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Very high frequency (VHF) is the radio frequency range from 30 MHz (wavelength 10 m) to 300 MHz (wavelength 1 m). 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. The general description of frequencies immediately below VHF is HF, and the next higher frequencies are known as Ultra high frequency (UHF). UHF and VHF are the most common frequency bands for television. 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 IL to Joplin MO directly, and to Austin TX 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 FL, New Orleans LA, Houston TX, 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). In the United Kingdom, the authorities chose to develop colour television exclusively on UHF, 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 StatesThe general services in the VHF band are:
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 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. Line of Sight FormulaAn 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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