Radio is the communication of information between distant points using radio waves, electromagnetic radio of wavelength between about 1 millimeter and 100 kilometers. Radio waves are also described in terms of their frequency – measured in hertz (Hz) and found by dividing the velocity of the waves (about 300 million meters/second) by their wavelength.
Radio communications system link transmitting stations with receiving stations. In a transmitting station a piezoelectric oscillator is used to generate a steady radio-frequency (RF) carrier wave. This is amplified and modulated (see modulation) 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, say, Morse code, but speech and music, entering the modulator as an audio-frequency (AF) signal from tape of a microphone, is made to interact with the carrier so that the shape of the audio wave determines either the amplitude of the carrier wave (amplitude modulation, AM) or its frequency within a small band on either side of the original carrier frequency (frequency modulation, FM). The modulated RF signal is then amplified (see amplifier) 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. The RF signal is amplified and the original audio signal is recovered (demodulation or detection). Detection and amplification often involve many stages including feedback and intermediate frequency (IF) circuits. A radio receiver must of course be able to discriminate between all the different signals acting at any one time on its antenna. This is accomplished with a tuning circuit which allows only the desired frequency to pass to the detector.
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. Programs are often produced centrally and distributed to a "network" of local broadcasting stations by wire or microwave link. Because there are potentially so many users of communications – aircraft, ships, police, amateur"hams" as well as broadcasting services – the 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 such as the US Federal Communications Commission (FCC) divide the RF spectrum into bands which it allocates to the various users. Public broadcasting in the US uses MF frequencies between 535 kHz and 1605 kHz (AM) and VHF bands between 88 MHz and 108 MHz (FM). VHF reception, though limited to line-of-sight transmissions, offers much higher fidelity of transmission and much greater freedom from interference. International broadcasting and local transmissions in other countries often use other frequencies in the LF, MF, and HF (short wave) bands.
The development of radio
The existence of radio waves was first predicted by James Clerk Maxwell in the 1860s but it was not until 1887 that Hertz succeeded in producing them experimentally. "Wireless" telegraphy was first demonstrated by Oliver Lodge in 1894 and Marconi made the first transatlantic transmission in 1901. Voice transmission was first achieved in 1900 but transmitter and amplifier powers were restricted before the invention of Lee de Forest's triode electron tube in 1906. Only the development of the transistor after 1948 had a great impact on radio technology. Commercial broadcasting began in the US in 1920.