Very high frequency

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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).

Common uses for VHF are FM radio broadcast, television broadcast (together with UHF), Amateur Radio, marine communications, air traffic control communications and terrestrial navigation systems (VOR in particular).

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.[citation needed] 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.

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).

Contents

[edit] 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 line-of-sight phenomenon.

An approximation to calculate the line-of-sight horizon distance is:

  • distance in miles = \sqrt{1.5 \times A_f} where Af is the height of the antenna in feet
  • distance in kilometres = \sqrt{12.7 \times A_m} where Am is the height of the antenna in metres
  • See: Radio horizon

[edit] By country

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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. [1]
  • 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; 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, meterological satellite [2]
  • 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 [3]
  • 162.40–162.55: NOAA Weather Stations, narrowband FM
  • 175-216 MHz television channels 7 - 13, known as "Band III" internationally
  • 174–216 MHz: professional wireless microphones (low power, certain exact frequencies only)
  • 216–222 MHz: land mobile,fixed, maritime mobile ,ref> Canadian table pg. 30 </ref>
  • 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

[edit] 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. [1]

[edit] 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 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.

[edit] See also

v • d • e
Radio spectrum

ELF
3 Hz
30 Hz

SLF
30 Hz
300 Hz

ULF
300 Hz
3 kHz

VLF
3 kHz
30 kHz

LF
30 kHz
300 kHz

MF
300 kHz
3 MHz

HF
3 MHz
30 MHz

VHF
30 MHz
300 MHz

UHF
300 MHz
3 GHz

SHF
3 GHz
30 GHz

EHF
30 GHz
300 GHz
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