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Radio, communication of information between distant points using electromagnetic radiation (radio waves). Radio waves are often described in terms of their frequency, which is measured in hertz (Hz) and found by dividing the velocity of the waves by their wavelength. Radio communications systems link transmitting stations with receiving stations. In a transmitting station a piezoelectric oscillator generates a steady radio-frequency (RF) “carrier” wave, which is amplified and “modulated” with a signal carrying the information to be communicated. The simplest method of modulation is to pulse (switch on and off) the carrier with a signal in, for example, Morse code. Speech and music enter the modulator as an audiofrequency (AF) signal from tape or a microphone, and can also interact with the carrier. The modulated RF signal is then amplified to a high power and radiated from an antenna. At the receiving station, another antenna picks up a minute fraction of the energy radiated from the transmitter, together with some background noise. This RF signal is amplified, and the original audio signal is recovered (demodulation, or detection). In point-to-point radio communications most stations can both transmit and receive messages, but in radio broadcasting a central transmitter broadcasts program sequences to a multitude of individual receivers. Because there are potentially so many users of radio communications, use of the RF portion of the electromagnetic spectrum is strictly controlled to prevent unwanted interference between signals having adjacent carrier frequencies. The International Telecommunication Union (ITU) and national agencies like the U.S. Federal Communications Commission (FCC) divide the RF spectrum into banks that they allocate to the various users.

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21st Century Webster's Family Encyclopedia21st Century Webster's Family Encyclopedia - Providence to Rafflesia