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A Yagi-Uda antenna, commonly known simply as a Yagi antenna, is a directional antenna consisting of multiple parallel dipole elements in a line, usually made of metal rods. Yagi-Uda antennas consist of a single driven element connected to the transmitter or receiver with a transmission line, and additional parasitic elements: a so-called ''reflector'' and one or more ''directors''.〔 It was invented in 1926 by Shintaro Uda of Tohoku Imperial University, Japan,〔 with a lesser role played by his colleague Hidetsugu Yagi〔 (This was the preface and notice in advance for a series of 11 papers with the same title by Uda between 1926-1929 on the antenna.)〕 The reflector element is slightly longer than the driven dipole, whereas the directors are a little shorter.〔 This design achieves a very substantial increase in the antenna's directionality and gain compared to a simple dipole. Also called a "beam antenna",〔 the Yagi is very widely used as a high-gain antenna on the HF, VHF and UHF bands.〔〔 It has moderate gain which depends on the number of elements used, typically limited to about 17 dBi,〔 linear polarization,〔 unidirectional (end-fire) beam pattern〔 with high front-to-back ratio of up to 20 db. and is lightweight, inexpensive and simple to construct.〔 The bandwidth of a Yagi antenna, the frequency range over which it has high gain, is narrow, a few percent of the center frequency, and decreases with increasing gain,〔〔 so it is often used in fixed-frequency applications. The largest and most well-known use is as rooftop terrestrial television antennas,〔 but it is also used for point-to-point fixed communication links,〔 in radar antennas,〔 and for long distance shortwave communication by shortwave broadcasting stations and radio amateurs.〔 == Origins == The antenna was invented in 1926 by Shintaro Uda of Tohoku Imperial University, Japan,〔 with a lesser role played by his colleague Hidetsugu Yagi.〔 However the "Yagi" name has become more familiar with the name of Uda often omitted. This appears to have been due to Yagi filing a patent on the idea in Japan without Uda's name in it, and later transferring the patent to the Marconi Company in the UK. Yagi antennas were first widely used during World War II in radar systems by the British, US and Germans.〔 After the war they saw extensive development as home television antennas. 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「Yagi-Uda antenna」の詳細全文を読む スポンサード リンク
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