Radio is the radiation (wireless transmission) of electromagnetic energy through space. The biggest use of radio waves is to carry information, such as sound, by systematically changing (modulating) some property of the radiated waves, such as their amplitude, frequency, phase, or pulse width. When radio waves strike an electrical conductor, the oscillating fields induce an alternating current in the conductor. The information in the waves can be extracted and transformed back into its original form.
Radio systems need a transmitter to modulate (change) some property of the energy produced to impress a signal on it, for example using amplitude modulation or angle modulation (which can be frequency modulation or phase modulation). Radio systems also need an antenna to convert electric currents into radio waves, and vice versa. An antenna can be used for both transmitting and receiving. The electrical resonance of tuned circuits in radios allow individual stations to be selected. The electromagnetic wave is intercepted by a tuned receiving antenna. A radio receiver receives its input from an antenna and converts it into a form usable for the consumer, such as sound, pictures, digital data, measurement values, navigational positions, etc. Radio frequencies occupy the range from a 3 kHz to 300 GHz, although commercially important uses of radio use only a small part of this spectrum.〔''The Electromagnetic Spectrum'', University of Tennessee, Dept. of Physics and Astronomy〕
A radio communication system sends signals by radio. The radio equipment involved in communication systems includes a transmitter and a receiver, each having an antenna and appropriate terminal equipment such as a microphone at the transmitter and a loudspeaker at the receiver in the case of a voice-communication system.
The term "radio" is derived from the Latin word ''radius'', meaning "spoke of a wheel, beam of light, ray". It was first applied to communications in 1881 when, at the suggestion of French scientist Ernest Mercadier, Alexander Graham Bell adopted "radiophone" (meaning "radiated sound") as an alternate name for his photophone optical transmission system.〔"Production of Sound by Radiant Energy" by Alexander Graham Bell, ''Popular Science Monthly'', July, 1881, pages 329-330: "()e have named the apparatus for the production and reproduction of sound in this way the "photophone", ''because an ordinary beam of light contains the rays which are operative.'' To avoid in future any misunderstandings upon this point, we have decided to adopt the term "''radiophone''", proposed by M. Mercadier, as a general term signifying the production of sound by any form of radiant energy..."〕 However this invention would not be widely adopted.
Following Heinrich Hertz's establishment of the existence of electromagnetic radiation in the late 1880s, a variety of terms were initially used for the phenomenon, with early descriptions of the radiation itself including "Hertzian waves", "electric waves", and "ether waves", while phrases describing its use in communications included "spark telegraphy", "space telegraphy", "aerography" and, eventually and most commonly, "wireless telegraphy". However, "wireless" included a broad variety of related electronic technologies, 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.
The first use of ''radio-'' in conjunction with electromagnetic radiation appears to have been by French physicist Édouard Branly, who in 1890 developed a version of a coherer receiver he called a ''radio-conducteur''.〔"The Genesis of Wireless Telegraphy" by A. Frederick Collins, ''Electrical World and Engineer'', May 10, 1902, page 811.〕 The radio- prefix was later used to form additional descriptive compound and hyphenated words, especially in Europe, for example, in early 1898 the British publication ''The Practical Engineer'' included a reference to "the radiotelegraph" and "radiotelegraphy",〔"Wireless Telegraphy", ''The Practical Engineer'', February 25, 1898, page 174. "Dr. O. J. Lodge, who preceded Marconi in making experiments in what may be called "ray" telegraphy or radiotelegraphy by a year or two, has devised a new method of sending and receiving the messages. The reader will understand that in the radiotelegraph electric waves forming the signals of the message start from the sending instrument and travel in all directions like rays of light from a lamp, only they are invisible."〕 while the French text of both the 1903 and 1906 Berlin Radiotelegraphic Conventions includes the phrases ''radiotélégraphique'' and ''radiotélégrammes''.
The use of "radio" as a standalone word dates back to at least December 30, 1904, when instructions issued by the British Post Office for transmitting telegrams specified that "The word 'Radio'... is sent in the Service Instructions".〔"Wireless Telegraphy", ''The Electrical Review'' (London), January 20, 1905, page 108, quoting from the British Post Office's December 30, 1904 ''Post Office Circular''.〕 This practice was universally adopted, and the word "radio" introduced internationally, by the 1906 Berlin Radiotelegraphic Convention, which included a Service Regulation specifying that "Radiotelegrams shall show in the preamble that the service is 'Radio'".
The switch to "radio" in place of "wireless" took place slowly and unevenly in the English-speaking world. 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 June 22, 1907 ''Electrical World'' about the need for legal restrictions warned that "Radio chaos will certainly be the result until such stringent regulation is enforced".〔"Interference with Wireless Messages", ''Electrical World'', June 22, 1907, page 1270.〕 The United States Navy would also play a role. Although its translation of the 1906 Berlin Convention used the terms "wireless telegraph" and "wireless telegram", by 1912 it began to promote the use of "radio" instead. The term started to become preferred by the general public in the 1920s with the introduction of broadcasting. ("Broadcasting" is based upon an agricultural term meaning roughly "scattering seeds widely".) British Commonwealth countries continued to commonly use the term "wireless" until the mid-20th century, though the magazine of the British Broadcasting Corporation in the UK has been called Radio Times since its founding in the early 1920s.
In recent years the more general term "wireless" has gained renewed popularity, even for devices using electromagnetic radiation, through the rapid growth of short-range computer networking, e.g., Wireless Local Area Network (WLAN), Wi-Fi, and Bluetooth, as well as mobile telephony, e.g., GSM and UMTS cell phones. Today, the term "radio" specifies the transceiver device or chip, whereas "wireless" refers to the lack of physical connections; thus equipment employs embedded ''radio'' transceivers, but operates as ''wireless'' devices over ''wireless'' sensor networks.
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