Genesis By late 1935, Robert Watt's development of what was then known as
Range and Direction Finding (RDF) at
Bawdsey Manor in
Suffolk on the east coast of England had succeeded in building a system able to detect large aircraft at ranges over . On 9 October, Watt wrote a memo calling for the construction of a chain of radar stations running down the east coast of England and Scotland, spaced about apart, providing
early warning for the entire British Isles. This became known as
Chain Home (CH), and soon the radars themselves became known by the same name. Development continued, and by the end of 1935 the range had improved to over , reducing the number of stations required. During 1936 the experimental system at Bawdsey was tested against a variety of simulated attacks, along with extensive development of interception theory carried out at
RAF Biggin Hill. One observer was
Hugh Dowding, initially as the director of research for the RAF, and later as the commander of
RAF Fighter Command. Dowding noted that the CH stations provided so much information that operators had problems relaying it to the pilots, and the pilots had problems understanding it. He addressed this through the creation of what is today known as the
Dowding system. The Dowding system relied on a private telephone network forwarding information from the CH stations,
Royal Observer Corps (ROC), and
pip-squeak radio direction finding (RDF) to a central room where the reports were plotted on a large map. This information was then telephoned to the four regional
Group headquarters, who re-created the map covering their area of operations. Details from these maps would then be sent to each Group's Sectors, covering one or two main airbases, and from there to the pilots via radio. This process took time, during which the target aircraft moved. As the CH systems were only accurate to about 1 km at best, subsequent reports were scattered and could not place a target more accurately than about . This was fine for daytime interceptions; the pilots would have normally spotted their targets within this range.
Night bombing Henry Tizard, whose
Committee for the Scientific Survey of Air Defence spearheaded development of the CH system, grew concerned that CH would be too effective. He expected that the
Luftwaffe would suffer so many losses that they would be forced to call off daylight attacks, and would turn to a night bombing effort. Their predecessors in
World War I did the same when the
London Air Defence Area successfully blocked daytime raids, and attempts to intercept German bombers at night proved comically ineffective. Tizard's concerns would prove prophetic; Bowen called it "one of the best examples of technological forecasting made in the twentieth century". Tizard was aware that tests showed an observer would only be able to see an aircraft at night at a range of about , perhaps under the very best moonlit conditions, an accuracy that the Dowding system could not provide. Adding to the problem would be the loss of information from the ROC, who would not be able to spot the aircraft except under the very best conditions. If the interception was to be handled by radar, it would have to be arranged in the short time between initial detection and the aircraft passing beyond the CH sites on the shoreline. Tizard put his thoughts in a 27 April 1936 letter to Hugh Dowding, who was at that time the
Air Member for Research and Development. He also sent a copy to Watt, who forwarded it to the researchers who were moving to their new research station at Bawdsey Manor. In a meeting at the Crown and Castle pub, Bowen pressed Watt for permission to form a group to study the possibility of placing a radar on the aircraft itself. This would mean the CH stations would only need to get the fighter into the general area of the bomber, the fighter would be able to use its own radar for the rest of the interception. Watt was eventually convinced that the staffing needed to support development of both CH and a new system was available, and the Airborne Group was spun off from the CH effort in August 1936.
Early efforts . Bowen started the
aircraft interception (AI) radar efforts by discussing the issue with two engineers at nearby
RAF Martlesham Heath, Fred Roland, and N.E. Rowe. He also made a number of visits to Fighter Command headquarters at
RAF Bentley Priory and discussed night fighting techniques with anyone who proved interested. The first criteria for an airborne radar, operable by either the pilot or an observer, included: • weight not to exceed , • installed space of or less, • maximum power use of 500 W (
watts), and • antennas of length or less. Bowen led a new team to build what was then known as RDF2, the original systems becoming RDF1. They began looking for a suitable receiver system, and immediately had a stroke of good luck;
EMI had recently constructed a prototype receiver for the experimental
BBC television broadcasts on 6.7 m wavelength (45 MHz). The receiver used seven or eight
vacuum tubes (valves) on a chassis only in height and about long. Combined with a CRT display, the entire system weighed only . Bowen later described it as "far and away better than anything which [had] been achieved in Britain up to that time." Only one receiver was available, which was moved between aircraft for testing. A transmitter of the required power was not available in portable form. Bowen decided to gain some familiarity with the equipment by building a ground-based transmitter. Placing the transmitter in Bawdsey's Red Tower and the receiver in the White Tower, they found they were able to detect aircraft as far as away.
RDF 1.5 holds title to two important firsts in radar history; it was the first aircraft to be detected by radar, and the first to carry a radar system. With the basic concept proven, the team then looked for a suitable aircraft to carry the receiver. Martlesham provided a
Handley Page Heyford bomber, a reversal of duties from the original
Daventry Experiment that led to the development of CH in which a Heyford was the target. One reason for the selection of this design was that its
Rolls-Royce Kestrel engines had a well-shielded ignition system which gave off minimal electrical noise. Mounting the receiver in the Heyford was not a trivial task; the standard
half-wave dipole antenna needed to be about long to detect wavelengths of 6.7 m. The solution was eventually found by stringing a cable between the Heyford's fixed
landing gear struts. A series of
dry cell batteries lining the aircraft floor powered the receiver, providing high voltage for the CRT through an
ignition coil taken from a
Ford car. When the system took to the air for the first time in the autumn of 1936, it immediately detected aircraft flying in the
circuit at Martlesham, away, in spite of the crudity of the installation. Further tests were just as successful, with the range pushed out to . It was around this time that Watt arranged for a major test of the CH system at Bawdsey with many aircraft involved. Dowding had been promoted to
Air Officer Commanding Fighter Command, and was on hand to watch. Things did not go well; for unknown reasons the radar did not pick up the approaching aircraft until they were far too close to arrange interception. Dowding was watching the screens intently for any sign of the bombers, failing to find one when he heard them pass overhead. Bowen averted total disaster by quickly arranging a demonstration of his system in the Red Tower, which picked out the aircraft as they re-formed away. The system, then known as RDF 1.5, would require a large number of ground-based transmitters to work in an operational setting. Moreover, good reception was only achieved when the target, interceptor, and transmitter were roughly in a line. Due to these limitations, the basic concept was considered unworkable as an operational system, and all effort moved to designs with both the transmitter and receiver in the interceptor aircraft. Bowen would later lament this decision in his book
Radar Days, where he noted his feelings about failing to follow up on the RDF 1.5 system: Another attempt to revive the RDF 1.5 concept, today known more generally as
bistatic radar, was made in March 1940 when a modified set was mounted in
Bristol Blenheim serial L6622. This set was tuned to the transmissions of the new
Chain Home Low transmitters, dozens of which were being set up along the
UK coastline. These experiments did not prove successful, with a detection range on the order of , and the concept was abandoned for good.
Giant acorns, shorter wavelengths, and ASV K8758, as seen from
K6260.
K6260 carried the radar unit while
K8758 acted as a target. The team received a number of
Western Electric Type 316A large
acorn vacuum tubes in early 1937. These were suitable for building transmitter units of about 20 W continual power for wavelengths of 1 to 10 m (300 to 30 MHz). Percy Hibberd built a prototype transmitter with pulses of a few hundred watts and fitted it to the Heyford in March 1937. In testing the transmitter proved only barely suitable in the air-to-air role, with short detection ranges due to its relatively low power. But to everyone's surprise, it was able to easily pick out the wharves and cranes at the
Harwich docks a few miles south of Bawdsey. Shipping appeared as well, but the team was unable to test this very well as the Heyford was forbidden to fly over water. After this success, Bowen was granted two
Avro Anson patrol aircraft,
K6260 and
K8758, along with five pilots stationed at Martlesham to test this ship-detection role. Early tests demonstrated a problem with noise from the
ignition system interfering with the receiver, but this was soon resolved by fitters at the
Royal Aircraft Establishment (RAE). Meanwhile, Hibberd had successfully built a new
push–pull amplifier using two of the same tubes but working in the
1.25-meter band, an upper-
VHF band (around 220 MHz); below 1.25 m the sensitivity dropped off sharply. Gerald Touch, originally from the
Clarendon Laboratory, converted the EMI receiver to this wavelength by using the existing set as the
intermediate frequency (IF) stage of a
superheterodyne circuit. The original 45 MHz frequency would remain the IF setting for many following radar systems. On its first test on 17 August, Anson
K6260 with Touch and Keith Wood aboard immediately detected shipping in the
English Channel at a range of . The team later increased the wavelength slightly to 1.5 m to improve sensitivity of the receiver, and this 200 MHz setting would be common to many radar systems of this era. After hearing of the success, Watt called the team and asked if they would be available for testing in September, when a combined fleet of
Royal Navy ships and
RAF Coastal Command aircraft would be carrying out
military exercises in the Channel. On the afternoon of 3 September the aircraft successfully detected the battleship , the aircraft carrier and the light cruiser , receiving very strong returns. The next day they took off at dawn and, in almost complete overcast, found
Courageous and
Southampton at a distance of . As they approached the ships and eventually became visible, they could see the
Courageous launching aircraft in a futile effort to intercept them. The promise of the system was not lost on observers;
Albert Percival Rowe of the Tizard Committee commented that "This, had they known, was the writing on the wall for the German Submarine Service." Airborne radar for detecting ships at sea came to be known as
air-to-surface-vessel (ASV) radar. Its successes led to continued demands for additional tests. Growing interest and increased efforts in ASV contributed to delays in airborne intercept sets; the team spent a considerable time in 1937 and 1938 working on the ASV problem.
ASV emerges Liberator GR Mk III. This made mounting large antennas easier than on night fighters. In May 1938 A.P. Rowe took over Bawdsey Manor from Watt, who had been appointed Director of Communications Development at the Air Ministry. The remainder of 1938 was taken up with practical problems in the development of ASV. One change was the use of the new Western Electric 4304 tubes in place of the earlier 316As. These allowed a further increase in power to pulses around 2 kW, which provided detection of ships at . Their test target was the
Cork Lightship, a small boat anchored about from the White Tower. This performance against such a small vessel was enough to prompt the Army to begin work on what would become the Coast Defence (CD) radars. The Army cell had first been set up on 16 October 1936 to develop the
Gun Laying radar systems. Another change was due to every part of the equipment having different power requirements. The tubes for the transmitter used 6 V to heat their filaments, but 4 V was needed for the receiver tubes and 2 V for the filament of the CRT. The CRT also needed 800 V for its
electron gun, but the transmitter tubes 1000 V for their modulators (drivers). At first, the team used
motor-generator sets placed in the Anson and Battle fuselages, or batteries connected in various ways as in the earliest sets in the Heyfords. Bowen decided the solution was to build a
power supply that would produce all of these
DC voltages from a single 240 V 50 Hz supply using transformers and rectifiers. This would allow them to power the radar systems using
mains power while the aircraft were on the ground. British aero engines were normally equipped with a
power take-off shaft that led to the rear of the engine. In twin engine aircraft like the Anson, one of these would be used for a
generator that powered the aircraft instruments at 24 V DC, the other would be left unconnected and available for use. Following a suggestion from Watt to avoid Air Ministry channels, in October Bowen flew one of the Battles to the
Metropolitan-Vickers (Metrovick) plant in Sheffield, where he pulled the DC generator off the engine, dropped it on the table, and asked for an AC
alternator of similar size and shape.
Arnold Tustin, Metrovick's lead engineer, was called in to consider the problem, and after a few minutes he returned to say that he could supply an 80 V unit at 1200 to 2400 Hz and 800 W, even better than the 500 W requested. Bowen had an order for 18 pre-production units placed as soon as possible, and the first units started arriving at the end of October. A second order for 400 more quickly followed. Eventually about 133,800 of these alternators would be produced during the war.
Working design offered fighter-like performance while still offering room for both a radar operator and observer. ,
K7033, the original Blenheim prototype. To better test the needs of AI, an aircraft with the speed needed to intercept a modern bomber was needed. In October 1938 the team was provided with two
Fairey Battle light bombers, which had performance and size more suited to the
night fighter role. Battles
K9207 and
K9208, and the crew to fly them, were sent to Martlesham;
K9208 was selected to carry the radar, while
K9207 was used as a target and support aircraft. By 1939, it was clear that war was looming, and the team began to turn their primary attention from ASV back to AI. A new set, built by combining the transmitter unit from the latest ASV units with the EMI receiver, first flew in a Battle in May 1939. The system demonstrated a maximum range that was barely adequate, around , but the too-long minimum range proved to be a far greater problem. The minimum range of any radar system is due to its
pulse width, the length of time that the transmitter is turned on before it turns off so the receiver can listen for reflections from targets. If the echo from the target is received while the transmitter is still sending, the echo will be swamped by the transmitted pulse backscattering off local sources. For instance, a radar with a pulse width of 1 μs would not be able to see returns from a target less than 150 m away, because the radar signal travelling at the
speed of light would cover the round trip distance of 300 m before that 1 μs interval had passed. In the case of ASV this was not a problem; aircraft would not approach a ship on the surface more closely than its altitude of perhaps a few thousand feet, so a longer pulse width was fine. But in the AI role, the minimum range was pre-defined by the pilot's eyesight, at 300 m or less for night interception, which demanded sub-microsecond pulse widths. This proved very difficult to arrange, and ranges under 1,000 feet were difficult to produce. Gerald Touch invested considerable effort in solving this problem and eventually concluded that a sub-1 μs transmitter pulse was possible. However, when this was attempted it was found that signals would leak through to the receiver and cause it to be blinded for a period, longer than 1 μs. He developed a solution using a
time base generator that both triggered the transmitter pulse as well as cut out the front-end of the receiver, causing it to become far less sensitive during this period. This concept became known as
squegging. In extensive tests in Anson
K6260, Touch finally settled on a minimum range of as the best compromise between visibility and sensitivity. Additionally, the sets demonstrated a serious problem with ground reflections. The broadcast antenna sent out the pulse over a very wide area covering the entire forward side of the aircraft. This meant that some of the broadcast energy struck the ground and reflected back to the receiver. The result was a solid line across the display at a distance equal to the aircraft's altitude, beyond which nothing could be seen. This was fine when the aircraft was flying at or more and the ground return was at about the maximum useful range, but meant that interceptions carried out at lower altitudes offered increasingly shorter range.
Dowding visits In May 1939 the unit was transferred to a Battle, and in mid-June "Stuffy" Dowding was taken on a test flight. Bowen operated the radar and made several approaches from various points. Dowding was impressed, and asked for a demonstration of the minimum range. He instructed Bowen to have the pilot hold position once they had made their closest approach on the radar scope so they could look up and see how close that really was. Bowen relates the outcome: Dowding's version of the same events differs. He states he was "tremendously impressed" by the potential, but pointed out to Bowen that the 1,000 foot minimum range was a serious handicap. He makes no mention of the close approach, and his wording suggests that it did not take place. Dowding reports that when they met again later in the day, Bowen stated that he had made a sensational advance, and the minimum range had been reduced to only . Dowding reports this uncritically, but the historical record demonstrates no such advance had been made. solved Dowding's concerns about armament, carrying both machine guns and a quartet of
20 mm cannon. On their return to Martlesham, Dowding outlined his concerns about night interceptions and the characteristics of a proper night fighter. Since the interceptions were long affairs, the aircraft needed to have long endurance. To ensure that
friendly fire was not an issue, pilots would be required to identify all targets visually. This meant a separate radar operator would be needed, so the pilot would not lose his night vision by looking at the CRTs. And finally, since the time needed to arrange an interception was so long, the aircraft required armament that could guarantee destruction of a bomber in a single pass—there was little chance a second interception could be arranged. Dowding later wrote a memo considering several aircraft for the role, rejecting the
Boulton Paul Defiant two-seat turret fighter due to its cramped rear turret area. He was sure the Bristol Beaufighter would be perfect for the role, but it would not be ready for some time. So he selected the Bristol Blenheim light bomber for the immediate term, sending two of the early prototypes to Martlesham Heath to be fitted with the radar from the Battles. Blenheim
K7033 was fitted with the radar, while
K7034 acted as the target. Both of these aircraft lost a propeller in flight but landed safely;
K7033s propeller was never found, but
K7034's was returned to Martlesham the next day by an irate farmer.
Mk. I Even at the 1.5 m wavelength, antennas of practical size had relatively low gain and very poor resolution; the transmitter antenna created a fan-shaped signal over 90 degrees wide. This was not useful for homing on a target, so some system of direction indication was required. The team seriously considered
phase comparison as a solution, but could not find a suitable phase shifting circuit. Instead, a system of multiple receiver antennas was adopted, each one located so that only a certain section of the sky was visible. Two horizontal receivers were mounted on either side of the
fuselage and only saw reflections from the left or right, slightly overlapping in the middle. Two vertical receivers were mounted above and below the wing, seeing reflections above or below the aircraft. Each pair of antennas was connected to a motorized switch that rapidly switched between the pairs, a technique known as
lobe switching. Both signals were then sent to a CRT for display, with one of them passing through a voltage inverter. If the target was to the left, the display would show a longer blip on the left than the right. When the target was dead ahead, the blips would be equal length. There was an inherently limited accuracy to such a solution, about five degrees but it was a practical solution in terms of limiting the antenna sizes. By this point the Air Ministry was desperate to get any unit into service. Satisfied with his visit in May, Dowding suggested that the Mk. I was good enough for operational testing purposes. On 11 June 1939, AI was given the highest priority and provisions were made to supply 11 additional Blenheims to
No 25 squadron at
RAF Hawkinge (for a total of 21). Since each of the parts came from different suppliers, and the fitters were unfamiliar with any of it, members of the AI team would have to hand-assemble the components as they arrived and instruct the fitters on the sets. Watt was waiting for the order, and in 1938 had arranged for production of the transmitters at Metrovick and receivers at
A.C. Cossor. These turned out to be the wrong products: Metrovick had been told to directly copy ("Chinese") the 1937 design by Percy Hibberd, but Bawdsey had delivered the wrong prototype to Metrovick, who copied it. The Cossor receivers were found to be unusable, weighing as much as the entire transmitter and receiver, and having sensitivity about half that of the EMI lash-up.
Pye strip It was at this point that the team had yet another stroke of luck. Bowen's former thesis advisor at
King's College, London, was
Edward Appleton, who had worked with Watt and
Harold Pye during the 1920s. Pye had since gone on to form his own radio company,
Pye Ltd., and was active in the television field. They had recently introduced a new television set based on an innovative vacuum tube developed by
Philips of Holland, the
EF50 pentode. Appleton mentioned the Pye design to Bowen, who found it to be a great improvement over the EMI version, and was happy to learn there had been a small production run that could be used for their experiments. The design became widely known as the
Pye strip. The Pye strip was such an advance on the EMI unit that the EF50 became a key strategic component. As a German invasion of the west loomed in 1940, the British contacted Philips and arranged a plan to remove the company's board of directors to the UK, along with 25,000 more EF50s and another 250,000 bases, onto which
Mullard, Philips's UK subsidiary, could build complete tubes. A destroyer, , was dispatched to pick them up in May, and left the Netherlands only days before the
German invasion of the country on 15 May 1940. The Pye strip, and its 45 MHz intermediate frequency, would be re-used in many other wartime radar systems. New Blenheims eventually arrived at Martlesham, these having been experimentally converted to
heavy fighters with the addition of four
.303 British (7.7 mm) Browning machine guns and four
20mm Hispano autocannon, while removing the mid-upper turret to reduce weight by and drag by a small amount. These arrived without any of the racking or other fittings required to mount the radar, which had to be constructed by local fitters. Further deliveries were not the Blenheim Mk. IF and IIF models originally provided, but the new Mk. IVF versions with a longer and redesigned nose. The gear had to be re-fitted for the new aircraft, and the receivers and CRTs were mounted in the enlarged nose, allowing the operator to indicate corrections to the pilot through hand signals as a backup if the intercom failed. By September, several Blenheims were equipped with what was now officially known as AI Mk. I and training of the crews began with No. 25 Squadron at
RAF Northolt.
Robert Hanbury Brown, a physicist who would later work on radar in the US, and Keith Wood joined them in August 1939, helping fitters keep the systems operational, and coming up with useful methods for interception. Near the end of August, Dowding visited the base and saw the radars in the nose and pointed out to Bowen that the enemy gunners would see the light from the CRTs and shoot the operator. The sets were re-fitted once again, returning to the rear of the fuselage, which caused more delays. With the units in the rear, the only communications method was via the intercom. Contemporary systems used the radio as the intercom as well, but the TR9D sets used in RAF aircraft used the voice channel for 15 seconds every minute for the pip-squeak system, blocking communications. Even when modified sets were supplied that addressed this, the radar was found to interfere strongly with the intercom. A
speaking tube was tried but found to be useless. Newer VHF radios being developed through this same period did not suffer these problems, and the Blenheims were moved to the front of the queue to receive these units.
Emergency move , not much larger than Bawdsey, was filled with students. Bawdsey, right on the eastern coast in a relatively secluded location, could not effectively be protected from air attack or even bombardment from boats offshore. The need to move the team to a more protected location on the opening of hostilities had been identified long before the war. During a visit to his
alma mater at
Dundee University, Watt approached the rector to ask about potentially basing the team there, on short notice. When the
Germans invaded Poland and war was declared on 3 September 1939, the research teams packed up and arrived in Dundee to find the rector only dimly recalling the conversation and having nothing prepared for their arrival. Students and professors had since returned after the summer break, and only two small rooms were available for the entire group. The AI group and their experimental aircraft of D Flight,
Aeroplane and Armament Experimental Establishment (A&AEE), moved to an airport some distance away at
Perth, Scotland. The airport was completely unsuitable for the fitting work, with only a single small hangar available for aircraft work while a second was used for offices and labs. This required most of the aircraft to remain outside while others were worked on inside. Nevertheless, the initial group of aircraft was completed by October 1939. With this success, more and more aircraft arrived at the airport to have the AI team fit radars, most of these being the ASV units for patrol aircraft like the
Lockheed Hudson and
Short Sunderland patrol aircraft, followed by experimental fittings to
Fleet Air Arm Fairey Swordfish torpedo bombers and
Supermarine Walrus.
Bernard Lovell joined the radar team at the personal suggestion of
Patrick Blackett, an original member of the Tizard Committee. He arrived at Dundee and met Sidney Jefferson, who told him he had been transferred to the AI group. The conditions at Perth were so crude that it was clearly affecting work, and Lovell decided to write to Blackett about it on 14 October. Among many concerns, he noted that; Blackett removed any direct reference to Lovell and passed it to Tizard, who discussed the issue with Rowe during his next visit to Dundee. Rowe immediately surmised who had written the letter and called Lovell in to discuss it. Lovell thought little of it at the time, but later learned that Rowe had written back to Tizard on 26 October: Rowe surmised from the conversation that the main problem was that Perth was simply not suitable for the work. He decided that most of the research establishment, now known as the Air Ministry Research Establishment (AMRE), would remain in Dundee while the AI team should be moved to a more suitable location. This time the chosen location was
RAF St Athan in Wales, about from
Cardiff. St Athan was a large base that also served as an RAF training ground, and should have been an ideal location. When the AI team arrived on 5 November 1939, they found themselves being housed in a disused hangar with no office space. A small amount of relief was found by using abandoned Heyford wings as partitions, but this proved largely useless as the weather turned cold. As the main doors of the hangar were normally left open during the day, it was often too cold to hold a screwdriver. Bowen complained that the conditions "would have produced a riot in a prison farm". Ironically, Bawdsey was ignored by the Germans for the entire war, while St Athan was attacked by a
Junkers Ju 88 only weeks after the team arrived. The single bomb struck the runway directly, but failed to explode.
Mk. II With October's deliveries, the Air Ministry began plans for a production AI Mk. II. This differed largely by the addition of a new
timebase system, which it was hoped would reduce the minimum range to a very useful . When the new units were installed, it was found the minimum range had increased to 1000 feet. This problem was traced to unexpectedly high
capacitance in the tubes, and with further work they were only able to return to the Mk. I's 800 feet. Blenheims from a number of squadrons were fitted with the Mk. II, with three aircraft each being allotted to No. 23, 25, 29, 219, 600 and 604 Squadrons in May 1940. Two experimental versions of the Mk. II were tested. The AIH unit used
GEC VT90 Micropup valves in place of the Acorns for additional power, the H standing for high power of about 5 kW. A test unit fitted to a Blenheim IF proved promising in March and a second was delivered in early April but development was ended for unknown reasons. The AIL had a
locking timebase, which improved maximum range, at the cost of a greatly increased minimum range of and work was abandoned. While aircraft were being delivered, Bowen, Tizard and Watt pressed the Air Ministry to appoint someone to command the entire night fighting system, from ensuring aircraft delivery and radar production to the training of pilots and ground crew. This led to the formation of the Night Interception Committee (so-named in July 1940) under the direction of
Richard Peirse. Peirse raised the Night Interception Unit at
RAF Tangmere on 10 April 1940; it was later renamed the
Fighter Interception Unit (FIU). Bowen led a series of lectures at Bentley Priory, on the theory of radar guided night interception and concluded that the fighter would require a speed advantage of 20 to 25% over its target. The main
Luftwaffe bombers—the Junkers Ju 88,
Dornier Do 17Z, and
Heinkel He 111—were capable of flying at about , at least with a medium load. This implied a fighter would need to fly at least and the Blenheim, fully loaded, was capable of only . Bowen's concerns over the poor speed of the Blenheim were proved right in combat.
Mk. III of No. 25 Sqn at Martlesham Heath run up on 25 July 1940. The aircraft on the right mounts the transmitter antenna in its original horizontal arrangement. in its nose. Note the Mk. IV antennas on either side. The Mk. IV guided the Havoc to close range and then the light was switched on, illuminating the target for other fighters to attack. The Mk. II was used for only a short time when the team replaced its transmitter section with one from the ASV Mk. I, which used the new Micropup valves. The new AI Mk. III sets were experimentally fitted to about twenty Blenheim IFs in April 1940, where they demonstrated an improved maximum range of . However, they still suffered from a long minimum range, from 800 to 1,500 ft depending on how the receiver was adjusted. This led to what Hanbury Brown describes as "the great minimum range controversy". From October 1939, working around the clock to install the remaining Mk. I sets at Perth and St Athan, the team had had no time for further development of the electronics. They were aware that the minimum range was still greater than was satisfactory but Bowen and Hanbury Brown were convinced there was a simple solution they could implement once the initial installations were completed. Meanwhile, the current sets continued to be installed, although all were aware of their problems. On 24 January 1940
Arthur Tedder, the Director General for Research, admitted to Tizard that: The issue of minimum range continued to be raised, working its way through the Air Ministry and eventually to
Harold Lardner, head of what was then known as the Stanmore Research Centre. Rowe and his deputy
Bennett Lewis were called to meet with Lardner to discuss the issue. Apparently without informing Lardner of Bowen and Hanbury Brown's potential solution, or the fact that they could not work on it due to the ongoing installations, they agreed to have Lewis investigate the matter. Lewis then sent a contract to EMI to see what they could do. According to both Bowen and Hanbury Brown, Rowe and Lewis instigated these events deliberately to pull control of the AI project from the AI team. At Dundee, Lewis raised the issue and two solutions to improving the range were considered. The Mk. IIIA consisted of a set of minor changes to the transmitter and receiver with the goal of reducing the minimum range to about . Lewis' own solution was the Mk. IIIB, which used a second transmitter that broadcast a signal that mixed with the main one to cancel it out during the end of the pulse. He believed this would reduce the minimum range to only . Two copies of the IIIA entered tests in May 1940 and demonstrated little improvement, with the range reduced to only , but at the cost of significantly reduced maximum range of only . Tests of the IIIB waited while the AI team moved from St Athan to
Worth Matravers in May, and were eventually overtaken by events. Development of both models was cancelled in June 1940. Word that Lewis was developing his own solutions to the minimum range problem reached the AI team at St Athan some time in early 1940. Bowen was extremely upset. He had become used to the way the researchers had been put into an ill-advised attempt at production but now Rowe was directly removing them from the research effort as well. Tizard heard of the complaints and visited Dundee in an attempt to smooth them over, which evidently failed. On 29 March 1940 a memo from Watt's DCD office announced a reorganization of the Airborne Group. Gerald Touch would move to the RAE to help develop production, installation and maintenance procedures for the Mk. IV, several other members would disperse to RAF airfields to help train the ground and air crews directly on the units, while the rest of the team, including Lovell and
Hodgkin, would re-join the main radar research teams in Dundee. Bowen was notably left out of the reorganization; his involvement in AI ended. In late July, Bowen was invited to join the
Tizard Mission, which left for the US in August 1940.
Prototype use Mk. III went into extensive testing at No. 25 Sqn in May 1940 and another troubling problem was found. As the target aircraft moved to the sides of the fighter, the error in the horizontal angle grew. Eventually, at about 60 degrees to the side, the target was indicated as being on the other side of the fighter. Hanbury Brown concluded that the problem was due to reflections between the fuselage and engine nacelles, due to the change to the long-nose IVF from the short-nose IF and IIF. In previous examples they had used the fuselage of the aircraft as the reflector, positioning and angling the antennas to run along the nose or wing leading edges. He tried moving the horizontal antennas to the outside of the nacelles, but this had little effect. Another attempt using vertically oriented antennas "completely cured the problem", and allowed the antennas to be positioned anywhere along the wing. When he later tried to understand why the antennas had always been horizontal, he found this had come from the ASV trials where it was found this reduced reflections from the waves. Given the parallel development of the ASV and AI systems, this arrangement had been copied to the AI side without anyone considering other solutions. At a meeting of the Night Interception Committee on 2 May it was decided that the bomber threat was greater than submarines, and the decision was made to move 80 of the 140 ASV Mk. I transmitters to AI, adding to 70 being constructed by
EKCO (E.K. Cole). These would be turned into 60 IIIA's and 40 IIIB's. At a further meeting on 23 May, Tizard, perhaps prompted by comments from Director of Signals (Air), suggested that the units were not suitable for operational use, especially due to low reliability, and should be confined to daylight training missions. By 26 July 70 Blenheims were equipped with Mk. III and the RAE wrote an extensive report on the system. They too had concerns about what they called "partially reliable" systems and pointed out that a significant problem was due to the unreliable antenna connections and cabling. But they went further and stated that the
self-exciting concept would simply not work for a production system. These systems used transmitter circuitry as an oscillator to produce the operating frequency, but they had the disadvantage of taking some time to stabilize and then shut down again. Hanbury Brown agreed with this assessment, as did Edmund Cook-Yarborough who had led work on the IIIB at Dundee.
Mk. IV The RAE's comments about the self-exciting transmitter were not random: they were referring to work that was just coming to fruition at EMI as a direct result of Lewis' earlier contract. EMI engineers
Alan Blumlein and Eric White had developed a system that dispensed with a self-exciting transmitter circuit and instead used a separate modulator that fed the signal into the transmitter for amplification. The oscillator signal was also sent to the receiver, using it to damp its sensitivity. The combined effect was to sharpen the transmitted pulse, while reducing 'ringing' in the receiver. In a test in May 1940, Hanbury Brown was able to clearly see the return at a range of , and could still make it out when they approached to 400. Touch, now at
RAE Farnborough and having delivered improved versions of ASV, quickly adapted the new oscillator to the existing Mk. III transmitter. Adapting the vertical transmitting "arrowhead",
folded twin-dipole antenna design on the nose of the aircraft, from Hanbury Brown's work with the Mk. III eliminated any remaining problems. In its first operational tests in July 1940, the new AI Mk. IV demonstrated the ability to detect another Blenheim at a range of and continued to track it down to a minimum of 500. Hanbury Brown stated that "it did everything that we had originally hoped that airborne radar would do for night-fighting". He went on to note that even though Mk. IV arrived only one year after the first Mk. I's, it felt like they had been working for ten years. A production contract for 3,000 units was immediately started at EMI, Pye, and EKCO. When they left for the US in August, the Tizard Mission team took a Mk. IV, ASV Mk. II and IFF Mk. II with them, via the
National Research Council (Canada). During the following discussions, it was agreed that the US would produce AI, while Canada would produce ASV. Western Electric arranged a production license for the Mk. IV in the US, where it was known as the SCR-540. Deliveries began for the P-70 (
A-20 Havoc) and
PV-1 aircraft in 1942. ==Operational use==