Extremely high frequency

Extremely high frequency (EHF) is the International Telecommunication Union (ITU) designation for the band of radio frequencies in the electromagnetic spectrum from 30 to 300 gigahertz (GHz). It lies between the super high frequency band, and the far infrared band, the lower part of which is also referred to as the terahertz gap. Radio waves in this band have wavelengths from ten to one millimetre, so it is also called the millimetre band and radiation in this band is called millimetre waves, sometimes abbreviated MMW or mmW or mmWave. Millimetre-length electromagnetic waves were first investigated in the 1890s by Indian scientist Jagadish Chandra Bose.ELF 3 Hz/100 Mm 30 Hz/10 MmSLF 30 Hz/10 Mm 300 Hz/1 MmULF 300 Hz/1 Mm 3 kHz/100 kmVLF 3 kHz/100 km 30 kHz/10 kmLF 30 kHz/10 km 300 kHz/1 kmMF 300 kHz/1 km 3 MHz/100 mHF 3 MHz/100 m 30 MHz/10 mVHF 30 MHz/10 m 300 MHz/1 mUHF 300 MHz/1 m 3 GHz/100 mmSHF 3 GHz/100 mm 30 GHz/10 mmEHF 30 GHz/10 mm 300 GHz/1 mmTHF 300 GHz/1 mm 3 THz/0.1 mm Extremely high frequency (EHF) is the International Telecommunication Union (ITU) designation for the band of radio frequencies in the electromagnetic spectrum from 30 to 300 gigahertz (GHz). It lies between the super high frequency band, and the far infrared band, the lower part of which is also referred to as the terahertz gap. Radio waves in this band have wavelengths from ten to one millimetre, so it is also called the millimetre band and radiation in this band is called millimetre waves, sometimes abbreviated MMW or mmW or mmWave. Millimetre-length electromagnetic waves were first investigated in the 1890s by 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. 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 propagate solely by line-of-sight paths. They are not reflected 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, it 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 limits 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 they 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 ITU 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. The 71–76, 81–86 and 92–95 GHz bands are also used for point-to-point high-bandwidth communication links. These higher frequencies do not suffer from oxygen absorption, but require a transmitting license in the US from the Federal Communications Commission (FCC). There are plans for 10 Gbit/s links using these frequencies as well. In the case of the 92–95 GHz band, a small 100 MHz range has been reserved for space-borne radios, limiting this reserved range to a transmission rate of under a few gigabits per second. The band is essentially undeveloped and available for use in a broad range of new products and services, including high-speed, point-to-point wireless local area networks and broadband Internet access. WirelessHD is another recent technology that operates near the 60 GHz range. Highly directional, 'pencil-beam' signal characteristics permit different systems to operate close to one another without causing interference. Potential applications include radar systems with very high resolution.