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Radio refers to the transmission and reception of electromagnetic waves in the radio frequency range. "Radio waves" are commonly used to transmit music, conversations, pictures, and other data through the air over significantly long distances. A transmitter radiates this field outward via an antenna; a receiver then picks up the field and translates it, for example, into the sounds heard through the radio. Radio waves are used in a range of everyday technologies, including the following:
- AM and FM radio broadcasts
- Cordless phones
- Garage door openers
- Wireless networks
- Radio-controlled toys
- Television broadcasts
- Cell phones
- GPS receivers
- Ham radios
- Satellite communications
- Police radios
- Wireless clocks
From its origins in the early twentieth century, broadcast radio provided news and entertainment with an immediacy, which was never before experienced. Between around 1920 to 1945 (until the adoption of television), radio developed into the first electronic mass medium. Broadcast radio remains a widely used electronic mass medium.
In AM (Amplitude Modulation) radio, the strength (amplitude) of the signal is changed (modulated) to produce the sounds. In FM (Frequency Modulation) radio, it is the speed (frequency) of the signal that is changed. Radio receivers have dials with numbers that indicate the kilo or megaHertz at which the signal is being broadcast. Hertz is a measurement of the number of wave cycles per second—AM is expressed in kiloHertz, while FM radio is expressed in megaHertz. The station power affects the range of the signal, or how far it can travel. The strongest AM power allowed in the United States is 50,000 watts.
The existence of radio waves was predicted by James Clark Maxwell in the early 1860s and was discovered by Heinrich Hertz in 1886. Italian inventor Guglielmo Marconi first developed the idea of a radio or wireless telegraph in the 1890s. In 1895, Marconi sent a wireless Morse Code message to a source more than a kilometer away, and he received a British patent for his radio in 1897. Other inventors in Russia and the United States had also been working on similar devices, and by 1900 there were four competing wireless systems. By October 1902, Marconi sent the letter "S" across the English Channel, marking the first successful transatlantic radiotelegraph message.
