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Very high frequency

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Very high frequency (VHF) radio signals are on frequencies between 30 and 300 megahertz (wavelengths of 10 and 1 metre respectively). The general description of frequencies immediately lower VHF is HF, and the next higher frequencies as known as ultra-high frenquencies (UHF). UHF and VHF are the most common frequency bands for television.

VHF frequencies' propagation characteristics are ideal for short-distance terrestrial communication. Unlike HF frequencies, the ionosphere does not reflect VHF radio (except on rare occasions during solar flares) and thus transmissions are restricted to the local area (and can't interfere with transmissions thousands of kilometres away) It 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. It was also easier to construct efficient transmitters, receivers, and antennas for it in the earlier days of radio. In most countries, the VHF spectrum is used for broadcast audio and television, as well as commercial two-way radios (such as that operated by taxis and police), marine two-way audio communications, and aircraft radios.

The large slice of 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).

In Britain colour television transmissions began on UHF in the late 1960s, instantly rendering VHF obsolete. The last British VHF TV transmitters closed down in 1986.



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