Super-high frequency

Super high frequency (SHF) is the ITU designation for radio frequencies (RF) in the range between 3 and 30 gigahertz (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 and for radar. This frequency range is used for most radar transmitters, wireless LANs, 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, K_u, K, or K_a band, or by similar NATO or EU designations.

Microwaves propagate entirely by line of sight; groundwave and ionospheric reflection (skywave or "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 at microwave frequencies that the antenna can be much larger than a wavelength, allowing highly directional (high gain) antennas to be built which can produce narrow beams. Therefore, they are used in point-to-point terrestrial communications links, limited by the visual horizon to 30–40 miles (48–64 km). Such high gain antennas allow frequency reuse by nearby transmitters. The wavelength of SHF waves allows strong 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 increase with frequency, limiting the use of high SHF frequencies for long range applications.