Microwaves are radio waves with wavelengths ranging from as long as one meter to as short as one millimetre, or equivalently, with frequencies between 300 MHz (0.3 GHz) and 300 GHz. 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).

Apparatus and techniques may be described qualitatively as "microwave" when the 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 give way to waveguides and stripline, and lumped-element tuned circuits are replaced by cavity resonators or resonant lines. Effects of reflection, polarization, scattering, diffraction, and atmospheric absorption usually associated with visible light are of practical significance in the study of microwave propagation. The same equations of electromagnetic theory apply at all frequencies.

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 light, terahertz radiation, microwaves, and ultra-high-frequency radio waves are fairly arbitrary and are used variously between different fields of study.

Microwave technology is extensively used for point-to-point telecommunications (i.e., non broadcast uses). Microwaves are especially suitable for this use since they are more easily focused into narrow beams than radio waves, and also their comparitively higher frequencies allow broad bandwidth and high data flow. Microwaves are the principal means by which data, TV, and telephone communications are transmitted between ground stations and to and from satellites. Microwaves are also employed in microwave ovens and in radar technology.

At about 20 GHz, decreasing microwave transmission through air is seen, due at lower frequencies from absorption from water and at higher frequencies from oxygen. A spectral band structure causes fluctuations in this behavior (see graph at right). Above 300 GHz, the absorption of microwave 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.

Light comparison
Name Wavelength Frequency (Hz) Photon Energy (eV)
Gamma ray less than 0.02 nm more than 15 EHz more than 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 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 meter 300 GHz – 300 MHz 1.24 meV – 1.24 µeV
Radio 1 mm – 100,000 km 300 GHz – 3 Hz 1.24 meV – 12.4 feV

Read more about Microwave:  Microwave Sources, Microwave Frequency Bands, Microwave Frequency Measurement, Health Effects, History and Research

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