17 - Cambridge Mullard Radio Observatory

Summary

The Early Years

Martin Ryle, Bernard Lovell and a number of other people who became leaders in British radio astronomy had worked for the British Telecommunications Research Establishment (TRE) during the Second World War. John (Jack) Ratcliffe, who had previously led a research group at Cambridge University's Cavendish Laboratory also worked at TRE as one of its top administrators. But in early 1945 Ratcliffe left TRE to rebuild his radio research group at Cambridge where he was able to attract Ryle, Derek Vonberg and F. Graham Smith amongst others.

When Ryle joined the Cavendish laboratory the sunspot cycle was on its way towards solar maximum. As a result Ratcliffe suggested to Ryle that it may be a good time to investigate the source of radio noise associated with sunspot activity that had been observed by Stanley Hey during the Second World War.(1) But unfortunately at that time a typical radar antenna, operating at a wavelength of 1.5 m, had a beamwidth of the order of 10° making it impossible to find the exact source of radio waves on the Sun. So in the winter of 1945–1946 Ryle decided to build a radio interferometer which should be able to locate the sources much more accurately. It consisted of two antennae working at 175 MHz (λ 1.7 m) at separations of up to 240 m or 140 wavelengths. Each antenna was made of eight half-wave dipoles mounted over a wire mesh reflector. With this system Ryle and Vonberg were able to show that short duration radio bursts from the Sun were often circularly polarised and came from discrete areas on the Sun's disc, and not from the disc as a whole.

Two years later Ryle and Smith used another Michelson interferometer at Cambridge to study the cosmic source Cyg A, which Hey, Parsons and Phillips had found in 1946 fluctuated over a period of a few seconds. Ryle and Smith's interferometer consisted of two groups of four Yagi antennae spaced 500 m apart, operating at 80 MHz (λ 3.75 m).(2)