See Also: Radio and Television Interference Noise, Radar
Transmission or reception of Electromagnetic Radiation in the radio Frequency range from one place to another without wires (see Conductor). For the propagation and interception of radio Waves, a transmitter and receiver are employed. A radio wave carries
information-bearing signals; the information may be encoded directly on the wave by
periodically interrupting its transmission (see Telegraph) or impressed on the carrier frequency by a process called Modulation, e.g., amplitude modulation (AM) or frequency modulation (FM). In its most
common form, radio transmits Sounds (voice and music) and pictures (Television). The sounds (or images) are converted into electrical signals by a Microphone (or camera tube), amplified (see Amplifier), and used to modulate a carrier wave that has been generated by a
transmitter. The modulated carrier is also amplified, then applied to an Antenna that converts the electrical signals to electromagnetic waves that radiate
into space at the Speed of Light. Receiving antennas intercept part of this radiation, convert it back into
electrical signals, and feed it to a receiver. Once the basic signals have been
separated from the carrier wave, they are fed to a Loudspeaker or Cathode-Ray Tube, where they are converted into sound and visual images, respectively. Some
celestial bodies and interstellar Gases (see States Of Matter) emit relatively strong radio waves that are observed with radio telescopes
composed of very sensitive receivers and large directional antennas (see Radio Astronomy). Long-range radio signals enable communications between astronauts and
ground-based controllers and carry information from space probes as they travel to
and encounter distant planets. The invention of the Transistor and other microelectronic devices (see Integrated Circuit, Semiconductor) led to the development of portable transmitters and receivers. Military
applications of radio include the proximity fuse and various types of
reconnaissance satellites (see also: Communications Satellite, Satellite - Artificial). Radio signals sent by navigation satellites can be used by special
receivers to determine one's position. Citizens band (CB) radios, operating at
frequencies near 27 megahertz, are used in vehicles for communication while traveling.
On August 28, 1922 radio station WEAF in New York broadcast a commercial
message for a real-estate developer. Though there are earlier examples of radio
stations agreeing to mention record stores in exchange for records, this may have
been the first instance of the purchase of Air time for a commercial message. (The noun commercial, an Americanism meaning a broadcast advertisement, dates (see Dating) from 1935.)