Microwave

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  • Microwave
definition
  • A very short electromagnetic wave. The portion of the electromagnetic spectrum lying between the far infrared and the conventional radio frequency portion. While not bounded by definition, it is commonly regarded as extending from 1 mm to 1 m in wavelength (300 GHz to 0.3 GHz frequency).

    Passive systems operating at these wavelengths sometimes are called microwave systems. Active systems are called radar, although the literal definition of radar requires a distance measuring capability not always included in active systems.

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Abstract from DBPedia
    Microwaves are a form of electromagnetic radiation with wavelengths ranging from one meter to one millimeter; with frequencies between 300 MHz (100 cm) and 300 GHz (0.1 cm). This broad definition includes both UHF and EHF (millimeter waves), and various sources use different boundaries. In all cases, microwave includes the entire SHF band (3 to 30 GHz, or 10 to 1 cm) at minimum, with RF engineering often restricting the range between 1 and 100 GHz (300 and 3 mm). The prefix micro- in microwave is not meant to suggest a wavelength in the micrometer range. It indicates that microwaves are "small", compared to waves used in typical radio broadcasting, in that they have shorter wavelengths. The boundaries between far infrared, terahertz radiation, microwaves, and ultra-high-frequency radio waves are fairly arbitrary and are used variously between different fields of study. There is broad absorption from water vapor from several hundred MHz, increasing in frequency to 40 GHz. Beginning at about 40 GHz, even dry atmosphere becomes less transparent to microwaves, at higher frequencies due to absorption from oxygen. A spectral band structure at even higher frequencies causes absorption peaks at specific frequencies (see graph at right). Above 100 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that it is in effect opaque, until the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges. The term microwave also has a more technical meaning in electromagnetics and circuit theory. Apparatus and techniques may be described qualitatively as "microwave" when the frequencies used are high enough that wavelengths of signals are roughly the same as the dimensions of the equipment, so that lumped-element circuit theory is inaccurate. As a consequence, practical microwave technique tends to move away from the discrete resistors, capacitors, and inductors used with lower-frequency radio waves. Instead, distributed circuit elements and transmission-line theory are more useful methods for design and analysis. Open-wire and coaxial transmission lines used at lower frequencies are replaced by waveguides and stripline, and lumped-element tuned circuits are replaced by cavity resonators or resonant lines. In turn, at even higher frequencies, where the wavelength of the electromagnetic waves becomes small in comparison to the size of the structures used to process them, microwave techniques become inadequate, and the methods of optics are used.

    マイクロ波(マイクロは、英: Microwave)は、電波の周波数による分類の一つである。「マイクロ」は、電波の中で最も短い波長域であることを意味する。 一般的には波長 1mから100μm、周波数 300MHzから3THzの電波(電磁波)を指し、この範囲には、デシメートル波 (UHF)、センチメートル波 (SHF)、ミリメートル波 (EHF)、サブミリ波が含まれる。しかし、明確な定義がある用語ではなく、より狭い範囲やより広い範囲に対して用いられることもある。 マイクロ波の発振には、マグネトロン、クライストロン、進行波管 (TWT)、ジャイロトロン、ガンダイオードを用いた回路などが用いられる。マイクロ波伝送線路には一般的に同軸ケーブルが使われるが、出力(電力・ワット数)の高いものには金属製の導波管が用いられる。また、近年ではマイクロストリップ線路など共に固体化(半導体)された発振器の利用も増えてきている。 マイクロ波の応用分野は広く、衛星テレビ放送、マイクロ波通信、レーダー、マイクロ波プラズマ、マイクロ波加熱(中でも最もポピュラーなものが電子レンジ)、マイクロ波治療、マイクロ波分光法、マイクロ波化学、マイクロ波送電、マイクロ波イメージングなどがある。これらの分野で必要とされる学問がマイクロ波工学である。その他の応用として、水洗便所の小便器にマイクロ波センサが組込まれ自動洗浄にも採用されている。 日本の地上波アナログテレビ放送では、2012年3月末まで難視聴地域用に第63チャンネルから第80チャンネルまで12GHz付近が割り当てられていた。(「チャンネル (テレビ放送)」の記事を参照)。

    (Source: http://dbpedia.org/resource/Microwave)

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