Radio
They are generated by an electronic device called a transmitter connected to an antenna which radiates oscillating electrical energy, often characterized as a wave.
[4] In the mid-1890s, building on techniques physicists were using to study electromagnetic waves, Italian physicist Guglielmo Marconi developed the first apparatus for long-distance radio communication,[5] sending a wireless Morse Code message to a recipient over a kilometer away in 1895,[6] and the first transatlantic signal on 12 December 1901.
It was first applied to communications in 1881 when, at the suggestion of French scientist Ernest Mercadier [fr], Alexander Graham Bell adopted radiophone (meaning "radiated sound") as an alternate name for his photophone optical transmission system.
Up until about 1910 the term wireless telegraphy also included a variety of other experimental systems for transmitting telegraph signals without wires, including electrostatic induction, electromagnetic induction and aquatic and earth conduction, so there was a need for a more precise term referring exclusively to electromagnetic radiation.
For example, in early 1898 the British publication The Practical Engineer included a reference to the radiotelegraph and radiotelegraphy.
Lee de Forest helped popularize the new word in the United States—in early 1907, he founded the DeForest Radio Telephone Company, and his letter in the 22 June 1907 Electrical World about the need for legal restrictions warned that "Radio chaos will certainly be the result until such stringent regulation is enforced.
On 11 November 1886, German physicist Heinrich Hertz, attempting to confirm Maxwell's theory, first observed radio waves he generated using a primitive spark-gap transmitter.
[19] In 1895, Guglielmo Marconi developed the first radio communication system, using a spark-gap transmitter to send Morse code over long distances.
[4][5][6][7] Marconi and Karl Ferdinand Braun shared the 1909 Nobel Prize in Physics "for their contributions to the development of wireless telegraphy".
Starting around 1908 industrial countries built worldwide networks of powerful transoceanic transmitters to exchange telegram traffic between continents and communicate with their colonies and naval fleets.
On 2 November 1920, the first commercial radio broadcast was transmitted by Westinghouse Electric and Manufacturing Company in Pittsburgh, under the call sign KDKA featuring live coverage of the Harding-Cox presidential election.
[21][22] They are generated artificially by time-varying electric currents, consisting of electrons flowing back and forth in a metal conductor called an antenna.
[25][26][27][28] Radio waves travel at the speed of light in vacuum[29] and at slightly lower velocity in air.
The wide use of radio waves for telecommunication is mainly due to their desirable propagation properties stemming from their longer wavelength.
[34] The modulated carrier is amplified in the transmitter and applied to a transmitting antenna which radiates the energy as radio waves.
Two radio transmitters in the same area that attempt to transmit on the same frequency will interfere with each other, causing garbled reception, so neither transmission may be received clearly.
[47] In order to adjust, maintain, or internally repair radiotelephone transmitters, individuals must hold a government license, such as the general radiotelephone operator license in the US, obtained by taking a test demonstrating adequate technical and legal knowledge of safe radio operation.
[48] Exceptions to the above rules allow the unlicensed operation by the public of low power short-range transmitters in consumer products such as cell phones, cordless phones, wireless devices, walkie-talkies, citizens band radios, wireless microphones, garage door openers, and baby monitors.
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 does not pay.
[52] Broadcasting uses several parts of the radio spectrum, depending on the type of signals transmitted and the desired target audience.
Longwave and medium wave signals can give reliable coverage of areas several hundred kilometers across, but have a more limited information-carrying capacity and so work best with audio signals (speech and music), and the sound quality can be degraded by radio noise from natural and artificial sources.
The shortwave bands have a greater potential range but are more subject to interference by distant stations and varying atmospheric conditions that affect reception.
[53][54] In the very high frequency band, greater than 30 megahertz, the Earth's atmosphere has less of an effect on the range of signals, and line-of-sight propagation becomes the principal mode.
[57] Television broadcasting is the transmission of moving images along with a synchronized audio (sound) channel by radio.
Digital television (DTV) transmission systems, which replaced older analog television in a transition beginning in 2006, use image compression and high-efficiency digital modulation such as OFDM and 8VSB to transmit HDTV video within a smaller bandwidth than the old analog channels, saving scarce radio spectrum space.
Unlike analog television, in which increasingly poor reception causes the picture quality to gradually degrade, in digital television picture quality is not affected by poor reception until, at a certain point, the receiver stops working and the screen goes black.
[126] Radar is a radiolocation method used to locate and track aircraft, spacecraft, missiles, ships, vehicles, and also to map weather patterns and terrain.
[133] Radiolocation is a generic term covering a variety of techniques that use radio waves to find the location of objects, or for navigation.
Uncrewed spacecraft are an example of remote-controlled machines, controlled by commands transmitted by satellite ground stations.
[175][176] US Federal law prohibits the nonmilitary operation or sale of any type of jamming devices, including ones that interfere with GPS, cellular, Wi-Fi and police radars.
