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Microwave radiation is a type of electromagnetic radiation. The prefix "micro-" in microwaves doesn't mean microwaves have micrometer wavelengths, but rather that microwaves have very small wavelengths compared with traditional radio waves (1 mm to 100,000 km wavelengths). In the electromagnetic spectrum, microwaves fall between infrared radiation and radio waves.
Microwave radiation has a frequency between 300 MHz and 300 GHz (1 GHz to 100 GHz in radio engineering) or a wavelength ranging from 0.1 cm to 100 cm. The range includes the SHF (super high frequency), UHF (ultra high frequency) and EHF (extremely high frequency or millimeter waves) radio bands.
While lower frequency radio waves can follow the contours of the Earth and bounce off layers in the atmosphere, microwaves only travel line-of-sight, typically limited to 30-40 miles on the Earth's surface. Another important property of microwave radiation is that it's absorbed by moisture. A phenomenon called rain fade occurs at the high end of the microwave band. Past 100 GHz, other gases in the atmosphere absorb the energy, making air opaque in the microwave range, although transparent in the visible and infrared region.
Because microwave radiation encompasses such a broad wavelength/frequency range, it is subdivided into IEEE, NATO, EU or other radar band designations:
|L band||1 to 2 GHz||15 to 30 cm||amateur radio, mobile phones, GPS, telemetry|
|S band||2 to 4 GHz||7.5 to 15 cm||radio astronomy, weather radar, microwave ovens, Bluetooth, some communication satellites, amateur radio, cell phones|
|C band||4 to 8 GHz||3.75 to 7.5 cm||long-distance radio|
|X band||8 to 12 GHz||25 to 37.5 mm||satellite communications, terrestrial broadband, space communications, amateur radio, spectroscopy|
|Ku band||12 to 18 GHz||16.7 to 25 mm||satellite communications, spectroscopy|
|K band||18 to 26.5 GHz||11.3 to 16.7 mm||satellite communications, spectroscopy, automotive radar, astronomy|
|Ka band||26.5 to 40 GHz||5.0 to 11.3 mm||satellite communications, spectroscopy|
|Q band||33 to 50 GHz||6.0 to 9.0 mm||automotive radar, molecular rotational spectroscopy, terrestrial microwave communication, radio astronomy, satellite communications|
|U band||40 to 60 GHz||5.0 to 7.5 mm|
|V band||50 to 75 GHz||4.0 to 6.0 mm||molecular rotational spectroscopy, millimeter wave research|
|W band||75 to 100 GHz||2.7 to 4.0 mm||radar targeting and tracking, automotive radar, satellite communication|
|F band||90 to 140 GHz||2.1 to 3.3 mm||SHF, radio astronomy, most radars, satellite tv, wireless LAN|
|D band||110 to 170 GHz||1.8 to 2.7 mm||EHF, microwave relays, energy weapons, millimeter wave scanners, remote sensing, amateur radio, radio astronomy|
Microwaves are used primarily for communications, include analog and digital voice, data, and video transmissions. They are also used for radar (RAdio Detection and Ranging) for weather tracking, radar speed guns, and air traffic control. Radio telescopes use large dish antennas to determine distances, map surfaces, and study radio signatures from planets, nebulas, stars, and galaxies. Microwaves are used to transmit thermal energy to heat food and other materials.
Cosmic microwave background radiation is a natural source of microwaves. The radiation is studied to help scientists understand the Big Bang. Stars, including the Sun, are natural microwave sources. Under the right conditions, atoms and molecules can emit microwaves. Man-made sources of microwaves include microwave ovens, masers, circuits, communication transmission towers, and radar.
Either solid state devices or special vacuum tubes may be used to produce microwaves. Examples of solid-state devices include masers (essentially lasers where the light is in the microwave range), Gunn diodes, field-effect transistors, and IMPATT diodes. The vacuum tube generators use electromagnetic fields to direct electrons in a density-modulated mode, where groups of electrons pass through the device rather than a stream. These devices include the klystron, gyrotron, and magnetron.
Microwave radiation is called "radiation" because it radiates outward and not because it's either radioactive or ionizing in nature. Low levels of microwave radiation are not known to produce adverse health effects. However, some studies indicate long-term exposure may act as a carcinogen.
Microwave exposure can cause cataracts, as dielectric heating denatures proteins in the eye's lens, turning it milky. While all tissues are susceptible to heating, the eye is particularly vulnerable because it doesn't have blood vessels to modulate temperature. Microwave radiation is associated with the microwave auditory effect, in which microwave exposure produces buzzing sounds and clicks. This is caused by thermal expansion within the inner ear.
Microwave burns can occur in deeper tissue-not just on the surface-because microwaves are more readily absorbed by tissue that contains a lot of water. However, lower levels of exposure produce heat without burns. This effect may be used for a variety of purposes. The United States military uses millimeter waves to repel targeted persons with uncomfortable heat. As another example, in 1955, James Lovelock reanimated frozen rats using microwave diathermy.
- 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.