Frequency range 300 MHz to 3 GHz Frequency range 300 MHz to 1 GHz | Wavelength range 1 m to 1 dm Wavelength range 1 m to 1 dm | |
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Related bands Related bands | ||
Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 MHz and 3 GHz, also known as the decimetre band as the wavelengths range from one meter to one decimetre. Radio waves with frequencies above the UHF band fall into the SHF (super-high frequency) or microwave frequency range. Lower frequency signals fall into the VHF (very high frequency) or lower bands. UHF radio waves propagate mainly by line of sight; they are blocked by hills and large buildings although the transmission through building walls is strong enough for indoor reception. They are used for television broadcasting, cell phones, satellite communication including GPS, personal radio services including Wi-Fi and Bluetooth, walkie-talkies, cordless phones, and numerous other applications.
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The IEEE defines the UHF radar band as frequencies between 300 MHz and 1 GHz. Two other IEEE radar bands overlap the ITU UHF band: the L band between 1 and 2 GHz and the S band between 2 and 4 GHz.
Propagation characteristics
Radio waves in the UHF band travel almost entirely by line-of-sight propagation (LOS) and ground reflection; there is very little reflection from the ionosphere (skywave propagation), or ground wave. They are blocked by hills and cannot travel far beyond the horizon, but can penetrate foliage and buildings for indoor reception. Since the wavelengths of UHF waves are comparable to the size of buildings, trees, vehicles and other common objects, reflection and diffraction from these objects can cause multipath propagation, especially in built-up urban areas. Atmospheric moisture reduces, or attenuates, the strength of UHF signals over long distances, and the attenuation increases with frequency. UHF TV signals are generally more degraded by moisture than lower bands, such as VHF TV signals.
Occasionally when conditions are right, UHF radio waves can travel long distances by tropospheric ducting as the atmosphere warms and cools throughout the day.
The length of an antenna is related to the length of the radio waves used. The UHF antenna is stubby and short; at UHF frequencies a quarter-wave monopole, the most common omnidirectional antenna is between 2.5 and 25 cm long for example. The short wavelengths make the VHF and UHF bands the first bands at which practical transmitting antennas are small enough to mount on handheld devices, thus these frequencies are used for portable wireless devices. UHF is widely used in cordless telephones, cell phones, walkie-talkies and other two-way radio systems from short range up to the visual horizon, about 40 miles (64 km). Their transmissions do not travel far, allowing frequency reuse. Public safety, business communications and personal radio services such as GMRS, PMR446, and UHF CB are often found on UHF frequencies as well as IEEE 802.11 wireless LANs ("Wi-Fi"). The widely adapted GSM and UMTS cellular networks use UHF cellular frequencies. Radio repeaters are used to retransmit UHF signals when a distance greater than the line of sight is required.
Antennas
UHF wavelengths are short enough that efficient transmitting antennas are small enough to mount on handheld and mobile devices, so these frequencies are used for two way land mobile radio systems, such as walkie-talkies, two way radios in vehicles, cordless phones, and cell phones. Omnidirectional UHF antennas used on mobile devices are usually short whips, sleeve dipoles or rubber ducky antennas. Higher gain omnidirectional UHF antennas can be made of collinear arrays of dipoles and are used for mobile base stations and cellular base station antennas.
The short wavelengths also allow high gain antennas to be conveniently small. High gain antennas for point-to-point communication links and UHF television reception are usually Yagi, log periodic, corner reflectors, or reflective array antennas. At the top end of the band slot antennas and parabolic dishes become practical. For satellite communication, helical, and turnstile antennas are used since satellites typically employ circular polarization which is not sensitive to the relative orientation of the transmitting and receiving antennas. For television broadcasting specialized vertical radiators that are mostly modifications of the slot antenna or helical antenna are used: the slotted cylinder, zig-zag, and panel antennas.
Applications
UHF television broadcasting fulfilled the demand for additional over-the-air television channels in urban areas. Today, much of the bandwidth has been reallocated to land mobile, trunked radio and mobile telephone use. UHF channels are still used for digital television.
UHF spectrum is used worldwide for land mobile radio systems for commercial, industrial, public safety, and military purposes. Many personal radio services use frequencies allocated in the UHF band, although exact frequencies in use differ significantly between countries.
Major telecommunications providers have deployed voice and data cellular networks in UHF/VHF range. This allows mobile phones and mobile computing devices to be connected to the public switched telephone network and public Internet.
UHF radars are said to be effective at tracking stealth fighters, if not stealth bombers.
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UHF channels are used for digital television broadcasting on both over the air channels and cable television channels. Since 1962, UHF channel tuners (at the time, channels 14-83) have been required in television receivers by the All-Channel Receiver Act. However, because of their more limited range, and because few sets could receive them until older sets were replaced, UHF channels were less desirable to broadcasters than VHF channels (and licenses sold for lower prices).
A complete list of US Television Frequency allocations can be found at North American Television Frequencies.
There is a considerable amount of lawful unlicensed activity (cordless phones, wireless networking) clustered around 900 MHz and 2.4 GHz, regulated under Title 47 CFR Part 15. These ISM bands – frequencies with a higher unlicensed power permitted for use originally by Industrial, Scientific, Medical apparatus – are now some of the most crowded in the spectrum because they are open to everyone. The 2.45 GHz frequency is the standard for use by microwave ovens, adjacent to the frequencies allocated for Bluetooth network devices.
The spectrum from 806 MHz to 890 MHz (UHF channels 70–83) was taken away from TV broadcast services in 1983, primarily for analog mobile telephony.
In 2009, as part of the transition from analog to digital over-the-air broadcast of television, the spectrum from 698 MHz to 806 MHz (UHF channels 52–69) was removed from TV broadcasting, making it available for other uses. Channel 55, for instance, was sold to Qualcomm for their MediaFLO service, which is resold under various mobile telephone network brands. Some US broadcasters had been offered incentives to vacate this channel early, permitting its immediate mobile use. The FCC's scheduled auction for this newly available spectrum was completed in March 2008.
The FCC has allowed Americans to connect any device and any application to the 22 MHz of radio spectrum that people are calling the 700 MHz band. The FCC did not include a wholesale condition, which would have required the owner of the band to resell bandwidth to third parties who could then service the end user. Google argued that the wholesale requirement would have stimulated internet competition. As of 2007, 96% of the country's broadband access was controlled by DSL and cable providers. A wholesale condition could have meant a third option for internet service.