radio [old school]

transmission or reception of ELECTROMAGNETIC RADIATION in the radio frequency range from one place to another without wires. 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 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 MICROELECTRONICS) led to the development of portable transmitters and receivers. Military applications of radio include the proximity fuse and various types of RECONNAISSANCE SATELLITES. 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. See also BROADCASTING; COMMUNICATIONS SATELLITES; RADAR.

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