Very high frequency


Very high frequency

Very high frequency (VHF) is the radio frequency range from 30 MHz to 300 MHz. Frequencies immediately below VHF are denoted High frequency (HF), and the next higher frequencies are known as Ultra high frequency (UHF). The frequency allocation is done by ITU. Common uses for VHF are FM radio broadcast, television broadcast, land mobile stations (emergency, business, and military), Amateur Radio, marine communications, air traffic control communications and air navigation systems (e.g. VOR, DME & ILS).

Propagation characteristics

VHF propagation characteristics are ideal 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 radio and thus transmissions are restricted to the local area (and don't interfere with transmissions thousands of kilometres away). VHF is also less affected by atmospheric noise and interference from electrical equipment than lower frequencies. Whilst it is more easily blocked by land features than HF and lower frequencies, it is less bothered by buildings and other less substantial objects than UHF frequencies.

Two unusual propagation conditions can allow much farther range than normal. The first, tropospheric ducting, can occur in front of and parallel to an advancing cold weather front, especially if there is a marked difference in humidities between the cold and warm air masses. A duct can form approximately 250 km (155 mi) in advance of the cold front, much like a ventilation duct in a building, and VHF radio frequencies can travel along inside the duct, bending or refracting, for hundreds of kilometers. For example, a 50 watt Amateur FM transmitter at 146 MHz can talk from Chicago, to Joplin, Missouri, directly, and to Austin, Texas, through a repeater. In a July 2006 incident, a NOAA Weather Radio transmitter in north central Wisconsin was blocking out local transmitters in west central Michigan, quite far out of its normal range. The second type, much more rare, is called Sporadic E, referring to the E-layer of the ionosphere. A sunspot eruption can pelt the Earth's upper atmosphere with charged particles, which may allow the formation of an ionized "patch" dense enough to reflect back VHF frequencies the same way HF frequencies are usually reflected (skywave). For example, KMID (TV Channel 2; 54–60 MHz) from Midland, Texas was seen around Chicago, pushing out Chicago's WBBM-TV. fact|date=August 2007 These patches may last for seconds, or extend into hours. FM stations from Miami, Florida; New Orleans, Louisiana; Houston, Texas and even Mexico were heard for hours in central Illinois during one such event. Mid summer 2006 central Iowa stations were heard in Columbus, NE and blocking out Omaha radio and TV stations for several days. Similar propagation effects can affect land-mobile stations in this band, rarely causing intereference well beyond the usual coverage area.

VHF Line of Sight Calculation

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:
*distance in miles = sqrt{2 imes A_f} where A_f is the height of the antenna in feet
*distance in kilometres = sqrt{17 imes A_m} where A_m is the height of the antenna in metres

These approximations are only valid for antennas at heights that are small compared to the radius of the Earth.

In engineered communications systems, more complex calculations are required to assess the probable coverage area of a proposed transmitter station.

By country

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.

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 required adjustment to enable tuning to the new channels.

Several TV stations were allocated to VHF channels 3, 4 and 5A, which were within the FM radio bands although not yet used for that purpose. A couple of notable examples were NBN Newcastle, WIN-4 Wollongong and ABC Illawarra on channel 5A. 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. 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 (eg. ABC in capital cities) but also for some new analogue services in regional areas.

New Zealand

*44–51, 54–68 MHz: Band I Television (channels 1–3)
*87.5–108 MHz: Band II Radio
*174–230 MHz: Band III Television (channels 4–11)

In New Zealand, the four main Free-to-Air TV stations still use the VHF Television bands (Band I and Band III) to transmit their programmes to New Zealand households. Other stations, including a variety of pay and regional free-to-air stations, broadcast their programmes in the UHF band, since the VHF band is very overloaded with four stations sharing a very small frequency band. In some areas, the band is so overcrowded, that the fourth television channel is not available.

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.

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:
*30–46 MHz: Licensed 2-way land mobile communication. [ The 42 MHz Segment is still currently used by the California Highway Patrol, New Jersey State Police, Tennessee Highway Patrol and other state law enforcement agencies. ]
*30–88 MHz: Military VHF-FM, including SINCGARS
*43–50 MHz: Cordless telephones, 49 MHz FM walkie-talkies and radio controlled toys, and mixed 2-way mobile communication. The FM broadcast band originally operated here (42-50 MHz) before moving to 88-108 MHz.
*50–54 MHz: Amateur radio 6 meter band; 50 MHz is an amateur radio band used for a variety of uses including DXing, FM repeaters and radio control
*55-72 and 77-88 MHz TV channels 2 through 6, known as "Band I" internationally; a tiny number of HDTV stations will appear here. See North American broadcast television frequencies
*72–76 MHz: Radio controlled models, industrial remote control, and other devices. Model aircraft operate on 72 MHz while surface models operate on 75 MHz, air navigation beacons 74.8-75.2 MHz.
*88–108 MHz: FM radio broadcasting (88–92 non-commercial, 92–108 commercial in the United States) (Known as "Band II" internationally)
*108–118 MHz: Air navigation beacons VOR
*118–137 MHz: Airband for air traffic control, AM, 121.5 MHz is emergency frequency
*137-138 Space research,space operations, meteorological satellite [ Industry Canada, "Canadian Table of Frequency Allocations 9 kHz - 275 GHz, 2005 Edition (revised February 2007)" pg. 29 ]
*138–144 MHz: Land mobile, auxiliary civil services, satellite, space research, and other miscellaneous services
*144–148 MHz: Amateur radio band 2 Meters
*148-150 Land mobile, fixed, satellite
*150–156 MHz: "VHF Business band," the unlicensed Multi-Use Radio Service (MURS), and other 2-way land mobile, FM
*156–158 MHz VHF Marine Radio; narrow band FM, 156.8 MHz (Channel 16) is the maritime emergency and contact frequency.
* 160-161 MHz Railways [ The 160 and 161 areas are AAR 99 channel railroad radios issued to the railroad (Sample, AAR 21 is 160.425 and that is issued to TVRM and other railroads that want AAR 21)]
*162.40–162.55: NOAA Weather Stations, narrowband FM
*175-216 MHz television channels 7 - 13, known as "Band III" internationally. A minority of HDTV channels may appear here.
*174–216 MHz: professional wireless microphones (low power, certain exact frequencies only)
*216–222 MHz: land mobile,fixed, maritime mobile , [ Canadian table pg. 30 ]
*222–225 MHz: 1.25 meters (US) ( Canada 219-220, 222-225 MHz) Amateur radio
*225 MHz and above: Military aircraft radio (225–400 MHz) AM, including HAVE QUICK, dGPS RTCM-104

The large technically and commercially valuable slice of the VHF spectrum taken up by television broadcasting has attracted the attention of many companies and governments recently, with the development of more efficient digital television broadcasting standards. In some countries much of this spectrum will likely become available (probably for sale) in the next decade or so (currently scheduled for 2009 in the United States).

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 audio for TV channel 6 is broadcast at 87.75 MHz.

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. [http://www.ofcom.org.uk/media/news/2006/11/nr_20061123b]

ee also

*Marine VHF radio
*Oldest radio station
*Apex (radio band)
*FM broadcast band
*Moving image formats
*Television channel frequencies
*Knife-edge effect

References


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Look at other dictionaries:

  • very high frequency — n. any radio frequency between 30 and 300 megahertz …   English World dictionary

  • Very High Frequency — Very High Frequency,   VHF …   Universal-Lexikon

  • very high frequency — noun 30 to 300 megahertz • Syn: ↑VHF • Hypernyms: ↑radio frequency • Part Holonyms: ↑electromagnetic spectrum * * * ˌvery high ˈfrequency [very high frequency] …   Useful english dictionary

  • very-high-frequency — adjective : of or relating to very high frequency or to a radio wave having such a frequency …   Useful english dictionary

  • very high frequency — labai aukštas dažnis statusas T sritis automatika atitikmenys: angl. very high frequency vok. Höchstfrequenz, f; Ultrakurzwellenfrequenz, f rus. очень высокая частота, f pranc. fréquence très haute, f …   Automatikos terminų žodynas

  • very high frequency — noun Date: 1920 a radio frequency between ultrahigh frequency and high frequency see radio frequency table …   New Collegiate Dictionary

  • very high frequency — The range of radio frequencies between 30 and 300 MHz. Portions of this band, 108 to 118 MHz, are used for certain NAVAIDs (navigational aids); 118 to 136 MHz are used for civil air to ground voice communications. Other frequencies in this band… …   Aviation dictionary

  • very high frequency — ver′y high fre′quency n. rtv any frequency between 30 and 300 megahertz Abbr.: VHF • Etymology: 1920–25 …   From formal English to slang

  • very high frequency — any frequency between 30 and 300 megahertz. Abbr.: VHF * * * …   Universalium

  • very high frequency — noun The frequency area from 30 MHz through 300 MHz, commonly used for radio and TV broadcasting. See Also: VHF …   Wiktionary


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