The Device that Won WW2 - The Cavity Magnetron

The Device that Won WW2 – The Cavity Magnetron

In Technology, Videos, Weapons by Paul ShillitoLeave a Comment


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 food quickly.

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.

Paul Shillito
Creator and presenter of Curious Droid Youtube channel and website

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