Copied from the UK Amateur Radio Newsgroup.

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"Joe McElvenney" wrote
Randall and Boot's cavity magnetron
didn't really come onto the scene until about 1940.

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R & B's cavity magnetron was developed at Birmingham University in the
midst of the air raids on that industrial city. We can imagine R & B
having to hide under the workbench whenever a descending bomb was
heard, culminating in a loud bang and broken glass.

The ingenious device generated a peak pulse power of 50 kilowatts at
3000 MHz. Pulse repetition frequency 400 Hz. Pulse width 1
microsecond. So far as I can remember there were 6 or 8 cavities
milled into the copper block. Alternate anodes surrounding the
cathode, and close to it, were strapped together at their ends via
copper bars. The block diameter was about 2" and about 1" thick. The
magnet was a U-shape with pole-pieces which closely fitted the flat
ends round the block such that the magnetic field was parallel to the
cathode.

Because the luftwaffer in 1940/1 had more bombers than the RAF, and in
view of its potential as a war weapon, Churchill personally banned
installation in RAF aircraft in case the top-secret device should be
shot down over Germany and fall into the hands of German scientists
and engineers.

So Churchill handed the cavity magnetron to Roosevelt as a free gift
in return for 50 rusty, old, WW1 destroyers. The manufacturing
capacity of the US radio industry far exceeded that of the UK. Not to
be outdone, the Americans soon produced a 10,000 MHz version.

I first held one in my hands in 1945 by which time centimetric radar
had been installed in RAF Catalina and Sunderland flying boats on
convoy-escort duties in the Battle of the Atlantic. By 1945 German
submarine crews were on suicide missions like kamikazi pilots, only 1
U-boat in 10 returned to base.

There are more than 100,000 merchant ship and U-boat crew-members
sharing Davy Jones locker at the silent bottom of the North Atlantic
Ocean. Thus was the ferocity of the war.

Once having detected a centimetric radar beam, and being accurately
located themselves, submarine commanders preferred to remain on the
surface, uncover the guns, and fight it out, day or night.

During most of the war there had been little effect on German
industrial production by RAF raids. Many bombs fell on open fields and
sometimes killed cattle. But by 1944 RAF navigatigators had maps of
rivers and cities laid before them. More than a 1000 heavy bombers,
Lancasters, could be put into the air, night after night.

With radar they couldn't miss whole cities and individual districts.
Nevertheless on one occasion more than 100 bombers, complete with
crews, failed to return to base. Such occurrences greatly exceeded the
capacity of factories to produce them and to train aircrews.

During the last 12 months of the war, radar equipped RAF bombers
killed more German civilians than died in the concentration camps.
Hiroshima and Nagasaki, which came shortly after, were just chicken
feed. Air Marshal "Bomber" Harris was never knighted for services
rendered.

The main beneficiaries of R & B's invention of the cavity magnetron,
done amid the high-explosives and incendiaries falling on Birmingam,
have been the Japanese and other Far Eastern peoples who have
manufactured many millions of cheap, reliable, microwave ovens.

And of course the many millions of people like you and I who benefit
from daily hot meals. I detest barbiques.

There was held in the Kensington, London, Science Museum, the original
prototype of the cavity magnetron without its magnet. It was in a
securely locked mahogany and glass case and looked, as I recollect,
like a small dirty can of baked beans with things sticking out of it.
It may still be there.

Makes a change from so-called SWR meters.
----
Reg, G4FGQ.