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EXTREMELY HIGH FREQUENCY
| extremely high frequency (EHF) |
Cycles per second: 30 GHz to 300 GHz
Wavelength: 10 mm to 1 mm
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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 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 100 MHz band has been reserved for space-borne radios, making this upper band limited to a transmission rate of only a few gigabits.
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.
See also
External links
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