1.
Low (band)
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Low is an American indie rock group from Duluth, Minnesota, formed in 1993. As of 2010, the group is composed of founding members Alan Sparhawk and Mimi Parker, previous bassists for the band include John Nichols from 1993 to 1994, Zak Sally from 1994 to 2005 and Matt Livingston from 2005 to 2008. The music of Low is characterized by slow tempos and minimalist arrangements, early descriptions sometimes referred to it as a rock subgenre called slowcore often compared to the band Bedhead, who played this style during the 1980s and early 1990s. However, Lows members ultimately disapproved of the term, the band formed in the spring of 1993. Sparhawk had been playing in the Superior, Wisconsin band Zen Identity and that band needed a new bassist and recruited future Low bassist John Nichols. At that time, Nichols was a senior at Superior Senior High School, Sparhawk taught Zen Identity songs to Nichols and during practices, the two started improvising with some very modest, quiet themes. As a joke, they wondered what would happen if they played such music in front of Duluth crowds. Soon, the became a serious thought. She was to use brushes almost exclusively rather than drum sticks, Lows debut album, I Could Live in Hope, was released on Virgin Records Vernon Yard imprint in 1994. It featured Nichols on bass, though he was replaced by Zak Sally, both I Could Live in Hope and Long Division were produced and recorded by Kramer. Long Division and its similar follow-up, 1996s The Curtain Hits the Cast, established the band as critical darlings, over the Ocean, a single drawn from The Curtain Hits the Cast, also became something of a hit on college radio. By the time of their next full-length album Low had moved to the independent label Kranky, in between, they released several singles and EPs. In 1999, Low joined forces with Dirty Three to record an In The Fishtank session for Konkurrent records, allmusic called the six-song disc some of the best material either unit has produced. Of particular note is the discs lengthy cover of Neil Youngs Down by the River,2001 saw the release of Things We Lost in the Fire. The following year saw the release of the bands final full-length on Kranky, in April 2003, Peter S. Scholtes of the Twin Cities weekly paper City Pages posted in his weblog that Sally had left Low. The following month, the posted a update to the news on their website. After sorting it out, the news is that Zak is remaining in the band. In July 2003, they toured Europe with Radiohead, Sally in tow, to tie up the loose ends of the era, Low released a three-disc rarities compilation on its own Chairkickers label in 2004
2.
Hertz
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The hertz is the unit of frequency in the International System of Units and is defined as one cycle per second. It is named for Heinrich Rudolf Hertz, the first person to provide proof of the existence of electromagnetic waves. Hertz are commonly expressed in SI multiples kilohertz, megahertz, gigahertz, kilo means thousand, mega meaning million, giga meaning billion and tera for trillion. Some of the units most common uses are in the description of waves and musical tones, particularly those used in radio-. It is also used to describe the speeds at which computers, the hertz is equivalent to cycles per second, i. e. 1/second or s −1. In English, hertz is also used as the plural form, as an SI unit, Hz can be prefixed, commonly used multiples are kHz, MHz, GHz and THz. One hertz simply means one cycle per second,100 Hz means one hundred cycles per second, and so on. The unit may be applied to any periodic event—for example, a clock might be said to tick at 1 Hz, the rate of aperiodic or stochastic events occur is expressed in reciprocal second or inverse second in general or, the specific case of radioactive decay, becquerels. Whereas 1 Hz is 1 cycle per second,1 Bq is 1 aperiodic radionuclide event per second, the conversion between a frequency f measured in hertz and an angular velocity ω measured in radians per second is ω =2 π f and f = ω2 π. This SI unit is named after Heinrich Hertz, as with every International System of Units unit named for a person, the first letter of its symbol is upper case. Note that degree Celsius conforms to this rule because the d is lowercase. — Based on The International System of Units, the hertz is named after the German physicist Heinrich Hertz, who made important scientific contributions to the study of electromagnetism. The name was established by the International Electrotechnical Commission in 1930, the term cycles per second was largely replaced by hertz by the 1970s. One hobby magazine, Electronics Illustrated, declared their intention to stick with the traditional kc. Mc. etc. units, sound is a traveling longitudinal wave which is an oscillation of pressure. Humans perceive frequency of waves as pitch. Each musical note corresponds to a frequency which can be measured in hertz. An infants ear is able to perceive frequencies ranging from 20 Hz to 20,000 Hz, the range of ultrasound, infrasound and other physical vibrations such as molecular and atomic vibrations extends from a few femtoHz into the terahertz range and beyond. Electromagnetic radiation is described by its frequency—the number of oscillations of the perpendicular electric and magnetic fields per second—expressed in hertz. Radio frequency radiation is measured in kilohertz, megahertz, or gigahertz
3.
Metre
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The metre or meter, is the base unit of length in the International System of Units. The metre is defined as the length of the path travelled by light in a vacuum in 1/299792458 seconds, the metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole. In 1799, it was redefined in terms of a metre bar. In 1960, the metre was redefined in terms of a number of wavelengths of a certain emission line of krypton-86. In 1983, the current definition was adopted, the imperial inch is defined as 0.0254 metres. One metre is about 3 3⁄8 inches longer than a yard, Metre is the standard spelling of the metric unit for length in nearly all English-speaking nations except the United States and the Philippines, which use meter. Measuring devices are spelled -meter in all variants of English, the suffix -meter has the same Greek origin as the unit of length. This range of uses is found in Latin, French, English. Thus calls for measurement and moderation. In 1668 the English cleric and philosopher John Wilkins proposed in an essay a decimal-based unit of length, as a result of the French Revolution, the French Academy of Sciences charged a commission with determining a single scale for all measures. In 1668, Wilkins proposed using Christopher Wrens suggestion of defining the metre using a pendulum with a length which produced a half-period of one second, christiaan Huygens had observed that length to be 38 Rijnland inches or 39.26 English inches. This is the equivalent of what is now known to be 997 mm, no official action was taken regarding this suggestion. In the 18th century, there were two approaches to the definition of the unit of length. One favoured Wilkins approach, to define the metre in terms of the length of a pendulum which produced a half-period of one second. The other approach was to define the metre as one ten-millionth of the length of a quadrant along the Earths meridian, that is, the distance from the Equator to the North Pole. This means that the quadrant would have defined as exactly 10000000 metres at that time. To establish a universally accepted foundation for the definition of the metre, more measurements of this meridian were needed. This portion of the meridian, assumed to be the length as the Paris meridian, was to serve as the basis for the length of the half meridian connecting the North Pole with the Equator
4.
Extremely low frequency
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Extremely low frequency is the ITU designation for electromagnetic radiation with frequencies from 3 to 30 Hz, and corresponding wavelengths of 100,000 to 10,000 kilometers, respectively. In atmospheric science, a definition is usually given, from 3 Hz to 3 kHz. In the related magnetosphere science, the lower frequency oscillations are considered to lie in the ULF range. ELF radio waves are generated by lightning and natural disturbances in Earths magnetic field, because of the difficulty of building antennas that can radiate such long waves, ELF frequencies have been used in only a very few human-made communication systems. ELF waves can penetrate seawater, which makes them useful in communication with submarines, the US, Russia, and India are the only nations known to have constructed ELF communication facilities. The U. S. facilities were used between 1985 and 2004 but are now decommissioned, ELF waves can also penetrate significant distances into earth or rock, and through-the-earth underground mine communication systems use frequencies of 300 to 3000 Hz. The frequency of alternating current flowing in electric grids,50 or 60 Hz, also falls within the ELF band. Some medical peer reviewed journal articles refer to ELF in the context of low frequency magnetic fields with frequencies of 50 Hz. United States Government agencies, such as NASA, describe ELF as non-ionizing radiation with frequencies between 0 and 300 Hz. Due to their long wavelength, ELF waves can diffract around large obstacles. ELF and VLF waves propagate long distances by an Earth-ionosphere waveguide mechanism, the Earth is surrounded by a layer of charged particles in the atmosphere at an altitude of about 60 km at the bottom of the ionosphere, called the D layer which reflects ELF waves. ELF waves have extremely low attenuation of 1 –2 dB per 1000 km. giving a single transmitter the potential to communicate worldwide, ELF waves can also travel considerable distances through lossy media like earth and seawater, which would absorb or reflect higher frequency radio waves. At certain frequencies these oppositely directed waves are in phase and add, in other words, the closed spherical Earth-ionosphere cavity acts as a huge cavity resonator, enhancing ELF radiation at its resonant frequencies. These are called Schumann resonances after German physicist Winfried Otto Schumann who predicted them in 1952, the United States Navy utilized extremely low frequencies as radio band and radio communications. The Submarine Integrated Antenna System was a research and development effort to communicate with submerged submarines, the Soviet/Russian Navy also utilized ELFs for submarine communications system, ZEVS. The Indian Navy has an operational ELF communication facility at the INS Kattabomman naval base to communicate with its Arihant class, because of its electrical conductivity, seawater shields submarines from most higher frequency radio waves, making radio communication with submerged submarines at ordinary frequencies impossible. Signals in the ELF frequency range, however, can penetrate much deeper, generally, ELF signals were used to order a submarine to rise to a shallow depth where it could receive some other form of communication. One of the difficulties posed when broadcasting in the ELF frequency range is antenna size, simply put, a 3 Hz signal would have a wavelength equal to the distance EM waves travel through a given medium in one third of a second
5.
Very low frequency
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Very low frequency or VLF is the ITU designation for radio frequencies in the range of 3 kHz to 30 kHz and corresponding wavelengths from 100 to 10 kilometres, respectively. The band is known as the myriametre band or myriametre wave as the wavelengths range from one to ten myriametres. Due to its bandwidth, audio transmission is highly impractical in this band. The VLF band is used for a few radio navigation services, government time radio stations, since VLF waves can penetrate at least 40 meters into saltwater, they are used for military communication with submarines. The main mode of long distance propagation is an Earth-ionosphere waveguide mechanism, the Earth is surrounded by a conductive layer of electrons and ions in the upper atmosphere, the ionosphere D layer at 60 km altitude, which reflects VLF radio waves. The conductive ionosphere and the conductive Earth, form a horizontal duct a few VLF wavelengths high, the waves travel in a zigzag path around the Earth, reflected alternately by the Earth and the ionosphere, in TM mode. Therefore, VLF transmissions are very stable and reliable, and are used for distance communication. Propagation distances of 5000 to 20000 km have been realized, however, atmospheric noise is high in the band, including such phenomena as whistlers, caused by lightning. VLF waves can penetrate seawater to a depth of at least 10 to 40 meters, depending on the frequency employed, VLF waves at certain frequencies have been found to cause electron precipitation. A major practical drawback to this band is that because of the length of the waves, vertical antennas must be used because VLF waves propagate in vertical polarization, but a quarter-wave vertical antenna at 30 kHz would be 2.5 kilometers high. So practical transmitting antennas are short, a small fraction of a wavelength long. Due to their low radiation resistance they are inefficient, radiating only 10% to 50% of the power at most. With the rest of the power dissipated in the antenna/ground system resistances, Very high power transmitters are required to radiate enough power for long distance communication. Transmitting antennas for VLF frequencies are very large antennas, up to a mile across. They consist of a series of radio masts, linked at the top with a network of cables. Either the towers themselves or vertical wires serve as monopole radiators, high power stations use variations on the umbrella antenna such as the delta and trideco antennas, or multiply-tuned flattop antennas. For low power transmitters, inverted-L and T antennas are used, a large loading coil is required at the antenna feed point to cancel the capacitive reactance of the antenna to make it resonant. To minimize power dissipated in the ground, these antennas require extremely low resistance ground systems, the high capacitance and inductance and low resistance of the antenna-loading coil combination, makes it act electrically like a high Q tuned circuit
6.
Medium frequency
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Medium frequency is the ITU designation for radio frequencies in the range of 300 kHz to 3 MHz. Part of this band is the medium wave AM broadcast band, the MF band is also known as the hectometer band or hectometer wave as the wavelengths range from ten to one hectometer. Frequencies immediately below MF are denoted low frequency, while the first band of frequencies is known as high frequency. MF is mostly used for AM radio broadcasting, navigational beacons, maritime ship-to-shore communication. A major use of frequencies is AM broadcasting, AM radio stations are allocated frequencies in the medium wave broadcast band from 526.5 kHz to 1606. Although these are medium frequencies,120 meters is generally treated as one of the shortwave bands, there are a number of coast guard and other ship-to-shore frequencies in use between 1600 and 2850 kHz. These include, as examples, the French MRCC on 1696 kHz and 2677 kHz, Stornoway Coastguard on 1743 kHz,2182 kHz is the international calling and distress frequency for SSB maritime voice communication. It is analogous to Channel 16 on the marine VHF band,500 kHz was for many years the maritime distress and emergency frequency, and there are more NDBs between 510 and 530 kHz. Navtex, which is part of the current Global Maritime Distress Safety System occupies 518 kHz and 490 kHz for important digital text broadcasts. Lastly, there are aeronautical and other mobile SSB bands from 2850 kHz to 3500 kHz, an amateur radio band known as 160 meters or top-band is between 1800 and 2000 kHz. Amateur operators transmit CW morse code, digital signals and SSB voice signals on this band, in recent years, some limited amateur radio operation has also been allowed in the region of 500 kHz in the US, UK, Germany and Sweden. Propagation at MF wavelengths is via ground waves and reflection from the ionosphere, ground waves follow the contour of the Earth. MF broadcasting stations use ground waves to cover their listening areas, however at certain times the D layer can be electronically noisy and absorb MF radio waves, interfering with skywave propagation. When this happens, MF radio waves can easily be received hundreds or even thousands of miles away as the signal will be refracted by the remaining F layer and this can be very useful for long-distance communication, but can also interfere with local stations. Due to the number of available channels in the MW broadcast band. On nights of good skywave propagation, the signals of distant stations may reflect off the ionosphere and interfere with the signals of local stations on the same frequency. These channels are called clear channels, and the stations, called stations, are required to broadcast at higher powers of 10 to 50 kW. Transmitting antennas commonly used on this band include monopole mast radiators, top-loaded wire monopole antennas such as the inverted-L and T antennas, ground wave propagation, the most widely used type at these frequencies, requires vertically polarized antennas like monopoles
7.
High frequency
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High frequency is the ITU designation for the range of radio frequency electromagnetic waves between 3 and 30 MHz. It is also known as the band or decameter wave as its wavelengths range from one to ten decameters. Frequencies immediately below HF are denoted medium frequency, while the band of higher frequencies is known as the very high frequency band. The HF band is a part of the shortwave band of frequencies. The band is used by shortwave broadcasting stations, aviation communication, government time stations, weather stations, amateur radio and citizens band services. By this method HF radio waves can travel beyond the horizon, around the curve of the Earth and it depends on the angle of incidence of the waves, it is lowest when the waves are directed straight upwards, and is higher with less acute angles. This means that at longer distances, where the waves graze the ionosphere at a blunt angle. The lowest usable frequency depends on the absorption in the layer of the ionosphere. This absorption is stronger at low frequencies and is stronger with increased solar activity. When all factors are at their optimum, worldwide communication is possible on HF, at many other times it is possible to make contact across and between continents or oceans. At worst, when a band is dead, no communication beyond the limited groundwave paths is possible no matter what powers, on such an open band, interference originating over a wide area affects many potential users. These issues are significant to military, safety and amateur radio users of the HF bands, other standards development such as STANAG5066 provides for error free data communications through the use of ARQ protocols. Noise, especially man-made interference from devices, tends to have a great effect on the HF bands. In recent years, concerns have risen among certain users of the HF spectrum over broadband over power lines Internet access and this is due to the frequencies on which BPL operates and the tendency for the BPL signal to leak from power lines. Some BPL providers have installed notch filters to block out certain portions of the spectrum, other electronic devices including plasma televisions can also have a detrimental effect on the HF spectrum. In aviation, HF communication systems are required for all trans-oceanic flights and these systems incorporate frequencies down to 2 MHz to include the 2182 kHz international distress and calling channel. The upper section of HF shares many characteristics with the part of VHF. The parts of this section not allocated to radio are used for local communications
8.
Very high frequency
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Very high frequency is the ITU designation for the range of radio frequency electromagnetic waves from 30 MHz to 300 MHz, with corresponding wavelengths of ten to one meters. Frequencies immediately below VHF are denoted high frequency, and the higher frequencies are known as ultra high frequency. Air traffic control communications and air navigation systems work at distances of 100 kilometres or more to aircraft at cruising altitude, some older DVB-T receivers included channels E2 to E4 but newer ones only go down to channel E5. VHF propagation characteristics are suited for terrestrial communication, with a range generally somewhat farther than line-of-sight from the transmitter. VHF waves are restricted to the radio horizon less than 100 miles. VHF is less affected by noise and interference from electrical equipment than lower frequencies. Unlike high frequencies, the ionosphere does not usually reflect VHF waves, the distance to the radio horizon is slightly extended over the geometric line of sight to the horizon, as radio waves are weakly bent back toward the Earth by the atmosphere. These approximations are only valid for antennas at heights that are compared to the radius of the Earth. They may not necessarily be accurate in mountainous areas, since the landscape may not be transparent enough for radio waves, in engineered communications systems, more complex calculations are required to assess the probable coverage area of a proposed transmitter station. The accuracy of calculations for digital TV signals is being debated. Portable radios usually use whips or rubber ducky antennas, while base stations usually use larger fiberglass whips or collinear arrays of vertical dipoles, for directional antennas, the Yagi antenna is the most widely used as a high gain or beam antenna. For television reception, the Yagi is used, as well as the log periodic antenna due to its wider bandwidth, helical and turnstile antennas are used for satellite communication since they employ circular polarization. For even higher gain, multiple Yagis or helicals can be mounted together to make array antennas, vertical collinear arrays of dipoles can be used to make high gain omnidirectional antennas, in which more of the antennas power is radiated in horizontal directions. Television and FM broadcasting stations use arrays of specialized dipole antennas such as batwing antennas. Certain subparts of the VHF band have the same use around the world, some national uses are detailed below. 108–118 MHz, Air navigation beacons VOR and Instrument Landing System localiser, 118–137 MHz, Airband for air traffic control, AM,121.5 MHz is emergency frequency 144–146 MHz, Amateur radio. Other capital cities and regional areas used a combination of these, the initial commercial services in Hobart and Darwin were respectively allocated channels 6 and 8 rather than 7 or 9. By the early 1960s it became apparent that the 10 VHF channels were insufficient to support the growth of television services and this was rectified by the addition of three additional frequencies—channels 0, 5A and 11
9.
Ultra high frequency
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Ultra high frequency is the ITU designation for radio frequencies in the range between 300 MHz and 3 GHz, also known as the decimetre band as the wavelengths range from one meter to one decimetre. Radio waves with frequencies above the UHF band fall into the SHF or microwave frequency range, lower frequency signals fall into the VHF or lower bands. UHF radio waves propagate mainly by line of sight, they are blocked by hills, the IEEE defines the UHF radar band as frequencies between 300 MHz and 1 GHz. Two other IEEE radar bands overlap the ITU UHF band, the L band between 1 and 2 GHz and the S band between 2 and 4 GHz. Radio waves in the UHF band travel almost entirely by propagation and ground reflection, there is very little reflection from the ionosphere. They are blocked by hills and cannot travel far beyond the horizon, atmospheric moisture reduces, or attenuates, the strength of UHF signals over long distances, and the attenuation increases with frequency. UHF TV signals are generally more degraded by moisture than lower bands, occasionally when conditions are right, UHF radio waves can travel long distances by tropospheric ducting as the atmosphere warms and cools throughout the day. The length of an antenna is related to the length of the radio waves used, the UHF antenna is stubby and short, at UHF frequencies a quarter-wave monopole, the most common omnidirectional antenna is between 2.5 and 25 cm long for example. UHF is widely used in telephones, cell phones, walkie-talkies and other two-way radio systems from short range up to the visual horizon. Their transmissions do not travel far, allowing frequency reuse, public safety, business communications and personal radio services such as GMRS, PMR446, and UHF CB are often found on UHF frequencies as well as IEEE802.11 wireless LANs. The widely adapted GSM and UMTS cellular networks use UHF cellular frequencies, radio repeaters are used to retransmit UHF signals when a distance greater than the line of sight is required. Omnidirectional UHF antennas used on mobile devices are usually short whips, higher gain omnidirectional UHF antennas can be made of collinear arrays of dipoles and are used for mobile base stations and cellular base station antennas. The short wavelengths also allow high gain antennas to be conveniently small, high gain antennas for point-to-point communication links and UHF television reception are usually Yagi, log periodic, corner reflectors, or reflective array antennas. At the top end of the band slot antennas and parabolic dishes become practical, for television broadcasting specialized vertical radiators that are mostly modifications of the slot antenna or helical antenna are used, the slotted cylinder, zig-zag, and panel antennas. UHF television broadcasting fulfilled the demand for additional over-the-air television channels in urban areas, today, much of the bandwidth has been reallocated to land mobile, trunked radio and mobile telephone use. UHF channels are used for digital television. UHF spectrum is used worldwide for mobile radio systems for commercial, industrial, public safety. Many personal radio services use frequencies allocated in the UHF band, major telecommunications providers have deployed voice and data cellular networks in UHF/VHF range
10.
Super high frequency
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Super high frequency is the ITU designation for radio frequencies in the range between 3 GHz and 30 GHz. This band of frequencies is known as the centimetre band or centimetre wave as the wavelengths range from one to ten centimetres. These frequencies fall within the band, so radio waves with these frequencies are called microwaves. This frequency range is used for most radar transmitters, wireless LANs, satellite communication, microwave relay 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. Microwaves propagate entirely by line of sight, groundwave and ionospheric reflection 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 larger than a wavelength. Therefore, they are used in point-to-point terrestrial communications links limited by the visual horizon, such high gain antennas allow frequency reuse by nearby transmitters. The size of SHF waves allows strong reflections from objects the size of automobiles, aircraft, and ships. 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 increase with frequency. Small amounts of energy are randomly scattered by water vapor molecules in the troposphere. This is used in communications systems, operating at a few GHz. A powerful microwave beam is aimed just above the horizon, as it passes through the some of the microwaves are scattered back to Earth to a receiver beyond the horizon. Distances of 300 km can be achieved and these are mainly used for military communication. The wavelengths of SHF waves are small enough that they can be focused into narrow beams by high gain antennas from a meter to five meters in diameter. Directive antennas at SHF frequencies are mostly aperture antennas, such as antennas, dielectric lens, slot. Large parabolic antennas can produce very narrow beams of a few degrees or less, for omnidirectional applications like wireless devices and cellphones, small dipoles or monopoles are used
11.
Extremely high frequency
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Extremely high frequency is the International Telecommunications Union designation for the band of radio frequencies in the electromagnetic spectrum from 30 to 300 gigahertz. It lies between the high frequency band, and the far infrared band which is also referred to as the terahertz gap. Radio waves in this band have wavelengths from ten to one millimetre, giving it the name millimetre band or millimetre wave, 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, 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, however the short propagation range allows smaller frequency reuse distances than lower frequencies. The short wavelength allows modest size antennas to have a beam width. 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. 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. They show optical propagation characteristics and can be reflected and focused by small metal surfaces around 1 ft. diameter, 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 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 security scanning. This band is used in radio astronomy and remote sensing. Ground-based radio astronomy is limited to high altitude such as Kitt Peak. Satellite-based remote sensing near 60 GHz can determine temperature in the 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 and it is used commonly in flat terrain. The 71-76, 81-86 and 92–95 GHz bands are used for point-to-point high-bandwidth communication links. These higher frequencies do not suffer from oxygen absorption, but require a license in the US from the Federal Communications Commission
12.
Terahertz radiation
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Photon energy in THz regime is less than band-gap of nonmetallic materials and thus THz beam can traverse through such materials. The transmitted THz beam is used for material characterization, layer inspection, terahertz radiation falls in between infrared radiation and microwave radiation in the electromagnetic spectrum, and it shares some properties with each of these. Like infrared and microwave radiation, terahertz radiation travels in a line of sight and is non-ionizing, like microwave radiation, terahertz radiation can penetrate a wide variety of non-conducting materials. Terahertz radiation can pass through clothing, paper, cardboard, wood, masonry, plastic, the penetration depth is typically less than that of microwave radiation. Terahertz radiation has limited penetration through fog and clouds and cannot penetrate liquid water or metal, THz is not ionizing yet can penetrate some distance through body tissue, so it is of interest as a replacement for medical X-rays. Due to its wavelength, images made using THz are low resolution. However, at distances of ~10 meters the band may still allow many useful applications in imaging and construction of high bandwidth wireless networking systems, terahertz radiation is emitted as part of the black-body radiation from anything with temperatures greater than about 10 kelvin. The opacity of the Earths atmosphere to submillimeter radiation restricts these observatories to very high altitude sites, and electronic oscillators based on resonant tunneling diodes have been shown to operate up to 700 GHz. There have also been solid-state sources of millimeter and submillimeter waves for many years, AB Millimeter in Paris, for instance, produces a system that covers the entire range from 8 GHz to 1000 GHz with solid state sources and detectors. Nowadays, most time-domain work is done via ultrafast lasers, in mid-2007, scientists at the U. S. The group was led by Ulrich Welp of Argonnes Materials Science Division, the device uses high-temperature superconducting crystals, grown at the University of Tsukuba in Japan. This alternating current induces an electromagnetic field, even a small voltage can induce frequencies in the terahertz range, according to Welp. In 2008, engineers at Harvard University achieved room temperature emission of several hundred nanowatts of coherent terahertz radiation using a semiconductor source, THz radiation was generated by nonlinear mixing of two modes in a mid-infrared quantum cascade laser. Previous sources had required cryogenic cooling, which limited their use in everyday applications. In 2009, it was discovered that the act of unpeeling adhesive tape generates non-polarized terahertz radiation, with a peak at 2 THz. In 2011, Japanese electronic parts maker Rohm and a team at Osaka University produced a chip capable of transmitting 1.5 Gbit/s using terahertz radiation. Such an antenna would broadcast in the frequency range. Unlike X-rays, terahertz radiation is not ionizing radiation and its low photon energies in general do not damage tissues, some frequencies of terahertz radiation can penetrate several millimeters of tissue with low water content and reflect back
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