Radio Métropolys

Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connected to an antenna which radiates the waves, and received by another antenna connected to a radio receiver. 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 (impressing an information signal on the radio wave by varying some aspect of the wave) 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 accepts 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 radio 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.

Applications of radio waves that do not involve transmitting the waves significant distances, such as RF heating used in industrial processes and microwave ovens, and medical uses such as diathermy and MRI machines, are not usually called ''radio''. The noun ''radio'' is also used to mean a broadcast radio receiver.

The existence of radio waves was first proven by German physicist Heinrich Hertz on November 11, 1886. In the mid 1890s, building on techniques physicists were using to study electromagnetic waves, Guglielmo Marconi developed the first apparatus for long-distance radio communication,Bondyopadhyay, Prebir K. (1995)
Guglielmo Marconi – The father of long distance radio communication – An engineer's tribute"
''25th European Microwave Conference: Volume 2'', pp. 879–85 sending a wireless Morse Code message to a source over a kilometer away in 1895, and the first transatlantic signal on December 12, 1901. The first commercial radio broadcast was transmitted on November 2, 1920 when the live returns of the Harding-Cox presidential election were broadcast by Westinghouse Electric and Manufacturing Company in Pittsburgh, under the call sign KDKA.

The emission of radio waves is regulated by law, coordinated by the International Telecommunication Union (ITU), which allocates frequency bands in the radio spectrum for different uses.

Technology

Radio waves are radiated by electric charges undergoing acceleration. They are generated artificially by time varying electric currents, consisting of electrons flowing back and forth in a metal conductor called an antenna.

As they travel farther from the transmitting antenna, radio waves spread out so their signal strength (intensity in watts per square meter) decreases, so radio transmissions can only be received within a limited range of the transmitter, the distance depending on the transmitter power, the antenna radiation pattern, receiver sensitivity, noise level, and presence of obstructions between transmitter and receiver. An omnidirectional antenna transmits or receives radio waves in all directions, while a directional antenna or high-gain antenna transmits radio waves in a beam in a particular direction, or receives waves from only one direction.

Radio waves travel at the speed of light in a vacuum.

The other types of electromagnetic waves besides radio waves, infrared, visible light, ultraviolet, X-rays and gamma rays, can also carry information and be used for communication. The wide use of radio waves for telecommunication is mainly due to their desirable propagation properties stemming from their large wavelength.

Radio communication

In radio communication systems, information is carried across space using radio waves. At the sending end, the information to be sent is converted by some type of transducer to a time-varying electrical signal called the modulation signal. The modulation signal may be an audio signal representing sound from a microphone, a video signal representing moving images from a video camera, or a digital signal consisting of a sequence of bits representing binary data from a computer. The modulation signal is applied to a radio transmitter. In the transmitter, an electronic oscillator generates an alternating current oscillating at a radio frequency, called the ''carrier wave'' because it serves to "carry" the information through the air. The information signal is used to modulate the carrier, varying some aspect of the carrier wave, impressing the information on the carrier. Different radio systems use different modulation methods:
* AM ( amplitude modulation) – in an AM transmitter, the amplitude (strength) of the radio carrier wave is varied by the modulation signal;
* FM (frequency modulation) – in an FM transmitter, the frequency of the radio carrier wave is varied by the modulation signal;
* FSK ( frequency-shift keying) – used in wireless digital devices to transmit digital signals, the frequency of the carrier wave is shifted between frequencies.
* OFDM (orthogonal frequency-division multiplexing) – a family of complicated digital modulation methods very widely used in high bandwidth systems such as Wi-Fi networks, cellphones, digital television broadcasting, and digital audio broadcasting (DAB) to transmit digital data using a minimum of radio spectrum bandwidth. It has higher spectral efficiency and more resistance to fading than AM or FM. In OFDM, multiple radio carrier waves closely spaced in frequency are transmitted within the radio channel, with each carrier modulated with bits from the incoming bitstream so multiple bits are being sent simultaneously, in parallel. At the receiver, the carriers are demodulated and the bits are combined in the proper order into one bitstream.

Many other types of modulation are also used. In some types, a carrier wave is not transmitted but just one or both modulation sidebands.

The modulated carrier is amplified in the transmitter and applied to a transmitting antenna which radiates the energy as radio waves. The radio waves carry the information to the receiver location.

At the receiver, the radio wave induces a tiny oscillating voltage in the receiving antenna which is a weaker replica of the current in the transmitting antenna. This voltage is applied to the radio receiver, which amplifies the weak radio signal so it is stronger, then demodulates it, extracting the original modulation signal from the modulated carrier wave. The modulation signal is converted by a transducer back to a human-usable form: an audio signal is converted to sound waves by a loudspeaker or earphones, a video signal is converted to images by a display, while a digital signal is applied to a computer or microprocessor, which interacts with human users.

The radio waves from many transmitters pass through the air simultaneously without interfering with each other because each transmitter's radio waves oscillate at a different rate, in other words, each transmitter has a different