AI Mark IV radar
Radar, Aircraft Interception, Mark IV, also produced in the USA as SCR-540, was the world's first operational air-to-air radar system. Early Mk. III units appeared in July 1940 on converted Bristol Blenheim light bombers, while the definitive Mk. IV reached widespread availability on the Bristol Beaufighter heavy fighter by early 1941. On the Beaufighter, the Mk. IV arguably played a role in ending the Blitz, the Luftwaffe's night bombing campaign of late 1940 and early 1941.
Early development was prompted by a 1936 memo from Henry Tizard on the topic of night fighting. The memo was sent to Robert Watson-Watt, director of the radar research efforts, who agreed to allow physicist Edward George "Taffy" Bowen to form a team to study the problem of air interception. The team had a test bed system in flights later that year, but progress was delayed for four years by emergency relocations, three abandoned production designs and Bowen's increasingly adversarial relationship with Watt's replacement, Albert Rowe. Ultimately, Bowen was forced from the team just as the system was finally maturing.
The Mk. IV series operated at a frequency of about 193 megahertz with a wavelength of 1.5 metres, and offered detection ranges against large aircraft up to. It had numerous operational limitations, including a maximum range that increased with the aircraft's altitude and a minimum range that was barely close enough to allow the pilot to see the target. Considerable skill was required of the radar operator to interpret the displays of its two cathode-ray tubes for the pilot. It was only with the increasing proficiency of the crews, along with the installation of new ground-based radar systems dedicated to the interception task, that interception rates began to increase. These roughly doubled every month through the spring of 1941, during the height of the Blitz.
The Mk. IV was used operationally for only a short period. The introduction of the cavity magnetron in 1940 led to rapid progress in microwave-frequency radars, which offered far greater accuracy and were effective at low altitudes. The prototype Mk. VII began to replace the Mk. IV at the end of 1941 and AI Mk. VIII largely relegated the Mk. IV to second-line duties by 1943. The Mk. IV's receiver, originally a television receiver, was used as the basis of the ASV Mk. II radar, Chain Home Low, AMES Type 7, and many other radar systems throughout the war.
Development
Genesis
By late 1935, Robert Watt's development of what was then known as Range and Direction Finding 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, 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, and pip-squeak radio direction finding 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 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, and
- antennas of length or less.
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
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.