Very high frequency
Very high frequency (VHF) is the radio frequency range from 30 MHz (wavelength 10 m) to 300 MHz (wavelength 1 m). Frequencies immediately below VHF is HF, and the next higher frequencies are known as Ultra high frequency (UHF).
Common uses for VHF are FM radio broadcast at 88-108 MHz and television broadcast (together with UHF). VHF is also commonly used for terrestrial navigation systems (VOR in particular) and aircraft communications.
VHF frequencies' 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 low 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 higher 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 150 miles (240 km.) 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 miles. For example, a 50-watt Amateur FM transmitter at 146 MHz can talk from Chicago, Illinois to Joplin, Missouri directly, and to Austin, Texas through a repeater. 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, TV channel 2 (54-60 MHz) from Midland, Texas was seen in Chicagoland, pushing out Chicago's own TV channel 2. 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.
It was also easier to construct efficient transmitters, receivers, and antennas for it in the earlier days of radio, as compared to UHF. In most countries, the VHF spectrum is used for broadcast audio and television, as well as commercial two-way radios (such as those operated by taxis and police), marine two-way audio communications, and aircraft radios.
The large technically and commercially valuable slice of the VHF spectrum taken up by television transmission 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 2008 in the United States).
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New Zealand
- 45-68 MHz: Band I Television (channels 1-3)
- 88-108 MHz: Band II Radio
- 175-225MHz: Band III Television (channels 7-11)
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 using the UHF band since the VHF band is very overloaded with four stations sharing a very small frequency band.
United Kingdom
In the United Kingdom, the authorities chose to develop colour television exclusively on UHF (channels 21-69), beginning in the late 1960s. The last British VHF TV transmitters closed down on January 3rd, 1985. VHF band III is now used in the UK for digital audio broadcasting.
United States
The general services in the VHF band are:
- 30-46 MHz: Licensed 2-way land mobile communication
- 30-88 MHz: Military VHF-FM, including SINCGARS
- 46-50 MHz: Cordless telephones, "49 MHz" FM walkie-talkies, and mixed 2-way mobile communication
- 50-54 MHz: Amateur radio "6-meter" band
- 54-72 MHz: TV channels 2, 3, and 4
- 72-76 MHz: Remote Control devices
- 76-88 MHz: TV channels 5 and 6
- 88-108 MHz: FM radio broadcasting (88-92 non-commercial, 92-108 commercial)
- 108-118 MHZ: Air navigation beacons VOR
- 118-132 MHz: Airband for Air Traffic Control, AM, 121.5 MHz is emergency frequency
- 132-144 MHZ: Auxiliary civil services,satellite, space research, and other miscellaneous services
- 144-148 MHz: Amateur "2-meter" band
- 148-174 MHz: "VHF Business Band", the new unlicensed Multi-Use Radio Service (MURS), and other 2-way land mobile, FM
- 156-174 MHz VHF Marine Radio, narrow band FM, 156.8 MHz (Channel 16) is emergency frequency
- 162.40-162.55: NOAA Weather Stations, FM
- 174-216 MHz: TV channels 7 through 13, and professional wireless microphones (low power, certain exact frequencies only)
- 216-222 MHz: mixed services
- 222-225 MHz: Amateur "1-1/4-meter" band
- above 225 MHz: Federal services, notably military aircraft radio (225-400 MHz) AM, including HAVE QUICK
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 microbroadcasting and sending output from CD or digital media players to radios without auxiliary-in jacks, though this is illegal in some other countries, most notably the United Kingdom.
Line of Sight Formula
An approximation to calculate the line-of-sight horizon distance is:
distance in miles = square root of ( 1.5 * height of antenna in feet )
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