Microwave 20097 226001810 2008-07-16T11:46:57Z El C 92203 Reverted edits by [[Special:Contributions/59.180.134.98|59.180.134.98]] ([[User talk:59.180.134.98|talk]]) to last version by Wtshymanski {{for|the appliance|Microwave oven}} [[Image:EM spectrum.svg|thumb|490px|right|Electromagnetic spectrum with light highlighted]] '''Microwaves''' are [[electromagnetic radiation|electromagnetic waves]] with [[wavelength]]s ranging from 1&nbsp;mm to 1&nbsp;m, or [[frequency|frequencies]] between 300&nbsp;[[hertz|MHz]] and 300&nbsp;GHz. Apparatus and techniques may be described qualitatively as "microwave" when the wavelengths of signals are roughly the same as the dimensions of the equipment, so that [[Lumped element model|lumped-element circuit theory]] is inaccurate. As a consequence, practical microwave technique tends to move away from the discrete [[resistor]]s, [[capacitor]]s, and [[inductor]]s used with lower frequency [[radio waves]]. Instead, distributed circuit elements and transmission-line theory are more useful methods for design, analysis. Open-wire and coaxial [[transmission line]]s give way to [[waveguide]]s, and lumped-element tuned circuits are replaced by cavity [[resonator]]s or resonant lines. Effects of [[reflection]], [[polarization]], [[scattering]], [[diffraction]], and atmospheric [[Absorption (electromagnetic radiation)|absorption]] usually associated with visible light are of practical significance in the study of microwave [[Radio propagation|propagation]]. The same [[Maxwell's equations| equations]] of electromagnetic theory apply at all frequencies. While the name may suggest a micrometer wavelength, it is better understood as indicating wavelengths very much smaller than those used in radio broadcasting. The boundaries between far [[infrared]] light, [[terahertz radiation]], microwaves, and [[ultra-high-frequency]] [[radio]] [[wave]]s are fairly arbitrary and are used variously between different fields of study. The term microwave generally refers to "[[alternating current]] signals with frequencies between 300 MHz (3×10<sup>8</sup>&nbsp;Hz) and 300 GHz (3×10<sup>11</sup>&nbsp;Hz)."<ref>Pozar, David M. (1993). ''Microwave Engineering'' Addison-Wesley Publishing Company. ISBN 0-201-50418-9.</ref> Both [[IEC]] standard 60050 and [[IEEE]] standard 100 define "microwave" frequencies starting at 1 GHz (30 cm wavelength). Electromagnetic waves longer (lower frequency) than microwaves are called "radio waves". Electromagnetic radiation with shorter wavelengths may be called "millimeter waves", [[terahertz radiation]] or even ''T-rays''. Definitions differ for millimeter wave band, which the IEEE defines as 110 GHz to 300 GHz. == Discovery == The existence of electromagnetic waves, of which microwaves are part of the electromagnetic spectrum, was predicted by [[James Clerk Maxwell]] in 1864 from his [[Maxwell's equations|equations]]. In 1888, [[Heinrich Hertz]] was the first to demonstrate the existence of electromagnetic waves by building an apparatus that produced and detected microwaves in the UHF region. The design necessarily used horse-and-buggy materials, including a horse trough, a wrought iron point spark, [[Leyden jar]]s, and a length of zinc gutter whose parabolic cross-section worked as a reflection antenna. In 1894 [[Jagdish Chandra Bose| J. C. Bose]] publicly demonstrated radio control of a bell using millimetre wavelengths, and conducted research into the propagation of microwaves. [[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&nbsp;mm. (simulated)]] == Frequency range == The microwave range includes [[ultra-high frequency]] (UHF) (0.3–3&nbsp;GHz), [[super high frequency]] (SHF) (3–30&nbsp;GHz), and [[extremely high frequency]] (EHF) (30–300&nbsp;GHz) signals. Above 300&nbsp;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. == Microwave Sources == [[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]], [[travelling wave tube]] (TWT), and [[gyrotron]]. These devices work in the [[density]] modulated mode, rather than the current modulated mode. This means that they work on the basis of clumps of electrons flying ballistically through them, rather than using a continuous stream. A [[maser]] is a device similar to a [[laser]], except that it works at microwave frequencies. Solid-state sources include the [[field-effect transistor]], at least at lower frequencies, [[tunnel diode]]s and [[Gunn diode]]s == Uses == [[Image:Microwave tower silhouette.jpg|thumb|A microwave telecommunications tower on Wrights Hill in [[Wellington]], [[New Zealand]]]] === Communication === * Before the advent of [[fiber optic]] transmission, most [[long distance]] [[telephone call]]s were carried via microwave point-to-point links through sites like the [[AT&T Long Lines]]. Starting in the early 1950's, [[frequency division multiplex]] was used to send up to 5,400 telephone channels on each microwave radio channel, with as many as ten radio channels combined into one antenna for the ''hop'' to the next site, up to 70&nbsp;km away. * [[Wireless LAN]] [[Protocol (computing)|protocol]]s, such as [[Bluetooth]] and the [[IEEE]] [[802.11]] specifications, also use microwaves in the 2.4&nbsp;GHz [[ISM band]], although [[802.11a]] uses [[ISM band]] and [[U-NII]] frequencies in the 5&nbsp;GHz range. Licensed long-range (up to about 25&nbsp;km) Wireless Internet Access services can be found in many countries (but not the USA) in the 3.5&ndash;4.0&nbsp;GHz range. * [[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&nbsp;GHz. The commercial implementations are in the 2.3GHz, 2.5 GHz, 3.5 GHz and 5.8 GHz ranges. * Wide Area [[Mobile Broadband]] Wireless Access: MBWA protocols based on standards specifications such as [[IEEE 802.20]] or ATIS/ANSI [[HC-SDMA]] (e.g. iBurst) are designed to operate between 1.6 and 2.3&nbsp;GHz to give mobility and in-building penetration characteristics similar to mobile phones but with vastly greater spectral efficiency. * [[Cable TV]] and [[Internet]] access on coaxial cable as well as [[broadcast television]] use some of the lower microwave frequencies. Some mobile phone networks, like [[Global System for Mobile Communications#Radio interface|GSM]], also use the lower microwave frequencies. * Microwave radio is used in [[broadcasting]] and [[telecommunication]] transmissions because, due to their short wavelength, highly directive antennas are smaller and therefore more practical than they would be at longer wavelengths (lower frequencies). There is also more [[bandwidth (signal processing)|bandwidth]] in the microwave spectrum than in the rest of the radio spectrum; the usable bandwidth below 300 MHz is less than 300 MHz while many GHz can be used above 300 MHz. Typically, microwaves are used in [[television news]] to transmit a signal from a remote location to a television station from a specially equipped van. === Remote Sensing === * [[Radar]] uses microwave radiation to detect the range, speed, and other characteristics of remote objects. Development of radar was accelerated during World War II due to its great military utility. Now radar is widely used for applications such as [[air traffic control]], navigation of ships, and [[speed limit]] enforcement. * A [[Gunn diode]] oscillator and waveguide are used as a motion detector for automatic door openers (although these are being replaced by ultrasonic devices). * Most [[radio astronomy]] uses microwaves. * Microwave imaging; ''see [[Photoacoustic imaging in biomedicine]]'' === Navigation === * [[Global Navigation Satellite System]]s (GNSS) including the American [[Global Positioning System]] (GPS) and the Russian ГЛОбальная НАвигационная Спутниковая Система ([[GLONASS]]) broadcast navigational signals in various bands between about 1.2 GHz and 1.6 GHz. === Power === * A [[microwave oven]] passes (non-ionizing) microwave radiation (at a frequency near 2.45 GHz) through food, causing [[dielectric heating]] by absorption of energy in the water, fats and sugar contained in the food. Microwave ovens became common kitchen appliances in Western countries in the late 1970s, following development of inexpensive [[cavity magnetron]]s. * Microwave heating is used in industrial processes for drying and curing products. * 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). * 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 [[Photovoltaic module|solar array]]s that would beam power down to the Earth's surface via microwaves. * [[Less-than-lethal]] weaponry exists that uses millimeter waves to heat a thin layer of human skin to an intolerable temperature so as to make the targeted person move away. A two-second burst of the 95 GHz focused beam heats the skin to a temperature of 130 F (54 C) at a depth of 1/64th of an inch (0.4 mm). The [[United States Air Force]] and [[United States Marine Corps|Marines]] are currently using this type of [[Active Denial System]].<ref>[http://www.raytheon.com/products/stellent/groups/public/documents/content/cms04_017939.pdf Raytheon's Silent Guardian millimeter wave weapon]</ref> ==Microwave frequency bands== The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1&nbsp;GHz to 1000&nbsp;GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40&nbsp;GHz range. Microwave frequency bands, as defined by the [[Radio Society of Great Britain]] (RSGB), are shown in the table below: {{MWband}} {| class="wikitable" |+Microwave frequency bands !Designation!!Frequency range |- |[[L band]]||1 to 2&nbsp;GHz |- |[[S band]]||2 to 4&nbsp;GHz |- |[[C band]]||4 to 8&nbsp;GHz |- |[[X band]]||8 to 12&nbsp;GHz |- |[[Ku band|K<sub>u</sub> band]]||12 to 18&nbsp;GHz |- |[[K band]]||18 to 26.5&nbsp;GHz |- |[[Ka band|K<sub>a</sub> band]]||26.5 to 40&nbsp;GHz |- |[[Q band]]||30 to 50&nbsp;GHz |- |[[U band]]||40 to 60&nbsp;GHz |- |[[V band]]||50 to 75&nbsp;GHz |- |[[E band]]||60 to 90&nbsp;GHz |- |[[W band]]||75 to 110&nbsp;GHz |- |[[F band]]||90 to 140&nbsp;GHz |- |[[D band]]||110 to 170&nbsp;GHz (Hot) |} The term [[P band]] is sometimes used for Ku Band. For other definitions see [http://www.jneuhaus.com/fccindex/letter.html Letter Designations of Microwave Bands] == Health effects == {{main article| Electromagnetic radiation and health}} Microwaves contain insufficient energy to directly chemically change substances by ionization, and so are an example of [[nonionizing]] radiation. The word "radiation" refers to the fact that energy can radiate, and not to the different nature and effects of different kinds of energy. Specifically, the term in this context is not to be confused with [[radioactivity]]. Due to this fact, it has not yet conclusively been shown that microwaves (or other [[nonionizing]] electromagnetic radiation) have any biological effects. This is separate from the risks associated with very high intensity exposure, which can cause thermal burns, in the same way that infrared emissions from a hot heating element can do so, and not due to any unique property of microwaves specifically. ==History and research== Perhaps the first, documented, formal use of the term ''microwave'' occurred in 1931: :"When trials with wavelengths as low as 18 cm were made known, there was undisguised surprise that the problem of the micro-wave had been solved so soon." ''Telegraph & Telephone Journal'' XVII. 179/1 Perhaps the first use of the word ''microwave'' in an astronomical context occurred in 1946 in an article "Microwave Radiation from the Sun and Moon" by [[Robert Dicke]] and [[Robert Beringer]]. For some of the history in the development of electromagnetic theory applicable to modern microwave applications see the following figures: <div class="references-small" style="-moz-column-count:2; column-count:2;"> * [[Hans Christian Ørsted]]. * [[Michael Faraday]]. * [[James Clerk Maxwell]]. * [[Heinrich Hertz]]. * [[Nikola Tesla]]. * [[Guglielmo Marconi]]. * [[Samuel F. B. Morse|Samuel Morse]]. * Sir [[William Thomson, 1st Baron Kelvin|William Thomson]], later Lord Kelvin. * [[Oliver Heaviside]]. * [[Lord Rayleigh]]. * [[Oliver Lodge]]. * [[Jagadish Chandra Bose]]. * [[Julius Lange]]. </div> Specific significant areas of research and work developing microwaves and their applications: {| class="wikitable" |+Specific work on microwaves !Work carried out by!!Area of work |- |[[Barkhausen]] and Kurz||Positive grid [[oscillator]]s |- |Hull||Smooth bore [[magnetron]] |- |Varian Brothers||Velocity modulated electron beam → [[klystron]] tube |- |Randall and Boot||Cavity magnetron |} ==See also== <div class="references-small" style="-moz-column-count:2; column-count:2;"> * [[Cosmic microwave background radiation]] * [[Diversity scheme]] * [[Electron cyclotron resonance]] * [[Home appliance]]s * [[Microwave oven]]s * [[Microwave auditory effect]] * [[Radio]] * [[Radiation]] * [[Rain fade]] * [[Optics]] * [[Microwave chemistry]] * [[Microwave radio relay]] * [[Microwave power transmission]] * [[Thing (listening device)]] * [[Tropospheric scatter]] * [[Microwave-related injury]] </div> ==References== {{reflist}} ==External links== * [http://www.emtalk.com EM Talk, Microwave Engineering Tutorials and Tools] * [http://www.metamaterials.net Microwave Irradiation for Negative Refraction by using Metamaterials] * [http://www.microwaves101.com Microwaves101, web resource covering the fundamental principles of microwave design] * [http://ieee.li/pdf/viewgraphs_applications_microwaves_medicine.pdf Applications of Microwaves in Medicine] * [http://homeguide123.com/videos/Microwave_Technology_Video.html Microwave Technology Video] * [http://www.anton-paar.com/sample-preparation/microwave_cmse-698d4d.en.0.jsp Theory of Microwave Technology] {{radio_spectrum}} {{Wireless video}} {{EMSpectrum}} [[Category:Electromagnetic spectrum]] [[Category:Radio]] [[Category:Microwave technology|*]] [[ar:موجات صغرية]] [[bs:Mikrovalno zračenje]] [[bg:Микровълни]] [[ca:Microones]] [[cs:Mikrovlny]] [[da:Mikrobølge]] [[de:Mikrowellen]] [[el:Μικροκύματα]] [[es:Microondas]] [[eo:Mikroondoj]] [[fa:ریزموج]] [[fr:Micro-onde]] [[gl:Microondas]] [[hr:Mikrovalovi]] [[id:Gelombang mikro]] [[it:Microonde]] [[he:מיקרוגל]] [[lv:Mikroviļņi]] [[lt:Mikrobangos]] [[li:Microgolf]] [[hu:Mikrohullám]] [[ms:Mikrogelombang]] [[nl:Microgolf]] [[ja:マイクロ波]] [[no:Mikrobølge]] [[nn:Mikrobølgjer]] [[pl:Mikrofale]] [[pt:Microondas]] [[ro:Microunde]] [[ru:Микроволновое излучение]] [[sq:Mikrovalët]] [[simple:Microwave]] [[sk:Mikrovlnné žiarenie]] [[sr:Микроталаси]] [[sh:Mikrotalasi]] [[fi:Mikroaallot]] [[sv:Mikrovågor]] [[th:ไมโครเวฟ]] [[vi:Vi ba]] [[tr:Mikrodalga]] [[yi:מיקראכוואליע]] [[zh:微波]]