ANTENNA
















Antenna
Device for sending and receiving radio waves: a metallic piece of equipment of variable shape, used in the sending and receiving of television or radio signals.
The antenna performs the directional transmission and reception of RF signals. The main parameters are frequency band, diameter and antenna gain.

Antenna (radio)

An antenna (or aerial) is a transducer that transmits or receives electromagnetic waves. In other words, antennas convert electromagnetic radiation into an electrical signal and vice versa. They are used to transmit and receive electromagnetic radiation of radio frequency, that is, radio waves, and are essential to the operation of all radio equipment. Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication, wireless LAN, cell phones, radar, and spacecraft communication.
Physically, an antenna is an arrangement of one or more conductors, usually called elements in this context. In transmission, an alternating current is created in the elements by applying a voltage at the antenna terminals, causing the elements to radiate an electromagnetic field. In reception, the inverse occurs: an electromagnetic field from another source induces an alternating current in the elements and a corresponding voltage at the antenna's terminals. In a parabolic antenna, a feed antenna of that sort is augmented by a much larger curved reflecting surface which creates a beam of radio waves similar to that of a searchlight. Since they are much larger than a wavelength, the parabolic and other so-called aperture antennas are widely used only at higher, especially microwave frequencies.
The first antennas were built in 1888 by Heinrich Hertz (1857–1894) in his pioneering experiments to prove the existence of electromagnetic waves predicted by the theory of James Clerk Maxwell. Hertz placed dipole antennas at the focal point of parabolic reflectors for both transmitting and receiving.
 TerminologyThe words antenna (plural: antennas[1]) and aerial are used interchangeably; but usually a rigid metallic structure is termed an antenna and a wire format is called an aerial. In the United Kingdom and other British English speaking areas the term aerial is more common, even for rigid types. The noun aerial is occasionally written with a diaeresis mark—aërial—in recognition of the original spelling of the adjective aërial from which the noun is derived.
The origin of the word antenna relative to wireless apparatus is attributed to Guglielmo Marconi. In 1895, while testing early radio apparatuses in the Swiss Alps at Salvan, Switzerland in the Mont Blanc region, Marconi experimented with early wireless equipment. A 2.5 meter long pole, along which was carried a wire, was used as a radiating and receiving aerial element. In Italian a tent pole is known as l'antenna centrale, and the pole with a wire alongside it used as an aerial was simply called l'antenna. Until then wireless radiating transmitting and receiving elements were known simply as aerials or terminals. Marconi's use of the word antenna (Italian for pole) would become a popular term for what today is uniformly known as the antenna.[2]
In common usage, the word antenna may refer broadly to an entire assembly including support structure, enclosure (if any), etc. in addition to the actual functional components. Especially at microwave frequencies, a receiving antenna may include not only the actual electrical antenna but an integrated preamplifier and/or mixer.

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