It was called “the most valuable cargo ever brought to the US shores” and in many ways
brought about many technological aspects of the world we live in today and yet this device
had a chequered beginning and appears to have been independently invented several times
in different countries.
A couple of years ago I did a video about the proximity fused shells which gave the
Allies a major advantage in WW2 but there was another piece of tech that worked hand
in hand with those which in some ways was more important to the war effort and these
are some of the things that came from development of this device.
The detection and tracking aircraft, spacecraft, missiles, even shells in flight, ships at
sea as well as insects and birds in the atmosphere, computerised vision systems, mapping the surface
of the earth from space and measuring properties of the atmosphere and oceans to monitor climate
change and the most popular way to cook
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The story of how this came to be is also unusual because although it was top secret British
innovation, it had actually been independently invented by several other countries before
the British finally created one that worked and could be put in mass production, and for
that, it ended up being given to the Americans as part of the so called Tizard mission in
August 1940.
This is the story of the cavity magnetron, a top secret device that put a stop to the
Atlantic U-boats that threatened to starve Britain of food and materials, changed how
the Allies took the air war to Germany and also enabled the TV diners of our convenience-led
lifestyles of today.
Even though it was in its infancy, the electronic warfare of WW2 was as equally important as
the guns, bombs and shells that were fired.
In the battle of Britain, radar played a key role for the RAF because they were at a numerical
disadvantage to the Luftwaffe.
Radar gave them the edge because they could see where the Germans were coming from, at
what height and the approximate strength of the attacking force, giving them a 15 min
head start.
Once the dispatchers of the RAF had that information, they could move the limited but more capable
fighters into positions ready to attack the German bomber formations.
Because they always seemed to be in the right place at the right time, it made the Germans
think the RAF had more aircraft and crews than they did.
It was estimated that the tactical advantage it gave the RAF was the equivalent of three
times the number of fighters than they actually had.
Whilst this British early warning radar worked well, it used a string of large 360ft or 110-meter
towers called chain home that covered the entire Europe-facing side of Great Britain.
By the time of the battle of Britain, the Germans had realized they were part of some
form of early warning system, which they went on to attack to try and destroy.
Although some of the equipment huts were damaged, the towers survived owing to their open steel
girder construction and the Luftwaffe concluded the stations were too difficult to damage
by just bombing and decided to leave them alone for the rest of the war.
The chain home radar used a wavelength between 10 and 13 meters which could see groups of
planes up to 100 miles or 160Km away but the wavelength was too long to see finer details.
There was an airborne version using a 1.5 meter wavelength in-development but Mark Oliphant
who was a member of the classified British radar program thought that a radar with a
10 centimeter or less wavelength and at least 1 kW of power would be much better for airborne
use.
The only problem was that they needed a device called a Magnetron capable of producing that,
but none were capable of delivering the results.
Back then, there were two ways to make a microwave signal, you could have a low power oscillator
and amplify it with a klystron, a specialized linear-beam vacuum tube or you could make
a high-power oscillator and use that directly.
Oliphant used a klystron to generate a 10cm or 3Ghz signal with a power of 400W but it
seemed that it would be impossible to create a pulsed sealed-off version that would be
suitable for airborne use.
Back then there were no semiconductors like we have today, everything was done with vacuum
tubes or valves as we call them in the UK.
The first types of vacuum tubes were Diodes and Triodes.
A diode valve allows current to flow in one direction from the heater cathode to the anode
but not in the other.
The triode added a grid between the anode and cathode, applying a small voltage to the
grid controlled the current flow from the heater cathode to the anode in proportion
to the grid voltage and thus become an amplifier.
However, this method of electrostatic control was patented in 1906 by the American Lee De
Forest, so the quest to find an alternative non-patented method was on and this is where
the Magnetron came about.
This was developed by Albert Hull in 1921 whilst working at GE to try and get around
the patent on the Triode Valve.
This used a magnetic field to control the flow of electrons and this is where the name
came from as the joining of “Magnetic” and “Electron” although the idea worked
it was very inefficient.
But Hull’s papers were picked up on by physicists in Germany in 1924 whom added an extra cathode
and then by Japanese physicists noting that it could produce microwave signals with more
developments in power output by the Russians.
But it would be the British physicist’s John Randall and Harry Boot at the University
of Birmingham, England in 1940 who took ideas from the Dutch engineer Klaas Posthumus who
had clarified the theoretical operation of the magnetron.
In this, they added 8 of cavities around the central cathode linked by small holes.
This effectively created a whole new device that worked in a very different way.
The output signal was determined entirely by the physical shape and size of the chambers
rather than any external circuits or fields.
This new device looked like the chamber of a Colt revolver, which they used the manufacturing
jigs of to make the prototypes with, and was called the cavity magnetron.
In this, the core is surrounded by a permanent magnet so the electrons produced by the heated
cathode are forced spin around the central cavity whilst electrons entering the surrounding
cavities and began to oscillate at a resonant frequency determined by the size of the cavities.
As more electrons were emitted from the cathode, the microwave frequency signal increased and
this was then channelled out of the device to an antenna or a waveguide.
This was a step change in technology and for the first time a device not much bigger than
the palm of your hand could produce a high-power sub 10cm signal that could be mounted into
a plane and show objects on a radar screen wherever the microwave beam was pointed.
The first prototypes produced a wavelength of 9.8 cm at 400W power output.
Work was handed over the General Electric Company Research Laboratories in Wembley,
London and within a couple of months they had a device producing 10kW of pulsed power.
They also fixed the frequency instability that had affected previous designs.
However, Britain at the time was alone in the war and pouring all it resources into
keeping the Germans at bay just across the English channel and the battle of Britain
was raging.
This meant that the development and production of new cutting-edge technologies were severely
limited.
The British government had been calling the US to join the war but they had refused but
they would supply food & hardware.
Henry Tizard, scientist and chairman of the Aeronautical Research Committee, which had
orchestrated the development of radar before the war was so concerned about the situation
that he persuaded Winston Churchill to authorize a secret delegation to be sent to the US to
offer Britain’s most advanced technology in return for development and manufacturing
on mass well away from the German threat and one of the devices taken was the last prototype
cavity magnetron made by GEC, serial number 12.
The ramifications of this were great and of all the ideas that the Tizard mission presented
to the US, including the plans for the atomic bomb, the cavity magnetron was the one the
Americans coveted the most.
This was something that was far in advance of what they had been working on and it astounded
the American scientists.
The British Cavity magnetron was a thousand times more powerful than the best American
microwave transmitter which used a klystron at the time and it produced accurate pulses.
American historian James Phinney Baxter III later said "When the members of the Tizard
Mission brought one cavity magnetron to America in 1940, they carried the most valuable cargo
ever brought to our shores."
Bell labs took the British sample and started making copies and by the end of 1940 the Radiation
Laboratory or Rad Lab at MIT had been setup to develop various types of radar based on
the cavity magnetron, from lightweight compact units for aircraft to early warning systems
that were carried around on five trucks.
However, so concerned that if the cavity magnetrons were to be used in aircraft they might fall
into the hands of the enemy their deployment was delayed for almost 2 years and it wasn’t
until 1943 that they were used by bombers over land.
Much like the early proximity fused shells, their use was over the sea in the battle of
the Atlantic as there was little chance of downed aircraft being recovered by the enemy
and later by RAF night fighters operating over the British mainland and English Channel.
By 1942, the effect on the war of the new radar was becoming dramatic for the Allies
and rather worse for the Germans with one of the biggest effects on the U-Boat fleets
or Wolfpacks which operated in the Atlantic and had sunk millions of tons of Allied shipping.
Now Allied aircraft using information from the recently broken enigma codes could search
in all weathers to look for the conning towers of surfaced U-boats and either attack directly
or warn the convoys where the U-boats were.
The Allies also knew that all the U-boats had to return to base at some point and these
were mostly on the Atlantic coast of occupied France.
During these trips close to the coast the U-boats surfaced to replenish their air supply
and recharge their batteries from diesel generators, something they did under cover of darkness.
Using the new aircraft-mounted radar, the British coastal command would look for U-boats
returning to and from their highly armored submarine pens under cover of darkness and
attack them, something which caught them by complete surprise.
Soon it became apparent to the Germans that just getting in and out of the submarine bases
was now far more dangerous and as their losses mounted, in late 1943 Admiral Doenitz was
forced to scale back the U-boat attacks from which they never recovered.
Doenitz’ once boasted that “an aircraft can no more kill a submarine than a crow can
kill a mole” something that the new aircraft-mounted radar was proving very wrong.
The strange thing is that at the beginning of the war, the Germans were leaders in radar
technology but through a decision made directly by Hitler after the fall of France and his
confidence that the German army could crush any opposition which was backed up by the
head of the Luftwaffe Hermann Goring, he decreed that no scientific research was to be undertaken
unless a conclusive result could be guaranteed within a year.
The German high command just didn’t believe that any kind of radar that allies would use
could have much of an impact on the German forces.
This would unknowingly put them at a considerable disadvantage just as the Allies were boosting
research.
The British H2S radar was the first “Blind targeting radar” capable of finding targets
on the ground without visual assistance and allowed much more accurate bombing even when
the target was obscured by could.
This used a cavity magnetron and a rotating dish mounted in the nose if the aircraft was
first used in January 1943.
A few nights later a Stirling bomber with an H2S onboard crashed near Rotterdam.
It was discovered intact by technicians working for Telefunken when the self-destruct explosive
failed to go off.
The Germans already knew about the principle of the cavity magnetron based on work published
in Leningrad in 1936 and concluded that the British design was similar to the Russian
one but their failure to follow up on it and develop their own microwave radar had given
the allies the upper hand.
In 1945 after the war, a German replica of the British H2S radar was sent back to Britain
for analysis and found that it was an almost identical copy of the first British version
with some errors but it was not used in combat operations.
This wasn’t because German technology was inferior but highlighted the differences in
how German scientists worked or more rather didn’t work well with the high command.
Whilst radar became fully integrated into the allied forces, in the German forces with
was almost non-existent.
Microwave gun-laying radars used along with the newly developed proximity fuse not only
made the big, allied battleship guns much more accurate but also anti-aircraft guns
for attacking aircraft.
During the V1 flying bomb attacks on London, British radar-controlled anti-aircraft batteries
using proximity fused shells were credited with taking down many of the flying bombs
before they reached their target.
The Allied use of radar based on the cavity magnetron would have a major impact on the
air war with the Luftwaffe and although it continues to be used in some radar systems,
due to the output signal changing from pulse to pulse, both in frequency and phase, the
cavity magnetron was replaced by high power klystrons and traveling-wave tubes for systems
that required highly accurate, high powered pulsed outputs but it wasn’t the end for
this device.
After the war, Percy Spencer an American inventor & engineer working for Raytheon noticed that
the chocolate bar he had in his pocket had melted when he approached a magnetron at the
Raytheon laboratories.
Although he didn’t know it at the time he suspected it had something to do with the
microwaves it was generating.
He started experimenting by holding a bag of popcorn kernels in front of the magnetron
and watched as they popped in the bag.
After a lot of effort, he filed a patent for the world’s first microwave oven on Oct
18th 1945 and although it took 20 years to come down in not only price but also size,
it started the revolution that would lead to the convenience cooking that we take for
granted today.
Today there are over one billion cavity magnetrons in use powering microwave ovens in the majority
of kitchens around the world and I wonder if the inventors of those first magnetrons
could have realized just how it would change the world for an altogether more peaceful
purpose.
So thank for watching and don’t forget to thumbs up share and subscribe and I’ll see
you in the next video.