Radio is the technology of signaling or communicating using radio waves.
The early history of radio is the history of technology that produces and uses radio instruments that use radio waves. Many people contributed theory and inventions in what became radio. Radio development began as "wireless telegraphy;" more recent history increasingly involves matters of broadcasting.
Radio waves are electromagnetic waves. They are generated by an electronic device called a transmitter connected to an antenna which radiates the waves, and received by a radio receiver connected to another antenna. Radio is very widely used in modern technology, in radio communication, radar, radio navigation, remote control, remote sensing and other applications. In radio communication, used in radio and television broadcasting, cell phones, two-way radios, wireless networking and satellite communication among numerous other uses, radio waves are used to carry information across space from a transmitter to a receiver, by modulating the radio signal in the transmitter. In radar, used to locate and track objects like aircraft, ships, spacecraft and missiles, a beam of radio waves emitted by a radar transmitter reflects off the target object, and the reflected waves reveal the object's location. In radio navigation systems such as GPS and VOR, a mobile receiver receives radio signals from navigational radio beacons whose position is known, and by precisely measuring the arrival time of the radio waves the receiver can calculate its position on Earth. In wireless remote control devices like drones, garage door openers, and keyless entry systems, radio signals transmitted from a controller device control the actions of a remote device.
Radio waves were first identified and studied by German physicist Heinrich Hertz in 1886. The first practical radio transmitters and receivers were developed around 1895-6 by Italian Guglielmo Marconi, and radio began to be used commercially around 1900. To prevent interference between users, the emission of radio waves is strictly regulated by law, coordinated by an international body called the International Telecommunications Union (ITU), which allocates frequency bands in the radio spectrum for different uses.
Broadcasting is the one-way transmission of information from a radio transmitter to receivers belonging to a public audience. Since the radio waves become weaker with distance, a broadcasting station can only be received within a limited distance of its transmitter. Systems which broadcast from satellites can generally be received over an entire country or continent.
Older terrestrial radio and television paid for by commercial advertising or governments. In subscription systems like satellite television and satellite radio the customer pays a monthly fee. In these systems the radio signal is encrypted and can only be decrypted by the receiver, which is controlled by the company and can be deactivated if the customer doesn't pay his bill.
Broadcasting uses several parts of the radio spectrum, depending on the type of signals transmitted and the desired target audience. Long-wave and medium-wave signals can give reliable coverage of areas several hundred kilometers across, but work only with audio signals (speech and music),and are subject to natural and artificial sources of interfering noise. The shortwave bands have greater potential range, but are more subject to interference by distant stations and varying atmospheric conditions that affect reception.