Ultra high frequency
UHF band spans 300 MHz to 3 GHz, enabling line-of-sight communications.
Ultra high frequency (UHF) is the ITU term for radio frequencies from 300 megahertz (MHz) up to 3 gigahertz (GHz). This range is also called the decimetre band because its wavelengths span one meter down to one tenth of a meter (one decimeter). Frequencies above UHF fall into the super-high frequency (SHF) or microwave range; those below belong to the VHF band or lower.
UHF radio waves travel mostly by line of sight and ground reflection. Unlike HF waves, they barely reflect off the ionosphere and have almost no ground wave propagation. Hills and large buildings block them, but they can penetrate walls well enough for indoor reception. Because UHF wavelengths are similar in size to buildings, trees, and vehicles, reflections and diffraction from these objects cause multipath fading, especially in cities. Atmospheric moisture weakens UHF signals over long distances, and this weakening grows with frequency—so UHF TV signals suffer more from moisture than VHF TV signals do.
The visual horizon limits typical UHF transmission range to 30 to 40 miles (48 to 64 km) or less, depending on terrain. This allows the same frequency channels to be reused in neighboring areas. When longer distances are needed, radio repeaters retransmit the signals. Occasionally, tropospheric ducting—caused by daily warming and cooling of the atmosphere—lets UHF waves travel far beyond the horizon.
UHF antennas are short because the wavelengths are short. A common quarter-wave monopole, for example, measures between 2.5 and 25 cm long. This compactness makes efficient transmitting antennas small enough for handheld and mobile devices, so UHF is widely used for two-way land mobile radios (walkie-talkies, vehicle radios), cordless phones, and cell phones. Omnidirectional antennas on mobile devices are often short whips, sleeve dipoles, rubber ducky antennas, or planar inverted F antennas (PIFAs). Higher-gain omnidirectional antennas use collinear arrays of dipoles and serve mobile and cellular base stations.
Short wavelengths also allow high-gain antennas to be conveniently small. Point-to-point links and UHF TV reception often use Yagi, log periodic, corner reflector, or reflective array antennas. At the top of the band, slot antennas and parabolic dishes become practical.
- Frequency range
- 300 MHz to 3 GHz
- Wavelength range
- 1 meter to 0.1 meter
- Typical line of sight range
- 30 to 40 miles (48 to 64 km)
- Common antenna length quarter wave monop
- 2.5 to 25 cm
- Ieee uhf radar band
- 300 MHz to 1 GHz
- Overlapping ieee bands
- L band (1–2 GHz) and S band (2–4 GHz)
Lore & Background
Radio waves in the UHF band travel almost entirely by line-of-sight propagation and ground reflection; unlike in the HF band there is little to no reflection from the ionosphere or ground wave. UHF radio waves are blocked by hills and cannot travel 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 fading due to multipath propagation, especially in built-up urban areas. Atmospheric moisture reduces the strength of UHF signals over long distances, and the attenuation increases with frequency. As the visual horizon sets the maximum range of UHF transmission to between 30 and 40 miles or less, the same frequency channels can be reused by other users in neighboring geographic areas. Radio repeaters are used to retransmit UHF signals when a distance greater than the line of sight is required. 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. Due to the short wavelengths, UHF antennas are conveniently stubby and short; at UHF frequencies a quarter-wave monopole is between 2.5 and 25 cm long. 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, and for portable wireless devices; cordless phones and cell phones. Omnidirectional UHF antennas used on mobile devices are usually short whips, sleeve dipoles, rubber ducky antennas or the planar inverted F antenna used in cellphones. 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. For television broadcasting specialized vertical radiators that are mostly modifications of the slot antenna or reflective array antenna are used: the slotted cylinder, zig-zag, and panel antennas.
Reader's Guide
The UHF band is notable for its extensive use in modern communications, particularly for television broadcasting, cell phones, satellite communication including GPS, and personal radio services including Wi-Fi and Bluetooth. Since at UHF frequencies transmitting antennas are small enough to install on portable devices, the UHF spectrum is used worldwide for land mobile radio systems, two-way radios used for voice communication for commercial, industrial, public safety, and military purposes. The most rapidly-expanding use of the band is Wi-Fi wireless LAN networks in homes, offices, and public places, with Wi-Fi IEEE 802.11 low band operating between 2412 and 2484 MHz. A second widespread use is for cellphones, allowing handheld mobile phones to be connected to the public switched telephone network and the Internet; current 3G and 4G cellular networks use UHF, the frequencies varying among different carriers and countries. Satellite phones also use this frequency in the L band and S band. The legacy of UHF includes its role in enabling digital television broadcasting, with much of the former bandwidth reallocated to land mobile radio system, trunked radio and mobile telephone use. In the United States, UHF channels have been used for digital television broadcasting on both over the air channels and cable television channels since 1962, when UHF channel tuners were 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. 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.
Did You Know?
- UHF is also known as the decimetre band because wavelengths range from one meter to one tenth of a meter.
- A quarter-wave monopole antenna at UHF frequencies is between 2.5 and 25 cm long.
- Wi-Fi IEEE 802.11 low band operates between 2412 and 2484 MHz in the UHF range.
More in Radio Spectrum 1-24
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