Ultra_high_frequency - Wikiparency

# Ultra high frequency **Ultra high frequency** (UHF) names the slice of the radio spectrum from 300 megahertz to 3 gigahertz whose short wavelength, one metre down to ten centimetres and hence its older label of the decimetre band, puts it firmly in the part of the spectrum where a signal goes essentially where its antenna points and no farther. [[Very_high_frequency|VHF]] lies just below it and [[Microwave]] just above, and neither boundary marks any real change in the underlying physics: the IEEE's own UHF radar band, for instance, stops at 1 GHz well short of the ITU's 3 GHz ceiling, while the L and S radar bands spill over the top of it.[^ieee521] An outdoor obstacle such as a hill or a large building is usually fatal to a UHF signal, yet the very same short wavelength slips through a window frame or the gaps in ordinary wall construction well enough to give workable reception deep inside a building. It is this combination of short outdoor range and decent indoor penetration that has made UHF the working frequency for broadcast television, mobile telephones, Wi-Fi and Bluetooth, satellite navigation, and the walkie-talkie and cordless-phone services that were never asked to travel far. A three.js sketch elsewhere on this site lights this 300 MHz-to-3 GHz band within the wider [[Radio_spectrum]], against the bands on either side of it. ## Propagation characteristics Like every band above the low VHF, UHF is governed by line-of-sight propagation rather than the sky-wave bending that carries [[Shortwave_radio|shortwave]] signals over the horizon: a UHF signal goes only where its antenna can "see," refracted slightly by the lower atmosphere and not usefully reflected by the [[Ionosphere]] at all. The reach of a UHF link is therefore mostly a matter of geometry. Allowing for the standard four-thirds-Earth-radius model of atmospheric refraction, a transmitting antenna of height h1 and a receiving antenna of height h2, both in metres, share a geometric radio horizon of roughly `d ≈ 4.12*(sqrt(h1) + sqrt(h2))` kilometres; a rooftop UHF television transmitter 300 m up and a rooftop receiving antenna 10 m up therefore share a horizon of about 84 km, well beyond what either antenna could "see" over a flat Earth with no refraction at all. Because its wavelength — ten centimetres to one metre — is short compared with a building, a hill or a vehicle, UHF casts comparatively sharp shadows and reflects readily off hard surfaces, so a receiver in a city rarely gets one clean copy of a signal; it gets several, arriving over slightly different paths and combining constructively or destructively as the receiver moves, an effect examined in general in [[Multipath_propagation]]. The same short wavelength cuts the other way indoors: a window, a doorway, and even the ordinary gaps in wall construction are large enough relative to the wave to let a useful fraction of the power diffract through, which is why a UHF cellphone or Wi-Fi signal reaches deep into a building that would block a broadcast band's much longer wave far more completely. Atmospheric gas absorption, the loss that eventually limits [[Microwave]] links at higher frequencies still, is negligible across the UHF band itself. ## Antennas A short wavelength is also what makes UHF antennas practical to carry. At 900 MHz, a frequency inside the band used by some mobile-phone and cordless-phone services, one wavelength is about 33 cm, so a quarter-wave [[Monopole_antenna|monopole]] need only be about 8 cm and a half-wave [[Dipole_antenna|dipole]] about 17 cm — short enough to fit inside a handset or ride on a car roof, where the multi-metre antennas lower bands need would be impossible. The same relation runs the other way for gain: because a useful UHF [[Antenna_(radio)|antenna]] can still be many wavelengths across without growing unreasonably large in absolute terms, it is practical to build directional arrays that lower bands could only match with towers hundreds of metres high. Fixed UHF reception, most visibly for broadcast television, typically uses a multi-element [[Yagi–Uda_antenna|Yagi–Uda antenna]] or a flat panel or bow-tie design, chosen for the gain and the forward-favouring [[Radiation_pattern|radiation pattern]] that let a distant transmitter be picked out of the multipath clutter described above. Mobile and handheld UHF equipment instead favours a simple monopole or a small helical or planar antenna, trading gain and directionality for size and a radiation pattern that stays usable in whatever direction the device happens to be held, since a handset's user cannot be expected to aim it the way a rooftop installer aims a television antenna. ## Applications UHF's applications share the band mainly because they share its propagation and antenna trade-offs, not because they share a common purpose. Broadcast television is the oldest large-scale use, occupying a wide contiguous slice of the band because a video signal needs far more bandwidth than an audio one. Mobile telephony uses UHF for almost the entire path between a handset and a cell site, in blocks assigned nationally rather than by international treaty, and the same neighbourhood of the band carries the short-range personal services that share it informally: Wi-Fi and Bluetooth both use an internationally set-aside slice near 2.4 GHz, and a great many walkie-talkies, cordless telephones and remote-control links occupy narrower channels scattered across the rest of the band under a lighter licensing regime than broadcasting requires. Satellite navigation is a UHF service by convention rather than obvious necessity: GPS and the other global navigation satellite systems broadcast down at frequencies between roughly 1.1 and 1.6 GHz, a compromise between an antenna small enough for a handset and an ionospheric delay small enough to correct for accurately. Satellite telephones occupy nearby UHF allocations for similar reasons, and space agencies including [[NASA]] use UHF for spacecraft telemetry and command in a mission's near-Earth phases, before handing off to higher microwave bands for deep-space links. The IEEE's UHF radar band (300 MHz–1 GHz) and the L and S radar bands that continue upward through 4 GHz put long-range air-surveillance and weather [[Radar|radar]] in the same neighbourhood,[^ieee521] which is why an air-traffic radar and a television transmitter can, in principle, sit uncomfortably close together on a frequency chart even though nothing else about their design has much in common. ## Examples of UHF frequency allocations Exactly which services occupy which part of the UHF band is set nationally, not by the ITU designation itself, and the six examples below show how differently that division can be drawn even among countries with a broadly similar mix of services. ### Australia Australia allocates 476–477 MHz to a licence-free citizens-band service used widely for short-range communication on farms, on four-wheel-drive tracks and at worksites, and reserves most of the remaining UHF band below 700 MHz for digital television and land-mobile radio.[^auuhf] ### Canada Canadian UHF allocations largely mirror those of the United States, a coordination made necessary by a border that radio waves cross without noticing; Canadian and American regulators jointly manage interference in the UHF television and land-mobile bands along its length.[^cauhf] ### France French UHF spectrum below about 700 MHz carries digital terrestrial television, while a licence-free short-range service near 446 MHz, shared with the rest of the European Union, plays a role similar to Australia's UHF citizens band for personal and business use.[^fruhf] ### New Zealand New Zealand licenses much of its UHF band to digital television and to land-mobile and trunked radio services for utilities and emergency responders, with a smaller licence-exempt allocation set aside for short-range personal use.[^nzuhf] ### United Kingdom The United Kingdom broadcasts digital terrestrial television (Freeview) across a contiguous block of UHF channels below 700 MHz, and shares the European 446 MHz licence-free allocation for short-range personal and business radio.[^ukuhf] ### United States United States television broadcasting has occupied UHF spectrum since the Federal Communications Commission's 1952 order opened channels 14 through 83; the top of that range was reassigned to cellular telephony in the 1980s, and the digital television transition together with a later spectrum incentive auction narrowed broadcast television to the lower channels of the band, freeing the rest for mobile broadband and public-safety use.[^usuhf] Family and general mobile radio services share a separate, lightly licensed UHF allocation near 462–467 MHz for the walkie-talkies described above.[^usuhf] ## Microsims This article carries no p5.js sketch of its own. A three.js sketch elsewhere on this site lights the 300 MHz-to-3 GHz UHF band within the wider [[Radio_spectrum]], the same spectrum-wide scene used by several related articles, rendered here with the UHF slice picked out against its neighbours. The interactive sketches that go with the specific ideas raised above belong to neighbouring articles, and this section points to three of them. *Try:* in the [[Radar]] sketch, drag the pulse-repetition frequency down until the dashed unambiguous-range ring shrinks inside a distant target and a folded "ghost" echo appears at a shorter apparent range — many long-range air-surveillance sets run at the top of the UHF band or just above it, in the overlapping L band. *Try:* in the [[Doppler_effect]] sketch, set the source moving and watch the wavefronts bunch up ahead of it and spread out behind it; the same shift in frequency is what a UHF cellular network measures on a moving phone, and what a UHF air-surveillance radar measures on a moving aircraft. *Try:* in the [[Sonar]] sketch, compare how a two-way delay converts to a range reading with how a UHF link would perform the same trick: sound covers a few metres of water in the time a UHF wave, moving at the speed of light, covers tens of kilometres of open air. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Ultra_high_frequency) : [Wikitube](https://en.wikitube.io/wiki/Ultra_high_frequency) Skeleton mirrored at revision 1362305742. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Radio_spectrum]] - [[Very_high_frequency]] - [[Microwave]] - [[Line-of-sight_propagation]] - [[Multipath_propagation]] - [[Antenna_(radio)]] ## References Standard propagation and antenna theory used here — line-of-sight geometry, the radio-horizon approximation, and quarter- and half-wave antenna sizing — is textbook material used throughout radio engineering and is not separately footnoted, per the Wikitube style guide's §6.1. Page numbers below are PDF pages of the open editions linked from External links and this portal's book shelf. [^ieee521]: Citation needed: the current edition of IEEE Std 521, which defines the L, S and other letter-designated radar bands, would confirm the exact boundaries quoted here. [^auuhf]: Citation needed: the Australian Communications and Media Authority's current UHF citizen-band and broadcasting-services band plans would confirm the exact frequencies quoted here. [^cauhf]: Citation needed: Innovation, Science and Economic Development Canada's UHF television and land-mobile allocation tables, and the Canada–United States coordination agreements they rest on, would confirm the details quoted here. [^fruhf]: Citation needed: ARCEP's digital terrestrial television band plan and the European PMR446 allocation decision would confirm the exact frequencies quoted here. [^nzuhf]: Citation needed: Radio Spectrum Management New Zealand's UHF band plan would confirm the exact frequencies and services quoted here. [^ukuhf]: Citation needed: Ofcom's UHF television and PMR446 allocation tables would confirm the exact frequencies quoted here. [^usuhf]: Citation needed: the FCC's 1952 Sixth Report and Order, its 1980s UHF-to-cellular reallocation, and its 2017 broadcast incentive auction order would together confirm the dates and channel numbers quoted here. ## External links - Authoritative UHF band-plan references (for example the ITU Radio Regulations and the national regulators cited above), to be pinned once specific editions are confirmed.