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    Very high frequency

    وی اِچ اِف


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    (Wikipedia) - Very high frequency   (Redirected from VHF) "VHF" redirects here. For other uses, see VHF (disambiguation).
    Very high frequency Frequency range Wavelength range
    30 to 300 MHz
    10 to 1 m
    ITU Radio Band Numbers ITU Radio Band Symbols NATO Radio bands IEEE Radar bands Television and radio bands

    1 2 3 4 5 6 7 8 9 10 11 12

    ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF THF

    A B C D E F G H I J K L M

    HF VHF UHF L S C X Ku K Ka V W mm

    I II III IV V VI

    v d e
    VHF television antennas used for broadcast television reception. These six antennas are a type known as a Yagi antenna, which is widely used at VHF frequencies

    Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves from 30 MHz to 300 MHz, with corresponding wavelengths of ten to one meters. Frequencies immediately below VHF are denoted high frequency (HF), and the next higher frequencies are known as ultra high frequency (UHF).

    Common uses for VHF are FM radio broadcasting, television broadcasting, land mobile stations (emergency, business, private use and military), long range data communication up to several tens of kilometres with radio modems, amateur radio, and marine communications. Air traffic control communications and air navigation systems (e.g. VOR, DME & ILS) work at distances of 100 kilometres or more to aircraft at cruising altitude.

    VHF was previously used for analog television stations in the US, and continues to be used, but not as much due to signal problems, for digital television.

    Contents

    Propagation characteristics

    VHF propagation characteristics are suited for short-distance terrestrial communication, with a range generally somewhat farther than line-of-sight from the transmitter (see formula below). Unlike high frequencies (HF), the ionosphere does not usually reflect VHF waves (called skywave propagation) so transmissions are restricted to the local radio horizon less than 100 miles. VHF is also less affected by atmospheric noise and interference from electrical equipment than lower frequencies. While it is blocked by land features such as hills and mountains, it is less affected by buildings and can be received indoors, although multipath television reception due to reflection from buildings can be a problem in urban areas.

    Line-of-sight calculation

    For analog TV, 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 near line-of-sight phenomenon. The distance to the radio 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.

    The accuracy of these calculations for digital TV signals is being debated.

    Antennas

    VHF is the first band at which wavelengths are small enough to make efficient transmitting antennas for vehicles and handheld devices, so the VHF and UHF wavelengths are used for two way radios in vehicles, aircraft, and handheld transceivers and walkie talkies. Portable radios usually use whips or rubber ducky antennas, while base stations usually use larger fiberglass whips or collinear arrays of vertical dipoles.

    For directional antennas, the Yagi antenna is the most widely used as a high gain or "beam" antenna. For television reception, the Yagi is used, as well as the log periodic antenna due to its wider bandwidth. Helical and turnstile antennas are used for satellite communication since they employ circular polarization. For even higher gain, multiple Yagis or helicals can be mounted together as an array antennas. Vertical collinear arrays of dipoles can be used to make high gain omnidirectional 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 as batwing antennas.

    Universal use

    Certain subparts of the VHF band have the same use around the world. Some national uses are detailed below.

    By countryA plan showing VHF use in television, FM radio, amateur radio, marine radio and aviation.Australia

    The VHF TV band in Australia was originally allocated channels 1 to 10 - with channels 2, 7 and 9 assigned for the initial services in Sydney and Melbourne, and later the same channels were assigned in Brisbane, Adelaide and Perth. Other capital cities and regional areas used a combination of these and other frequencies as available. The initial commercial services in Hobart and Darwin 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 were NBN-3 Newcastle, WIN-4 Wollongong and ABC Newcastle on channel 5. While some Channel 5 stations were moved to 5A in the 1970s and 80s, beginning in the 1990s, the Australian Broadcasting Authority began a process to move these stations to UHF 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 frequencies, 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.

    Refer to Australasian television frequencies for more information.

    New Zealand

    In New Zealand, the four main Free-to-Air TV stations still use the VHF Television bands (Band I and Band III) to transmit to New Zealand households. Other stations, including a variety of pay and regional free-to-air stations, are forced to broadcast in the UHF band, since the VHF band is very overloaded with four stations sharing a very small frequency band, which can be so overcrowded that one or more channels, more often than not one of the MediaWorks-owned channels (TV3 and FOUR), is unavailable in some smaller towns.

    However, by the end of 2013, there will be no television channels broadcasting on the VHF bands.

    Refer to Australasian television frequencies for more information.

    United Kingdom

    British television originally used VHF band I and band III. Television on VHF was in black and white with 405-line format (although there were experiments with all three colour systems—NTSC, PAL, and SECAM—adapted for the 405-line system in the late 1950s and early 60s).

    British colour television was broadcast on UHF (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 of BBC2 (which had always broadcast solely on UHF). The last British VHF TV transmitters closed down on January 3, 1985. VHF band III is now used in the UK for digital audio broadcasting, and VHF band II is used for FM radio, as it is in most of the world.

    Unusually, the UK has an amateur radio allocation at 4 metres, 70-70.5 MHz.

    United States and Canada

    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 for radio astronomy. The general services in the VHF band are:

    VHF television

    The U.S.''s FCC allocated television broadcasting to a channelized roster as early as 1938 with 19 channels. That changed 3 more times: in 1940 when Channel 19 was deleted and several channels changed frequencies, then in 1946 with television going from 18 to 13 channels with different frequencies again, and finally in 1948 with the removal of Channel 1 (channels 2-13 remain as they are today).

    The move from VHF for analog television to UHF for digital television has made the large technically and commercially valuable slice of the UHF spectrum formerly taken up by television broadcasting available for relicensing or sale after a transition period, which ended June 12, 2009 in the United States; see Effect on existing analog technology for dates in other countries.

    87.5-87.9 MHz

    87.5-87.9 MHz is a radio frequency which, in most of the world, is used for FM broadcasting. In North 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, most notably those joining the Pulse 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.

    87.9 MHz is normally off-limits for FM audio broadcasting except for displaced class D stations which have no other frequencies in the normal 88.1-107.9 MHz subband on which to move. So far, only 2 stations have qualified to operate on 87.9 MHz: 10-watt KSFH in Mountain View, California and 34-watt translator K200AA in Sun Valley, Nevada.

    Unlicensed operation

    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 as micro-broadcasting and sending output from CD 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.

    Tags:ABC, Adelaide, American, Australia, Australian, Brisbane, British, California, Canada, Canadian, Earth, Landing, Melbourne, NATO, New Zealand, Newcastle, North America, Radio, Sun, Sydney, UHF, UK, US, USA, United Kingdom, United States, Universal, VHF, Wikipedia


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