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Extremely high frequency is the highest radio frequency band. EHF runs the range of frequencies from 30 to 300 gigahertz, above which electromagnetic radiation is considered to be low (or far) infrared light, also referred to as Terahertz radiation. This band has a wavelength of ten to one millimetre, giving it the name millimeter band or millimetre wave. Radio signals in this band are extremely prone to atmospheric attenuation, making them of very little use over long distances. Even over relatively short distances, rain fade is a serious problem, caused when absorption by rain reduces signal strength.
Applications This band is commonly used in radio astronomy. In the United States, the band 38.6 - 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 (gigabits per second). This is a Civil Air Patrol frequency, as well. 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 frequencies, as opposed to the 60 GHz frequency, require a transmitting license in the US from the FCC, though they do not suffer from the effects of oxygen absorption as the 60 GHz does. 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, making this reserved range limited 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. Highly directional, "pencil-beam" signal characteristics permit systems in these bands to be engineered in close proximity to one another without causing interference. Potential applications include radar systems with very high resolution. A recent development has been imagers for security applications as clothing and other organic materials that are translucent in some mm-wave atmospheric windows. Uses of the millimeter wave bands includes point-to-point communications, and point-to-multipoint communications. Because of shorter wavelengths, the band permits the use of smaller antennas than would be required for similar circumstances in the lower bands, to achieve the same high directivity and high gain. The immediate consequence of this high directivity, coupled with the high free space loss at these frequencies, is the possibility of a more efficient use of the spectrum for point-to-multipoint applications. Since a greater number of high directive antennas can be placed than less directive antennas in a given area, the net result is higher reuse of the spectrum, and higher density of users, as compared to lower frequencies. Furthermore, due to the fact that one can place more voice channels or broadband information using a higher frequency to transmit the information, this spectrum could potentially be used as a replacement for or supplement to fiber optics. Applications also include radar systems with high resolution. A recent development has been imagers for security applications as clothing and other organic materials are translucent in some mm-wave atmospheric windows *. Privacy issues Privacy advocates are concerned about the use of this technology because it allows screens to see airport passengers without clothing. These machines are currently deployed in the UK to scan travelers on the bus system, and the TSA is planning to deploy several machines in airports for testing in the United States in early Spring. These machines have been deployed in the Jersey City PATH train system as well. Currently the technology does not mask any part of the bodies of the people who are being scanned and proposed remedies for privacy concerns include only scanning people who are detected to be carrying an object that may be a weapon and developing technology to mask genitals and other 'private parts', and an article about the PATH system states that an unnamed government official stated this technology is already in place, leading the journalist to conclude "there are no...privacy issues for travelers". According to Farran Technologies, a manufacturer of on model of the millimeter wave scanner, the technology exists to extend the search area to as far as 50 meters beyond the scanning area which would allow security workers to scan a large number of people without their awareness that they are being scanned *. Notes See also Academic Commercial | ||||||||
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