UHF television broadcasting
UHF television broadcasting is the use of ultra high frequency 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 1 to 13, and UHF channels numbered 14 to 83. Compared with an equivalent VHF television transmitter, to cover the same geographic area with a UHF transmitter requires a higher effective radiated power, implying a more powerful transmitter or a more complex antenna. However, the additional channels allow more broadcasters in a given region without causing objectionable mutual interference.
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 incompatible 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 sensitive at UHF band reception, and the signals are also subject to more environmental interference. Additionally, the signals are less susceptible to diffraction effects, which can improve reception at long range. 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 giving the impression that channels were disappearing, digital broadcast systems have a virtual channel concept, allowing stations to display 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 reassigned channels 70 through 83 to the 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 cellular band. Additionally, in 2019 the US removed channels 38 through 50 to use them for cellular phone service. Thus UHF TV in the US now only includes channels 14 through 36.
UHF vs VHF
The most common type of antennas rely on the concept of resonance. Conductors, normally metal wires or rods, are cut to a length so that the desired radio signal will create a standing wave of electrical current within them. This means that antennas have a natural size, normally of a wavelength long, which maximizes performance. Antennas designed to receive the same signal will almost always have similar dimensions.Because the antenna size is based on the wavelength, UHF broadcasting can be received with much smaller antennas than VHF while still having the same gain. For instance, Channel 2 in the North American television frequencies is at 54 MHz, which corresponds to a wavelength of 5.5 m, and thus requires dipole antenna about 2.75 m across. In comparison, the lowest channel in the UHF band, Channel 14, is on 470 MHz, a wavelength of 64 cm, or a dipole length of only 32 cm. 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 bedspring 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 the spaces between 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 stations with strong signals, UHF antennas mounted beside the television are relatively useful, and medium-distance signals away 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 readily 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 stations 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 316 kW, and UHF to 5 MW, well over 10 times the power of the low-VHF transmitter power limit. This greatly increased the cost of transmitting in these frequencies, both in electrical cost as well as the upfront cost of the equipment needed to reach those power levels.
The introduction of digital television changed the relative outcome of these effects. DTV systems use a system known as forward error correction which adds information to the signal to allow it to correct errors. This works well if the error rate is well known, in which case a fixed amount of extra information is added to the signal to correct for these errors. This works well with the type of constant low-level interference found on UHF, which FEC can effectively eliminate. In comparison, VHF noise is largely unpredictable, consisting of periods of little noise followed by periods of almost 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. This would have required a large number of stations to move out of their current VHF channel assignments. 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 maintain the same service area. DTV offsets the latter to a great degree, with the current FCC power limitations at 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. For this reason, channels in the high-VHF region were kept for television use. The power of the stations on these channels was also reduced, to 160 kW, about one-third of the earlier limit. Stations making the transition generally acquired a second channel allocation in the upper UHF region to test their new equipment, and then 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.
Australia
In Australia, UHF was first anticipated in the mid-1970s with TV channels 27–69. The first UHF TV broadcasts in Australia were operated by Special Broadcasting Service on channel 28 in Sydney and Melbourne starting in 1980, and translator stations for the Australian Broadcasting Corporation. The UHF band is now used extensively as ABC, SBS, commercial and public-access television services have expanded, particularly through regional areas.Canada
The first Canadian television network was publicly owned Radio-Canada, the Canadian Broadcasting Corporation. Its stations, as well as that of the first private networks, are primarily VHF. More recent third-network operators that initially signed on in the 1970s or 1980s were often relegated to UHF, or to reduced power or stations in outlying areas. Canada's VHF spectrum was already crowded with both domestic broadcasts and numerous American TV stations along the border.The use of UHF to provide programming that otherwise would not be available, such as province-wide educational services,, has therefore become common. Third networks such as Quatre-Saisons or Global often will rely heavily on UHF stations as repeaters or as a local presence in large cities where VHF spectrum is largely already full. The original digital terrestrial television stations were all UHF broadcasts, although some digital broadcasts returned to VHF channels after the digital transition was completed in August 2011.
Digital Audio Broadcasting, deployed on a very limited scale in Canada in 2005 and largely abandoned, uses UHF frequencies in the L band from 1452 to 1492 MHz. There are currently no VHF Band III digital radio stations in Canada as, unlike in much of Europe, these frequencies are among the most popular for use by television stations.