Science Fair Project Encyclopedia
Ultra high frequency
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 microwave or higher bands, while lower frequency signals fall into the VHF or lower bands. See electromagnetic spectrum for a full listing of frequency bands.
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; q.v. digital audio broadcasting for details.
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, convering 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.
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 a weaker signal than their VHF counterparts (channels 2–13). The movie UHF, starring Weird Al Yankovic, parodied this phenomenon.
However, with the emergence of additional broadcast television networks (Fox, UPN, WB, and Pax), the concentration of media ownership, and the proliferation of cable television, digital television, and DSS in the 1990s, the distinction between UHF and VHF stations has virtually disappeared in the United States. Most high-definition TV 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.
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 (analog 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.
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 "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 - 806 MHz: TV channels 21-69 (channels 53-69 to be auctioned)
- 806 - 824 MHz: pocket pagers and Nextel SMR band
- 824 - 849 MHz: Cellular phones, A & B franchises, mobile phone
- 849 - 869 MHz: public safety 2-way (fire, police, ambulance)
- 869 - 894 MHz: cellular phones, A & B franchises, base station
- 902 - 928 MHz: ISM band: cordless phones and stereo, datalinks, Amateur radio
- 928 - 960 MHz: mixed Studio-Transmitter Links, mobile 2-way, other
- 1240 - 1300 MHz: Amateur radio
- 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, shared with below:
- 2400 - 2483.5 MHz: ISM, IEEE 802.11, 802.11b, 802.11g Wireless LAN
- around 2450 MHz: Microwave oven
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