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UHF television broadcasting is the use of ultra high frequency (UHF) radio for over the air transmission of television signals. UHF frequencies are used for both analog and digital television broadcasts. UHF channels are typically given higher channel numbers, like the US arrangement with VHF channels 2 to 13, and UHF channels numbered 14 to 83. UHF broadcasting became possible due to the introduction of new high-frequency vacuum tubes developed by Philips immediately prior to the opening of World War II. These were used in experimental television receivers in the UK in the 1930s, and became widely used during the war as radar receivers. Surplus tubes flooded the market in the post-war era. At the same time, the development of color television was taking its first steps, initially based on different transmission systems. The US FCC set aside a block of the then-unused and now-practical UHF frequencies for color television use. The introduction of the backward compatible NTSC standard led to these channels being released for any television use in 1952. Early receivers were generally less efficient at UHF band reception, and the signals are also subject to more environmental interference.〔(【引用サイトリンク】title=A Guide to UHF Television Reception )〕 Additionally, the signals are less susceptible to diffraction effects, which can improve reception at long range.〔(【引用サイトリンク】title=Choosing a mounting site )〕 UHF therefore has to be broadcast at much higher powers to provide the same reception as VHF. UHF generally had less clear signals, and for some markets, became the home of smaller broadcasters who were not willing to bid on the more coveted VHF allocations. These issues are greatly reduced with digital television, and today most over-the-air broadcasts take place on UHF, while VHF channels are being retired. To avoid the appearance of disappearing channels, digital broadcast systems have a ''virtual channel'' concept, allowing stations to keep their original VHF channel number while actually broadcasting on a UHF frequency. Over time a number of former television channels in the upper UHF band have been re-designated for other uses. Channel 37 was never used in the US and some other countries in order to prevent interference with radio astronomy. In 1983, the US FCC removed channels 70 through 83 and reassigned them to Land Mobile Radio System. In 2009, with the move to digital television complete in the US, channels 52 through 69 were reallocated as the 700 MHz band for cellular telephone service. In 2011, Channel 51 was removed to prevent interference with the 700 MHz band. The US UHF channel map now includes channels 14 through 36 and 38 through 50. ==UHF vs VHF== One advantage to UHF broadcasting is that antenna size for a given gain varies with wavelength, meaning that higher frequency broadcasts can be received with equal gain on smaller antennas. A powerful VHF antenna using the log-periodic design might be as long as 3 m, while a UHF Yagi antenna with similar gain is often found placed in front of it, occupying perhaps 1 m. Modern UHF-only antennas often use the bedspread array and are less than a meter on a side. Another effect due to the shorter wavelength is that UHF signals can pass through smaller openings than VHF. These openings are created by any metal in the area, including lines of nails or screws in the roof and walls, electrical wiring, and the frames of doors and windows. A metal-framed window will present almost no barrier to a UHF signal, while a VHF signal may be attenuated or strongly diffracted. For strong signals, UHF antennas mounted beside the television are relatively useful, and medium-distance signals, , can often be picked up by attic mounted antennas.〔 On the downside, higher frequencies are less susceptible to diffraction. This means that the signals will not bend around obstructions as rapidly as a VHF signal. This is a particular problem for receivers located in depressions and valleys. Normally the upper edge of the landform acts as a knife-edge and causes the signal to diffract downwards. VHF signals will be seen by antennas in the valley, whereas UHF bends about as much, and far less signal will be received. The same effect also makes UHF signals more difficult to receive around obstructions. VHF will quickly diffract around trees and poles and the received energy immediately downstream will be about 40% of the original signal. In comparison, UHF blockage by the same obstruction will result on the order of 10% being received.〔 Another difference is the nature of the electrical and radio noise encountered on the two frequency bands. UHF bands are subject to constant levels of low-level noise that appear as "snow" on an analog screen. VHF more commonly sees impulse noise that produces a sharp "blip" of noise, but leaves the signal clear at other times. This normally comes from local electrical sources, and can be mitigated by turning them off. This means that at a given ''received'' power, a UHF analog signal will appear worse than VHF, often significantly.〔 For these reasons, in order to allow UHF channels to provide the same ground coverage as VHF, ideally about , the FCC allowed UHF broadcasters to operate at much higher power levels. For analog signals in the United States, VHF signals on channels 2 to 6, the ''low-VHF'' range, were limited to 100 kW, ''high-VHF'' on channels 7 to 13 to 315 kW, and UHF to 5 MW, well over 10 times the power. This greatly increased the cost of transmitting in these frequencies, both in electrical costs as well as the upfront cost of the equipment needed to reach these power levels.〔 The introduction of digital television (DTV) changed the relative outcome of these effects. DTV systems use a system known as forward error correction to add additional information to the signal to allow it to correct errors. This works well if the error rate is well known, in which case the signal has to add a fixed amount of correction. This works perfectly well with constant low-level interference found on UHF, which this method can essentially eliminate. In comparison, VHF noise is largely unpredictable, consisting of periods of little noise followed by periods of complete signal loss. Forward error correction cannot easily address this situation. For this reason, DTV broadcasting was initially going to take place entirely on UHF.〔 In the US, the FCC initially wanted to move all stations to UHF, auctioning off the frequencies for cell phone use. This required a large number of stations to move out of the abandoned channels.〔 Moving from one UHF channel to another is a fairly simple exercise and generally costs little to accomplish. Moving from VHF to UHF is a much more expensive proposition, generally requiring all new equipment, and a dramatic increase in power in order to keep the same range. DTV offsets the later to a great degree, with the current FCC power limitations at 160 kW for high-VHF and 1 MW for UHF, the former limits. Nevertheless, moving from a 100 kW low-VHF analog signal to a 1 MW UHF signal is still a considerable change, which some broadcasters estimated could cost up to $4 million per station (although most estimates were much lower, on the order of $400,000). For this reason, channels in the high-VHF region were kept for television use. Channels making the transition generally acquired a second channel allocation in the upper UHF region to test their new equipment, and the moved into the low-UHF or high-VHF once the conversion period was over. This adds some complexity to the system as a whole, as the antennas needed to receive VHF and UHF are very different. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「UHF television broadcasting」の詳細全文を読む スポンサード リンク
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