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141-519: AIRPASS was a British aircraft interception radar and fire-control radar system developed by Ferranti . It was the world's first airborne monopulse radar system and fed data to the world's first head-up display . The name is an acronym for "Aircraft Interception Radar and Pilot's Attack Sight System". In the Royal Air Force (RAF) it was given the official name Radar, Aircraft Interception, Mark 23 , normally shortened to AI.23 . AIRPASS

282-707: A C-scope display on the CRT. In the post-war period the Mk. X became one of the UK's most widely used fighter radars, largely because a lack of foreign exchange to purchase newer designs, and the poor economy in general which required the RAF to have a "make do" attitude. The Mk. X would go on to equip the first jet-powered night fighters, including the Vampire NF.10 and Meteor NF.11 . Small numbers remained in service as late as 1957. For

423-508: A Junkers Ju 88 A-5 near Chichester . Several advanced versions of the Mk. IV were also produced, which offered direct readings for the pilot and options to allow use in single seat aircraft. However, these developments were overtaken by the rapid improvements in microwave systems, and both the Mark V and Mark VI saw only limited production and service. In February 1940, John Randall and Harry Boot at Birmingham University successfully ran

564-736: A friendly fire incident, killing him and destroying the only prototype. This so greatly delayed the program that the Air Ministry asked Jackson to test the US SCR-720 unit as a stop-gap measure. This proved to be able to pick the bomber from the window, and work on the Mk. IX was given low priority while the UK version of the SCR-720, known as the Mk. X, was purchased. With the night fighter force certain of its ability to continue operating successfully if needed, Bomber Command received clearance to begin using window on 16 July 1943. Work on

705-420: A numerical control system to mill the waveguides from single blocks of aluminium. The signal was sent and received from feedhorns that were split vertically to produce two outputs, one on either side of the reflector centerline. The reflector was shaped as two partial paraboloids, so that the two signals re-combined in space in front of the aircraft. The entire assembly was mounted on a servo system that allowed

846-494: A DC power supply , as a demodulator of amplitude modulated (AM) radio signals and for similar functions. Early tubes used 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

987-582: A Mosquito NF.II was upgraded to the Mk. VIII, serving as the pattern for the Mosquito NF.XII. Starting in December, Beaufighter units were upgraded to the similar Mk. VIIIA, an interim type awaiting production quantities of the VIII. Although the precise origins of the concept are unknown, on 8 March 1941 Lovell mentions the concept of "lock-follow" for the first time in his notes. This was a modification to

1128-649: 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

1269-501: 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

1410-586: 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

1551-505: 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 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

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1692-415: A gentle curve. This pattern is rotated to follow the aircraft's velocity vector. The radar scanned in a U-shaped pattern, taking accurate measurements of the altitude and range to objects in front and slightly to either side of the aircraft. The computer compared the range and altitude of objects in the radar to the pre-calculated path, and then rotated the path so that the terrain feature would be overflown at

1833-661: A ground-mapping display. The AI.18R added modes to support the Red Top missile . The AI Mark 20 was an X-band radar developed by EKCO Electronics for single seat fighters. Code named "Green Willow" by the MoS, it was intended to be a backup system to the AI.23 being developed for the English Electric Lightning (see below). It is believed that the 1953 contract was awarded to EKCO due to their already existing work on

1974-431: 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 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

2115-440: A helical scan instead of spiral. The radar antenna was spun around a vertical axis through an entire 360 degrees 10 times a second, with the transmitter switching off when the antenna was pointed back towards the aircraft. This provided a 150 degree scan in front of the aircraft. As it spun, the antenna slowly nodded up and down to provide altitude coverage between +50 and -20 degrees. The resulting scanning pattern naturally produced

2256-723: A lock at as much as 75° in roll. The dish was unique in that it included a fibreglass ring around the outer rim as a stiffener. Mk. 18 was able to detect the English Electric Canberra at 28 nautical miles (52 km) at altitudes over 20,000 feet (6,100 m) and a closing speed of 900 knots (1,700 km/h). It could detect the Boeing B-47 at 38 nautical miles (70 km) under the same conditions, and could lock-follow after closing to about 25 nautical miles (46 km). When set to its longest range, 100 miles (160 km), it also offered sea surface search, and

2397-400: 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

2538-780: A memo on the topic in 1936, indicating that the Germans would likely begin a night campaign if the daylight campaign went as poorly as he believed it would due to Chain Home. The obvious solution would be to mount a small radar on the aircraft, one able to cover the range between the Dowding system's 5-mile accuracy and the average visual spotting range, about 500 to 1,000 feet (150–300 m). As early as August 1936 "Taffy" Bowen , one of Robert Watson-Watt 's hand-picked radar development team, personally requested that he be allowed to start research into an airborne radar set for this role. This

2679-414: 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

2820-414: A pre-set altitude between 60 and 300 metres (200 and 980 ft). This was relayed to the pilot as a dot in the heads-up display, and by following the dot the aircraft would attempt to maintain the selected altitude by continually raising or lowering the dot as the terrain moved. The ski-shaped curve was selected to ensure any required manoeuvres were half-gee, lowering loads on the crew. The concept became

2961-412: 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

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3102-514: A receiver fit to a Handley Page Heyford bomber, with an antenna consisting of a wire strung between the fixed landing gear . A working transmitter was first fit to the Heyford and flew in March 1937. In spite of this success, the system's antennas were still too large to be practical, and work continued on versions working at shorter wavelengths. A new system working at 1.25 m (220 MHz)

3243-509: 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

3384-520: A separate squegging oscillator was used to produce pulses of the carrier signal using a timer. This timer also muted down the receiver, solving the ringing issue. Minimum range was reduced to about 400 feet. The resulting AI Mk. IV went into production in July 1940 and all units were sent to newly arriving Bristol Beaufighters . The Beaufighter/AI Mk. IV achieved its first victory on the night of 15/16 November 1940, when an aircraft from No. 604 destroyed

3525-407: 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

3666-419: 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

3807-473: A system that displayed the resulting data as a map, opened the possibility of producing a terrain-following radar guidance system. Ferranti developed this concept extensively through the 1960s, first with their Dakota and Canberra aircraft, and later with the Buccaneer. The idea was simple; the computer calculated a ski-ramp shaped ideal trajectory, flat directly under the aircraft and then sloping upward in

3948-405: 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, 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

4089-456: 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 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

4230-400: A very high plate voltage away from lower voltages, and accommodating one more electrode than allowed by 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

4371-429: 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

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4512-445: 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

4653-406: 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 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

4794-469: Is not heated and does not emit electrons. The filament has a dual function: it emits electrons when heated; and, together with 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

4935-410: 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

5076-571: Is the AI.24 radar of the Tornado ADV . These radars were often given common names as well, and generally better known by these; the AI.24 is almost universally referred to as "Foxhunter". Other widely used post-war examples include the AI.18 used on the de Havilland Sea Vixen , and the AI.23 Airpass on the English Electric Lightning . This article will use Mk. or AI. depending on which is most commonly used in available references. In order to provide

5217-564: 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

5358-416: 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

5499-564: 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

5640-636: 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 ,

5781-429: 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

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5922-480: 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

6063-482: 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

6204-689: The Bristol Beaufighter by early 1941. The Mk. IV helped end the Blitz , the Luftwaffe ' s night bombing campaign of late 1940 and early 1941. Starting with the AI Mk. VII , AI moved to microwave frequencies using the cavity magnetron , greatly improving performance while reducing size and weight. This gave the UK an enormous lead over their counterparts in the Luftwaffe , an advantage that

6345-510: The Fairey Fireflash missile illumination radar. AI.20 was significantly simpler than the AI.23, being much closer in design to an upgraded AI.17 than the much more advanced AI.23. It used a simple spiral scan system driven at 10,000 RPM, scanning out to 45 degrees and then back every 2.25 seconds. Testing started in 1955, and the AI.20 demonstrated its ability to lock-on to a Hawker Hunter sized target at 7 miles (11 km) 95% of

6486-592: The Fairey Firefly , which had the size to carry a radar operator and the performance to operate as a fighter. Some were also used on the Mosquito. Considerably later, a single Meteor, EE348 , was fit with an APS-4 in a nose mounting as a test vehicle. The APS-6 was a modification of the APS-4 specifically for the interception role. It replaced the side-to-side scan with a spiral-scan system largely identical to

6627-571: The Fleet Air Arm , the TRE developed a series of AI radars operating at the even shorter 3 cm wavelength, the X band , which further reduced the size of the antennas. The original model was the Mark XI, followed by the improved Mark XII and lightened Mark XIII. It is not clear if any of these models saw service, and few references mention them even in passing. These designations were given to

6768-526: The H2S radar project and was replaced by Arthur Ernest Downing. This delayed the project just long enough that it got caught up in a great debate that broke out in the summer of 1942 about the use of window , today known as chaff . Window caused false returns on radar displays that made it difficult to tell where the bombers were amid a sea of blips. Bomber Command had been pressing to use window over Germany to reduce their losses, which were beginning to mount as

6909-505: The HOTAS (Hands On Throttle-And-Stick) system whereby the radar and gun sight controls were situated on the control column and throttle lever instead of elsewhere in the cockpit, eliminating the need for the pilot to take his hands off the controls while making an interception. The radar entered service with the RAF in 1960 in the English Electric Lightning interceptor. The next version of

7050-540: The Mirage III . The radar of AIRPASS I weighed around 90 kg. Aircraft interception radar The term was first used circa 1936, when a group at the Bawdsey Manor research center began considering how to fit a radar system into an aircraft. This work led to the AI Mk. IV radar , the first production air-to-air radar system. Mk. IV entered service in July 1940 and reached widespread availability on

7191-663: The Panavia Tornado ADV , an interceptor development of the Tornado that provides long-range defense against bomber-like targets. Development of the ADV began in 1976 and the radar system contract was eventually won by a curious combined bid; Marconi and Elliot Automation would provide most of the design, while Ferranti built the transmitter section and Antenna Platform. Vacuum tube A vacuum tube , electron tube , valve (British usage), or tube (North America)

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7332-487: 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

7473-467: 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

7614-496: The rainbow codename Blue Parrot ), was modified to handle low-level scanning by eliminating the reflections from waves. Since the waves reflected away much of the signal, to make up for these losses the new version used a more powerful 250 kW transmitter and a larger Cassegrain antenna . During tests of the monopulse systems, Ferranti engineers noticed that the systems produced high quality ranging information of ground reflections. Older systems without monopulse processing made

7755-413: 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

7896-672: The AI.20 primarily in the details of the operation and visual presentation. This was later upgraded to the Mark 2 model that equipped the V-force for most of its lifetime. As the Javelin ran into delays, it was decided to increase the useful life of the existing Meteor and Vampire night fighters with a new radar. After considering three US designs, they chose the Westinghouse AN/APS-57 . Its 200 kW transmitter improved range to as much as 25 miles (40 km) although this

8037-476: The AI.23B, added additional small antennas for S-band reception of signals from ground based radars. This allowed the aircraft to seek out targets with its own radar transmitter turned off. The signal was displayed in A-scope fashion along the bottom of the radar display, which the pilot could use to seek out targets while under ground direction. When a peak of the required height appeared, the pilot would take over

8178-718: The AIRPASS design were used on many subsequent radars from Ferranti, while its head-up display was licensed for use in the United States , where it was quickly adopted for many aircraft. Development of the monopulse radar underlying AIRPASS began in 1951 at Ferranti's Ferry Road location in Edinburgh . The AIRPASS system was announced to the public in late June 1958. It was initially tested on Douglas DC-3 TS423 (later civilian registered as G-DAKS) and later on an English Electric Canberra WJ643 for higher speed trials, replacing

8319-485: 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

8460-551: The FAW.6. The last AI.17-equipped Javelin FAW.9's ended their service in Singapore in 1968. Having lost the contest for the Javelin, GEC submitted an updated version of the Mk. 16 for the contest for the de Havilland Sea Vixen . This produced the Mk. 18. Mk. 18 operated in the X band with a 180 kW peak power, using a 29 inches (740 mm) parabolic dish that could be pointed ±100° in azimuth, +50/-40° in elevation, and could keep

8601-474: The German defensive network improved. Fighter Command was concerned that if Bomber Command used it over Germany, the Germans would return the favour and use it over the UK. A series of tests carried out in September 1942 by Wing Commander Derek Jackson suggested that some changes to the display systems might solve the problems with window on the Mk. VIII. At this point it was suggested that the Mk. IX might ignore

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8742-419: The Mk. IV, but as the timebase now spun, they drew short arcs on the display during the period the antenna was pointed in that direction. Like the Mk. IV, the distance from the center of the CRT indicated the range. As the target moved closer to the centreline of the aircraft, the beam spent more time painting the target, and the arc spread out, becoming a ring when dead ahead. First introduced in March 1941, it

8883-482: The Mk. IX continued, but it never saw operational service. In testing in 1944 it was found to be marginally better than the US SCR-720, but with the SCR-720 expected to arrive at any moment, the demand for another radar was not pressing. Instead, the Mk. IX was given more time to mature. Further development led to more testing in 1948, but it was again passed up for production and cancelled the next year. The Mark X

9024-639: The Mk. VII, requiring very large amount of aircraft space for the install. Conversions on the Beaufighter began in December 1941. This run was followed by the production Mark VIII that included the new "strapped magnetron" of 25 kW, improving range to about 5.5 miles (8.9 km). This version also had several major clean-ups in the electronics, support for IFF Mark III which caused a sunrise pattern to appear when aimed at friendly aircraft, and beacon tracking allowing it to home in on ground-based transmitters emplaced by friendly units. In September 1942

9165-494: The Mk. X to soldier on while a definitive jet-powered night fighter evolved. This effort underwent similar delays and setbacks before finally emerging as the Gloster Javelin . Two radar sets competed for the design, the Mk. 16 and Mk. 17. The later went into production, and is better known as the AI.17. General Electric Company 's Mark 16 was one of two similar designs competing to equip the Gloster Javelin . The contest

9306-740: The NF.14, which started deliveries in June. Likewise the de Havilland Venom received the Mk. 21 to become the Venom NF.3, also entering service in June, but was withdrawn by the end of 1957. The Sea Venom flew the Mk. 21 until 1959, and in second-line duty until 1970. The Mark 22 was the British version of the US AN/APQ-43 , This consisted of two radar antennas driven from a common magnetron transmitter. One used spiral-scan to search for targets, while

9447-627: The US AN/APS-4 and AN/APS-6 radars, small under-wing X band radars used primarily by naval aircraft. The APS-4 was originally developed as the ASH, a forward-aimed surface-search system. It was packaged into an underwing pod so it could be used on single-engine aircraft like the TBM Avenger . It proved to have a useful interception function, and was modified to be able to scan up and down as well as just side to side. The Fleet Air Arm mounted it on

9588-462: 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 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

9729-400: 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

9870-435: 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 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

10011-404: The antenna assembly to be pointed in two dimensions. On reception of a pulse, the signal was sent into a klystron local oscillator and then into two conventional superheterodyne receivers with an intermediate frequency of 30 MHz. The monopulse technique requires the signals from the two channels to be compared in strength, so the output of the amplifiers must be precisely matched. This

10152-625: The approach and then turn on their own radar when the conditions appeared correct. This allowed the aircraft to make accurate approaches without signalling their presence or inviting jamming from the target. The same system was used in the E/F-band to provide home-on-jam. In the late 1950s, Ferranti won the contract to supply radars for the Blackburn Buccaneer aircraft in Royal Navy service. This version, AIRPASS II (also known by

10293-479: 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 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

10434-521: The basis for the ill-fated BAC TSR-2 project, which used another modified version of the AIRPASS hardware, now extensively transistorized . As part of a proposal for the Swiss Air Force , Saab AB modified a single Saab 35 Draken by replacing its relatively simple radar system with an AIRPASS II radar. This produced the Saab J35H (H for Helvetia ), but the contract was ultimately won by

10575-404: The cathode slam into the anode (plate) and heat it; this can occur even in an idle amplifier due to 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

10716-536: 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

10857-400: 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 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

10998-418: The determination of range difficult as the radar returned signals were from the entire beamwidth, meaning that it received signals that were from the ground that was closer and further from the aircraft. Monopulse processing allowed the beam to be discriminated vertically and thus range against a single feature very accurately. The ability of the radar to produce highly accurate range measurements, combined with

11139-428: 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 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

11280-445: 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 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

11421-546: 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 a certain sound or tone). Not all electronic circuit valves or electron tubes are vacuum tubes. Gas-filled tubes are similar devices, but containing

11562-417: 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; 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

11703-425: 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

11844-419: 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 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:

11985-427: The first cavity magnetron , eventually generating 1 kW at 9.8 cm (3,060 MHz). Supported by GEC, the device quickly developed into a practical 10 kW system, and several test units were available by May 1940. Microwave wavelengths are so much shorter than the Mk. IV's 1.5 m, fifteen times, that the dipole antennas required for reasonable gain were only a few inches long. This dramatically reduced

12126-444: 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 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

12267-435: The gunsight, as well as computer-calculated cueing information that located both the target and the proper position to fly to engage based on the selected weapon. For instance, when using missiles, the system guided the aircraft not toward its target, but a point behind it where the missile could be fired. This gave the system its name, AIRPASS , an acronym for aircraft interception radar and pilot's attack sight system. AI.23

12408-499: The headquarters staff at the University of Dundee attempted to develop their own solutions to the problem. This led to considerable strife and in-fighting between the two groups. The AI group was eventually broken up at the end of March 1940, leaving Bowen out of the AI effort. A solution was eventually provided by EMI who had developed a new type of transmitter that was not based on the common self exciting principle. Instead,

12549-418: The infamous 1957 Defence White Paper , but by this time the interim English Electric Lightning design, the P.1, had progressed to the point where development was undertaken anyway (along with TSR.2). This led to continued development of the AI.23 for this aircraft (and Mk. 20, see above), and it was given the official designation "ARI 5897". The system was mounted entirely in a single bullet-shaped housing that

12690-411: 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

12831-628: The maximum possible warning time of an incoming raid, the RAF's Chain Home (CH) radar stations had been positioned as far forward as possible, right on the coastline. These systems could only see targets in front of them, over the English Channel . Tracking over land fell to the Royal Observer Corps (ROC) using visual means. In testing it was found that the two different reporting systems provided information that varied enough to make tracking targets confusing and error prone, and

12972-561: 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

13113-445: The nose sections of these aircraft. After testing use, WJ643 was renamed T.Mk 11 and used as a trainer aircraft for the radar operators of the Gloster Javelin . Several further T.Mk 11 were produced, but these mounted the AI.17 from the Javelin. The first flight on the English Electric Lightning took place on airframe XG312 on 29 December 1958. It was initially linked to the de Havilland Firestreak air-to-air missile . It introduced

13254-431: 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,

13395-435: 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

13536-541: The one in the Mk. VIII. It also included a switch that reduced the scanning pattern to a 15 degree cone in front of the aircraft, producing a C-scope view used during the final approach. This was paired with a new and much smaller display, allowing it to be fit to smaller single-seat aircraft. It was widely used on the F6F Hellcat and F4U Corsair . With Mk. IX cancelled in 1949, the Ministry of Supply (MoS) allowed

13677-528: 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

13818-399: 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

13959-448: 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,

14100-532: 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

14241-461: The radar could only see targets directly in front of the antenna, unlike the Mk. IV which could see anything in the entire volume in front of the aircraft. To solve this problem, the dish was mounted on a bearing system from Nash & Thompson that allowed it to be rotated in a spiral pattern. The cockpit display was modified to spin the timebase at the same speed as the antenna, 17 times a second. The display still produced blips similar to those on

14382-480: The receiver causing it to oscillate or ring for a period. While this powerful signal was dying down, reflections from nearby aircraft were lost in the noise. Numerous solutions had been attempted, but were of limited use. Starting in late 1939 the development team was asked to fit the existing Mk. III design, of limited use, to aircraft. This ended further attempts to address the minimum range issue while they worked on installations. While their development effort ended,

14523-403: The same target into a single track. Telephone operators, or "tellers", would then forward this information to group headquarters who would re-create the map, and then from group to the sector HQs who would give instructions to the fighter pilots. Due to delays in the flow of information between the various centres, and inherent inaccuracies in the reports coming from multiple sources, this system

14664-634: The second used conical scanning for tracking at close range. This was one of the earliest radars to offer track while scan (TWS) operation, although it did so through the use of what was essentially two radars. The APQ-43 was one of three designs also considered for updated versions of the Meteor and Venom, the others being the AN/APQ-35 which also had two-dish TWS, and the AN/APS-57. The -35 and -43 proved too large to install in these aircraft, forcing

14805-454: The selected weapon, and presented the results in the reflector gunsight mechanism. The system also read data from various aircraft systems like the altimeter and air speed indicator and combined this into the same display. The result was the world's first heads-up display , a concept that was soon licensed by US manufacturers. AIRPASS had an average detection range against a Tupolev Tu-95 "Bear" bomber of about 40 miles (64 km). This

14946-474: The selection of the -57 as the Mk. 21. The two TWS units proved interesting, and the -43 was considered for the Javelin. These were used in small numbers in the FAW.2 and FAW.6 models. Ferranti 's Mark 23 was an X band design originally designed for the modified Fairey Delta 2 proposed for the Ministry of Supply's Operational Requirement F.155 for a modern interceptor aircraft . Work on F.155 ended with

15087-565: The sheer volume of information could be overwhelming. Hugh Dowding addressed this through the creation of what is today known as the Dowding system , networking together the radars and observation centres by telephone to a central station. Here, in the Fighter Command 's "filter room" at RAF Bentley Priory , operators would plot the map coordinates sent to them on a single large map, which allowed them to correlate multiple reports of

15228-436: The size of the system, allowing it to fit entirely in the nose of the aircraft. While a team under Herbert Skinner developed the electronics, Bernard Lovell was put in charge of examining the use of a parabolic dish to improve the directionality of the signal. The resulting beam was so sharply focussed, spanning about 10 degrees, that it easily avoided ground reflections at even low altitudes. The narrow beam also meant that

15369-455: The spiral-scan system that allowed it to track targets automatically without further manual operation. This became known as AIF. "Freddie" Williams joined the effort, and by the autumn of 1941 the system was basically functional and plans began to introduce it as the Mark IX. Several unrelated events conspired to greatly delay further progress. On 1 January 1942 Lovell was sent to work on

15510-440: 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

15651-441: The system was called AIRPASS II, or " Blue Parrot ", and was a system optimised for use at low-level and originally developed for the cancelled BAC TSR.2 and subsequently used in the Blackburn Buccaneer . AIRPASS was based on a magnetron source which provided pulses of about 100 kW peak. Pulses were about one microsecond in duration and sent 1000 times a second. To make the system as compact as possible, Ferranti invested in

15792-471: The time, excellent performance for that era. Nevertheless, as AI.23 began successful trials the same year, further work on AI.20 was cancelled. The next year the MoS published a requirement for a new tail warning radar for the V bomber force, replacing the original Orange Putter , and quickly chose the AI.20 as its basis. This was developed into the ARI-5919 Red Steer , which differed from

15933-458: 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

16074-407: 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

16215-646: 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

16356-404: The tubes' heaters to be supplied from 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

16497-482: 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

16638-406: The vacuum, a process called thermionic emission . This can produce a controllable unidirectional current though 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

16779-421: 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

16920-435: The war. Practical ASV radars were operational in 1940, but the AI developments proved much more difficult. It was not until 1939, with the war obviously looming, that the team was once again moved back to AI development full-time. A lingering problem was that the minimum range remained around 1,000 feet, too long to allow easy interception. This was due to the transmitter signal not turning off sharply, leaking through to

17061-399: The window completely, as the light metal strips rapidly dispersed from the target being tracked, faster than the radar could follow. Further testing by Jackson demonstrated the opposite was true, and that the Mk. IX almost always locked-on to the window instead. Arthur Downing quickly implemented several changes to fix this problem. He was personally operating the system when he was shot down in

17202-525: Was a version of the Airpass dedicated to low-level flying, especially target detection, fitted to the Blackburn Buccaneer . Further development led to the terrain following radar used in the BAC TSR.2 . Many other variants were proposed for a wide variety of projects. The final radar in the UK series of AI designs to see deployment was the Mark 24, better known as "Foxhunter". Foxhunter was developed for

17343-510: Was able to detect and track a Bear-sized bomber at 40 miles (64 km), allowing the Lightning to accomplish fully independent interceptions with only the minimum of ground assistance. A version with fully automated guidance that would have flown the aircraft into range and fired its missiles automatically was cancelled in 1965. Further development of Airpass led to AI.23 Airpass II, code named "Blue Parrot" and also known as ARI 5930. This

17484-410: Was accomplished with a highly advanced automatic gain control system with 100 dB range that adjusted the pulse-to-pulse outputs. To this point the system was entirely analogue, using miniaturized vacuum tubes cooled by forced air. Behind the analog section was the analog computer portion of the system. This took the outputs from the radar system, calculated the proper intercept course based on

17625-408: Was accurate to perhaps 5 miles (8.0 km). Within 5 miles the fighters would normally be able to spot their targets visually and complete the interception on their own. Interception rates over 80% was common, and on several occasions the system succeeded in getting every fighter launched into position for an attack. While the Dowding system proved invaluable inputs during daylight attacks, it

17766-449: 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

17907-564: Was approved, and the small aircraft interception team set up shop in Bawdsey Manor 's two towers. At the time, radar development was in its infancy and the other teams were working with long- wavelength transmitters operating around 7 meters. An efficient antenna requires it to be about 1 ⁄ 2 the wavelength or more, which demanded antennas at least 3 metres (9.8 ft) long, impractical for an aircraft. Additionally, available transmitters were large, heavy and fragile. The first AI experiments thus used ground-based transmitters and

18048-405: 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

18189-574: Was decided to produce another version of the Javelin with the US AN/APQ-43, which on paper appeared to be a better system. In RAF service the APQ-43 became the AI.22, and produced the Javelin FAW.2. In practice, the two systems offered similar performance and the AI.17 quality issues were soon addressed. Future versions of the Javelin mostly mounted the AI.17, although the AI.22 was also used on

18330-617: 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

18471-486: Was essentially useless against night raids. Once the enemy aircraft passed the coastline they could not be seen by the radars, and the ROC could not see at night except under ideal conditions with bright moonlight, no cloud cover, and considerable luck. Even when tracks could be developed, the difficulty of spotting a target from the cockpit of an aircraft while flying it at night proved to be equally difficult. Henry Tizard wrote

18612-434: Was eventually won by AI.17. AI.17 was essentially a version of the Mk. IXC with a number of detail cleanups and a 200 kW magnetron, as well as the ability to cue the "Blue Jay" missile that was then under development. It could detect a Javelin-sized target at about 20 nautical miles (37 km; 23 mi). AI.17 entered service with the Javelin in early 1956. Early sets had considerable reliability problems and it

18753-487: Was found that the ground reflection created a sort of artificial horizon on the bottom of the display, a surprising side-effect which proved very useful. However, the limited power of the magnetron, about 5 kW, provided range of about 3 miles (4.8 km), not a great improvement over the Mk. IV. Performance of the system at low altitude was so improved over the Mk. IV that it was decided to make an initial run of 100 units out of what were essentially prototype systems as

18894-437: Was more than enough to allow the Lightning to be directed into the general area of the target through ground controlled interception and then use AIRPASS to hunt it down. Some consideration was given to sending the aircraft into the right area using commands sent from the ground to the aircraft's autopilot , allowing the pilot to concentrate solely on their radar display, but this project was ultimately cancelled. Later models,

19035-519: Was rarely achieved in practice. It also included various beacon homing modes, as well as an air-to-surface mode for detecting ships. This was modified to add a British strobe unit and variable pulse repetition frequency , becoming the Mark 21. The Mk. 21 was first used on the Meteor NF.12 and flew for the first time on the 21 April 1953, entering service in January 1954. Small improvements produced

19176-442: Was ready by August 1937 and fitted to Avro Anson K6260 at RAF Martlesham Heath . This unit demonstrated the ability to detect aircraft at the range of about 1 mile (1.6 km) in the air-to-air mode, but also demonstrated the ability to detect ships on the ocean at ranges up to 3 miles (4.8 km). This ability led to the split between AI and air-to-surface-vessel (ASV) radar systems, both of which would be widely used during

19317-423: Was suspended within the Lightning's circular nose air intake. The AI.23 was the world's first operational aircraft interception monopulse radar system. The monopulse method allows higher resolution and is far more resistant to common forms of jamming . AI.23 also included all of the features of earlier AI radars, and more. Among the highlights were an automatic lock-follow system which fed ranging information to

19458-402: Was the UK version of the SCR-720. This was originally promised for delivery in the summer of 1942, but ran into delays and only started arriving in December 1943. These were fit to the Mosquito to produce the NF.XVII and later versions. Conversions at operational units began in January 1944, and the Mk. X remained in service through the rest of the war. Compared to the Mk. VIII, the SCR-720 used

19599-498: Was to exist for the remainder of World War II . By the end of the war, over a dozen AI models had been experimented with, and at least five units widely used in service. This included several US-built models, especially for the Fleet Air Arm. The AI naming convention was used in the post-war era as well, but these generally dropped the "Mk." when written in short form and used numbers instead of Roman numerals . A good example

19740-401: Was used on the English Electric Lightning throughout its lifetime. The basic AIRPASS electronics system was later adapted as the basis for a terrain-following radar for navigation and targeting for air-to-ground attacks. This AIRPASS II was originally intended for the BAC TSR.2 , but when that aircraft was cancelled in 1965, it was subsequently used in the Blackburn Buccaneer . Elements of

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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