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. 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.
An approximation to calculate the line-of-sight horizon distance (on Earth) is:
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 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.
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.
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.
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).
87.9 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.
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DEFENSE SECURITY COOPERATION AGENCY NOTIFIES CONGRESS OF POSSIBLE FOREIGN MILITARY SALE TO PHILIPPINES OF HARRIS HIGH FREQUENCY-VERY HIGH FREQUENCY (VHF) RADIOS
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