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This article is about the electromagnetic wave. For the cooking appliance, see Microwave oven. For other uses, see Microwaves (disambiguation).
The atmospheric attenuation of microwaves in dry air with a precipitable water vapor level of 0.001 mm. The downward spikes in the graph correspond to frequencies at which microwaves are absorbed more strongly. The right half of this graph includes the lower ranges of infrared by some standards

Microwaves are a form of electromagnetic radiation with wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz (0.3 GHz) and 300 GHz.[1][2] 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 putting the lower boundary at 1 GHz (30 cm), and the upper around 100 GHz (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.

Beginning at about 40 GHz, the atmosphere becomes less transparent to microwaves, due at lower frequencies to absorption from water vapor and at higher frequencies from oxygen. A spectral band structure 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.

The electromagnetic spectrum[edit]

Electromagnetic spectrum
Name Wavelength Frequency (Hz) Photon energy (eV)
Gamma ray less than 0.02 nm more than 15 EHz more than 62.1 keV
Visible 390 nm – 750 nm 770 THz – 400 THz 3.2 eV – 1.7 eV
Microwave 1 mm – 1 meter 300 GHz – 300 MHz 1.24 meV – 1.24 µeV
Radio 1 m – 100,000 km 300 MHz3 Hz 1.24 µeV – 12.4 feV

Microwave sources[edit]

Microstrip techniques become increasingly necessary at higher frequencies, as in this antenna splitter.


Cutaway view inside a cavity magnetron as used in a microwave oven


All warm objects emit low level microwave black body radiation, depending on their temperature, so in meteorology and remote sensing microwave radiometers are used to measure the temperature of objects or terrain .[4] The sun[5] 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 astronomers using receivers called radio telescopes.[4] The cosmic microwave background radiation (CMBR), for example, is a weak microwave noise filling empty space which is a major source of information on cosmology's Big Bang theory of the origin of the Universe.

Microwave uses[edit]





Metropolitan area network (MAN) protocols, such as WiMAX (Worldwide Interoperability for Microwave Access) are based on standards such as IEEE 802.16, designed to operate between 2 to 11 GHz. Commercial implementations are in the 2.3 GHz, 2.5 GHz, 3.5 GHz and 5.8 GHz ranges.






Microwaves cannot be carried with usable efficiency in ordinary transmission lines but require waveguide (metal pipe), like this example from an air traffic control radar.

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, weather forecasting, navigation of ships, and speed limit enforcement.

A Gunn diode oscillator and waveguide are used as a motion detector for automatic door openers.

Radio astronomy[edit]

Atacama Large Millimeter Array uses microwaves to create images of the universe beyond clouds of dust and gas.

Most radio astronomy uses microwaves. Usually the naturally-occurring microwave radiation is observed, but active radar experiments have also been done with objects in the solar system, such as determining the distance to the Moon or mapping the invisible surface of Venus through cloud cover.

Cosmic background radiation of the Big Bang mapped with increasing resolution.


Global Navigation Satellite Systems (GNSS) including the Chinese Beidou, the American Global Positioning System (GPS) and the Russian GLONASS broadcast navigational signals in various bands between about 1.2 GHz and 1.6 GHz.

Heating and power application[edit]

A microwave oven passes (non-ionizing) microwave radiation (at a frequency near 2.45 GHz) through food, causing dielectric heating primarily by absorption of the energy in water. Microwave ovens became common kitchen appliances in Western countries in the late 1970s, following the development of less expensive cavity magnetrons. Water in the liquid state possesses many molecular interactions that broaden the absorption peak. In the vapor phase, isolated water molecules absorb at around 22 GHz, almost ten times the frequency of the microwave oven.



Microwave frequencies typically ranging from 110 – 140 GHz are used in stellarators and more notably in tokamak experimental fusion reactors to help heat the fuel into a plasma state. The upcoming ITER Thermonuclear Reactor[7] is expected to range from 110–170 GHz and will employ Electron Cyclotron Resonance Heating (ECRH).[8]


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 Marines are currently using this type of active denial system in fixed installations.[9]



Microwave frequency bands[edit]


The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 100 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. One set of microwave frequency bands designations by the Radio Society of Great Britain (RSGB), is tabulated below:

ITU Radio Band Numbers

1 2 3 4 5 6 7 8 9 10 11 12

ITU Radio Band Symbols


NATO Radio bands


IEEE Radar bands


Television and radio bands


Microwave frequency bands
Letter Designation Frequency range Wavelength range Typical uses
40 to 60 GHz 5.0 mm to 7.5 mm
V band 50 to 75 GHz 4.0 mm to 6.0 mm millimeter wave radar research and other kinds of scientific research

P band is sometimes used for Ku Band. "P" for "previous" was a radar band used in the UK ranging from 250 to 500 MHz and now obsolete per IEEE Std 521, see[10] and.[11][12]

When radars were first developed at K band during World War II, it was not realized that there was a nearby absorption band (due to water vapor and oxygen at the atmosphere). to avoid this problem, the original K band was split into a lower band, Ku, and upper band, Ka see.[13]

Microwave frequency measurement[edit]

Wavemeter for measuring in the Ku band

Microwave frequency can be measured by either electronic or mechanical techniques.




Effects on health[edit]

Microwaves do not contain sufficient energy to chemically change substances by ionization, and so are an example of non-ionizing radiation. The word "radiation" refers to energy radiating from a source and not to radioactivity. It has not been shown conclusively that microwaves (or other non-ionizing electromagnetic radiation) have significant adverse biological effects at low levels. Some, but not all, studies suggest that long-term exposure may have a carcinogenic effect.[14] This is separate from the risks associated with very high intensity exposure, which can cause heating and burns like any heat source, and not a unique property of microwaves specifically.

During World War II, it was observed that individuals in the radiation path of radar installations experienced clicks and buzzing sounds in response to microwave radiation. This microwave auditory effect was thought to be caused by the microwaves inducing an electric current in the hearing centers of the brain.[15] Research by NASA in the 1970s has shown this to be caused by thermal expansion in parts of the inner ear. In 1955 Dr. James Lovelock was able to reanimate rats frozen at 0 °C using microwave diathermy.[16]


History and research[edit]

Electromagnetic spectrum with visible light highlighted

The existence of radio waves was predicted by James Clerk Maxwell in 1864 from his equations. In 1888, Heinrich Hertz was the first to demonstrate the existence of radio waves by building a spark gap radio transmitter that produced 450 MHz microwaves, in the UHF region. The equipment he used was primitive, including a horse trough, a wrought iron point spark, and Leyden jars. He also built the first parabolic antenna, using a zinc gutter sheet. In 1894 Indian radio pioneer Jagdish Chandra Bose publicly demonstrated radio control of a bell using millimeter wavelengths, and conducted research into the propagation of microwaves.[17]

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

In 1943, the Hungarian engineer Zoltán Bay sent ultra-short radio waves to the moon, which, reflected from there, worked as a radar, and could be used to measure distance, as well as to study the moon.

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. This same article also made a showing in the New York Times issued in 1951.

In the history of electromagnetic theory, significant work specifically in the area of microwaves and their applications was carried out by researchers including:

Specific work on microwaves
Work carried out by Area of work
Barkhausen and Kurz Positive grid oscillators
Hull Smooth bore magnetron
Varian Brothers Velocity modulated electron beam → klystron tube
Randall and Boot Cavity magnetron

See also[edit]


  3. ^ Microwave Oscillator notes by Herley General Microwave
  5. ^ Liou, Kuo-Nan (2002). An introduction to atmospheric radiation. Academic Press. p. 2. ISBN 0-12-451451-0. Retrieved 12 July 2010. 
  7. ^ "the way to new energy". ITER. 2011-11-04. Retrieved 2011-11-08. 
  8. ^ "Electron Cyclotron Resonance Heating (ECRH)". Ipp.mpg.de. Retrieved 2011-11-08. 
  9. ^ Silent Guardian Protection System. Less-than-Lethal Directed Energy Protection. raytheon.com
  10. ^ "eEngineer – Radio Frequency Band Designations". Radioing.com. Retrieved 2011-11-08. 
  11. ^ PC Mojo – Webs with MOJO from Cave Creek, AZ (2008-04-25). "Frequency Letter bands – Microwave Encyclopedia". Microwaves101.com. Retrieved 2011-11-08. 
  12. ^ For other definitions see Letter Designations of Microwave Bands.
  14. ^ Goldsmith, JR (December 1997). "Epidemiologic evidence relevant to radar (microwave) effects". Environmental Health Perspectives 105 (Suppl. 6): 1579–1587. doi:10.2307/3433674. JSTOR 3433674. PMC 1469943. PMID 9467086. 
  15. ^ Philip L. Stocklin, U.S. Patent 4,858,612, December 19, 1983
  16. ^ Andjus, R.K.; Lovelock, J.E (1955). "Reanimation of rats from body temperatures between 0 and 1 °C by microwave diathermy". The Journal of Physiology 128 (3): 541–546. PMC 1365902. PMID 13243347. 

External links[edit]

Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Microwave — Please support Wikipedia.
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This nifty little invention plugs RIGHT into your computer to keep you posted on how much longer you have to wait to chow down, and it also comes with a scanable fork that actually TELLS THE MICROWAVE what you're putting in it and then it decides how ...

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Wed, 27 Aug 2014 13:02:00 -0700

Too busy to walk all the way to the kitchen to heat up a meal? The prototype for the BrainWave desktop microwave is the answer. It's exactly what it sounds like: a phone book-size microwave to heat up your frozen lunch, at your desk. The BrainWave is ...


Tue, 26 Aug 2014 12:07:30 -0700

PORTLAND, Maine (NEWS CENTER) -- Two teenage girls have each been sentenced to 100 hours of community service for putting an 8-week-old kitten in a microwave last year. Both girls, who were 16 at the time, were originally charged with a felony after ...
KCCI Des Moines
Wed, 27 Aug 2014 08:10:55 -0700

District officials said the actual annual energy cost of running each of the appliances is $153.30 for a small refrigerator, $123.04 for a coffee pot, and $32.86 for a microwave oven. The fee is applied district wide. Des Moines Public Schools had more ...
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Thu, 28 Aug 2014 11:45:00 -0700

It was spotted again the next morning, sheltering beneath a microwave which rested on a kitchen counter, at which point Will County Animal Control was notified and captured the bat. Rabies was confirmed Aug. 25, and post-exposure rabies treatments were ...
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Wed, 27 Aug 2014 03:52:30 -0700

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