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Ultra-high frequency (UHF) designates the ITU radio frequency range of electromagnetic waves between 300 MHz and 3 GHz (3,000 MHz), also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres (10 cm to 1 metre). Radio waves with frequencies above the UHF band fall into the SHF (super-high frequency) and EHF (extremely high frequency) bands, all of which fall into the microwave frequency range. Lower frequency signals fall into the VHF (very high frequency) or lower bands. See Electromagnetic spectrum and Radio spectrum for a full listing of frequency bands.
The point to point transmission and reception of TV and radio signals is affected by many variables. Atmospheric moisture; solar wind; physical obstructions, such as mountains and buildings; and time of day all affect the signal transmission and the degradation of signal reception. All radio waves are partly absorbed by atmospheric moisture. Atmospheric absorption reduces, or attenuates, the strength of radio signals over long distances. The effects of attenuation degradation increases with frequency. UHF TV signals are generally more degraded by moisture than lower bands, such as VHF TV signals. The ionosphere, a layer of the Earth's atmosphere, is filled with charged particles that can reflect some radio waves. Amateur radio enthusiasts primarily use this quality of the ionosphere to help propagate lower frequency HF signals around the world: the waves are trapped, bouncing around in the upper layers of the ionosphere until they are refracted down at another point on the Earth. This is called skywave transmission. UHF TV signals are not carried along the ionosphere but can be reflected off of the charged particles down at another point on Earth in order to reach farther than the typical line-of-sight transmission distances; this is the skip distance. UHF transmission and reception are enhanced or degraded by tropospheric ducting as the atmosphere warms and cools throughout the day.
The main advantage of UHF transmission is the physically short wave that is produced by the high frequency. The size of transmission and reception antennas is related to the size of the radio wave. The UHF antenna is stubby and short. Smaller and less conspicuous antennas can be used with higher frequency bands.
The major disadvantage of UHF is its limited broadcast range and reception, often called line-of-sight between the TV station's transmission antenna and customer's reception antenna, as opposed to VHF's very long broadcast range and reception, which is less restricted by line of sight.
UHF is widely used in two-way radio systems and cordless telephones, whose transmission and reception antennas are closely spaced. UHF signals travel over line-of-sight distances. Transmissions generated by two-way radios and cordless telephones do not travel far enough to interfere with local transmissions. Several public-safety and business communications are handled on UHF. Civilian applications, such as GMRS, PMR446, UHF CB, 802.11b ("WiFi") and the widely adapted GSM and UMTS cellular networks, also use UHF cellular frequencies. A repeater propagates UHF signals when a distance greater than the line of sight is required.
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 still used for digital television.
UHF spectrum is used world-wide for land mobile radio systems for commercial, industrial, public safety, and military purposes. Many personal radio services use frequencies allocated in the UHF band, although exact frequencies in use differ significantly between countries.
The Family Radio Service and General Mobile Radio Service use the 462 and 467 MHz areas of the UHF spectrum. There is a considerable amount of lawful unlicensed activity (cordless phones, wireless networking) clustered around 900 MHz and 2.4 GHz. These ISM bands – open frequencies with a higher unlicensed power permitted for use originally by Industrial, Scientific, Medical apparatus – are now becoming some of the most crowded in the spectrum because they are open to everyone.
The 2.45 GHz frequency is the standard for use by microwave ovens. The spectrum from 806 MHz to 890 MHz (UHF channels 70–83) was taken away from TV broadcast services in 1983, primarily for analogue mobile telephony. In 2009, as part of the transition from analog to digital over-the-air broadcast of television, the spectrum from 698 MHz to 806 MHz (UHF channels 52–69) was also no longer used for TV broadcasting. Channel 55, for instance, was sold to Qualcomm for their MediaFLO service, which is resold under various mobile telephone network brands. Some US broadcasters had been offered incentives to vacate this channel early, permitting its immediate mobile use.
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