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141-462: 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

282-412: A thermionic tube or thermionic valve utilizes thermionic emission of electrons from a hot cathode for fundamental electronic functions such as signal amplification and current rectification . Non-thermionic types such as a vacuum phototube , however, achieve electron emission through the photoelectric effect , and are used for such purposes as the detection of light intensities. In both types,

423-413: 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. The power of the stations on these channels was also reduced, to 160 kW, about one-third of

564-404: A beam of electrons for display purposes (such as the television picture tube, in electron microscopy , and in electron beam lithography ); X-ray tubes ; phototubes and photomultipliers (which rely on electron flow through a vacuum where electron emission from the cathode depends on energy from photons rather than thermionic emission ). A vacuum tube consists of two or more electrodes in

705-550: A blower, or water-jacket. Klystrons and magnetrons often operate their anodes (called collectors in klystrons) at ground potential to facilitate cooling, particularly with water, without high-voltage insulation. These tubes instead operate with high negative voltages on the filament and cathode. Except for diodes, additional electrodes are positioned between the cathode and the plate (anode). These electrodes are referred to as grids as they are not solid electrodes but sparse elements through which electrons can pass on their way to

846-649: A blue glow. Finnish inventor Eric Tigerstedt significantly improved on the original triode design in 1914, while working on his sound-on-film process in Berlin, Germany. Tigerstedt's innovation was to make the electrodes concentric cylinders with the cathode at the centre, thus greatly increasing the collection of emitted electrons at the anode. Irving Langmuir at the General Electric research laboratory ( Schenectady, New York ) had improved Wolfgang Gaede 's high-vacuum diffusion pump and used it to settle

987-401: A certain sound or tone). Not all electronic circuit valves or electron tubes are vacuum tubes. Gas-filled tubes are similar devices, but containing a gas, typically at low pressure, which exploit phenomena related to electric discharge in gases , usually without a heater. One classification of thermionic vacuum tubes is by the number of active electrodes . A device with two active elements

1128-501: A combination of a triode with a hexode and even an octode have been used for this purpose. The additional grids include control grids (at a low potential) and screen grids (at a high voltage). Many designs use such a screen grid as an additional anode to provide feedback for the oscillator function, whose current adds to that of the incoming radio frequency signal. The pentagrid converter thus became widely used in AM receivers, including

1269-466: A common circuit (which can be AC without inducing hum) while allowing the cathodes in different tubes to operate at different voltages. H. J. Round invented the indirectly heated tube around 1913. The filaments require constant and often considerable power, even when amplifying signals at the microwatt level. Power is also dissipated when the electrons from the cathode slam into the anode (plate) and heat it; this can occur even in an idle amplifier due to

1410-609: A drop that was only partially compensated for by field upgrades or the availability of external UHF converters for separate purchase. Plummeting inclusion of UHF tuners in sets placed VHF–UHF intermixture at grave risk of failure. On the transmission side, UHF stations were also found to have issues involving transmission distance and strength. The FCC tried solving this problem by allowing the lower-powered UHF stations to broadcast with more power, but VHF continued to have more stations. Advertisers soon caught on to this and did most of their business with VHF stations since UHF tuner adoption

1551-586: A far superior and versatile technology for use in radio transmitters and receivers. At the end of the 19th century, radio or wireless technology was in an early stage of development and the Marconi Company was engaged in development and construction of radio communication systems. Guglielmo Marconi appointed English physicist John Ambrose Fleming as scientific advisor in 1899. Fleming had been engaged as scientific advisor to Edison Telephone (1879), as scientific advisor at Edison Electric Light (1882), and

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1692-511: A huge setback after a forced move from a 42–50 MHz allocation to an 88–108 MHz allocation in 1946. This had rendered all pre-1946 FM transmitters and receivers obsolete, and there was heavy resistance to moving FM a second time. Aeronautical radio is located above 108 MHz; military aeronautical radio used 225–400 MHz. Additional public safety, commercial land-mobile, and amateur radio services had allocations in Band II . It

1833-400: A low potential space charge region between the anode and screen grid to return anode secondary emission electrons to the anode when the anode potential is less than that of the screen grid. Formation of beams also reduces screen grid current. In some cylindrically symmetrical beam power tubes, the cathode is formed of narrow strips of emitting material that are aligned with the apertures of

1974-497: 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

2115-605: A paid UHF station offers foreign programs not shown on local TV and commencing regular service in January 1993, but it was closed down as a result from intense competition from the rival Sky Cable . From 2001 to the present, more channels were established, regional stations are established in the provinces which specialize news, public service and free programming. With Digital TV was introduced, all UHF channels will allocate their frequencies and can be served for broadcast companies such as ABS-CBN, GMA Network and TV5 , among others as

2256-414: A pair of beam deflection electrodes which deflected the current towards either of two anodes. They were sometimes known as the 'sheet beam' tubes and used in some color TV sets for color demodulation . The similar 7360 was popular as a balanced SSB (de)modulator . A beam tetrode (or "beam power tube") forms the electron stream from the cathode into multiple partially collimated beams to produce

2397-412: A printing instrument was needed. As a result of experiments conducted on Edison effect bulbs, Fleming developed a vacuum tube that he termed the oscillation valve because it passed current in only one direction. The cathode was a carbon lamp filament, heated by passing current through it, that produced thermionic emission of electrons. Electrons that had been emitted from the cathode were attracted to

2538-509: A relatively low-value resistor is connected between the cathode and ground. This makes the cathode positive with respect to the grid, which is at ground potential for DC. However C batteries continued to be included in some equipment even when the "A" and "B" batteries had been replaced by power from the AC mains. That was possible because there was essentially no current draw on these batteries; they could thus last for many years (often longer than all

2679-647: A satellite feed directly from their U.S. military bases in Japan), at the time when Mount Pinatubo erupted and became abandoned. Commercial UHF stations began in May, 1992, as DWCP-TV on channel 21 became the first local UHF TV station in Metro Manila by the Southern Broadcasting Network as SBN-21 (then Talk TV ) and commenced free programing, the second channel, DWKC-TV (on channel 31) of

2820-414: A significant impact on transistor performance (such as under a MOS gate or in a bipolar base region), the output signal can be substantial. These defects can be caused by manufacturing processes, such as heavy ion implantation , or by unintentional side-effects such as surface contamination. Individual op-amps can be screened for burst noise with peak detector circuits, to minimize the amount of noise in

2961-407: A simple oscillator only requiring connection of the plate to a resonant LC circuit to oscillate. The dynatron oscillator operated on the same principle of negative resistance as the tunnel diode oscillator many years later. The dynatron region of the screen grid tube was eliminated by adding a grid between the screen grid and the plate to create the pentode . The suppressor grid of the pentode

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3102-419: A small-signal vacuum tube are 1 to 10 millisiemens. It is one of the three 'constants' of a vacuum tube, the other two being its gain μ and plate resistance R p or R a . The Van der Bijl equation defines their relationship as follows: g m = μ R p {\displaystyle g_{m}={\mu \over R_{p}}} The non-linear operating characteristic of

3243-464: A vacuum inside an airtight envelope. Most tubes have glass envelopes with a glass-to-metal seal based on kovar sealable borosilicate glasses , although ceramic and metal envelopes (atop insulating bases) have been used. The electrodes are attached to leads which pass through the envelope via an airtight seal. Most vacuum tubes have a limited lifetime, due to the filament or heater burning out or other failure modes, so they are made as replaceable units;

3384-429: A wide range of frequencies. To combat the stability problems of the triode as a radio frequency amplifier due to grid-to-plate capacitance, the physicist Walter H. Schottky invented the tetrode or screen grid tube in 1919. He showed that the addition of an electrostatic shield between the control grid and the plate could solve the problem. This design was refined by Hull and Williams. The added grid became known as

3525-551: A year". The freeze would give the FCC and broadcasting interests time to address questions such as the allocation of additional channel frequencies, and the selection of a color television standard. At the time of the freeze, less than 100 stations were on the air, but stations already under construction would be allowed to complete work. All but one of these was on the VHF band; on December 29, 1949, KC2XAK of Bridgeport, Connecticut , became

3666-445: Is a current . Compare this to the behavior of the bipolar junction transistor , in which the controlling signal is a current and the output is also a current. For vacuum tubes, transconductance or mutual conductance ( g m ) is defined as the change in the plate(anode)/cathode current divided by the corresponding change in the grid to cathode voltage, with a constant plate(anode) to cathode voltage. Typical values of g m for

3807-489: Is a diode , usually used for rectification . Devices with three elements are triodes used for amplification and switching . Additional electrodes create tetrodes , pentodes , and so forth, which have multiple additional functions made possible by the additional controllable electrodes. Other classifications are: Vacuum tubes may have other components and functions than those described above, and are described elsewhere. These include as cathode-ray tubes , which create

3948-413: 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, 1 ⁄ 5 the former limits. Nevertheless, moving from

4089-597: Is a type of electronic noise that occurs in semiconductors and ultra-thin gate oxide films. It is also called random telegraph noise ( RTN ), popcorn noise , impulse noise , bi-stable noise , or random telegraph signal ( RTS ) noise. It consists of sudden step-like transitions between two or more discrete voltage or current levels, as high as several hundred microvolts , at random and unpredictable times. Each shift in offset voltage or current often lasts from several milliseconds to seconds, and sounds like popcorn popping if hooked up to an audio speaker. Burst noise

4230-523: 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

4371-410: Is not important since they are simply re-captured by the plate. But in a tetrode they can be captured by the screen grid since it is also at a positive voltage, robbing them from the plate current and reducing the amplification of the tube. Since secondary electrons can outnumber the primary electrons over a certain range of plate voltages, the plate current can decrease with increasing plate voltage. This

UHF television broadcasting - Misplaced Pages Continue

4512-611: Is one of a loosely knit group of free commercial terrestrial television stations that is not a member of the major national networks keyed in Tokyo and Osaka . Japan's original broadcasters were VHF. Although some experimental broadcasts were made as early as 1939, NHK (founded in 1926 as a radio network modeled on the BBC) began regular VHF television broadcasting in 1953. Its two terrestrial television services ( NHK General TV and NHK Educational TV ) appear on VHF 1 and 3, respectively, in

4653-488: Is operated mostly by local governments or metropolitan newspapers with less outside control. Compared with major network stations, Japan's UHF independents have more restrictive programming acquisition budgets and lower average ratings; they are also more likely to broadcast single episode or short-series UHF anime (many of which serve to promote DVD's or other product tie-ins) and brokered programming such as religion and infomercials . Japanese terrestrial television

4794-564: Is the Loewe 3NF . This 1920s device has three triodes in a single glass envelope together with all the fixed capacitors and resistors required to make a complete radio receiver. As the Loewe set had only one tube socket, it was able to substantially undercut the competition, since, in Germany, state tax was levied by the number of sockets. However, reliability was compromised, and production costs for

4935-416: Is the dynatron region or tetrode kink and is an example of negative resistance which can itself cause instability. Another undesirable consequence of secondary emission is that screen current is increased, which may cause the screen to exceed its power rating. The otherwise undesirable negative resistance region of the plate characteristic was exploited with the dynatron oscillator circuit to produce

5076-889: Is usually allocated for VCRs, decoder units (i.e. the ASTRO and MiTV set top boxes ) and other devices that have an RF signal generator (i.e. game consoles). Refer to Australian and New Zealand television frequencies for more information. UHF broadcasting was introduced in the Philippines in the early 1960s when FEN Philippines began broadcasts on channel 17 in Pampanga and Zambales (as in Subic and Clark bases), and channel 43 in Bulacan and also in Metro Manila on Channel 50 until 1991 (most of its programs and newscasts are from

5217-564: The Edison effect , that became well known. Although Edison was aware of the unidirectional property of current flow between the filament and the anode, his interest (and patent ) concentrated on the sensitivity of the anode current to the current through the filament (and thus filament temperature). It was years later that John Ambrose Fleming applied the rectifying property of the Edison effect to detection of radio signals, as an improvement over

5358-636: The plate ( anode ) when the plate was at a positive voltage with respect to the cathode. Electrons could not pass in the reverse direction because the plate was not heated and not capable of thermionic emission of electrons. Fleming filed a patent for these tubes, assigned to the Marconi company, in the UK in November 1904 and this patent was issued in September 1905. Later known as the Fleming valve ,

5499-429: The screen grid or shield grid . The screen grid is operated at a positive voltage significantly less than the plate voltage and it is bypassed to ground with a capacitor of low impedance at the frequencies to be amplified. This arrangement substantially decouples the plate and the control grid , eliminating the need for neutralizing circuitry at medium wave broadcast frequencies. The screen grid also largely reduces

5640-480: The 6GH8 /ECF82 triode-pentode, quite popular in television receivers. The desire to include even more functions in one envelope resulted in the General Electric Compactron which has 12 pins. A typical example, the 6AG11, contains two triodes and two diodes. Some otherwise conventional tubes do not fall into standard categories; the 6AR8, 6JH8 and 6ME8 have several common grids, followed by

5781-482: The 6SN7 , is a "dual triode" which performs the functions of two triode tubes while taking up half as much space and costing less. The 12AX7 is a dual "high mu" (high voltage gain ) triode in a miniature enclosure, and became widely used in audio signal amplifiers, instruments, and guitar amplifiers . The introduction of the miniature tube base (see below) which can have 9 pins, more than previously available, allowed other multi-section tubes to be introduced, such as

UHF television broadcasting - Misplaced Pages Continue

5922-557: The American Broadcasting Company and DuMont Television Network , the need for additional TV channels in major markets was urgent. For proponents of educational TV broadcasting, the difficulties in competing with commercial broadcasters for the increasingly scarce VHF channels was also a problem. Allocating more of the VHF band (30 to 300 MHz ) by moving existing radio communication users away seemed to be impossible. FM radio broadcasting had already suffered

6063-675: The Australian Broadcasting Corporation (ABC). The UHF band is now used extensively as ABC , SBS, commercial and public-access television services have expanded, particularly through regional areas. 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 ( CTV and TVA , created in 1961), are primarily VHF. More recent third-network operators that initially signed on in

6204-676: The National Telecommunications Commission plans to migrate all VHF channels to digital UHF channels before December 31, 2015, though this was delayed several times. Digital terrestrial television services are currently in development by the major broadcasting companies before the Implementing Rules and Regulations (IRR) will be passed by law. South Africa only received analog TV service from 1976 onwards. There were three TV channels: TV1 (now SABC1), TV2 (now SABC2), TV3 (now SABC3), and later came

6345-783: The Radio Mindanao Network was launched on October 31 of the same year as CTV-31 from 1992 to 2000 (then E! from 2000 to 2003 and BEAM in 2011). The third channel, DZRJ-TV (channel 29) was also launched in 1993 for the Rajah Broadcasting Network, Inc. which specializes niche programing (mostly infomercials, foreign shows and cartoons). Two more channels include DWDB-TV (channel 27) of GMA Network, Inc. (as Citynet Television from 1995 to 1999 and EMC from 1999 to 2001) and DWAC-TV (channel 23) of ABS-CBN (as Studio 23 ) between August 27, 1995 and October 12, 1996, as fourth and fifth UHF stations, and

6486-737: The Republic of Ireland , UHF was introduced in 1978 to augment the existing RTÉ One VHF 625-line transmissions and to provide extra frequencies for the new RTÉ Two channel. The first UHF transmitter site was Cairn Hill in County Longford , followed by Three Rock Mountain in South County Dublin . These sites were followed by Clermont Carn in County Louth and Holywell Hill in County Donegal in 1981. Since

6627-514: 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. In

6768-467: The magnetic detector . Amplification by vacuum tube became practical only with Lee de Forest 's 1907 invention of the three-terminal " audion " tube, a crude form of what was to become the triode . Being essentially the first electronic amplifier , such tubes were instrumental in long-distance telephony (such as the first coast-to-coast telephone line in the US) and public address systems , and introduced

6909-468: The quiescent current necessary to ensure linearity and low distortion. In a power amplifier, this heating can be considerable and can destroy the tube if driven beyond its safe limits. Since the tube contains a vacuum, the anodes in most small and medium power tubes are cooled by radiation through the glass envelope. In some special high power applications, the anode forms part of the vacuum envelope to conduct heat to an external heat sink, usually cooled by

7050-462: The spark gap transmitter for radio or mechanical computers for computing, it was the invention of the thermionic vacuum tube that made these technologies widespread and practical, and created the discipline of electronics . In the 1940s, the invention of semiconductor devices made it possible to produce solid-state devices, which are smaller, safer, cooler, and more efficient, reliable, durable, and economical than thermionic tubes. Beginning in

7191-443: The 1930s with regular commercial broadcasting in cities such as New York and Chicago in 1941. Efforts at TV broadcasting on any channel were drastically curtailed once World War II began, due largely to lack of available receivers. The upper five VHF channels were removed from the FCC allocation list during the war with those frequencies re-allocated for military use, leaving thirteen channels (1 through 13) as of May 1945. The end of

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7332-434: The 1970s or 1980s were often relegated to UHF, or (if they were to attempt to deploy on VHF) 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 (BC's Knowledge: channel, or TVOntario -

7473-413: The 19th century, telegraph and telephone engineers had recognized the need to extend the distance that signals could be transmitted. In 1906, Robert von Lieben filed for a patent for a cathode-ray tube which used an external magnetic deflection coil and was intended for use as an amplifier in telephony equipment. This von Lieben magnetic deflection tube was not a successful amplifier, however, because of

7614-593: The 82 channels possible under the standard 6 MHz bandwidth. CBS Vice President Adrian Murphy told the FCC: "I would say that it would be better to have two networks in color" instead of the four or more networks possible with narrower bandwidths in UHF. In October 1948, the Federal Communications Commission stopped accepting applications for new stations, a freeze expected to last "six months to

7755-607: The 82 new UHF-TV stations in the United States broadcasting as of June 1954, only 24 of them remained a year later. The majority of the 165 UHF stations to begin telecasting between 1952 and 1959 did not survive. Not until the passage of the 1962 All-Channel Receiver Act did FCC regulations require all new TV sets sold in the U.S. to have built-in UHF tuners that could receive channels 14–83. Even though that requirement came into effect on April 30, 1964, there were only about 170 full-service UHF stations in operation in 1971. In

7896-485: The Audion for demonstration to AT&T's engineering department. Dr. Harold D. Arnold of AT&T recognized that the blue glow was caused by ionized gas. Arnold recommended that AT&T purchase the patent, and AT&T followed his recommendation. Arnold developed high-vacuum tubes which were tested in the summer of 1913 on AT&T's long-distance network. The high-vacuum tubes could operate at high plate voltages without

8037-823: The Tokyo region. Privately owned Japanese VHF TV stations were most often built by large national newspapers with Tokyo stations exerting a large degree of control over national programming. The number of VHF broadcasters varied depending on the prefecture. For example, in the Kanto region , there were seven VHF channels available. Outside of Tokyo, Osaka , Nagoya , and Fukuoka , most prefectures had four privately owned television stations, with three of them broadcasting on UHF. Almost all prefectures had at least one privately owned VHF television station (except for Saga ). The independent stations broadcast in analogue UHF, unlike major networks, which were historically broadcast primarily in analogue VHF . The loose coalition of UHF independents

8178-580: 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

8319-430: The US now only includes channels 14 through 36. 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 1 ⁄ 2 of a wavelength long, which maximizes performance. Antennas designed to receive

8460-468: The United States, UHF stations gained a reputation for local ownership, nonprofessional operations, small audiences and weak signal propagation. While UHF-TV was available to American TV broadcasters in today's form since 1952, affiliates of the 1950s four largest American TV networks ( NBC , CBS , ABC , and DuMont ) preferentially transmitted on VHF wherever it was available. All available VHF-TV allocations were already in use in most large TV markets by

8601-468: 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 (DTV) changed

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8742-493: The United States, television stations in or near state capital cities are important because they closely covered the operations of state governments and spread information to residents across their state. Vacuum tube A vacuum tube , electron tube , valve (British usage), or tube (North America) is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. The type known as

8883-406: The VHF band because of their bandwidth demands; more significantly, it offered the possibility for sufficient numbers of conventional 6 MHz channels to support the FCC's goals of a "truly nationwide and competitive service". CBS was not trying to maximize broadcast (or network) competition through freer market entry. Instead CBS's 16 MHz channels would have allowed only 27 UHF channels versus

9024-399: The VHF stations, losing $ 10,500,000 in 1953. More stations left the air than opened, and sixty percent of television industry losses from 1953 to 1956 came from UHF stations. TV network affiliations were difficult to get in many locations; the UHF stations with major-network affiliation would often lose these affiliations in favor of any viable new VHF TV station that entered the same market. Of

9165-400: The allied military by 1916. Historically, vacuum levels in production vacuum tubes typically ranged from 10 μPa down to 10 nPa (8 × 10   Torr down to 8 × 10  Torr). The triode and its derivatives (tetrodes and pentodes) are transconductance devices, in which the controlling signal applied to the grid is a voltage , and the resulting amplified signal appearing at the anode

9306-623: The analogue television switchoff on October 24, 2012, all digital terrestrial TV is on UHF only, although VHF allocations exist. The UHF band has been used in parts of Ireland for television deflector systems bringing British television signals to towns and rural areas that cannot receive these signals directly. However, since the introduction of free to air satellite transmission of UK TV channels these deflectors have largely ceased operation. In Japan, an Independent UHF Station ( ja:全国独立UHF放送協議会 , Zenkoku Dokuritsu Yū-eichi-efu Hōsō Kyōgi-kai , literally National Independent UHF Broadcasting Forum)

9447-431: The base assumption that a UHF television station was functionally equivalent to a VHF one. To allocate four to as many as seven VHF channels to each of the largest cities would mean forcing the smaller, intervening cities completely onto UHF channels, while an allocation scheme that sought to assign one or two VHF channels in each smaller city would force VHF and UHF stations to compete in most markets. The largest cities with

9588-409: The base. There was even an occasional design that had two top cap connections. The earliest vacuum tubes evolved from incandescent light bulbs , containing a filament sealed in an evacuated glass envelope. When hot, the filament in a vacuum tube (a cathode ) releases electrons into the vacuum, a process called thermionic emission . This can produce a controllable unidirectional current though

9729-536: The cathode, no direct current could pass from the cathode to the grid. Thus a change of voltage applied to the grid, requiring very little power input to the grid, could make a change in the plate current and could lead to a much larger voltage change at the plate; the result was voltage and power amplification . In 1908, de Forest was granted a patent ( U.S. patent 879,532 ) for such a three-electrode version of his original Audion for use as an electronic amplifier in radio communications. This eventually became known as

9870-617: The channel crowding problem, the following cities were never allocated any VHF-TV stations at all, due to technical reasons found by the FCC: Huntsville, Alabama ; Peoria, Illinois ; Fort Wayne, Indiana ; South Bend, Indiana , Lexington, Kentucky ; Springfield, Massachusetts ; Elmira, New York ; Youngstown, Ohio ; Scranton / Wilkes-Barre, Pennsylvania ; and Yakima, Washington . Other cites were able to receive only one VHF broadcast station. The entire state of New Jersey would receive only one VHF broadcast station of its own (which

10011-481: The complex over-lapping led to the fifth and final network having a significantly reduced national coverage compared to the other networks, with reduced picture quality in many areas and the use of wide-band aerials often required. The launch of digital terrestrial television in 1998 saw the continued use of UHF for television, with six multiplexes allocated for the service, all within the UHF band. Analogue transmissions have ceased completely since 2012 after which

10152-409: The control grid, reducing control grid current. This design helps to overcome some of the practical barriers to designing high-power, high-efficiency power tubes. Manufacturer's data sheets often use the terms beam pentode or beam power pentode instead of beam power tube , and use a pentode graphic symbol instead of a graphic symbol showing beam forming plates. Burst noise Burst noise

10293-722: 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. 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 (SBS) on channel 28 in Sydney and Melbourne starting in 1980, and translator stations for

10434-418: The electrode leads connect to pins on the tube's base which plug into a tube socket . Tubes were a frequent cause of failure in electronic equipment, and consumers were expected to be able to replace tubes themselves. In addition to the base terminals, some tubes had an electrode terminating at a top cap . The principal reason for doing this was to avoid leakage resistance through the tube base, particularly for

10575-426: The electrons are accelerated from the cathode to the anode by the electric field in the tube. The simplest vacuum tube, the diode (i.e. Fleming valve ), was invented in 1904 by John Ambrose Fleming . It contains only a heated electron-emitting cathode and an anode. Electrons can flow in only one direction through the device – from the cathode to the anode. Adding one or more control grids within

10716-425: The exception of early light bulbs , such tubes were only used in scientific research or as novelties. The groundwork laid by these scientists and inventors, however, was critical to the development of subsequent vacuum tube technology. Although thermionic emission was originally reported in 1873 by Frederick Guthrie , it was Thomas Edison's apparently independent discovery of the phenomenon in 1883, referred to as

10857-403: The filament as the cathode; this is called a "directly heated" tube. Most modern tubes are "indirectly heated" by a "heater" element inside a metal tube that is the cathode. The heater is electrically isolated from the surrounding cathode and simply serves to heat the cathode sufficiently for thermionic emission of electrons. The electrical isolation allows all the tubes' heaters to be supplied from

10998-565: The first UHF originating station in Canada), Télé-Québec , French language programming outside Québec and ethnic/multilingual television 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

11139-411: The first and only pre-1950 UHF television station to operate on a regular daily schedule. Existing FCC rules at the time of the freeze had designated 42 UHF channels, designated 14–55, between 475–890 MHz. Ultimately, the question the FCC faced of how to allocate bandwidth for new television licenses would not take "months" to resolve, but several years. To newer entrants into TV broadcasting such as

11280-483: The first commercial UHF television station on the air was KPTV , Channel 27, in Portland, Oregon , on September 18, 1952. Early in 1953, 35% of televisions sold contained a UHF tuner compliant with 1952 rules, lending hope to the idea that intermixture of UHF and VHF stations might succeed. Several problems with early UHF tuners became evident. One was poor image frequency rejection in superheterodyne receivers with

11421-553: The fourth, Etv. In the UK, UHF television began in 1964 following a plan by the General Post Office to allocate sets of frequencies for 625-lined television to regions across the country, so as to accommodate four national networks with regional variations (the VHF allocations allowed for only two such networks using 405 lines ). The UK UHF channels would range from 21 to 68 (later extended to 69) and regional allocations were in general grouped close together to allow for

11562-400: The high impedance grid input. The bases were commonly made with phenolic insulation which performs poorly as an insulator in humid conditions. Other reasons for using a top cap include improving stability by reducing grid-to-anode capacitance, improved high-frequency performance, keeping a very high plate voltage away from lower voltages, and accommodating one more electrode than allowed by

11703-463: 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,

11844-538: The independent UHF station phenomenon; a fictional UHF station was also parodied in the 1980 film Pray TV . Some cities did develop successful independent UHF stations, many of these located in or near state capital cities, or served by nearby major rural regions. These included Montgomery , Alabama ; Frankfort , Kentucky ; Dover , Delaware ; Lincoln , Nebraska ; Topeka , Kansas ; Jefferson City , Missouri ; Lansing , Michigan ; Harrisburg , Pennsylvania ; Madison , Wisconsin ; and Springfield , Illinois . In

11985-411: The influence of the plate voltage on the space charge near the cathode, permitting the tetrode to produce greater voltage gain than the triode in amplifier circuits. While the amplification factors of typical triodes commonly range from below ten to around 100, tetrode amplification factors of 500 are common. Consequently, higher voltage gains from a single tube amplification stage became possible, reducing

12126-400: The late 1950s and early 1960s) and the fall in television prices saw most households use a UHF set by the end of that decade. With the second and last VHF television service having launched in 1955, VHF TV was finally decommissioned for good in 1985 with no plans for it to return to use. The launch of Channel 5 in 1997 added a fifth national television network to UHF, requiring deviation from

12267-566: The late 1990s, freeing up the frequency for other uses. UHF was not commonly used in the Klang Valley until 1994 (despite TV3's signal also being available over UHF Channel 29, as TV3 transmitted over VHF Channel 12 in the Klang Valley). 1994 saw the introduction of the channel MetroVision (which ceased transmission in 1999, got bought over by TV3's parent company – System Televisyen Malaysia Berhad – and relaunched as 8TV in 2004). This

12408-498: The license freeze enacted in 1949 to end. Through the entire three-and-a-half-year freeze period, KC2XAK remained the only UHF television station in regular operation. When the FCC television license freeze ended in 1952, a huge backlog of potential stations applied, many allocated to the UHF band as defined by the 1952 rules. The first commercially licensed UHF television station was WWLP in Springfield, Massachusetts ; however,

12549-405: 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

12690-463: The meantime, these shortcomings led to "UHF taboos", which in effect limited each metropolitan area to only moderately more UHF stations than VHF ones, despite the much higher number of channels. Television sets in the United States were not mandated to include UHF tuners until 1964. With UHF's reputation for reception problems, the fraction of new TV receivers that were factory-equipped with all-channel tuners dropped from 35% in early 1953 to 9% by 1958,

12831-537: The mid-1950s, owing to FCC spacing rules to avoid co-channel and adjacent channel interference between VHF TV stations in the same or nearby cities. Two VHF TV stations on the same channel needed to be 160 or more miles apart, and two VHF TV stations on adjacent channel frequencies needed to be 60 or more miles apart. UHF stations in major population centers of the United States were usually either educational network or independent TV stations. The movie UHF (starring "Weird Al" Yankovic and Michael Richards ) parodied

12972-573: The mid-1960s, thermionic tubes were being replaced by the transistor . However, the cathode-ray tube (CRT) remained the basis for television monitors and oscilloscopes until the early 21st century. Thermionic tubes are still employed in some applications, such as the magnetron used in microwave ovens, certain high-frequency amplifiers , and high end audio amplifiers, which many audio enthusiasts prefer for their "warmer" tube sound , and amplifiers for electric musical instruments such as guitars (for desired effects, such as "overdriving" them to achieve

13113-498: The miniature tube version of the " All American Five ". Octodes, such as the 7A8, were rarely used in the United States, but much more common in Europe, particularly in battery operated radios where the lower power consumption was an advantage. To further reduce the cost and complexity of radio equipment, two separate structures (triode and pentode for instance) can be combined in the bulb of a single multisection tube . An early example

13254-496: The most sets in use benefitted most from VHF allocations. For example, New York City, Washington-Baltimore, Los Angeles, and San Francisco received seven VHF stations apiece, and Chicago was allocated five, with the other two of those channels going to Milwaukee, Wisconsin and Rockford, Illinois . FCC rules published on April 11, 1952, defined the final modern-day UHF allocation of 70 channels, 14 through 83, with 6 MHz separation. It used standard NTSC standards. This would allow

13395-458: The neighbouring region. Initial uptake of UHF television was very slow: Differing propagation characteristics between VHF and UHF meant new additional transmitters needed to be built, often at different locations to the then-established VHF sites, and in general with a larger number of relay stations to fill the greater number of gaps in coverage that came with the new band. This led to poor picture quality in bad coverage areas, and many years before

13536-431: The number of external pins (leads) often forced the functions to share some of those external connections such as their cathode connections (in addition to the heater connection). The RCA Type 55 is a double diode triode used as a detector, automatic gain control rectifier and audio preamplifier in early AC powered radios. These sets often include the 53 Dual Triode Audio Output. Another early type of multi-section tube,

13677-435: The number of tubes required. Screen grid tubes were marketed by late 1927. However, the useful region of operation of the screen grid tube as an amplifier was limited to plate voltages greater than the screen grid voltage, due to secondary emission from the plate. In any tube, electrons strike the plate with sufficient energy to cause the emission of electrons from its surface. In a triode this secondary emission of electrons

13818-403: The original frequency allocation plan of the early 1960s and the allocation of UHF frequencies previously not used for television (such as UK Channels 35 and 37, previously reserved for RF modulators in devices such as domestic videocassette recorders , requiring an expensive VCR re-tuning programme funded by the new network). A lack of capacity within the band to accommodate a fifth service with

13959-421: 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

14100-528: The oscillation valve was developed for the purpose of rectifying radio frequency current as the detector component of radio receiver circuits. While offering no advantage over the electrical sensitivity of crystal detectors , the Fleming valve offered advantage, particularly in shipboard use, over the difficulty of adjustment of the crystal detector and the susceptibility of the crystal detector to being dislodged from adjustment by vibration or bumping. In

14241-652: The plate current, possibly changing the output by hundreds of volts (depending on the circuit). The solid-state device which operates most like the pentode tube is the junction field-effect transistor (JFET), although vacuum tubes typically operate at over a hundred volts, unlike most semiconductors in most applications. The 19th century saw increasing research with evacuated tubes, such as the Geissler and Crookes tubes . The many scientists and inventors who experimented with such tubes include Thomas Edison , Eugen Goldstein , Nikola Tesla , and Johann Wilhelm Hittorf . With

14382-471: The plate, it creates an electric field due to the potential difference between them. Such a tube with only two electrodes is termed a diode , and is used for rectification . Since current can only pass in one direction, such a diode (or rectifier ) will convert alternating current (AC) to pulsating DC. Diodes can therefore be used in a DC power supply , as a demodulator of amplitude modulated (AM) radio signals and for similar functions. Early tubes used

14523-405: The plate. The vacuum tube is then known as a triode , tetrode , pentode , etc., depending on the number of grids. A triode has three electrodes: the anode, cathode, and one grid, and so on. The first grid, known as the control grid, (and sometimes other grids) transforms the diode into a voltage-controlled device : the voltage applied to the control grid affects the current between the cathode and

14664-414: The plate. When held negative with respect to the cathode, the control grid creates an electric field that repels electrons emitted by the cathode, thus reducing or even stopping the current between cathode and anode. As long as the control grid is negative relative to the cathode, essentially no current flows into it, yet a change of several volts on the control grid is sufficient to make a large difference in

14805-399: The power used by the deflection coil. Von Lieben would later make refinements to triode vacuum tubes. Lee de Forest is credited with inventing the triode tube in 1907 while experimenting to improve his original (diode) Audion . By placing an additional electrode between the filament ( cathode ) and plate (anode), he discovered the ability of the resulting device to amplify signals. As

14946-537: The prefectures of Iwate , Miyagi , and Fukushima were switched off on 31 March 2012. UHF broadcasting was used outside Kuala Lumpur and the Klang Valley by private TV station TV3 in the late 1980s, with the government stations only transmitting in VHF (Bands 1 and 3) and the 450 MHz range being occupied by the ATUR cellular phone service operated by Telekom Malaysia . The ATUR service ceased operation in

15087-448: The present-day C cell , for which the letter denotes its size and shape). The C battery's positive terminal was connected to the cathode of the tubes (or "ground" in most circuits) and whose negative terminal supplied this bias voltage to the grids of the tubes. Later circuits, after tubes were made with heaters isolated from their cathodes, used cathode biasing , avoiding the need for a separate negative power supply. For cathode biasing,

15228-532: The question of thermionic emission and conduction in a vacuum. Consequently, General Electric started producing hard vacuum triodes (which were branded Pliotrons) in 1915. Langmuir patented the hard vacuum triode, but de Forest and AT&T successfully asserted priority and invalidated the patent. Pliotrons were closely followed by the French type ' TM ' and later the English type 'R' which were in widespread use by

15369-465: The relative outcome of these effects. DTV systems use a system known as forward error correction (FEC) 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

15510-497: 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,

15651-500: The service achieved full national coverage. In addition to this, the only exclusively UHF service, BBC2, would run for only a few hours a day and run alternative programming for minority audiences in contrast to the more populist schedules of BBC1 and ITV. However the 1970s saw a large increase in UHF TV viewing while VHF took a significant decline: The appeal of colour, which was never introduced to VHF (despite preliminary plans to do so in

15792-467: 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 60 miles (97 km), the FCC allowed UHF broadcasters to operate at much higher power levels. For analog signals in

15933-434: 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

16074-505: The sixth and the last, DWDZ-TV (channel 47) of the Associated Broadcasting Company in 1999, but it was silent in 2003. UHF channels in Metro Manila were used as an alternative to cable television which offered free programing for households in the target markets and became popular in the 1990s. Similarly, pay services were also introduced in late 1992, when DWBC-TV on channel 68 began initial transmissions as

16215-464: The standard intermediate frequency of 45.75 MHz. Another was very poor adjacent-channel rejection and channel selectivity by early tuner designs and manufactures. These problems were so significant that UHF-TV stations in the same geographic area were usually assigned a minimum of six channels apart from one another. Technical problems with the design of vacuum tubes for operation at high UHF frequencies were beginning to be addressed in 1954, but in

16356-440: The suppressor grid wired internally to the cathode (e.g. EL84/6BQ5) and those with the suppressor grid wired to a separate pin for user access (e.g. 803, 837). An alternative solution for power applications is the beam tetrode or beam power tube , discussed below. Superheterodyne receivers require a local oscillator and mixer , combined in the function of a single pentagrid converter tube. Various alternatives such as using

16497-406: The television are relatively useful, and medium-distance signals 25–50 kilometres (16–31 mi) 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

16638-402: The time of the 1948 FCC freeze. With the knowledge that UHF channel allocation would be necessary to expand television coverage, and with the knowledge that by 1949 VHF television was an entrenched standard, the FCC proposed intermixture , licensing both VHF and UHF stations in a single city. Intermixture would rely on consumers rapidly adopting television sets with UHF tuning capability, and on

16779-458: The triode caused early tube audio amplifiers to exhibit harmonic distortion at low volumes. Plotting plate current as a function of applied grid voltage, it was seen that there was a range of grid voltages for which the transfer characteristics were approximately linear. To use this range, a negative bias voltage had to be applied to the grid to position the DC operating point in the linear region. This

16920-407: The triode. De Forest's original device was made with conventional vacuum technology. The vacuum was not a "hard vacuum" but rather left a very small amount of residual gas. The physics behind the device's operation was also not settled. The residual gas would cause a blue glow (visible ionization) when the plate voltage was high (above about 60 volts). In 1912, de Forest and John Stone Stone brought

17061-464: The tube allows the current between the cathode and anode to be controlled by the voltage on the grids. These devices became a key component of electronic circuits for the first half of the twentieth century. They were crucial to the development of radio , television , radar , sound recording and reproduction , long-distance telephone networks, and analog and early digital computers . Although some applications had used earlier technologies such as

17202-646: The tube were much greater. In a sense, these were akin to integrated circuits. In the United States, Cleartron briefly produced the "Multivalve" triple triode for use in the Emerson Baby Grand receiver. This Emerson set also has a single tube socket, but because it uses a four-pin base, the additional element connections are made on a "mezzanine" platform at the top of the tube base. By 1940 multisection tubes had become commonplace. There were constraints, however, due to patents and other licensing considerations (see British Valve Association ). Constraints due to

17343-482: The tubes) without requiring replacement. When triodes were first used in radio transmitters and receivers, it was found that tuned amplification stages had a tendency to oscillate unless their gain was very limited. This was due to the parasitic capacitance between the plate (the amplifier's output) and the control grid (the amplifier's input), known as the Miller capacitance . Eventually the technique of neutralization

17484-440: 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 1 ⁄ 10 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

17625-411: The use of aerials designed to receive a specific sub-band with greater efficiency than wider-band aerials could. Aerial manufacturers would therefore divide the band into over-lapping groups; A (channels 21–34), B (39–53), C/D (48–68) and E (39–68). The first service to use UHF was BBC2 in 1964 followed by BBC1 and ITV (both already broadcast on VHF) in 1969 and Channel 4 / S4C in 1982. PAL colour

17766-541: The vacated capacity was used for additional digital television services and put into alternative use, such as mobile telecommunications or internet services. Bandwidth for television in the United States was allocated by the Federal Communications Commission (FCC) in 1937, solely in the VHF (Very High Frequency) band, across 18 channels. American television broadcasting began experimentally in

17907-420: The vacuum known as the Edison effect . A second electrode, the anode or plate , will attract those electrons if it is at a more positive voltage. The result is a net flow of electrons from the filament to plate. However, electrons cannot flow in the reverse direction because the plate is not heated and does not emit electrons. The filament has a dual function: it emits electrons when heated; and, together with

18048-421: The voltage applied to the control grid (or simply "grid") was lowered from the cathode's voltage to somewhat more negative voltages, the amount of current from the filament to the plate would be reduced. The negative electrostatic field created by the grid in the vicinity of the cathode would inhibit the passage of emitted electrons and reduce the current to the plate. With the voltage of the grid less than that of

18189-514: The war brought rapid expansion in the nascent broadcast television industry. Thirteen VHF channels was found to be insufficient to support the desired expansion of broadcast television across the United States. Interference and channel crowding in densely populated areas (such as the eastern mid-Atlantic states) was a particular problem. This bandwidth crunch was made even worse by the need to re-allocate VHF Channel 1 to land-mobile radio systems in 1948 due to radio-interference problems. To illustrate

18330-449: Was also technical consultant to Edison-Swan . One of Marconi's needs was for improvement of the detector , a device that extracts information from a modulated radio frequency. Marconi had developed a magnetic detector , which was less responsive to natural sources of radio frequency interference than the coherer , but the magnetic detector only provided an audio frequency signal to a telephone receiver. A reliable detector that could drive

18471-405: Was called the idle condition, and the plate current at this point the "idle current". The controlling voltage was superimposed onto the bias voltage, resulting in a linear variation of plate current in response to positive and negative variation of the input voltage around that point. This concept is called grid bias . Many early radio sets had a third battery called the "C battery" (unrelated to

18612-479: Was converted entirely to digital UHF starting in December 2003, with all analogue television signals (both VHF and UHF) being terminated between 2010 and 2012. The analogue translators in northeastern Ishikawa Prefecture were shut down as part of a technical trial on 24 July 2010; analogue signals in the rest of that prefecture and 43 other prefectures were terminated on 24 July 2011. The analogue transmitters in

18753-617: Was developed whereby the RF transformer connected to the plate (anode) would include an additional winding in the opposite phase. This winding would be connected back to the grid through a small capacitor, and when properly adjusted would cancel the Miller capacitance. This technique was employed and led to the success of the Neutrodyne radio during the 1920s. However, neutralization required careful adjustment and proved unsatisfactory when used over

18894-436: Was first observed in early point contact diodes , then re-discovered during the commercialization of one of the first semiconductor op-amps ; the 709. No single source of burst noise is theorized to explain all occurrences, however the most commonly invoked cause is the random trapping and release of charge carriers at thin film interfaces or at defect sites in bulk semiconductor crystal. In cases where these charges have

19035-425: Was flagging. While the more-established broadcasters were operating profitably on VHF channels as affiliates of the two largest TV networks (at the time, NBC and CBS), most of the original UHF local stations of the 1950s soon went bankrupt, limited by the range their signals could travel, the lack of UHF tuners in most TV sets and the paucity of advertisers willing to spend money on them. UHF stations fell quickly behind

19176-574: Was followed by Ntv7 in 1998 (also acquired by TV3's parent company in 2005) and recently Channel 9 (which started in 2003, ceased transmission in 2005, was also acquired by TV3's parent company shortly after, and came back as TV9 in early 2006). At current count, there are 6 distinct UHF signals receivable by an analog TV set in the Klang Valley: Channel 27 (8TV), Channel 29 (TV3 UHF transmission), Channel 37 (NTV7), Channel 42 (TV9), Channel 55 (TV Alhijrah) and Channel 39 (WBC). Channel 35

19317-416: Was impractical and uneconomic to require these well-established VHF users to move to other frequencies, such as the 300 MHz – 3 GHz UHF band. All of this made expansion of broadcast television channels into the UHF band inevitable, though the technology and broadcasting characteristics of UHF was at this time largely unproven. Even the television standard to use to broadcast on UHF was in question at

19458-403: Was introduced on UHF only in 1967 (for BBC2) and 1969 (for BBC1 & ITV). As a consequence of achieving maximum national coverage, signals from one region would typically over-lap with that of another, which was accommodated for by allocating a different set of channels in each adjacent area, often resulting in greater choice for viewers when a network in one region aired different programmes to

19599-476: 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 efficient at UHF band reception, and the signals are also subject to more environmental interference. Additionally,

19740-525: Was to ultimately become WNET 13 Newark ). Similarly, Delaware also had only one VHF station. Meanwhile, UHF broadcasting until 1949 was designated as experimental. In the fall of 1944, the Columbia Broadcasting System proposed a high-definition black and white system on the UHF band employing 750–1,000 scanning lines that offered the possibility of higher-definition monochrome and color broadcasting, both then were precluded from

19881-401: Was usually connected to the cathode and its negative voltage relative to the anode repelled secondary electrons so that they would be collected by the anode instead of the screen grid. The term pentode means the tube has five electrodes. The pentode was invented in 1926 by Bernard D. H. Tellegen and became generally favored over the simple tetrode. Pentodes are made in two classes: those with

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