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Microwave is a kind of electromagnetic radiation with wavelengths ranging from roughly one meter to one millimeter and frequencies ranging from 300 MHz to 300 GHz. Microwaves are defined differently by different sources; the wide definition above encompasses both UHF and EHF (millimeter wave) bands. The range between 1 and 100 GHz is a more frequent definition in radio–frequency engineering (wavelengths between 0.3 m and 3 mm). Microwaves must cover the whole SHF range (3 to 30 GHz, or 10 to 1 cm) in all instances. The IEEE radar band designations S, C, X, Ku, K, or Ka band, as well as corresponding NATO or EU designations, are used to describe microwave frequencies.
In microwave, the prefix micro- does not imply a wavelength in the micrometer range. Rather, it denotes that microwaves are “little” (with shorter wavelengths) as compared to previous radio waves. The distinctions between far infrared, terahertz, microwaves, and ultra-high-frequency radio waves are rather arbitrary, and they are employed differently in different fields of research.
Microwaves move in a straight line, unlike lower-frequency radio waves, and do not diffract over hills, follow the earth’s surface as ground waves, or reflect off the ionosphere. As a result, terrestrial microwave communication lines are restricted to around 40 miles by the visible horizon (64 km). They are absorbed by gases in the atmosphere at the upper end of the spectrum, limiting practical communication range to roughly a kilometer.
Microwaves are widely used in modern technology, including 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 microwave oven cooking.
Microwaves move in a straight line, unlike lower frequency radio waves, they do not travel as ground waves that follow the curvature of the Earth or reflect off the ionosphere (skywaves). Although they can penetrate through building walls enough for practical reception at the low end of the band, rights of way cleared to the first Fresnel zone are normally necessary.
As a result, microwave communication lines on Earth’s surface are restricted to around 30″40 miles (48″64 kilometers) by the optical horizon. Moisture in the atmosphere absorbs microwaves, and attenuation rises with frequency, becoming a considerable influence (rain fade) towards the high end of the spectrum.
At roughly 40 GHz, atmospheric gases begin to absorb microwaves, limiting microwave transmission to a few kilometers above this frequency. Absorption peaks at various frequencies are caused by a spectral band structure (see graph at right). Above 100 GHz, the Earth’s atmosphere absorbs so much electromagnetic radiation that it becomes opaque, until the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges.