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The making of the E1T or The making of the Z550M/ZM1050 or The importance of the emergency landing of a German seaplane in 1917 For the early development of Philips Research. |
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The EF50 RF penthode constitutes a landmark in the history of the radio tube. Before the EF50, all radio tubes were fabricated using technology which was directly derived from the technology of making light bulbs. The EF50 in contrast had, like all “modern” radio tubes, a base made from pressed glass. This enabled the designers at Philips to combine superior performance with a low-cost high volume production technology in a single tube. Through a remarkable chain of events the EF50 played an important role in radar in World War II. Several accounts of the history of the EF50 have been given, both in literature as well as on the Web [1]. On this page I have tried to gather all this information, and to go as much as possible directly to the sources. On this page you will find an account of the history of radar and companies like PYE, Mullard and Philips in as far as it is relevant to the story of the EF50. Using unique Philips sources, special attention will be given to the novel fabrication technologies that were used for the EF50. Finally, and for me personally most interestingly, I have tried as much as possible to reveal the people behind the technology.
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Edward George “Taffy” Bowen
Edward George “Taffy” Bowen was born the 14th January 1911 in Cockett near Swansea, Wales [2]. Being highly intelligent, he was able to get a good education by winning scholarships. From a very early age he developed a strong interest in radio (and cricket). He studied physics at Swansea University College and graduated with First-Class Honors degree in 1930. He completed his doctorate under Professor E.V. Appleton at Kings College London. [3] In 1926 Appleton had proven the existence of the first ionospheric layer (now called the E-layer) by the reflection of radio waves. In 1947 he was to receive the Nobel prize for his contributions to exploring the ionosphere.
As part of his research, Bowen spent a large part of 1933 and 1934 at the Radio Research Station at Slough. In the early months of 1935 an advertisement for a job position at the Radio Research Station was issued and Bowen decided to apply. After a very relaxed interview with Watson Watt, the superintendant of the institute whom he already knew from his PhD work, he was given the position. He joined the staff of the Radio Research Station towards the end of April 1935 as a Junior Scientific Officer. By then he was still completely unaware of the fact that the Radio Research Station harbored a great secret. But that was soon to change. On his very first day he was introduced to the provisions of the Official Secrecy Act, which stated the penalty for the slightest deviation from the most meticulous standards of security: “to be hanged by the neck until life was extinct!” After he had signed the contract, a highly impressed Bowen was told of the secrets of radar.
Figure 1. left: Sir Edward Victor Appleton (1892-1965), right: Sir Henry Tizard (1885-1959)
Radar had been "in the works" in Germany as early as 1933. Not long after that it started being developed in England, France, Holland, Italy, Russia, and the United States. Only England, however, really pushed radar and its practical use. By the time the Nazis were ready to start the blitz of England in July 1940, England had 29 radar stations making an invisible curtain along its southern and eastern coasts. According to Bowen, this was largely due to the vision of one man, Sir Henry Tizard.
Tizard was a chemist by training and he had spend a year working in Nernst’s laboratory in Berlin in 1908. As such he knew the German people and he had a first-hand experience of that country preparing to go to war. During World War I Tizard served in the Royal Flying Corps and in 1917 he was in charge of the scientific work of the Aeroplane and Armaments Experimental Station. Between the wars Tizard held a variety of governmental posts, perhaps the most important of which was as Secretary of the Department of Scientific and Industrial Research. As such Tizard was very much aware of the length of time which must elapse between initiating something at a research level and its practical application.
In 1934 Tizard was the rector of Imperial College. Towards the end of 1934 he began to think about the problems of air defense and what could be done about it. He realized that in a few years’ time Britain would be subjected to a devastating air attack and , as things stood, the country was defenseless against it. Most likely on initiative of Tizard himself a committee was formed to consider this problem. The committee consisted of P.M.S. Blackett [5], A.V. Hill [6] and H.E. Wimperis, with Tizard as Chairman. It became known as the “Tizard Committee.”
Figure 2. left: Sir Robert Watson Watt (1912-1973), right: Arnold F. Wilkins.
There were at that time numerous rumors that Germany had developed a “Death ray.” Engines would stop and animals and people would drop death when subjected to such a death ray. To assess the truth of these rumors the Tizard committee decided to consult Watson Watt. Watson Watt was at that time the Superintendent of the National Physical Laboratory (NPL). As a physicist Watson Watt had worked on the detection of dangerous thunderstorms. To do this he had designed an elementary radio direction finder which on a cathode-ray tube gave the direction of thunderstorm activity. Watson Watt passed the “death ray” question on to Arnold Wilkins, and he asked him to calculate how much energy would be required to damage an aircraft or adversely affect the crew. The answer came out at an impossibly high figure. However, in the course of making his calculations, Wilkins realized that if radio waves were send in the direction of an approaching aircraft, while no damage would be done, there was a possibility of something quite different, namely detection of radio waves reflected back from that aircraft.
Figure 3. left: Watson Watt’s apparatus for studying waveforms of ionospherics (ca. 1924),
right: Pulse Ionosonde receiver built by Bainbridge-Bell (1933).
This proposal was put forward in the famous memorandum “Detection and Location of Aircraft by Radio Methods” which Watson Watt submitted to the Air Ministry on th 12th of February 1935 [7]. By a happy coincidence, the Air Member for Research and Development at that time was the legendary Air Vice-Marshal Sir Hugh Dowding, the man who was destined to play a vital role as Commander-in-Chief during the Battle of Britain. In his gruff way, his first reaction to Watson Watt’s memorandum was that he was not much impressed by calculations, but if a practical demonstration were given he might be convinced. This led to the historic occasion on the 26th February 1935 when A.F. Wilkins demonstrated the reflection of radio-waves originating from the BBC short-wave transmitter at Daventry using a Heyford bomber as target. It is completely to the credit and vision of Hugh Dowding that he granted the initial sum of £ 10000 for initial work and in this way secured the victory of Britain in the upcoming war. It was at this point that Taffy Bowen came into the picture as the most junior of the three scientists assigned to the task: Wilkins, Bainbridge-Bell and Bowen.
Figure 4. From left to right: Air Vice-Marshal Sir Hugh Dowding, the RAF Heyford bomber of the type that was the first radar target in England in February 1935, Arnold Wilkins in the van radio van that was used in the first radar demonstration.
In order to preserve secrecy, it was decided to erect the first radar system not at Slough, but at a remote site at Orfordness. This was a remote split of salt marsh, already owned by the Air Ministry that could be reached by boat from Orford. The group of three complimented by George Willis, the technical assistant of Bainbridge-Bell arrived at Orfordness on the 13th of May 1935. During the first weeks this small group worked like slaves to have the first system installed. The receiver and the cahode-ray indicator were basically identical to the set-up for detecting thunderstorms at slough and were assembled by Wilkins and Bainbridge-Bell. The transmitter was of a completely new design and the responsibility of Bowen. It was basically a couple of NT46 valves, the highest transmitting power valves used by the British Navy, used in a push-pull arrangement. With the filament consuming some 20 amps at 20 volts and the anode voltage pushed to 12000 volts, the transmitter eventually reached an output power of 200 kilowatts. Within a month on Monday the 17th of June the first echo’s from a Scapa flying boat were received from a distance of 17 miles.
Figure 5. The NT46, the transmitter tube used in the first radar experiments in Orfordness in may 1935. The valve measures 500x105 mm and nominally draws 40A of filament current at 15V and is rated at a maximum anode voltage of 10000V.
From that moment the progress was very rapid. Martlesham airbase assumed responsibility for the test flights of the new radar system. With such flights on a daily basis, the performance of the equipment rapidly improved until it reached more than 100 miles in the early part of 1936. The successes in Orfordness prompted the Air Staff to ask for a setoff five stations to provide air warning over the Thames estuary at the expense of a sum of £ 1000000. It was clear that this task was far beyond the small staff at Orfordness so it was decided to increase the staff and to move to a more favorable site. Bawdsey Manor located not far from Orfordness at a location on the coast some 30 meters above sea level proved to be the ideal place. The Manor was purchased by the Air Ministry and in March 1936 the radar group gradually took possession over the estate.
Figure 6. Map showing the Orfordness, Bawdsey and Martlesham area as it was in 1935. On the right, the receiver hut on Orfordness now.
Bawdsey Manor became an exceedingly busy place. The fast growing staff worked hard, often well after midnight. But there was also time for a swim before lunch or a game of cricket before dinner. The stimulating atmosphere has been compared to that of Oxford or Cambridge. Enormous 80 meter high antennas were erected and the white tower was converted in a laboratory. As the construction of the radar stations proceeded, another vital component was added to the scheme. The Tizard Committee, alert as always to the problems in hand, pointed out that it was not sufficient just to get a warning of approach of enemy aircraft. There was a need to coordinate the information coming from different stations, for decisions to be taken on which squadrons should be deployed against the enemy, and for precise instructions to be given to the fighters. As a result when the final air defence system came into being it was, in contrast the radar systems in other countries, a fully integrated one. It consisted of the air warning network, a filtering process for assessing and collating the data and an efficient communications system for alerting the fighters and guiding them to their targets. The performance of the stations was so good that before the end of 1936 plans were made to extend the air warning network to a chain of 19 stations along the whole of the east coast, later extended by another 6 stations to cover the south coast. The Battle of Britain could never have been won without “Home Chain” as the radar system was called by then. It was a tribute to the genius of Tizard who foresaw the problem, and to Watson Watt who provided the solution.
Figure 7. Bawdsey Manor, the headquarters of radar research in Britain from 1936 until the outbreak of the war in 1939.
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The success of the integrated Home Chain radar system prompted Tizard and his committee to start thinking again about the next steps to be taken. Tizar argued that once the RAF had a much better chance of repelling the German Bombers at daytime, they would undoubtedly increase their night bombing efforts. At night, the Home Chain radar system was much less effective. The range at which an enemy aircraft could be seen was less than 300m, and it was clearly beyond the capability of the Home Chain system to guide a fighter that accurate to its target. What was required was a radar small enough to be installed in a night fighter, which would enable a pilot on his own initiative to close a range of 4 or 5 miles down to the required 150 to 300m.
Figure 8. Left, Bawdsey Manor anno 2008 the red tower with the white tower behind it. Right, the transmitter towers at Bawdsey of the Home Chain system during World War II.
No one was very optimistic about the feasibility of such a miniature radar system at that time. The original wavelength of the radar system during the first trials in Orfordness was 50 meters (6 MHz). Watson-Watt had chosen this wavelength on the assumption that the best reflected signals from an aircraft would come on the wavelength at which the wing span acted as a dipole resonator. Since many bombers of that period had a wing span of about 25 meters, they would thus resonate at 50 meters wavelengths. Due to interference with commercial traffic this wavelength was later reduced to 26 meters and again later to the final wavelength of 10 to 13 meters. To achieve a reasonable antenna size for airborne systems, the operating wavelength would have to be reduced to one or two meters (150-300 MHz), which was clearly beyond what was possible at that time. In addition the pulse width would have to be reduced from 20 to 1 microsecond which was strictly unknown territory. On top of that the size and weight of the system had to be such that it would fit into an airplane.
Figure 9. Left A.G. Touch, middle ‘Perc’ Hibbert and right Keith Wood members of the original airborne radar team.
Despite all the obvious difficulties Bowen managed to persuade Watson-Watt to embark on the airborne project around mid-1936. A small team was formed consisting of Bowen, Gerald Touch, Sidney Jefferson and Perc Hibberd. One of the most important designs restrictions for the airborne radar system was that the antenna should not cause too much aerodynamic drag eliminating any type of long wire antenna. The most that could be tolerated was a stub antenna, half a meter or so in length, demanding a wavelength of about a meter (300MHz). This was on edge of what was possible in 1936. In his book “Radar Days” Bowen recollected a striking story from this period:
| ‘About this time, we stumbled on a gem beyond price; it was a tuned radio frequency (TRF) receiver designed by EMI for their projected television service from Alexandra Palace. It operated on a frequency of 45 megacycles per second or a wavelength of 6.7 meters and had a bandwidth of 1 Mhz. It had 7 or 8 valves on a chassis about 3 inches wide by 15 or 18 inches long. I cannot quote the sensitivity, but it was far and away better than anything which had been achieved in Britain up to that time. This receiver formed the basis of our whole airborne radar experimental programme for the next two years. I have never discovered exactly how we came by that particular set, but I suspect it came through the back door of the EMI Company; they had no idea of the use to which it was being put. During the next few years we made strenuous efforts to obtain additional receivers of the same design; but although the negotiations were conducted by Watson-Watt himself, he failed to produce another chassis. This account being written some 50 years later and it is difficult to believe that until the end of 1938, when we had two Ansons and the Battles as experimental aircraft and at least a number of radar transmitters to share between them, we only had the one receiver chassis which was switched from one aircraft to the other as occasion demanded!’ |
Figure 10. Left, the Western Electric 316A “giant Acorn” or “door-knob” valve that was used in the first 6.7 meter airborne radar transmitter. Middle, the 4304 CB (alias NT58, VT62, CV2761). Right, an RCA Acorn tube that was used in the down converter of the first 1.5 meter radar system.
Rapid progress was being made in the airborne group however, and by March 1937 Perc Hibbert had built a 6.7 meters transmitter using two Western Electric 316A “giant Acorn” or “door-knob” valves as they were called. Although the output power could not have exceeded a few hundreds of watts at a pulse length of 2 to 3 microseconds and a repetition frequency of a 1000Hz, it enabled the first fully airborne radar system with a detection range of three to four miles. The system was installed in the Heyford, and in March 1937 the first ground objects were detected during a trial flight. This success generated considerable enthusiasm and Bowen and his team were given free choice for an aircraft especially devoted to the airborne radar program. They choose and Anson, and much to their surprise they were allocated not one but two aircraft.
Figure 11. Left, the Avro Anson K8758 aircraft acting as target during the initial airborne radar experiments photographed from the radar equipped K6260. Middle, a Fairey Battle. Right, the Blenheim MK IV.
During the summer of 1937 Bowen and his team worked hard getting the wavelength of the airborne radar system down. By converting the transmitter to push-pull operation using two 316As, they managed to reduce the wavelength to 1.25 meters at a pulse width of one microsecond. Below 1.25 meters the output power of the transmitter dropped sharply. Touch rebuilt the receiver to operate at 1.25 meter by converting it to a super-heterodyne, using acorn valves from RCA in a mixer ahead of the one and only EMI chassis. The latter became the IF-amplifier, still on its design frequency of 45 MHz. It is interesting to note that this is the reason how the transmit frequency of the original Alexandra Palace television system of 45 MHz became the standard IF frequency for all airborne and many other radar systems for the remainder of the war, and probably a long time afterwards! The 1.25 meters system was installed in one of the Ansons and on the first flight targets were detected. It was soon found that a small increase in wavelength to 1.5 meters (200 MHz) greatly increased the sensitivity. This was the moment that the 200 MHz radar was born, which was to remain the most widely used frequency band for airborne, shipborne and groundbased radar during the whole of the war.
A few days after the successful demonstration of the 1.5 meter system, Bowen and his team were invited into an exercise that was planned starting on the 4th of September (1937), during which Coastal Command would search for the British Fleet in the North Sea. The fleet would follow a zigzag coarse and, naturally would try to avoid discovery. A total of 48 aircraft, none equipped with radar of course, since that was obviously still in development and highly secret, were scheduled to search for them. Bowen and his team in the radar equipped Anson managed to find the fleet within a few hours. They were then overtaken by bad weather. By the help of their radar they managed to safely find their way back. When they returned they learned that they indeed had spotted the fleet in the right location but also that the exercise had been cancelled due to bad weather. It was something of a landmark in the history of airborne radar. They had found the fleet under conditions which had grounded Coastal Command, they had detected other aircraft for the first time with a self-contained radar and, simply by returning home in one piece, had demonstrated some of its navigational capabilities. From then on their path was much easier and they were besieged for flight demonstrations, none the least for Sir Henry Tizard, for whom this was a particular nostalgic occasion.
Klick here to hear Keith Wood’s verbal account of the exercise.
Figure 12.
By the end of 1937 the team worked on two projects – Air to Surface Vessel (ASV) and Air Interception (AI). Much of 1938 was actually spent on the development of the ASV system. During 1938, the principal instrumental improvement in the airborne radar system was the substitution of Western Electric 4304 valves for the ‘giant acorns’ in the transmitter. They gave an increase in peak power to 1 or 2 kilowatts. In the course of 1938 the Anson aircrafts, which were becoming rapidly obsolete, were replaced by two Fairy Battles. Towards the end of 1938 it became clear that AI and ASV would soon come into service and the problem of supply them with electrical power became an important consideration. The standard RAF generator at that time was a 500 watt, 25 DC machine. Most of this power was taken up by the existing aircraft equipment and on top of that the radar equipment was going to require such a variety of voltages that an alternating supply was essential. A way out was to fit an AC generator on the spare generator shaft of the second engine. The story of this generator came into service is an excellent example of prewar decisiveness and pragmatic engineering [10].
Towards the beginning of 1939 it became clear that the ranging performance of the air-to-air radar was more or less adequate. The most urgent task therefore was to add directional information in azimuth and elevation. After considering a number of alternatives it was decided to indicate bearing and elevation in the way depicted in Fig. xx. In total four antennas were used, two with overlapping antenna patterns for azimuth and two for elevation. The data was represented on two CRT tubes. The image on the CRT tubes showed the direct pulse from the transmitter and, at the far end, the reflection from the ground (the so called Xmas tree). The simulation on the left gives an impression of how an approaching aircraft would be observed by the radar operator. A mechanical switch designed by Touch was used to multiplex the receiver and transmitter over the four antennas of the aircraft.
Following the Air Ministries decision to adopt the Benheim as a night fighter, an extensive series of flight trials using Blenheims K7033 and K7034 as night fighter and target respectively was began. In July 1939 things were rapidly coming to a head in Europe and Bowen and his team had to start thinking about producing airborne radars in quantities. Vickers was selected as a contractor for the transmitter but the receiver possed to be a greater problem. Unbelievably, Bowen and his men at that time still only had one receiver; the original 45 MHz EMI chassis. The obvious choice for a contractor would obviously have been EMI but Bowen was under some kind of restraint not to talk to them. Instead Cossor, who had built receivers for the Home Chain system, was asked to produce some 45 MHz receiver samples. They turned out to be a complete failure. They did not come within a tenth of the required sensitivity, and their weight was astronomical, weighing more than their complete system at that time. The receiver was rapidly becoming a problem.
Quite by chance in April or May of 1939, Bowen heard some encouraging news from Edward Appleton, his old Professor at King’s College and now the Jacksonian Professor of Physics at Cambridge. He told Bowen that the Pye company, still hoping that there would be a television industry in Britain, had set up a production line for 45 MHz chassis and had actually made a trial run! Bowen immediately went to Cambridge to see B.J. Edwards, the Technical Director of Pye, and was rewarded with a remarkable sight – he had scores of TRF chassis of just the type that they were looking for!
Figure 1.
Figure 1.
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I would like to thank the following people for their contributions:
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