Very_high_frequency - Wikiparency

# Very high frequency **Very high frequency** (VHF) is the ITU-designated band of radio waves running from 30 to 300 megahertz, with wavelengths from ten metres down to one metre, sitting between high frequency (HF) below it and [[Ultra_high_frequency|ultra high frequency]] (UHF) above, itself one rung on the ladder the [[Radio_spectrum|radio spectrum]] article lays out in full. A wave in this band propagates mainly by line of sight: short enough that it no longer diffracts usefully around hills the way a much longer high-frequency wave does, yet long enough that the atmosphere barely absorbs it, so the practical limit on range is mostly geometry rather than loss. That single fact shapes everything else here, from the antennas VHF equipment carries to the reach of an FM station or an aircraft's navigation receiver. A three.js companion sketch, already built for the aviation portal's own treatment of VHF navigation and communication, is reused here unchanged. The band is put to work almost everywhere: FM and digital radio, television, two-way land-mobile radio, marine and aviation communication and navigation, and amateur radio all share it, allocated in slightly different ways from one country to the next. ## Propagation characteristics A radio wave's ability to bend around an obstacle depends on how the obstacle's size compares with the wave's own wavelength, and at VHF that comparison usually goes against the wave: a ten-metre or one-metre wavelength diffracts around a building or a low hill only weakly, nothing like the much longer wavelengths that let [[Shortwave_radio|shortwave]] broadcasting diffract around terrain and hop off the ionosphere instead. The practical result is [[Line-of-sight_propagation|line-of-sight propagation]]: a VHF signal reaches a receiver mainly along a fairly direct path, unlike the surface-hugging [[Ground_wave|ground wave]] that carries much lower frequencies, and a hill or a building squarely in that direct path blocks it much as it would block visible light. The atmosphere does the wave one favour that pure geometry does not: its refractive index falls slightly with altitude, bending the wave's path very gently downward, in a weaker version of the same bending [[Snell's_law|Snell's law]] describes at a sharp interface, and stretching the usable range somewhat beyond the geometric horizon. Occasionally the lower end of the band reflects off a sporadic, patchy layer of the [[Ionosphere|ionosphere]] instead, carrying a signal hundreds of kilometres by a brief, unpredictable [[Skywave|skywave]] hop that has nothing to do with the band's normal behaviour and cannot be relied on for a working link. Man-made background noise also falls away through this band faster than the wave itself does: one engineering model of urban interference puts it at roughly 1,120,000 kelvin at 30 MHz in a city centre against about 3,960 kelvin in a quiet rural area at the same frequency, a gap that narrows well before VHF gives way to UHF and one reason VHF broadcasting and mobile radio stay usable in cities where a lower frequency would struggle.[^noise30] ## Line-of-sight calculation Because propagation is dominated by geometry, the range of a VHF link can be estimated from nothing more than the height of the two antennas above the ground. Treating the Earth as a smooth sphere of radius 6,371 kilometres, an antenna of height h metres sees a pure geometric horizon at a distance of about `d = 3.57·√h` kilometres. Atmospheric refraction extends that reach: radio engineers commonly fold the bending described above into the geometry itself by pretending the Earth's radius is four-thirds its real value, which raises the constant to `d = 4.12·√h`, sometimes called the radio horizon, or [[Radar_horizon|radar horizon]] in a radar context, to distinguish it from the purely optical one. For a link between two raised points, an antenna and a receiver, the two horizon distances simply add: `d_total = 4.12·(√h1 + √h2)`. A worked example shows how far this reaches in practice: an aircraft cruising at 10,700 metres and a ground station's antenna at 3 metres give a radio horizon of roughly 4.12 times the sum of the square roots of those two heights, or about 433 kilometres, well beyond the roughly 100 kilometres or so that a link between two low, ground-based antennas usually manages, which is exactly why aviation communication and navigation, discussed below, gets so much more range out of the same band than a car radio does. ## Antennas VHF's wavelengths, from one to ten metres, sit in the range where a resonant antenna is neither so long that it becomes awkward, as at high frequency, nor so short that its precise placement barely matters, as at microwave frequencies; a half-wave [[Dipole_antenna|dipole]] is a manageable half a metre to five metres long, and a quarter-wave [[Monopole_antenna|monopole]] whip about half that, both practical to mount on a vehicle roof, a mast or a handheld radio. Where a signal needs to be sent or received from mostly one direction rather than all around, a [[Yagi–Uda_antenna|Yagi–Uda antenna]] strings several such elements in a line to concentrate the [[Radiation_pattern|radiation pattern]] into a narrower beam, the standard rooftop television aerial shape for decades; where many antennas work together instead, an [[Antenna_array|antenna array]] can steer or shape that pattern electrically rather than by pointing the whole structure at all. The antenna's practical, human-scaled size at this band, more than any other single factor, is why VHF equipment ranges so easily from a handheld radio to a broadcast tower without changing the basic antenna design very much. ## Universal use Some VHF uses are close to universal, appearing in some form in nearly every country with a radio regulator at all. [[Frequency_modulation|FM]] and digital radio broadcasting occupy the same general part of the band nearly everywhere, even where the exact edges differ slightly by region.[^fmband] Marine VHF reserves a single internationally agreed channel, channel 16, for distress, safety and calling, monitored by ships and coast stations regardless of nationality so that a vessel in trouble can be heard by whoever is listening nearby.[^ch16] Aviation leans on the band for both voice communication with air traffic control and for navigation aids such as VOR, which gives a pilot a bearing rather than a message, both operating in adjacent slices of VHF chosen for the long, reliable line-of-sight range the calculation above explains. Amateur radio operators worldwide share at least a "two-metre" band around 144 to 148 megahertz, one of the most consistently allocated amateur bands on Earth, used for everything from local repeater conversations to bouncing a signal off the Moon. ## By country National regulators fit their own broadcasting, land-mobile and amateur allocations into the internationally agreed VHF band in ways that differ in detail even when the underlying physics does not. ### Australia Australian FM broadcasting occupies the upper part of the VHF band, and VHF television, once the country's only television band, has been substantially reallocated as broadcasters moved to digital transmission; the exact channel plan and the timetable of that transition are not sourced here.[^auvhf] ### New Zealand New Zealand's VHF allocations follow a broadly similar pattern to Australia's, with FM broadcasting in the same general part of the band and a television band likewise reshaped by the digital transition; specific channel numbers and dates are not sourced here.[^nzvhf] ### United Kingdom United Kingdom FM broadcasting occupies Band II, the same 87.5-to-108-megahertz range used across most of Europe, while the country's original television service, broadcast in VHF's Band I and Band III on a 405-line standard, was switched off in favour of UHF broadcasting decades before the eventual move to digital; Band III carries the VHF leg of digital audio broadcasting today rather than television.[^uktv] ### United States and Canada United States and Canadian FM broadcasting runs from 88 to 108 megahertz, immediately above the television band that shares the lower part of VHF with it. #### VHF television Analog television in the United States and Canada once occupied two separate ranges within VHF, channels 2 through 6 at the low end of the band and channels 7 through 13 above the FM band, with UHF channels carrying the rest; the digital television transition led many stations to move to UHF for better building penetration, though some broadcasters kept or returned to a VHF channel number even where the actual digital carrier frequency differs from the old analog assignment.[^vhftv] #### 87.5–87.9 MHz A narrow strip just below the official 88-megahertz start of the FM band, corresponding to the audio carrier of the old analog television channel 6, has historically let a small number of low-power stations in the United States be received on an ordinary FM radio tuned just below its normal range, a quirk of the two services' adjacent frequency assignments rather than a deliberate allocation to broadcasting.[^lpfm] ## Unlicensed operation Not every use of VHF requires an individual licence. In the United States, the Multi-Use Radio Service sets aside a handful of channels around 151 to 154 megahertz for short-range, license-free two-way radios at modest power, intended for the same kind of business and personal use that citizens-band radio serves at high frequency.[^murs] Listening, as opposed to transmitting, is unlicensed almost everywhere in any case: a marine VHF receiver monitoring channel 16, or a scanner tuned to a VHF broadcast or public-safety frequency, needs no licence at all in most jurisdictions, since it puts nothing back onto the air. ## Microsims This article carries no p5.js sketch of its own. Its three.js companion is not new: it is the sketch already built for the aviation portal's treatment of VHF navigation and communication, reused here unchanged rather than rebuilt as a variant. The interactive sketches that go with the ranging and propagation ideas raised above belong to neighbouring articles, and this section points to three of them. *Try:* in the [[Radar]] sketch, note how the antenna beam sweeps in a straight line at every azimuth; the same geometric line-of-sight limit this article calculates for a VHF link is what keeps a ground radar's own coverage bounded by the horizon, not merely by its transmitter power. *Try:* in the [[Doppler_effect]] sketch, set the source moving toward the observer and read the frequency it reports; a VHF signal from a moving vehicle or aircraft shifts by the same mechanism, small enough at these frequencies to be a minor correction rather than the dominant effect it becomes higher up the spectrum. *Try:* in the [[Sonar]] sketch, compare its underwater range, limited by absorption over distance, with a VHF link's range, limited instead by the horizon calculated above; the two systems fail for opposite reasons even though both are, at bottom, a timed echo or a received wave. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Very_high_frequency) : [Wikitube](https://en.wikitube.io/wiki/Very_high_frequency) Skeleton mirrored at revision 1360694137. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Line-of-sight_propagation]] - [[Radio_spectrum]] - [[Ultra_high_frequency]] - [[Shortwave_radio]] - [[Ionosphere]] - [[Yagi–Uda_antenna]] ## Notes Country-specific channel numbers, transition dates and station examples in By country are stated only in general terms; where a precise figure could not be confirmed against a current regulatory source for this pass, the claim is footnoted as citation needed rather than given an invented number. ## References [^noise30]: Ellingson, S. *Radio Systems Engineering - Revised First Edition*. 2023, pp. 106-107 (PDF pages). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. [^fmband]: Citation needed: a current comparative source for FM broadcast band edges by ITU region and country. [^ch16]: Citation needed: the current international regulation (for example an ITU or IMO instrument) fixing marine VHF channel 16 (156.8 MHz) as the common distress and calling channel, with edition and year. [^auvhf]: Citation needed: a current Australian regulatory source (ACMA) for the VHF FM and television band plan and the digital-transition timetable. [^nzvhf]: Citation needed: a current New Zealand regulatory source for the VHF FM and television band plan and the digital-transition timetable. [^uktv]: Citation needed: a primary UK broadcasting-history source confirming the 405-line VHF television service's band allocation and shutdown date. [^vhftv]: Citation needed: a current FCC source for United States VHF television channel assignments and the digital-transition channel changes described. [^lpfm]: Citation needed: a current FCC rule citation (for example Part 15 or the LPFM rules) for operation near 87.5-87.9 MHz, and named examples of stations using it. [^murs]: Citation needed: the current FCC Part 95 rule citation for Multi-Use Radio Service frequencies and power limits.