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An antenna (plural antennae or antennas), or aerial, is an electrical device which converts electric power into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver. In transmission, a radio transmitter supplies an electric current oscillating at radio frequency (i.e. a high frequency alternating current (AC)) to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves (radio waves). In reception, an antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage at its terminals, that is applied to a receiver to be amplified. Antennas are essential components of all equipment that uses radio. They are used in systems such as radio broadcasting, broadcast television, two-way radio, communications receivers, radar, cell phones, and satellite communications, as well as other devices such as garage door openers, wireless microphones, Bluetooth-enabled devices, wireless computer networks, baby monitors, and RFID tags on merchandise. Typically an antenna consists of an arrangement of metallic conductors (elements), electrically connected (often through a transmission line) to the receiver or transmitter. An oscillating current of electrons forced through the antenna by a transmitter will create an oscillating magnetic field around the antenna elements, while the charge of the electrons also creates an oscillating electric field along the elements. These time-varying fields radiate away from the antenna into space as a moving transverse electromagnetic field wave. Conversely, during reception, the oscillating electric and magnetic fields of an incoming radio wave exert force on the electrons in the antenna elements, causing them to move back and forth, creating oscillating currents in the antenna. Antennas can be designed to transmit and receive radio waves in all horizontal directions equally (omnidirectional antennas), or preferentially in a particular direction (directional or high gain antennas). In the latter case, an antenna may also include additional elements or surfaces with no electrical connection to the transmitter or receiver, such as parasitic elements, parabolic reflectors or horns, which serve to direct the radio waves into a beam or other desired radiation pattern. The first antennas were built in 1888 by German physicist Heinrich Hertz 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. He published his work in ''Annalen der Physik und Chemie'' (vol. 36, 1889). ==Terminology== The words ''antenna'' (plural: ''antennas''〔In the context of electrical engineering and physics, the plural of ''antenna'' is ''antennas'', and it has been this way since about 1950 (or earlier), when a cornerstone textbook in this field, ''Antennas'', was published by the physicist and electrical engineer John D. Kraus of The Ohio State University. Besides in the title, Dr. Kraus noted this in a footnote on the first page of his book. Insects may have "antennae", but this form is not used in the context of electronics or physics.〕 in US English, although both "antennas" and "antennae" are used in International English〔For example http://www.telegraph.co.uk/science/science-news/7810454/British-scientists-launch-major-radio-telescope.html; http://www.ic.gc.ca/eic/site/smt-gst.nsf/eng/sf09377.html; http://www.ska.ac.za/media/meerkat_cad.php〕) and ''aerial'' are used interchangeably. Occasionally the term "aerial" is used to mean a wire antenna. However, note the important international technical journal, the ''IEEE Transactions on Antennas and Propagation''. In the United Kingdom and other areas where British English is used, the term aerial is sometimes used although 'antenna' has been universal in professional use for many years. The origin of the word ''antenna'' relative to wireless apparatus is attributed to Italian radio pioneer Guglielmo Marconi. In the summer of 1895, Marconi began testing his wireless system outdoors on his father's estate near Bologna and soon began to experiment with long wire "aerials". Marconi discovered that by raising the "aerial" wire above the ground and connecting the other side of his transmitter to ground, the transmission range was increased.〔Marconi, "(Wireless Telegraphic Communication: Nobel Lecture, 11 December 1909. )" Nobel Lectures. Physics 1901–1921. Amsterdam: Elsevier Publishing Company, 1967: 196–222. p. 206.〕 Soon he was able to transmit signals over a hill, a distance of approximately . In Italian a tent pole is known as ''l'antenna centrale,'' and the pole with the wire was simply called ''l'antenna.'' Until then wireless radiating transmitting and receiving elements were known simply as aerials or terminals. Because of his prominence, Marconi's use of the word ''antenna'' (Italian for ''pole'') spread among wireless researchers, and later to the general public. 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 or mixer. An antenna, in converting radio waves to electrical signals or vice versa, is a form of transducer. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Antenna (radio)」の詳細全文を読む スポンサード リンク
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