Microwave: Difference between revisions
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[[Vacuum tube]] based devices operate on the ballistic motion of electrons in a vacuum under the influence of controlling electric or magnetic fields, and include the [[magnetron]], [[klystron]], [[traveling wave tube]] (TWT), and [[gyrotron]]. |
[[Vacuum tube]] based devices operate on the ballistic motion of electrons in a vacuum under the influence of controlling electric or magnetic fields, and include the [[magnetron]], [[klystron]], [[traveling wave tube]] (TWT), and [[gyrotron]]. |
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==Uses== |
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[[Image:Atmospheric microwave transmittance at mauna kea(simulated).gif|thumb|right|300px|Plot of the zenith atmospheric transmission on the summit of [[Mauna Kea]] throughout the entire gigahertz range of the electromagnetic spectrum at a precipitable water vapor level of 0.001 mm. (simulated)]] |
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* A [[microwave oven]] uses a [[magnetron]] microwave generator to produce microwaves at a frequency of approximately 2.45 GHz for the purpose of [[cooking]] food. Microwaves cook food by causing [[molecule]]s of [[water]] and other compounds to vibrate or rotate. The vibration creates heat which warms the food. Since organic matter is made up primarily of water, food is easily cooked by this method. |
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* Microwaves are used in [[broadcasting]] transmissions because microwaves pass easily through the earth's atmosphere with less interference than longer wavelengths. There is also much more [[bandwidth]] in the microwave spectrum than in the rest of the radio spectrum. Typically, microwaves are used in [[television news]] to transmit a signal from a remote location to a television station from a specially equipped van. |
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* [[Radar]] also uses microwave radiation to detect the range, speed, and other characteristics of remote objects. |
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* [[Wireless LAN]] [[Protocol (computing)|protocol]]s, such as [[Bluetooth]] and the [[IEEE]] [[802.11g]] and [[802.11b|b]] specifications, also use microwaves in the 2.4 GHz [[ISM band]], although [[802.11a]] uses an [[ISM band]] in the 5 GHz range. Licensed long-range (up to about 25 km) Wireless Internet Access services can be found in many countries (but not the USA) in the 3.5–4.0 GHz range. |
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* [[Metropolitan Area Network]]s - MAN protocols, such as [[WiMAX]] (Worldwide Interoperability for Microwave Access) based in the [[IEEE]] [[802.16]] specification. The IEEE 802.16 specification was designed to operate between 2 to 11 GHz. The commercial implementations are in the 2.5 GHz, 3.5 GHz and 5.8G Hz ranges. |
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* [[Cable TV]] and [[Internet]] access on coax cable as well as [[broadcast television]] use some of the lower microwave frequencies. Some mobile phone networks also use the lower microwave frequencies. |
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* Many [[Fabrication (semiconductor)|semiconductor processing]] techniques use microwaves to generate [[plasma physics|plasma]] for such purposes as [[reactive ion etching]] and plasma-enhanced [[chemical vapor deposition]] (PECVD). |
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* Microwaves can be used to [[microwave power transmission|transmit power]] over long distances, and post-[[World War II]] research was done to examine possibilities. [[NASA]] worked in the 1970s and early 1980s to research the possibilities of using [[Solar power satellite]] (SPS) systems with large [[solar array]]s that would beam power down to the Earth's surface via microwaves. |
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* A [[maser]] is a device similar to a [[laser]], except that it works at microwave frequencies. |
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==Microwave frequency bands== |
==Microwave frequency bands== |
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Revision as of 17:40, 14 March 2006
- This page is about the radiation; for the appliance, see microwave oven.
Microwaves are electromagnetic waves with wavelengths longer than those of infrared light, but relatively short for radio waves.
Microwaves have wavelengths approximately in the range of 30 cm (frequency = 1 GHz) to 1 mm (300 GHz). However, the boundaries between far infrared light, microwaves, and ultra-high-frequency radio waves are fairly arbitrary and are used variously between different fields of study. A credible definition comes from Pozar's text "Microwave Engineering", which states that the term microwave "refers to alternating current signals with frequencies between 300 MHz (3 x 108 Hz) and 300 GHz (3 x 1011 Hz)."
The existence of electromagnetic waves, of which microwaves are part of the higher frequency spectrum, was predicted by James Clerk Maxwell in 1864 from his famous Maxwell's equations. In 1888, Heinrich Hertz was the first to demonstrate the existence of electromagnetic waves by building apparatus to produce radio waves.
The microwave range includes ultra-high frequency (UHF) (0.3-3 GHz), super high frequency (SHF) (3-30 GHz), and extremely high frequency (EHF) (30-300 GHz) signals.
Above 300 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that it is effectively opaque , until the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges.
Generation
Microwaves can be generated by a variety of means, generally divided into two categories: solid state devices and vacuum-tube based devices. Solid state microwave devices are based on semiconductors such as silicon or gallium arsenide, and include field-effect transistors (FET's), bipolar junction transistors (BJT's), Gunn diodes, and IMPATT diodes. Specialized versions of standard transistors have been developed for higher speed which are commonly used in microwave applications. Microwave variants of BJT's include the heterojunction bipolar transistor (HBT), and microwave variants of FET's include the MESFET, the HEMT (also known as HFET), and LDMOS transistor. Vacuum tube based devices operate on the ballistic motion of electrons in a vacuum under the influence of controlling electric or magnetic fields, and include the magnetron, klystron, traveling wave tube (TWT), and gyrotron.
Microwave frequency bands
The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 1000 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. Microwave Frequency Bands are defined in the table below:
| Designation | Frequency range |
|---|---|
| L band | 1 to 2 GHz |
| S band | 2 to 4 GHz |
| C band | 4 to 8 GHz |
| X band | 8 to 12 GHz |
| Ku band | 12 to 18 GHz |
| K band | 18 to 26.5 GHz |
| Ka band | 26.5 to 40 GHz |
| Q band | 30 to 50 GHz |
| U band | 40 to 60 GHz |
| V band | 50 to 75 GHz |
| E band | 60 to 90 GHz |
| W band | 75 to 110 GHz |
| F band | 90 to 140 GHz |
| D band | 110 to 170 GHz |
The above table reflects Radio Society of Great Britain (RSGB) usage. The term P band is sometimes used for UHF frequencies below L-band. For other definitions see Letter Designations of Microwave Bands
History and research
Perhaps the first use of the term microwave occurred in 1931:
References
- Pozar, David M. (1993). Microwave Engineering Addison-Wesley Publishing Company. ISBN 0-201-50418-9.
See also
- Cosmic microwave background radiation.
- Home appliances.
- Microwave auditory effect.
- Radio.
- Optics.
- microwave chemistry.
- Microwave radio relay.
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