Ultra high frequency
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Ultra high frequency (UHF) designates a range (band) of electromagnetic waves with frequencies 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 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 for a full listing of frequency bands.
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[edit] Uses
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 because of 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 WiFi, Bluetooth and US cordless phones 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.
Some radio frequency identification (RFID) tags utilize UHF. These tags are commonly known as UHFID's or Ultra-HighFID's (Ultra-High Frequency Identification).
[edit] 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.
- See: Radio horizon
[edit] History
[edit] Australia
In Australia, UHF was first anticipated in the mid 1970s with TV 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.
[edit] Ireland
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 initially, although VHF allocations exist. 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
[edit] Malaysia
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).
[edit] United Kingdom
In the UK, UHF television began in 1964 following a plan by the GPO to allocate sets of frequencies for 625-lined television to regions across the country, so as to accommodate four national networks with regional variations (the VHF allocations only allowed for only two such networks using 405 lines). The UK UHF channels would range from 21 to 68 (later extended to 69) and regional allocations were generally grouped close together to allow for the use of aerials designed to receive a specific sub-band with greater efficiency than wider-band aerials could. Aerial manufacturers would therefore divide the band into over-lapping groups; A (channels 21-34), B (39-53), C/D (48-68) and E (39-68). The first service to use UHF was BBC2 in 1964 followed by BBC1 and ITV (already broadcast on VHF) in 1969 and Channel 4/S4C in 1982. PAL colour was introduced on UHF only in 1967 (for BBC2) and 1969 (for BBC1 & ITV).
As a consequence of achieving maximum national coverage, signals from one region would typically over-lap with that of another, which was accommodated for by allocating a different set of channels in each adjacent area, often resulting in greater choice for viewers when a network in one region aired different programmes to the neighbouring region.
Initial uptake of UHF television was very slow: Differing propagation characteristics between VHF and UHF meant new additional transmitters needed to be built, often at different locations to the then-established VHF sites, and generally with a larger number of relay stations to fill the greater number of gaps in coverage that came with the new band. This led to poor picture quality in bad coverage areas, and many years before the service achieved full national coverage. In addition to this, the only exclusively UHF service, BBC2, would run for only a few hours a day and comprised of alternative programming for minority audiences in contrast to the more popularist schedules of BBC1 and ITV. However the 1970s saw a large increase in UHF TV viewing, and VHF take a significant decline: The appeal of colour, which was never introduced to VHF (despite preliminary plans to do so in the late 1950s and early 1960s) and the fall in television prices saw most households use a UHF set by the end of that decade. With the second and last VHF television service having launched in 1955, VHF TV was finally decommissioned for good in 1985 with no plans for it to return to use.
The launch of Channel 5 in 1997 added a fifth national television network to UHF, requiring deviation from the original frequency allocation plan of the early 1960s and the allocation of UHF frequencies previously not used for television (such as UK Channels 35 and 37 which was previously reserved for domestic VCRs requiring an expensive VCR re-tuning programme funded by the new network). A lack of capacity within the band to accommodate a fifth service with the complex over-lapping led to the fifth and final network having a significantly reduced national coverage compared to the other networks, with reduced picture quality in many areas and the use of wide-band aerials often required.
The launch of digital terrestrial television in 1998 saw the continued use of UHF for television, with six multiplexes allocated for the service, all within the UHF band. However analogue transmissions have been planned to cease completely by 2012 after which time it is uncertain as to whether the vacated capacity will be used for additional digital television services or put into alternative use, such as mobile telecommunications or internet services.
[edit] United States
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 [1]
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, 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 and thus were independent stations. ABC and DuMont, being smaller and 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 licenses from the FCC, because of interference from would-be overlapping broadcast ranges. Instead, the FCC granted many mid- and small-sized cities only UHF licenses (even for the big three networks), because of the vastly greater number of available channels (56 total UHF channels vs. only 12 total VHF).
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, MyNetworkTV, Univision, Telemundo 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 in the UHF band.
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.
In February 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) will no longer be used for TV broadcasting. The FCC's scheduled auction for this newly-available spectrum was completed in March 2008. [2]
[edit] Frequency allocation
[edit] Australia
- UHF Citizens Band: 476.425–477.400 MHz
[edit] United Kingdom
- 380–395 MHz: Terrestrial Trunked Radio (TETRA) service for emergency use
- 430–440 MHz: Amateur radio (ham - 70 cm band)
- 606–614 MHz: Protected for radio-astronomy
- 470–862 MHz: TV channels 21–69 (channel 36 used for radar, channel 38 used for radio astronomy, channel 69 used for licenced and licence exempt wireless microphones, channels 31-40 and 63-68 to be released and may be made available for other uses by Ofcom. Public consultation due December 2006)
- 1240–1316 MHz: Amateur radio (ham - 23 cm band)
- 1880–1900 MHz: DECT Cordless telephone
- 2310–2450 MHz: Amateur radio (ham - 13 cm band)
[edit] United States
A brief summary of some UHF frequency use:
- 300–420 MHz: Government use, including meteorology and federal two-way use
- 420–450 MHz: Government radiolocation and amateur radio (ham - 70 cm band)
- 450–470 MHz: UHF business band, General Mobile Radio Service, and Family Radio Service 2-way "walkie-talkies", public safety
- 470–512 MHz: TV channels 14–20
- 512–698 MHz: TV channels 21–51 (channel 34 used sometimes for radar, channel 37 used for radio astronomy)
- 698–806 MHz: TV channels 52–69 (was auctioned in March 2008; bidders will get full use once conversion to digital TV has been accomplished which is scheduled for February 2009)
- 806–824 MHz: Pagers (formerly TV channels 70–72)
- 824–849 MHz: AMPS A & B franchises, terminal (mobile phone) (formerly TV channels 73–77)
- 849–869 MHz: Public safety 2-way (fire, police, ambulance — formerly TV channels 77–80)
- 869–894 MHz: AMPS A & B franchises, base station (formerly TV channels 80–83) (one translator is still in use on channel 83)
- 902–928 MHz: ISM band: cordless phones and stereo, radio frequency identification, datalinks, amateur radio (33 cm band)
- 928–960 MHz: Mixed studio-transmitter links, mobile 2-way, paging, other.
- 1240–1300 MHz: Amateur radio (ham - 23 cm band)
- 1850–1910 MHz: PCS mobile phone—note below
- 1920–1930 MHz: DECT Cordless telephone
- 1930–1990 MHz: PCS base stations—note below
- note: order is A, D, B, E, F, C blocks. A, B, C = 15 MHz; D, E, F = 5 MHz
- 2300–2310 MHz: Amateur radio (ham - 13 cm band, lower segment)
- 2310–2360 MHz: Satellite radio (Sirius and XM)
- 2390–2450 MHz: Amateur radio (ham - 13 cm band, upper segment)
- 2400–2483.5 MHz: ISM, IEEE 802.11, 802.11b, 802.11g Wireless LAN, IEEE 802.15.4, Bluetooth
- around 2450 MHz: Microwave oven
[edit] See also
- Broadcast television systems
- Moving image formats
- Oldest television station
- Television channel frequencies
- Very High Frequency
- U.S. cable television channel frequencies
- TVTower.com - Commercial Television Frequencies
[edit] External links
- Tomislav Stimac, "Definition of frequency bands (VLF, ELF... etc.)". IK1QFK Home Page (vlf.it).
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