Editing Microwave (section)

== Sources ==
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 |image1=Magnetron section transverse to axis.JPG
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 |footer=Cutaway view inside a [[cavity magnetron]] as used in a [[microwave oven]] ''(left)''. Antenna splitter: [[microstrip]] techniques become increasingly necessary at higher frequencies ''(right)''.
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[[File:Radar speed gun internal works.jpg|thumb|upright=1.2|Disassembled [[radar speed gun]]. The grey assembly attached to the end of the copper-colored [[horn antenna]] is the [[Gunn diode]] which generates the microwaves.]]

High-power microwave sources use specialized [[vacuum tube]]s to generate microwaves.  These devices operate on different principles from low-frequency vacuum tubes, using the ballistic motion of electrons in a vacuum under the influence of controlling electric or magnetic fields, and include the [[magnetron]] (used in [[microwave oven]]s), [[klystron]], [[traveling-wave tube]] (TWT), and [[gyrotron]]. These devices work in the [[density]] modulated mode, rather than the [[Electric current|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 of electrons.

Low-power microwave sources use solid-state devices such as the [[field-effect transistor]] (at least at lower frequencies), [[tunnel diode]]s, [[Gunn diode]]s, and [[IMPATT diode]]s.<ref>[http://www.herley.com/index.cfm?act=app_notes&notes=oscillators Microwave Oscillator] {{Webarchive|url=https://web.archive.org/web/20131030115909/http://www.herley.com/index.cfm?act=app_notes&notes=oscillators |date=2013-10-30 }} notes by [[Herley Industries|Herley General Microwave]]</ref> Low-power sources are available as benchtop instruments, rackmount instruments, embeddable modules and in card-level formats.  A [[maser]] is a solid-state device that amplifies microwaves using similar principles to the [[laser]], which amplifies higher-frequency light waves.

All warm objects emit low level microwave [[black-body radiation]], depending on their [[temperature]], so in meteorology and [[remote sensing]], [[microwave radiometer]]s are used to measure the temperature of objects or terrain.<ref name="Sisodia">{{cite book |last=Sisodia |first=M. L. |title=Microwaves : Introduction To Circuits, Devices And Antennas |publisher=New Age International |date=2007 |pages=1.4–1.7 |url=https://books.google.com/books?id=iEvgmwH1esgC&q=microwaves&pg=SA1-PA4 |isbn=978-8122413380}}</ref>  The sun<ref name="Liou">{{cite book |last=Liou |first=Kuo-Nan |title=An introduction to atmospheric radiation |url=https://books.google.com/books?id=6xUpdPOPLckC&q=microwaves%20from%20Sun&pg=PR13 |page=2 |year=2002 |publisher=Academic Press |isbn=978-0-12-451451-5 |access-date=12 July 2010}}</ref> and other astronomical radio sources such as [[Cassiopeia A]] emit low level microwave radiation which carries information about their makeup, which is studied by [[radio astronomer]]s using receivers called  [[radio telescope]]s.<ref name="Sisodia" />  The [[cosmic microwave background radiation]] (CMBR), for example, is a weak microwave noise filling empty space which is a major source of information on [[Physical cosmology|cosmology]]'s [[Big Bang]] theory of the origin of the [[Universe]].