A telecommunications tower with a variety of dish antennas for microwave relay links on Frazier Peak, Ventura County, California. The apertures of the dishes are covered by plastic sheets (radomes) to keep out moisture.
The atmospheric attenuation of microwaves and far infrared radiation 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. This graph includes a range of frequencies from 0 to 1 THz; the microwaves are the subset in the range between 0.3 and 300 gigahertz.

Microwaves are a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter; with frequencies between 300 MHz (1 m) and 300 GHz (1 mm).[1][2][3][4][5] Different sources define different frequency ranges as microwaves; the above broad definition includes both UHF and EHF (millimeter wave) bands. A more common definition in radio engineering is the range between 1 and 100 GHz (wavelengths between 0.3 m and 3 mm).[2] In all cases, microwaves include the entire SHF band (3 to 30 GHz, or 10 to 1 cm) at minimum. Frequencies in the microwave range are often referred to by their IEEE radar band designations: S, C, X, Ku, K, or Ka band, or by similar NATO or EU designations.

The prefix micro- in microwave is not meant to suggest a wavelength in the micrometer range. Rather, it indicates that microwaves are "small" (having shorter wavelengths), compared to the radio waves used prior to microwave technology. 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.

Microwaves travel by line-of-sight; unlike lower frequency radio waves they do not diffract around hills, follow the earth's surface as ground waves, or reflect from the ionosphere, so terrestrial microwave communication links are limited by the visual horizon to about 40 miles (64 km). At the high end of the band they are absorbed by gases in the atmosphere, limiting practical communication distances to around a kilometer. Microwaves are widely used in modern technology, for example in point-to-point communication links, wireless networks, microwave radio relay networks, radar, satellite and spacecraft communication, medical diathermy and cancer treatment, remote sensing, radio astronomy, particle accelerators, spectroscopy, industrial heating, collision avoidance systems, garage door openers and keyless entry systems, and for cooking food in microwave ovens.

Electromagnetic spectrum

Microwaves occupy a place in the electromagnetic spectrum with frequency above ordinary radio waves, and below infrared light:

Electromagnetic spectrum
Name Wavelength Frequency (Hz) Photon energy (eV)
Gamma ray < 0.02 nm > 15 EHz > 62.1 keV
X-ray 0.01 nm – 10 nm 30 EHz – 30 PHz 124 keV – 124 eV
Ultraviolet 10 nm – 400 nm 30 PHz – 750 THz 124 eV – 3 eV
Visible light 390 nm – 750 nm 770 THz – 400 THz 3.2 eV – 1.7 eV
Infrared 750 nm – 1 mm 400 THz – 300 GHz 1.7 eV – 1.24 meV
Microwave 1 mm – 1 m 300 GHz – 300 MHz 1.24 meV – 1.24 µeV
Radio 1 mm – 100 km 300 GHz3 kHz 1.24 µeV – 12.4 feV

In descriptions of the electromagnetic spectrum, some sources classify microwaves as radio waves, a subset of the radio wave band; while others classify microwaves and radio waves as distinct types of radiation. This is an arbitrary distinction.

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