Radio

Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves — the same phenomenon as visible light, but at a different frequency. The definition of radio waves sets the range between 3 hertz and 300 gigahertz. A hertz is one wave cycle per second, so 3 hertz means three complete cycles per second, and 300 gigahertz means three hundred billion. That range is wide enough to include waves that behave very differently from one another, but all of them can be generated, transmitted, and received using the same fundamental principle.

That principle is this: an electronic device called a transmitter, connected to an antenna, generates the waves and radiates them into space. The waves travel outward at the speed of light. Another antenna, connected to a receiver, picks them up. What moves between the two is the wave itself, not a current through a wire. That is what makes radio different from telegraphy or telephony over a cable: the signal crosses empty space.

To carry information rather than just a pulse, the transmitter modulates the wave. Modulation means varying some aspect of the wave — its amplitude, its frequency, or its phase — in a pattern that encodes sound, data, or an image. The receiver decodes that variation back into the original information. This is the mechanism behind radio and television broadcasting, mobile phones, wireless networking, and satellite communication.

Radio waves are also used for purposes other than communication. In radar, a transmitter sends out a beam of radio waves that reflects off an object — an aircraft, a ship, a missile — and the reflected waves return to a receiver, typically located with the transmitter. By measuring the time the waves take to return, the system calculates the object's distance and location. In radio navigation systems, a receiver picks up signals from multiple beacons whose positions are known, and by measuring the arrival time of the waves, it calculates its own position on Earth. In remote control devices, radio signals transmitted from a controller direct the actions of a remote device, such as a drone or a garage door opener.

The existence of radio waves was proved by Heinrich Hertz, a German physicist, on 11 November 1886. In the mid-1890s, Guglielmo Marconi, an Italian physicist, built on techniques physicists were using to study electromagnetic waves and developed apparatus for long-distance radio communication. He sent a wireless Morse Code message to a recipient over a kilometre away in 1895, and sent a transatlantic signal on 12 December 1901. The discovery date recorded for radio as a technology is 1894, which falls between Hertz's proof of the waves and Marconi's development of practical apparatus.

The emission of radio waves is regulated by law. Frequency bands within the radio spectrum are allocated for different uses, coordinated by the International Telecommunication Union.