Early Magnetron Development - E1189 (E1189)

The following extract from Metres to Microwaves - Callick, gives some of the background to the developments that lead to a practical device capable of mass production in England and the USA.

After Randall and Boot's demonstration of their 6-cavity 10 cm magnetron,
which was fitted with a small diameter tungsten filament cathode and therefore
basically a device for continuous operation, Megaw and his group at GEC made
essential and fundamental changes to the original Birmingham design, and in
mid-1940 developed E1189, the first magnetron suitable for use as a radar
transmitter*. This had a large diameter oxide cathode with end discs, similar to
one used earlier by H. Gutton of SFR in his 8- and 12-segment interdigital
magnetrons. Some years previously, Megaw, who was in close contact with
Gutton, had identified back bombardment of the cathode and the importance of
-secondary emission as the major source of operating current. He had also
ventured the opinion that magnetrons would operate efficiently with cathodes
significantly greater in diameter than the tungsten filaments and small spirals in
common use at that time. It was Gutton who demonstrated the manyfold increase
in power which could be obtained by replacing a tungsten spiral by a large oxide
cathode, and that cathodes of this type could withstand operation with very high
voltages. Megaw himself acknowledges the great importance of Gutton's innovation,
which was the key to development of multicavity magnetrons giving peak
power outputs from ten to several hundred kilowatts. Without the invention of
'strapping' by J. Sayers of Birmingham University and the refinement of this
technique by S. Devons ofTRE , who originated 'strap setting', the multicavity
magnetron would have continued to be inherently unstable in frequency and
difficult to operate. The addition ofstraps also increased the efficiency and power
output by many times. Without them, development of the 'megawatt' S-band
magnetron would have been delayed by many years. The addition of straps also
increased productivity in manufacture. Later on, B.V. Rollin of the Clarendon
Laboratory, who had concentrated on designs for wavelengths down to 1.25 cm,
invented the 'rising sun' resonator system. This required no straps, and so was
much easier to fabricate than the ring-strapped system. (The rising sun design
was also used by Columbia University, New York in their X- and K-band
magnetrons. It is almost certain that they originated their own design independently)
• In a letter to Dr. Megaw's secretary shortly after his death in 1956, Sir Edward Appleton, who
in 1940 was a member of the Committee for the Scientific Survey of Air Defence (the 'Tizard'
Committee), wrote 'Those who were in the business know how much the practical development
ofthe cavity magnetron - the development that made it something that could go into operational
use - was due to Dr. Megaw.' [The Tizard Committee reported to and made recommendations
for action by the Subcommittee on Air Defence Research which reported to the Committee of
Imperial Defence.]

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