The specific frequencies for many police and fire dispatch centers could be dialed in directly and kept in a memory bank for future scanning. This also allowed radio scanners to become more compact, resulting in handheld models which could be easily concealed. As with many other technologies, however, radio scanners began to be used for illicit purposes -- eavesdropping, cordless phone tapping, surveillance of police activities and the like. Many emergency services switched to encrypted broadcasts to avoid detection by criminals.
Owning radio scanners is not technically illegal under most circumstances. There is no danger of radio scanners being used as illegal transmitters or electronic signal jammers. Users are simply receiving radio signals broadcast in open air frequencies. It is illegal, however, to unscramble encrypted messages, so radio scanners should never be modified with descrambling accessories. Conversations overheard on radio scanners cannot be transcribed or repeated publicly. Some police agencies consider possession of radio scanners while committing a crime to be worthy of additional charges.
Most radio scanners pick up little more than routine chatter and dispatches, since many organizations have already switched over to indecipherable encrypted channels. Broadcasts from high-profile emergency service organizations can also be monitored through certain websites. As communication technology changes, the capabilities of many radio scanners may be reduced even further. It is best to look for radio scanners with broad bandwidth parameters and programmable memory banks in order to find the most usable frequencies.