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Ultra high frequency
Ultra high frequency (UHF) designates a range (band) of electromagnetic waves whose frequency is between 300 MHz and 3 GHz (3,000 MHz). Also known as the decimeter band or decimeter wave as the wavelengths range from ten to one decimeters. Radio waves whose frequency is above the UHF band fall into the SHF (Super high frequency) and EHF (Extremely high frequency) bands; all of which fall under the Microwave frequency range. Lower frequency signals fall into the VHF or lower bands. See electromagnetic spectrum for a full listing of frequency bands.
UHF and VHF are the most commonly used frequency bands for transmission of television signals. Modern mobile phones also transmit and receive within the UHF spectrum. UHF is widely used by public service agencies for two-way radio communication, usually using narrowband frequency modulation, but digital services are on the rise. There has traditionally been very little radio broadcasting in this band until recently; see digital audio broadcasting for details. The Global Positioning System also uses UHF.
One uncommon use of UHF waves is for the detection of partial discharges. Partial discharges occur due to the sharp geometries created in high voltage insulated equipment. The advantage of UHF detection is that this method can be used to localize the source of the discharge. A drawback to UHF detection is the extreme sensitivity to external noise. UHF detection methods are used in the field, especially for large distribution transformers.
2.45 GHz, now mainly used for cordless phones and WiFi, has been proposed for Wireless energy transfer. Some pilot experiments have been performed, but it is not used on a large scale.
Amateur radio operators also operate in several UHF bands.
Characteristics and advantages
The point to point transmission of radio waves is affected by many variables. Atmospheric moisture, the stream of particles from the sun called solar wind, and time of day will all have an effect on the signal transmission. All radio waves are partially absorbed by atmospheric moisture. Atmospheric absorption reduces, or attenuates, the strength of radio signals over long distances. The effects of attenuation increases according to the frequency. UHF signals are generally more degraded by moisture than lower bands such as VHF. The layer of the Earth's atmosphere called the ionosphere is filled with charged particles that can reflect radio waves. The reflection of radio waves can be helpful in transmitting a radio signal over long distances as the wave repeatedly bounces from the sky to the ground. UHF benefits less from the effects of reflection than lower (VHF, etc.) frequencies. UHF transmissions may be enhanced 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 equipment, (particularly antennas), is related to the size of the radio wave. Smaller and less conspicuous antennas can be used with higher frequency bands.
UHF is widely used in two-way radio systems and cordless telephones. 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. A number of public safety and business communications are handled on UHF. Civilian applications such as GMRS, PMR446, UHF CB, and 802.11b ("WiFi") are popular uses of UHF frequencies. A repeater is used to propagate UHF signals when a distance that is greater than the line-of-sight is required.
On December 29, 1949, KC2XAK of Bridgeport, Connecticut, became the first UHF television station to operate on a regular daily schedule. The first commercially licensed UHF television station on the air was KPTV/Channel 27 (now VHF Channel 12) in Portland, Oregon, on September 18, 1952. The station even used much of the equipment, including the transmitter, from KC2XAK, which was delivered by high-speed freight train.
Hopes that UHF would allow dozens of television stations in every media market were thwarted by poor image frequency rejection in superheterodyne tuners with 45.75 MHz Intermediate frequency. This shortcoming led to "UHF taboos" which limited each area to only modestly more UHF than VHF stations, despite the much larger number of channels. See 
In the United States, UHF stations (broadcast channels above 13) originally gained a reputation for being locally owned, less polished and professional, not as popular, and having weaker signal propagation than their VHF counterparts (channels 2 through 13). The movie UHF, starring "Weird Al" Yankovic and Michael Richards (of Seinfeld fame), parodied the phenomenon. In the late 1940s/early 1950s, affiliates of the four major US television networks (NBC, CBS, ABC and DuMont) transmitted their programs primarily through VHF. Most UHF stations that operated in major population centers of the USA were unable to obtain network affiliation, thus were independent stations. ABC and Dumont, being later and (at that time) less prosperous networks, did have a number of UHF affiliates, and the later PBS had even more. Conversely, many mid-sized and small television markets within the United States were too close to the outer fringe of the broadcast range of VHF stations from major population centers to qualify for their own VHF license from the FCC, because of interference from would-be overlapping broadcast ranges. Instead, the FCC granted many mid-sized and small-sized cities only UHF licenses (even for the big three networks), because the broadcast range of UHF is shorter, so fewer overlaps would occur between the mid-size market's UHF stations and the rather far-away big-city UHF stations.
A minority of UHF TV transmitters have no programming or commercial identity of their own, and merely retransmit the signal of a VHF station that owns it, to a smaller area poorly covered by the VHF signal. Such transmitters are called "translators" rather than “stations”.
The distinction between UHF and VHF characteristics has declined in importance with the emergence of additional broadcast television networks (Fox, The CW, The Tube, MyNetworkTV, and ION), and the decline of direct OTA reception. The concentration of media ownership, the proliferation of cable television, digital television and DBS have contributed to the quality equalization of VHF and UHF broadcasts. Most HDTV stations broadcast over-the-air signals using UHF bands.
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 (legal) unlicensed activity (cordless phones, wireless networking) clustered around 900 MHz and 2.4 GHz frequencies.
In February 2009, as part of the transition from analog to digital over-the-air broadcast of television, the spectrum from 698MHz to 806MHz (UHF channels 52-69) will no longer be used for TV broadcasting. The FCC has scheduled Auction 73 for some of the newly-available spectrum to be held in January 2008.  Google and other companies are bidding, probably to use UHF to carry wireless internet signals.
In Britain, UHF television began with the launch of BBC2 in 1964. BBC1 and ITV soon added their own services on UHF (British channels 21 to 69) on November 15 1969, and PAL colour was introduced on UHF only in 1967 (for BBC2) & 1969 (for BBC1 & ITV). VHF only carried BBC1 & ITV and was only ever in 405 line Black & White (though early on, there were plans to add colour to the existing 405-line services). VHF television was closed in January 1985. Today all British terrestrial (non-satellite) television channels (analogue and digital) are on UHF. A drawback to this is the very large number of small relay transmitters needed to fill in gaps in the main transmitters' coverage, which would not have been necessary with a VHF system due to its different propagation characteristics.
In Australia, UHF was first anticipated in the mid 1970s with channels 28 to 69. The first UHF TV broadcasts in Australia were operated by Special Broadcasting Service (SBS) on channel 28 in Sydney and Melbourne starting in 1980, and translator stations for the Australian Broadcasting Corporation (ABC). The UHF band is now used extensively as ABC, SBS, commercial and community (public access) television services have expanded particularly through regional areas.
Australia also provides the UHF CB service for general-purpose two-way communications.
In the Republic of Ireland, UHF was introduced in 1978 to augment the existing RTÉ One VHF 625-line transmissions and to provide extra frequencies for the new RTÉ Two channel. The first UHF transmitter site was Cairn Hill in Co. Longford, followed by Three Rock Mountain in South Co. Dublin. These sites were followed by Clermont Carn in Co. Louth and Holywell Hill in Co. Donegal in 1981. Elsewhere in Ireland, both the RTÉ channels are available on VHF. Since then RTÉ have migrated nearly all their low-power relay sites to UHF. TV3 and TG4 are transmitted entirely in UHF only. When Digital Terrestrial TV is introduced, it is intended to broadcast this on UHF only. VHF TV is likely to cease whenever the existing analogue broadcasts are switched off. The UHF band is also used in parts of Ireland for Television deflector systems bringing British television signals to towns and rural areas which cannot receive these signals directly
UHF broadcasting was used outside Kuala Lumpur and the Klang Valley by private TV station TV3 in the late 80s, with the government stations only transmitting in VHF (Bands 1 and 3) and the 450 MHz range being occupied by the ATUR cellular phone service operated by Telekom Malaysia. The ATUR service ceased operation in the late 90s, freeing up the frequency for other uses. UHF was not commonly used in the Klang Valley until 1994 (despite TV3's signal also being available over UHF Channel 29, as TV3 transmitted over VHF Channel 12 in the Klang Valley). 1994 saw the introduction of the channel MetroVision (which ceased transmission in 1999, got bought over by TV3's parent company - System Televisyen Malaysia Berhad - and relaunched as 8TV in 2004). This was followed by Ntv7 in 1998 (also acquired by TV3's parent company in 2005) and recently Channel 9 (which started in 2003, ceased transmission in 2005, was also acquired by TV3's parent company shortly after, and came back as TV9 in early 2006). At current count, there are 4 distinct UHF signals receivable by an analog TV set in the Klang Valley: Channel 25 (8TV), Channel 29 (TV3 UHF transmission), Channel 37 (NTV7) and Channel 39 (TV9). Channel 35 is usually allocated for VCRs, decoder units (i.e. the ASTRO and MiTV set top boxes) and other devices that have an RF signal generator (i.e. game consoles).
A brief summary of some UHF frequency usage:
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Ultra_high_frequency". A list of authors is available in Wikipedia.|