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Ultra high frequency

"UHF" redirects here. For other uses see UHF (disambiguation).
ultra high frequency (UHF)
Cycles per second: 300 MHz to 3000 MHz

Wavelength: 1 m to 100 mm

Ultra high frequency (UHF) designates a range (band) of electromagnetic waves whose frequency is between 300 MHz and 3.0 GHz. 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.


Contents

Uses

UHF and VHF are the most common frequency bands for television. Modern mobile phones also transmit and receive within the UHF spectrum, and UHF is widely used for two-way radio communication (usually using narrowband frequency modulation, but digital services are on the rise) by both public service agencies and the general public. Though television broadcasting is common on UHF, there has traditionally been very little radio broadcasting in this band until fairly recently; see digital audio broadcasting for details.

One uncommon use of UHF waves is in the detection of partial discharges. These discharges occur due to the sharp geometries created in high voltage insulated equipment. The advantage is that this method can be used to localize the source of the discharge, but it is extremely sensitive to external noise. Nonetheless, such detection methods are used in the field especially for large distribution transformers.

Characteristics and advantages

Main article: radio propagation

The transmission of radio waves from one point to another is affected by many variables such as atmospheric moisture, the stream of particles from the sun called solar wind, and time of day. All radio waves are somewhat absorbed by atmospheric moisture. This reduces, or attenuates, the strength of radio signals over long distances. However, this effect increases according to the frequency: UHF signals are generally more degraded by moisture than lower bands such as VHF. As well, the layer of the Earth's atmosphere called the ionosphere is filled with charged particles that can reflect radio waves. This can be helpful in transmitting a radio signal, since the wave bounces from the sky to the ground over and over, covering long distances. However, UHF benefits less from this effect than lower (VHF, etc.) frequencies. As the atmosphere warms and cools throughout the day, UHF transmissions may be enhanced by tropospheric ducting.

The main advantage of UHF transmission is that its high frequency means it has a physically short wave. Since the size of transmission and reception equipment (particularly antennas) is related to the size of the wave, smaller, less conspicuous antennas can be used than with VHF or lower bands.

UHF is also widely used in two-way radio systems and cordless phones due to the fact that since UHF signals essentially travel over line-of-sight distances, distant transmissions cannot travel far enough to interfere with local transmissions. A great number of public safety and business communications are handled on UHF, and civilian applications such as GMRS, PMR446, and UHF CB are extremely popular. Where communications greater than line-of-sight are required, a repeater is used to propagate signals that otherwise would not reach their destinations.

History

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 true commercially licensed UHF television station on the air was KPTV/Channel 27 (now VHF Channel 12) in Portland, Oregon on September 18, 1952.

In the United States, UHF stations (broadcast channels above 13) originally gained a reputation for being more locally owned, less polished, less professional, less popular, and for having weaker signal propagation than their VHF counterparts (channels 2–13). The movie UHF, starring "Weird Al" Yankovic and Michael Richards (of Seinfeld fame), parodied this phenomenon. In the late 1940s/early 1950s, the four major TV networks (NBC, CBS, ABC and DuMont) transmitted their programs mainly through VHF and the independent stations mainly through UHF (Although ABC and particularly Dumont had a large number of UHF affiliates).

However, with the emergence of additional broadcast television networks (Fox, UPN, WB, and i), the concentration of media ownership, and the proliferation of cable television, digital television, and DBS in the 1990s, the distinction between UHF and VHF stations has virtually disappeared in the United States. Most HDTV stations broadcast their over-the-air signal 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 also a great amount of unlicensed activity (cordless phones, wireless networking, and the like) clustered around 800 MHz and 2.4 GHz.

United Kingdom

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 February 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.

Australia

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.

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 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.

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 VLF and VHF and the 450MHz 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).

Frequency allocation

United States

A brief summary of some UHF frequency usage:

United Kingdom

Australia

See also

Radio spectrum
ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF
3 Hz 30 Hz 300 Hz 3 kHz 30 kHz 300 kHz 3 MHz 30 MHz 300 MHz 3 GHz 30 GHz
30 Hz 300 Hz 3 kHz 30 kHz 300 kHz 3 MHz 30 MHz 300 MHz 3 GHz 30 GHz 300 GHz


The Electromagnetic Spectrum
(Sorted by wavelength, short to long)
Gamma ray | X-ray | Ultraviolet | Visible spectrum | Infrared | Terahertz radiation | Microwave | Radio waves
Visible (optical) spectrum: Violet | Blue | Green | Yellow | Orange | Red
Microwave spectrum: W band | V band | K band: Ka band, Ku band | X band | C band | S band | L band
Radio spectrum: EHF | SHF | UHF | VHF | HF | MF | LF | VLF | ULF | SLF | ELF
Wavelength designations: Microwave | Shortwave | Mediumwave | Longwave


Categories


Articles with unsourced statements | Radio spectrum

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