Ultra high frequency

Ultra high frequency (UHF) designates a range (band) of electromagnetic waves whose frequency is between 300 MHz and 3 GHz. 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.

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. Television broadcasting is common on UHF. 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 has been proposed for Wireless energy transfer, some pilot experiments have been performed but is is not used on a large scale.

Characteristics and advantages

Main article: radio propagation

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 than with lower 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-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.

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 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 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, the four major US TV networks (NBC, CBS, ABC and DuMont) transmitted their programs primarily through VHF. The independent stations primarily transmitted using UHF. ABC and Dumont did have a number of UHF affiliates.

The distinction between UHF and VHF characteristics has disappeared with the emergence of additional broadcast television networks (Fox, UPN, WB and ION Television). 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. Many 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 unlicensed activity (cordless phones, wireless networking) clustered around 900 MHz and 2.4 GHz frequencies.

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 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).

Frequency allocation

United States

A brief summary of some UHF frequency usage:

• 300–420 MHz: government use, including meteorology and Military/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 Use.
• 470–512 MHz: TV channels 14–20, public safety
• 512–698 MHz: TV channels 21–51 (channel 34 used for radar^{[citation needed]}, channel 37 used for radio astronomy)
• 698–806 MHz: TV channels 52–69 (to be auctioned for other uses once conversion to digital TV has been accomplished)
• 806–824 MHz: pocket pagers and Nextel SMR band (formerly TV channels 70–72)
• 824–849 MHz: Cellular phones, A & B franchises, 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: cellular phones, A & B franchises, base station (formerly TV channels 80–83)
• 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, 929/931MHz 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
• 2310–2360 MHz: Satellite radio (Sirius and XM)
• 2390–2450 MHz: Amateur radio (ham - 13 cm band)
• 2400–2483.5 MHz: ISM, IEEE 802.11, 802.11b, 802.11g Wireless LAN, IEEE 802.15.4
• around 2450 MHz: Microwave oven

United Kingdom

• 380–395 MHz: Terrestrial Trunked Radio (TETRA) service for emergency use
• 430–440 MHz: Amateur radio (ham - 70 cm band)
• 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)

Australia

• UHF Citizens Band: 476.425 - 477.400 MHz Refer: http://www.manilla2007.com/Default.aspx?tabid=145

See also

• Channel 37
• Television channel frequencies

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