Frequency range | 30MHz to 300MHz |
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Wavelength range | 10 to 1 m |
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Very high frequency (VHF) is theITU designation[1][2][3] for the range ofradio frequencyelectromagnetic waves (radio waves) from 30 to 300megahertz (MHz), with corresponding wavelengths of ten meters to one meter. Frequencies immediately below VHF are denotedhigh frequency (HF), and the next higher frequencies are known asultra high frequency (UHF).
VHF radio waves propagate mainly byline-of-sight, so they are blocked by hills and mountains, although due to refraction they can travel somewhat beyond thevisual horizon out to about 160 km (100 miles). Common uses for radio waves in the VHF band areDigital Audio Broadcasting (DAB) andFM radio broadcasting,television broadcasting,two-wayland mobile radio systems (emergency, business, private use and military), long rangedata communication up to several tens of kilometers withradio modems,amateur radio, andmarine communications.Air traffic control communications and air navigation systems (e.g.VOR andILS) work at distances of 100 kilometres (62 miles) or more to aircraft at cruising altitude.
In the Americas and many other parts of the world, VHFBand I was used for the transmission ofanalog television. As part of the worldwidetransition to digital terrestrial television most countries require broadcasters to air television in the VHF range using digital, rather than analog encoding.
Radio waves in the VHF band propagate mainly byline-of-sight and ground-bounce paths; unlike in theHF band there is only some reflection at lower frequencies from theionosphere (skywave propagation).[4] They do not follow the contour of the Earth asground waves and so are blocked by hills and mountains, although because they are weakly refracted (bent) by the atmosphere they can travel somewhat beyond thevisual horizon out to about 160 km (100 miles). They can penetrate building walls and be received indoors, although in urban areas reflections from buildings causemultipath propagation, which can interfere with television reception. Atmosphericradio noise and interference (RFI) from electrical equipment is less of a problem in this and higher frequency bands than at lower frequencies. The VHF band is the first band at which efficient transmitting antennas are small enough that they can be mounted on vehicles and portable devices, so the band is used fortwo-wayland mobile radio systems, such aswalkie-talkies, andtwo way radio communication with aircraft (Airband) and ships (marine radio). Occasionally, when conditions are right, VHF waves can travel long distances bytropospheric ducting due to refraction by temperature gradients in the atmosphere.
VHF transmission range is a function of transmitter power, receiver sensitivity, and distance to the horizon, since VHF signals propagate under normal conditions as a nearline-of-sight phenomenon. The distance to theradio horizon is slightly extended over the geometric line of sight to the horizon, as radio waves are weakly bent back toward the Earth by the atmosphere.
An approximation to calculate the line-of-sight horizon distance (on Earth) is:
These approximations are only valid for antennas at heights that are small compared to the radius of the Earth. They may not necessarily be accurate in mountainous areas, since the landscape may not be transparent enough for radio waves.
In engineered communications systems, more complex calculations are required to assess the probable coverage area of a proposed transmitter station.[7]
VHF is the first band at which wavelengths are small enough that efficient transmitting antennas are short enough to mount on vehicles and handheld devices, aquarter wave whip antenna at VHF frequencies is 25 cm to 2.5 meter (10 inches to 8 feet) long. So the VHF and UHF wavelengths are used fortwo-way radios in vehicles, aircraft, and handheldtransceivers andwalkie-talkies. Portable radios usually usewhips orrubber ducky antennas, while base stations usually use larger fiberglass whips orcollinear arrays of vertical dipoles.
For directional antennas, theYagi antenna is the most widely used as ahigh gain or "beam" antenna. For television reception, the Yagi is used, as well as thelog-periodic antenna due to its wider bandwidth.Helical andturnstile antennas are used forsatellite communication since they employcircular polarization. For even higher gain, multiple Yagis or helicals can be mounted together to makearray antennas. Verticalcollinear arrays of dipoles can be used to make high gainomnidirectional antennas, in which more of the antenna's power is radiated in horizontal directions. Television and FM broadcasting stations use collinear arrays of specialized dipole antennas such asbatwing antennas.
Certain subparts of the VHF band have the same use around the world. Some national uses are detailed below.
The VHF TV band in Australia was originally allocated channels 1 to 10-with channels 2, 7 and 9 assigned for the initial services inSydney andMelbourne, and later the same channels were assigned inBrisbane,Adelaide andPerth. Other capital cities and regional areas used a combination of these and other frequencies as available. The initial commercial services inHobart andDarwin were respectively allocated channels 6 and 8 rather than 7 or 9.
By the early 1960s it became apparent that the 10 VHF channels were insufficient to support the growth of television services. This was rectified by the addition of three additional frequencies-channels 0, 5A and 11. Older television sets using rotary dial tuners required adjustment to receive these new channels. Most TVs of that era were not equipped to receive these broadcasts, and so were modified at the owners' expense to be able to tune into these bands; otherwise the owner had to buy a new TV.
Several TV stations were allocated to VHF channels 3, 4 and 5, which were within the FM radio bands although not yet used for that purpose. A couple of notable examples wereNBN-3Newcastle,WIN-4Wollongong andABCNewcastle on channel 5. While some Channel 5 stations were moved to 5A in the 1970s and 80s, beginning in the 1990s, theAustralian Broadcasting Authority began a process to move these stations toUHF bands to free up valuable VHF spectrum for its original purpose of FM radio. In addition, by 1985 the federal government decided new TV stations are to be broadcast on the UHF band.
Two new VHF channels, 9A and 12, have since been made available and are being used primarily for digital services (e.g.ABC in capital cities) but also for some new analogue services in regional areas. Because channel 9A is not used for television services in or near Sydney, Melbourne, Brisbane, Adelaide or Perth,digital radio in those cities are broadcast on DAB frequencies blocks 9A, 9B and 9C.
VHF radio is also used for marine Radio[8] as per its long-distance reachability comparing UHF frequencies.
Example allocation of VHF–UHF frequencies:[9]
Until 2013, the four main free-to-air TV stations inNew Zealand used the VHF television bands (Band I andBand III) to transmit to New Zealand households. Other stations, including a variety of pay and regional free-to-air stations, were forced to broadcast in theUHF band, since the VHF band had been very overloaded with four stations sharing a very small frequency band, which was so overcrowded that one or more channels would not be available in some smaller towns.
However,at the end of 2013, all television channels stopped broadcasting on the VHF bands, as New Zealand moved to digital television broadcasting, requiring all stations to either broadcast on UHF or satellite (where UHF was unavailable) utilising theFreeview service.[10]
Refer toAustralasian television frequencies for more information.
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British television originally used VHFband I andband III. Television on VHF was in black and white with405-line format (although there were experiments with all three colour systems-NTSC,PAL, andSECAM-adapted for the 405-line system in the late 1950s and early 1960s).
British colour television was broadcast onUHF (channels 21–69), beginning in the late 1960s. From then on, TV was broadcast on both VHF and UHF (VHF being a monochromatic downconversion from the 625-line colour signal), with the exception ofBBC2 (which had always broadcast solely on UHF). The last British VHF TV transmitters closed down on January 3, 1985. VHFband III is now used in the UK fordigital audio broadcasting, and VHFband II is used forFM radio, as it is in most of the world.
Unusually, the UK has anamateur radio allocation at4 metres, 70–70.5 MHz.
Frequency assignments between US and Canadian users are closely coordinated since much of the Canadian population is within VHF radio range of the US border. Certain discrete frequencies are reserved forradio astronomy.The general services in the VHF band are:
Cable television, though not transmitted aerially, uses a spectrum of frequencies overlapping VHF.[13]
The U.S.FCC allocated television broadcasting to a channelized roster as early as 1938 with 19 channels. That changed three more times: in 1940 when Channel 19 was deleted and several channels changed frequencies, then in 1946 with television going from 18 channels to 13 channels, again with different frequencies, and finally in 1948 with the removal ofChannel 1 (analog channels 2–13 remain as they were, even oncable television).[14] Channels 14–19 later appeared on the UHF band, while channel 1 remains unused.
87.5–87.9 MHz is a radio band which, in most of the world, is used forFM broadcasting. InNorth America, however, this bandwidth is allocated to VHF television channel 6 (82–88 MHz). The analog audio for TV channel 6 is broadcast at 87.75 MHz (adjustable down to 87.74). Several stations, known asFrankenstations, most notably those joining thePulse 87 franchise, have operated on this frequency as radio stations, though they use television licenses. As a result, FM radio receivers such as those found in automobiles which are designed to tune into this frequency range could receive the audio for analog-mode programming on the local TV channel 6 while in North America. The practice largely ended with theDTV transition in 2009, although some still exist.
The FM broadcast channel at 87.9 MHz is normally off-limits for FM audio broadcasting; it is reserved for displaced class D stations which have no other frequencies in the normal 88.1–107.9 MHz subband to move to. So far, only two stations have qualified to operate on 87.9 MHz: 10–wattKSFH inMountain View, California and 34–watt translatorK200AA inSun Valley, Nevada.
In some countries, particularly the United States and Canada, limited low-power license-free operation is available in the FM broadcast band for purposes such asmicro-broadcasting and sending output fromCD or digital media players to radios without auxiliary-in jacks, though this is illegal in some other countries. This practice was legalised in the United Kingdom on 8 December 2006.[15]
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