Radar, Airborne Interception, Mark IV (AI Mk. IV), produced by 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 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 Percival 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 (MHz) 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 (CRTs) 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 in the front lines 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 the 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 (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.