Chain Home Radar - A Personal Reminiscence
1. The scene is an artist's impression
of radar pioneers Wilkins and Watson-Watt about to perform their historic experiment
on 26 February 1935
The experiment at Weedon, which took less than 24 hours from arrival on site, is portrayed in fig. 1. A. R Wilkins was one of the four men present, and (one suspects) the architect of the experiment, the other three being A. R Rowe, R. Watson-Watt (both higher-ranking than Wilkins) and the driver of the grandiloquently-titled travelling laboratory in which the receiver and its display were housed. The scattered signal was detectable even when the aircraft was more than 12km distant, a result which was taken to be very satisfactory.
This account of the Weedon (or Daventry) experiment is based on a little-publicized account by Neale(1), who in turn got it first-hand from Wilkins many years after the event. It was Wilkins who had done the calculations which showed, first, that the idea of a radio 'death-ray' (that is, a ray which would disable or destroy an aircraft) was impracticable, and, second, that the detection of aircraft by some radio-based system might be possible. The Weedon experiment was designed to verify this second set of calculations: unfortunately, there appears to be no record of the calculations themselves.
By 1935, of course, there was abundant evidence that unseen objects scattered incident radio waves sufficiently to be detected. In his initial work on radio waves in the 1880s, Hertz had shown that they were subject to reflection, refraction and interference, just as light was. In a speech in 1922, Marconi commented that:
'I have noticed the effects of reflection of these waves by metallic objects miles away',
and went on to suggest that a ship, suitably equipped, could detect the presence and bearing of other ships. Moreover, in the 1920s, the presence and height of the F and F-layers of the ionosphere had been detected and measured by radio techniques closely analogous to radar. It may be of interest to note that the height of the E-layer is 100km, while the F-layer shows a maximum in the range 200 - 400km. Thus the range of the F-layer corresponds roughly to the minimum radar range useful for early warning (giving 20 - 30 minutes warning of the approaching aircraft at the cruising speeds of the 1930s), while 200 - 400 km is the maxi-mum range which might be expected, allowing for the curvature of the Earth, and depending on the height of the aircraft.
All these facts, and others leading to the same conclusion, were well known in 1935. Why then was the experiment needed? Perhaps to convince some bureaucrat who held the purse strings? Neale suggests that the experiment was needed to verify Wilkins' calculations, which, despite the ionospheric work and the rather casual comments of Marconi, were the only quantitative estimates of what might be possible.
For whatever reason, the experiment was carried out - after all, it cannot have been costly - and the results were received euphorically - Watson-Watt is said to have remarked that Great Britain had once more become an island!
