Cooke-Yarborough eventually ended up in the testing lab for the Airborne Interception radar, the AI Mark III radar, which was having trouble tracking aircraft at less than 1000 feet. It was the Unit's job to bring that distance down to at least 500 feet so visual contact could be made by intercepting aircraft.

This meant reducing the intensity of the tail of the transmitting pulse faster, through dampening, so that the receiver would be able to detect the weaker reflected pulse from an aircraft that was within 1,000 feet. (See diagram).

This idea was tried and tested at RAF Leuchars, and the results showed that the performance had been improved to about 600ft.

In May of 1940 this research was moved to Worth Matravers near Swanage where flight trials continued at Christchurch. However an EMI team, under the leadership of Blumlein, were also located at Worth Matravers researching the same problem. The EMI team developed a pulse modulator, which was more reliable than Cooke-Yarborough's device, this was based on Blumlein's work with TV circuits in the pre-war years. This device became the AI Mark IV radar.

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