Radio

A radio tower

Radio is a way to send electromagnetic signals over a long distance, to deliver information from one place to another. A machine that sends radio waves is called a transmitter, while a machine that "picks up" the signals is called a receiver or antenna. A machine that does both jobs is a "transceiver". When radio signals are sent out to many receivers at the same time, it is called a broadcast.

Television also uses radio signals to send pictures and sound.

Airplanes and other things may be used under radio control. Radio signals can be used to lock and unlock the doors in a car from a distance.

Sound can be sent by radio, sometimes through Frequency Modulation (FM) or Amplitude Modulation (AM).^[1]

History of radio

Many people worked to make radio possible. After James Clerk Maxwell predicted them, Heinrich Rudolf Hertz in Germany first showed that radio waves exist.^[2] Guglielmo Marconi in Italy made radio into a practical tool of telegraphy,^[3] used mainly by ships at sea. He is sometimes said to have invented radio. Later inventors learned to transmit voices, which led to broadcasting of news, music and entertainment.

Uses of radio

Radio was first created as a way to send telegraph messages between two people without wires. Then two-way radio brought voice communication, including walkie-talkies and eventually mobile phones.

Radio waves are still used to send messages between people. Talking to someone with a radio is different than "talk radio". Citizens band radio and amateur radio use specific radios to talk back and forth. Policemen, firemen and other people who help in emergency use a radio emergency communication system to communicate (talk to each other). It is like a mobile phone, (which also uses radio signals) but the distance they reach is shorter and both people must use the same kind of radio.

The word "radio" is sometimes used to mean only voiceband broadcasting. Most voiceband broadcasting uses lower frequency and longer wavelength than most television broadcasting. Voiceband broadcasting sends music, news and entertainers including "talk radio". Radio shows were used before there were TV programs. In the 1930s, the US president started sending a message about the country every week to the American people. Companies that make and send radio programming are called radio stations. These are sometimes run by governments, and sometimes by private companies, who make money by sending advertisements. Other radio stations are supported by local communities. These are called community radio stations. In the early days, manufacturing companies would pay to broadcast complete stories on the radio. These were often plays or dramas. Because companies who made soap often paid for them, these were called "soap operas".

Microwaves have even higher frequency; shorter wavelength. They also are used to transmit television and radio programs, and for other purposes. Communications satellites relay microwaves around the world.

A radio receiver does not need to be directly in view of the transmitter to receive programme signals. Low frequency radio waves can bend around hills by diffraction, although repeater stations are often used to improve the quality of the signals.

Shortwave radio frequencies are also reflected from an electrically charged layer of the upper atmosphere, called the Ionosphere. The waves can bounce between the ionosphere and the earth to reach receivers that are not in the line of sight because of the curvature of the Earth's surface. They can reach very far, sometimes around the world.

Radio telescopes receive radio waves from the sky to study astronomical objects. Satellite navigation uses radio to determine location, and radar uses it to find and track things.

Radio Media

An antenna farm hosting various radio antennas on Sandia Peak near Albuquerque, New Mexico, United States
Radio communication. Information such as sound is converted by a transducer such as a microphone to an electrical signal, which modulates a radio wave produced by the transmitter. A receiver intercepts the radio wave and extracts the information-bearing modulation signal, which is converted back to a human usable form with another transducer such as a loudspeaker.
Comparison of AM and FM modulated radio waves
Frequency spectrum of a typical modulated AM or FM radio signal. It consists of a component C at the carrier wave frequency f_c with the information (modulation) contained in two narrow bands of frequencies called sidebands (SB) just above and below the carrier frequency.
Volkswagen's RNS-510 receiver supports Sirius Satellite Radio.
Satellite phones, showing the large antennas needed to communicate with the satellite
Motorola SCR-536 from WW2, the first walkie-talkie
Firefighter using modern walkie-talkie
VHF marine radio on a ship