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super high frequency
Super high frequency (SHF) is the ITU designation for radio frequencies (RF) in the range between 3 GHz and 30 GHz. This band of frequencies is also known as the centimetre band or centimetre wave as the wavelengths range from one to ten centimetres. These frequencies fall within the microwave band, so radio waves with these frequencies are called microwaves. The small wavelength of microwaves allows them to be directed in narrow beams by aperture antennas such as parabolic dishes, so they are used for point-to-point communication and data links〔 This article from the beginning of the microwave era predicted the future value of microwaves for point-to-point communication.〕 and for radar. This frequency range is used for most radar transmitters, microwave ovens, wireless LANs, cell phones, satellite communication, microwave radio relay links, and numerous short range terrestrial data links. Wireless USB technology is anticipated to use approximately one-third of this spectrum.
Frequencies in the SHF 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.
==Propagation==
Microwaves propagate entirely by line of sight; groundwave and skywave ("skip") propagation do not occur. Although in some cases they can penetrate building walls enough for useful reception, unobstructed rights of way cleared to the first Fresnel zone are usually required. Wavelengths are small enough that they can be focused into narrow beams by parabolic and horn antennas from a half meter to five meters in diameter. Therefore, they are used in point-to-point terrestrial communications links limited by the visual horizon (about 40 miles). Such high gain antennas allow frequency reuse by nearby transmitters. The size of SHF waves allow large reflections from metal objects the size of automobiles, aircraft, and ships, and other vehicles. Thus, the narrow beamwidths possible with high gain antennas and the low atmospheric attenuation as compared with higher frequencies make SHF the main frequencies used in radar. Attenuation and scattering by moisture in the atmosphere is a factor at the high end of the band.
Although they are not reflected by the ionosphere, small amounts of microwave energy are randomly scattered by water vapor molecules in the troposphere. This is used in troposcatter communications systems, operating at a few GHz, to communicate beyond the horizon. A powerful microwave beam is aimed just above the horizon; as it passes through the tropopause some of the microwaves are scattered back to Earth to a receiver beyond the horizon. Distances of 300 km can be achieved. These are mainly used for military communication.
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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