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Extremely high frequency
Extremely high frequency (EHF) is the ITU designation for the band of radio frequencies in the electromagnetic spectrum from 30 to 300 gigahertz, above which electromagnetic radiation is considered to be low (or far) infrared light, also referred to as terahertz radiation. Radio waves in this band have wavelengths from ten to one millimetre, giving it the name millimetre band or millimetre wave, sometimes abbreviated MMW or mmW. Millimetre length electromagnetic waves were first investigated in the 1890s by pioneering Indian scientist Jagadish Chandra Bose.
Compared to lower bands, radio waves in this band have high atmospheric attenuation; they are absorbed by the gases in the atmosphere. Therefore, they have a short range and can only be used for terrestrial communication over about a kilometer. In particular, signals in the 57–64 GHz region are subject to a resonance of the oxygen molecule and are severely attenuated. Even over relatively short distances, rain fade is a serious problem, caused when absorption by rain reduces signal strength. In climates other than deserts absorption due to humidity also has an impact on propagation. While this absorption limits potential communications range, it also allows for smaller frequency reuse distances than lower frequencies. The short wavelength allows modest size antennas to have a small beam width, further increasing frequency reuse potential.
==Propagation==
Millimeter waves travel solely by line-of-sight, and are blocked by building walls and attenuated by foliage. The high free space loss and atmospheric absorption limits propagation to a few kilometers.〔 Thus they are useful for densely packed communications networks such as personal area networks that improve spectrum utilization through frequency reuse.〔
They show "optical" propagation characteristics and can be reflected and focused by small metal surfaces around 1 ft. diameter, and diffracted by building edges. At millimeter wavelengths, surfaces appear rougher so diffuse reflection increases.〔 Multipath propagation, particularly reflection from indoor walls and surfaces, causes serious fading. Doppler shift of frequency can be significant even at pedestrian speeds.〔 In portable devices shadowing due to the human body is a problem. Since they penetrate clothing and their small wavelength allows them to reflect from small metal objects they are used in millimeter wave scanners for airport security scanning.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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