 On 21 February 1940 , John Randall and Harry Boot first ran their cavity magnetron at 10 cm ( 3 GHz ) . In April , GEC was told of their work and asked if they could improve the design . They introduced new sealing methods and an improved cathode , delivering two examples capable of generating 10 kW of power at 10 cm , an order of magnitude better than any existing microwave device . At this wavelength , a half @-@ dipole antenna was only a few centimetres long , and allowed Lovell 's team to begin looking at parabolic reflectors , producing a beam only 5 degrees wide . This had the enormous advantage of avoiding ground reflections by simply not pointing the antenna downwards , allowing the fighter to see any target at its altitude or above it . 
