Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter (one decimeter). Radio waves with frequencies above the UHF band fall into the super-high frequency (SHF) 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, satellite phones, and numerous other applications. 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; unlike in the HF band there is little to no reflection from the ionosphere (skywave propagation), 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, 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. As the visual horizon sets the maximum range of UHF transmission to between 30 and 40 miles (48 to 64 km) or less, depending on local terrain, the same frequency channels can be reused by other users in neighboring geographic areas (frequency reuse). 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.
Antennas
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, the most common omnidirectional antenna 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 (PIFA) 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 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 reflective array antenna are used: the slotted cylinder, zig-zag, and panel antennas.
Applications UHF television broadcasting channels are used for digital television, although much of the former bandwidth has been reallocated to land mobile radio system, trunked radio and mobile telephone use. 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. Examples of personal radio services are GMRS, PMR446, and UHF CB. The most rapidly-expanding use of the band is Wi-Fi (wireless LAN) networks in homes, offices, and public places. Wi-Fi IEEE 802.11 low band operates between 2412 and 2484 MHz. A second widespread use is for cellphones, allowing handheld mobile phones 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.
Examples of UHF frequency allocations
Australia 406–406.1 MHz: Mobile satellite service 450.4875–451.5125 MHz: Fixed point-to-point link 457.50625–459.9875 MHz: Land mobile service 476–477 MHz: UHF citizens band (land mobile service) 503–694 MHz: UHF channels for television broadcasting
Canada 430–450 MHz: Amateur radio (70 cm band) 470–806 MHz: Terrestrial television (with select channels in the 600 and 700 MHz bands left vacant) 1452–1492 MHz: Digital Audio Broadcasting (L band) Many other frequency assignments for Canada and Mexico are similar to their US counterparts
France 380-400 MHz: Terrestrial Trunked Radio for Police 430-440 MHz: Amateur radio (70 cm band) 446.0–446.2 MHz : European unlicensed PMR service, PMR446 470-694 MHz: Terrestrial television
New Zealand 406.1–420 MHz: Land mobile service 430–440 MHz: Amateur radio (70 cm band) and amateur radio satellite 476–477 MHz: PRS Personal Radio Service (Land mobile service) 485–502 MHz: Analog and P25 emergency services use 510–622 MHz: Terrestrial television 960–1215 MHz: Aeronautical radionavigation 1240–1300 MHz: Amateur radio (23 cm band)
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