Early Centimetric Ground Radars - A Personal Reminiscence
In all these cases, however, the resonant circuit was external to the vacuum envelope. For further details of this early work on magnetrons, the reader is referred to Willshaw(7) and Harvey(8).
The crucial breakthrough was made by Boot and Randall early in 1940, and consisted in the use, in Willshaw's words,
"of six resonant circuits machined out of a solid copper block forming the anode, through the centre of which a wire cathode was mounted. Power was extracted through a concentric line coupled to a loop in one of the cavities".
This prototype magnetron was water-cooled, continuously pumped and run in a large electromagnet at a high field: it was hardly a practical proposition for use in the field, much less in an aircraft. However, for the first time, a useful amount of power became available at a wave-length of 10 cm.
The Boot-Randall prototype, developed at the University of Birmingham, was shown to Megaw and his team at the Hirst Research Centre of GEC at Wembley. Megaw had been working on magnetrons for some years and was in touch with Gutton's developments in France. He quickly designed a sealed-off version of the Boot-Randall valve, and then proceeded to various improvements, as detailed by Willshaw. This gave, by July 1940, what was (except for one important step yet to come) virtually the final design, and many valves of this type were made in the laboratories at Wembley and in the BTH labs at Rugby, both for military services use and for further study and evaluation.
Many of these early cavity magnetrons suffered from the defect that they were apt to jump discontinuously in frequency with slight changes in operating conditions. This problem was solved (the one important step mentioned above) by Sayers, also of Birmingham University, in 1941. Sayers showed that by 'strapping' alternate cavities together, the frequency was stabilized and efficiency improved by a factor of five or six. The strapped cavity magnetron was a relatively docile device which was produced in its tens of thousands through the war years and for many years thereafter, mainly on wavelengths of 10cm and 3cm. Until its advent, no country had produced useful power at a wavelength below 50 cm; British radars had been restricted to wavelengths of l.5m, and that only at low efficiency.
The availability of useful power at wavelengths of 10 cm and below spawned a whole new technology - 'microwaves'. Thus waveguides, which had been discussed by Lord Rayleigh in 1897, and by several other authors in the mid-1930s, at last became a practical proposition, combining low loss and high power-handling capability. Also, for the first time, it became possible to form a radio beam from a single launch point and a suitably shaped reflector, rather like a searchlight, instead of from an elaborate array of dipoles: moreover, for the same overall size of antenna, the beam would be much narrower. It was now possible to think of a narrow-beam radar in an aircraft as small as a night-fighter: in a large aircraft, such as a heavy bomber or a long-range maritime reconnaissance aircraft, a centimetric radar system would fit relatively easily.
The Centimetre Radar Course at Yatesbury
The Yatesbury radar course was, as usual, intensive: eight hours a day in classroom or practical sessions, and six days a week. It was an achievement to have assembled such a course in the two years or so since Sayer's discovery of strapping, before which, I suspect, the magnetron, major breakthrough though it undoubtedly was, must have been a doubtful proposition for services use. There was little attempt to go deeply into theory, rather to give a working 'picture' of what went on. For instance, Yatesbury gave a pictorial view of the operation of a waveguide, whereas the textbooks, Lamont(9), for example, usually develop the theory from Maxwell's equations. As far as the magnetron itself went, there was not then, or for many years thereafter, a fully satisfactory theory: instead, we were invited to visualize a cloud of electrons rotating in the anode-cathode space and giving up their energy to the cavities machined in the anode.
Only
one radar set was taught on the course, the Type 277. This had originally been
developed by the British Admiralty, and "could be used both for fighter
direction in carriers and for target indication for the anti-aircraft weapons
in other large ships"(10). In 1943, it was installed in
trailer cabins for coastal defence. The RAF installed it at the top of 60 m
towers, and used it to watch over coastal shipping, reporting to naval plotting
rooms: in RAF terms, this was a radar Type 52 (fig. 2).
There could hardly be a greater contrast than between
CH (Type 1) and Naval Type 277 (RAF Type 52) radars.
Some of the differences are outlined in table 1; however, a mere table does
scant justice to the contrast. The 277 used one transmitter valve (a magnetron
CV76, which could be carried on the palm of one hand (see fig. 3) to produce
as much peak power as, and more mean power than, the CH transmitter, which used
three very large valves, heavy enough to need pulleys and chains to lift them.
A CH Station had nine towers (three for the main transmitters, over 100m high,
four for the main receivers, and two for the buried reserve station): the main
transmitters and receivers were in relatively large permanent buildings, each
surrounded by an anti-blast wall and bank of earth.
