(Redirected from UHF)
| Ultra High Frequency (UHF) |
|---|
Cycles per second: 300 MHz to 3 GHz
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
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. 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
microwave ovens, has been proposed for
Wireless energy transfer, some pilot experiments have been performed but it is not used on a large scale.
Characteristics and advantages
Main articles: 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. 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 (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.
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 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.
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 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
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).
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 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)
★ 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)
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
External links
★ Tomislav Stimac, "''
Definition of frequency bands (VLF, ELF... etc.)''". IK1QFK Home Page (vlf.it).
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