Extremely high frequency

Extremely high frequency (EHF) is the International Telecommunication Union (ITU) designation for the band in the electromagnetic spectrum from 30 to 300 gigahertz (GHz). It is in the microwave part of the radio spectrum, between the super high frequency band and the terahertz band. Radio waves in this band have wavelengths from ten to one millimeter, so it is also called the millimeter band and radiation in this band is called millimeter waves, sometimes abbreviated MMW or mmWave. Some define mmWaves as starting at 24 GHz, thus covering the entire FR2 band (24.25 to 71 GHz), among others.

Compared to lower bands, radio waves in this band have high atmospheric attenuation: they are absorbed by the gases in the atmosphere. Absorption increases with frequency until at the top end of the band the waves are attenuated to zero within a few meters. Absorption by humidity in the atmosphere is significant except in desert environments, and attenuation by rain (rain fade) is a serious problem even over short distances. However the short propagation range allows 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. Millimeter waves are used for military fire-control radar, airport security scanners, short range wireless networks, and scientific research.

In a major new application of millimeter waves, certain frequency ranges near the bottom of the band are being used in the newest generation of cell phone networks, 5G networks. The design of millimeter-wave circuit and subsystems (such as antennas, power amplifiers, mixers and oscillators) also presents severe challenges to engineers due to semiconductor and process limitations, model limitations and poor Q factors of passive devices.

Millimeter waves propagate solely by line-of-sight paths. They are not refracted by the ionosphere nor do they travel along the Earth as ground waves as lower frequency radio waves do. At typical power densities they are blocked by building walls and suffer significant attenuation passing through foliage. Absorption by atmospheric gases is a significant factor throughout the band and increases with frequency. However, this absorption is maximum at a few specific absorption lines, mainly those of oxygen at 60 GHz and water vapor at 24 GHz and 184 GHz. At frequencies in the "windows" between these absorption peaks, millimeter waves have much less atmospheric attenuation and greater range, so many applications use these frequencies. Millimeter wavelengths are the same order of size as raindrops, so precipitation causes additional attenuation due to scattering (rain fade) as well as absorption. The high free space loss and atmospheric absorption limit useful 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.

Millimeter waves show "optical" propagation characteristics and can be reflected and focused by small metal surfaces and dielectric lenses around 5 to 30 cm (2 inches to 1 foot) diameter. Because their wavelengths are often much smaller than the equipment that manipulates them, the techniques of geometric optics can be used. Diffraction is less than at lower frequencies, although millimeter waves can be 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 the waves 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.

This band is commonly used in radio astronomy and remote sensing. Ground-based radio astronomy is limited to high altitude sites such as Kitt Peak and Atacama Large Millimeter Array (ALMA) due to atmospheric absorption issues.

Satellite-based remote sensing near 60 GHz can determine temperature in the upper atmosphere by measuring radiation emitted from oxygen molecules that is a function of temperature and pressure. The International Telecommunication Union non-exclusive passive frequency allocation at 57–59.3 GHz is used for atmospheric monitoring in meteorological and climate sensing applications and is important for these purposes due to the properties of oxygen absorption and emission in Earth's atmosphere. Currently operational U.S. satellite sensors such as the Advanced Microwave Sounding Unit (AMSU) on one NASA satellite (Aqua) and four NOAA (15–18) satellites and the special sensor microwave/imager (SSMI/S) on Department of Defense satellite F-16 make use of this frequency range.

In the United States, the band 36.0–40.0 GHz is used for licensed high-speed microwave data links, and the 60 GHz band can be used for unlicensed short range (1.7 km) data links with data throughputs up to 2.5 Gbit/s. It is used commonly in flat terrain.