Radio frequency refers to an alternating electrical current with certain properties that allow it to be broadcast from an antenna. If the current generates an electromagnetic field or wave at a frequency that is suitable for broadcasting television or radio signals, then it is considered a radio frequency. These frequencies are part of the electromagnetic spectrum and are located just beyond the infrared side of visible light.
Any frequency between about nine hertz -- meaning nine cycles per second -- and 300 gigahertz -- meaning 300 billion cycles per second -- can be considered a radio wave, although only frequencies near the middle of this range are used in actual radio broadcasts. The rest of the range of radio frequencies is used by military and scientific personnel, mostly.
Most of us are familiar with AM and FM radio, but radios are only some of the wireless devices that use a radio frequency to operate. Television broadcasts received over the air are a form of radio waves, as are satellite communications, citizens’ band radios, and cordless and cellular telephones. Indeed, every wireless technology available utilizes its own radio frequency.
The majority of radios and wireless devices serve a single purpose, such as to receive AM radio, or to transmit sound and images over a short distance on a single radio frequency, such as is the case with a baby monitor. However, there are also radio receivers that have access to a very wide range of frequencies, and these are known as scanners. Many people use scanners to tune into the radio frequencies used by police and fire departments, or air traffic controllers. Scanners can be used to tune into just one station, or set to search the airwaves in the area for activity, and stop when a transmission is detected.
One of the lesser-known uses of radio frequencies is as a visual tool in astronomy. Objects in outer space often emit large amounts of energy other than visible light, such as x-rays and radio waves. In fact, some of the static that we hear between stations as we turn a radio dial, especially at night in deserted areas, is actually from interstellar radio waves. Although these radio waves are very weak by the time they reach earth, they can be used by astronomers to form a more complete picture of the cosmos than can be seen with the eye alone, even with the aid of a telescope.