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| ultra high frequency (UHF) |
|---|
| Cycles per second: 300MHz to 3000MHz | Wavelength: 1m to 100mm
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
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 through VHF and the independent stations through UHF.
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 467MHz areas of the UHF spectrum; there is also a great amount of unlicensed activity (cordless phones, wireless networking, and the like) clustered around 900MHz and 2.4GHz.
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), and PAL colour was introduced on UHF only in 1967–1969. VHF was phased out by 1986. 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.
Frequency Allocation - United States
A brief summary of some UHF frequency usage:
- 300–420 MHz: government use, including meteorology
- 420–450 MHz: radiolocation and Amateur radio (ham - 70 cm band)
- 450–470 MHz: UHF business band, GMRS, and FRS 2-way "walkie-talkies"
- 470–512 MHz: TV channels 14–20, public safety
- 512–698 MHz: TV channels 21–51 (channel 34 used for radar, 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, RFID, datalinks, Amateur radio (33 cm band)
- 928–960 MHz: mixed Studio-Transmitter Links, mobile 2-way, other
- 1240–1300 MHz: Amateur radio (ham - 23 cm band)
- 1850–1910 MHz: PCS mobile phone—note below
- 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
- around 2450 MHz: Microwave oven
Frequency Allocation - United Kingdom
- 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 auctioned for other uses)
- 1240–1316 MHz: Amateur radio (ham - 23 cm band)
- 2310–2450 MHz: Amateur radio (ham - 13 cm band)
See also
External articles
- Tomislav Stimac, "Definition of frequency bands (VLF, ELF... etc.)". IK1QFK Home Page (vlf.it).
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