The Device That Won WW2: A History Of The Cavity Magnetron

[Curious Droid] is back with a history lesson on one of the most important inventions of the 20th century: The cavity magnetron. Forged in the fighting of World War II, the cavity magnetron was the heart of radar signals used to identify attacking German forces.

The magnetron itself was truly an international effort, with scientists from many countries providing scientific advances. The real breakthrough came with the work of  [John Randall] and [Harry Boot], who produced the first working prototype of a cavity magnetron. The device was different than the patented klystron, or even earlier magnetron designs. The cavity magnetron uses physical cavities and a magnetic field to create microwave energy.  The frequency is determined by the size and shape of the cavities.

While the cavity magnetron had been proven to work, England was strapped by the war effort and did not have the resources to continue the work. [Henry Tizzard] brought the last prototype to the USA where it was described as “the most valuable cargo ever brought to our shores”. The cavity magnetron went on to be used throughout the war in RADAR systems both air and sea.

Today, many military RADAR systems use klystrons or traveling wave tube amplifiers due to requirements for accurate frequency pulses.  But the cavity magnetron still can be found in general and commercial aviation RADAR systems, as well as the microwave ovens we all know and love.

Check the video out after the break.

55 thoughts on “The Device That Won WW2: A History Of The Cavity Magnetron

  1. In the junk room/archive of Boston’s State house there’s a handful sized crossed-field device that seems too small to be a magnetron, unless it’s a millimeter wave one. Maybe it’s a klystron. It’s unlabeled and undated, but pretty rusty. I’d post a picture if it were possible …. maybe after we get an edit function.

    There’s a Radar Museum in London, Ontario which remembers the secret training centre for the Chain Home operators. Lots of history, but not too much hardware or descriptions of the radars.

  2. The British were rapidly developing and deploying the critical cavity magnetron based radar systems throughout the war, and produced the important air interception radars, anti shipping radars (ASV Mk3), and ground mapping radars (H2S) well before the later US copies and alternative designs. When the USAAF finally accepted the need for ground mapping radars for their 8th airforce they took British H2S radars.
    The idea that the Tizard mission was to obtain US development help is a modern distortion. The reality is Churchill was trying to bring forward US interest and prevent a repeat of the experience of WW1, when the US forces were almost entirely dependant on the UK and France for military technology (aircraft, artillery, tanks, machine guns etc).
    People seem to have forgotten the reality of US military technology in 1940.

    1. “People seem to have forgotten the reality of US military technology in 1940.”

      Oh yeah? Rifles on both sides of the Channelwent PEW *cric crac* PEW *cric crac* PEW *cric crac*.

      Meanwhile M1 Garands were like PEW PEW PEW PEW PEW PEW PEW PEW *cling*. Rinse and repeat.

        1. It’s a common myth, thanks to people being able to talk about things they know nothing about across the entire globe on the internet, that M1 thumb could occur when loading one, which is silly to think that the US or any military would accept a rifle capable of smashing your thumb while loading it in the heat of battle, M1 thumb occurred when a nervous recruit would fumble the “Inspection Arms” drill in basic training

      1. Garrand was the best rifle, yes.
        But USA had NO real LMG!! No decent AT gun. No tanks. No decent fighters (except naval). No jeep. No radar SYSTEM.
        Your great skill was in catching up very very fast and scaling up massively.

        1. To an extent though, the USA had different options that were meant to cover the same needs. E.G. the M1918 BAR worked well enough even if it was limited on capacity, and there were a bunch of well-liked medium and heavy machine guns to mount to things. They had a few cannons to mount to things which don’t seem awful for the time they were used. Everyone developed rapidly to the point where kinetic anti-tank rifles that the US lacked very quickly needed to be replaced by explosives, and there was the bazooka from mid-1942 on. As for fighters, the P-40’s don’t seem terrible, even if things got a lot better later on. P-39’s too. They weren’t good at high altitude, though.

          1. If one was a Japanese soldier on Guadalcanal, you hated the P-39. Especially being caught in the open. With its armament it made hamburger of enemy soldier on the ground. Even battle hardened Marines were sick when they saw what it would due to an enemy ground formation.

        2. We had the best dive bomber. Ask the Japanese about it sfter the Battle of Midway, the most important 5 minutes of the war. We slaughtered their aircraft carriers snd they never relaced one of them. At war’s end, they had none and we had 120 of all kinds.
          How stupid they were to bomb Pearl Harbor, sinking ships in 25 feet of water which we raised, repaired, re-armed and then used against them at the Battle of Leyte Gulf, Iwo Jima, Okinawa and bombing the Japanese mainland.

    2. The UK lacked a good receiver for cm radar which MIT had. The US lacked a good way to generate cm radar which the UK had. The UK had another issue in that they couldn’t manufacture it in large numbers. The UK design required a lot of very precise machining. The US turned it over to RCA and they figured out how to replace the machining with stamping making them much easier to mass produce.
      On of the bad choices the US made was to really cut back on their Radar systems thinking the UK was farther ahead. Chain Home was very privative and the US radar the SCR-270 at the time was much better. But Chain Home was their and installed and the UK had developed great tactics to use it which the US lacked. And the UK invented the PPI. The US however invented the proximity fuse.
      So what did we learn? The UK and the US were allies and worked together and by doing got more done than they would have alone.

      1. I’ll just make a couple of points, as I agree with the intention of your post.
        The US and the British Empire were not allies at this time, which is what made the Tizard mission so powerful.
        The proximity fuze was acknowledged to have been invented, developed and tested by the UK, the United States Naval Institute proceedings of September 1968 describes the debate in the article ‘The Proximity Fuze: Whose Brainchild?’:

        The US then went on to productionise the technology for it’s 5 inch shells to great effect.

        The SCR-270 is an interesting case as it does illustrate the state of reality, while technical merits are one thing it was the system, not the sensor (the radar set) that made the technology useful. When Chain Home had been demonstrating an effective and resilient early warning system including IFF for well over one year, the system surrounding the SCR-270 at Pearl Harbor, not so much.

        Having studied at the physics department of the University of Birmingham, I met one of the participants in both the development of both microwave radar and nuclear weapons, so I have a preference for history being set in prespective.

        1. When in Ann Arbor in the mid 90’s I was tasked with transferring some color film of all things from Hawaii from an estate of a photographer for the military that included yep color footage of radar being used I believe for the first time to bounce off the Moon if that makes sense I remember it showed the radar the inside of the room and I believe the vehicle and it showed the screen all in color..

          If anyone knows who that photographer was and is that photography available still now thank you

          1. The project to bounce RADAR off the moon was called Project Diana and is the base on which radar astronomy (as distinct from radio astronomy) was built and one reason the now destroyed 405m dish at Arecibo was built.

          2. The first “moon bounce” experiment took place at Camp Evans in New Jersey in 1946:,_1946. Footage of that event, or the initiation of the Opana station, also an IEEE Milestone,,_1941, would be exciting additions to the documentary and artifact record. The question is who was the cameraman, and what his descendants did with the original film and the (U-matic?) transfer. The National Archives should have a copy in its Signal Corps audio-visual collections, which may be difficult but not impossible to locate.

        2. That’s interesting. My father studied at Birmingham, and Sayers was one of his lecturers. Years later my father had to review one of Sayers books and was not complementary !

      1. Ok, the tube alloys project is a ‘lets manufacture a bomb’ offshoot of the MAUD committee ‘we
        can design a practical bomb and here is a road map’, which was initiated from the Frisch–Peierls memorandum ‘look we believe a bomb is possible’. This is the set of UK projects that discover that it is possible to build a practical U-235 device and examine and start testing small scale production methods.

        The Frisch–Peierls memorandum is what the Tizard mission provides in 1940.

        The understanding that a U-235 bomb is possible and also that a plutonium device requires a touchy implosion technology had both been missed by the following US project, which was focused on reactors producing plutonium. Mark Oliphant (University of Birmingham) pushes the US to recognize the path to a viable device when he pursues the baffling lack of US response to the MAUD committee report in Sept 1941. This then results in the merging of projects from August 1943.

        So if someone was making a stupidly simplistic description of the path to the devices used in 1945 it is CLOSER to US manufacture of a British concept with joint development than the current popular history.

        But the reality is hopelessly entwinned, there are british empire scientists in the manhattan project, even in the plutonium device development that the US seems keen on restricting british access to. Ernest Titterton is a good example, as he is deeply involved in the trigger circuitry for the implosion device, probably because he developed the pulse timing circuitry for the Cavity Magnetron at the University of Birmingham physics department (yay, I finally got back to the topic!). He is also probably the only person you have seen triggering a nuclear weapon.

    3. Yep, Britain was way ahead in Technology, leading up to the Second World War. More advanced in Radar, Electronics, Computers, Aviation, Aero and Astronautical Engineering and Atomic Development and Medicine than Any Country in the World. Then the Second World War came along, and Because the Germans nearly destroyed Britain and Cost the Country dearly to rebuild, they had to give all their Technology to the Yanks as payment for their help. And So Britain went backwards and lost its Technological Edge and the Financial and Economic Rewards that came with handing over its know how to the Americans so they could become the World’s Techological and Economical Powerhouse.

    4. Yes, and anyone truly interested in the story of the magnetron should find a copy of Tuxedo Park a story of a man named Al Lumis. Who has possibly the most interesting and compelling figure tied to Lawrence Labs and MIT’s Rad Labs. Check Ebay or Amazon. You well be helpless to put the book down

  3. Some time ago I did some reading about radar origins. A resource list I compiled of the more popular titles:

    The Tizard Mission: The Top-secret Operation That Changed the Course of World War II
    Phelps, Stephen

    A Summer Bright and Terrible
    Winston Churchill, Lord Dowding, Radar, and the Impossible Triumph of the Battle of Britain
    Fisher, David E.

    The Invention That Changed the World: How a Small Group of Radar Pioneers Won the Second World War and Launched a Technical Revolution
    Buderi, Robert

    The Wizard War: British Scientific Intelligence 1939-1945
    Jones, R. V. (Key person in development of radar)
    Published in UK as ‘Most Secret War’

    1. I had the very great honour of hosting RV Jones, as I organised a lecture by him on the radar war in the late 80’s. Only lecture ever where the speaker gave an encore

  4. Still this annoying war rhetoric after such a long time, while providing no useful information about the functionality.

    Wikipedia has a much more informative introduction:

    A cavity magnetron generates microwaves using the interaction of a stream of electrons with a magnetic field, while moving past a series of cavity resonators, which are small, open cavities in a metal block. Electrons pass by the cavities and cause microwaves to oscillate within, similar to the functioning of a whistle producing a tone when excited by an air stream blown past its opening.

  5. Radar won the war, the Atomic Bomb ended the war.. My father William “Bud” Uanna had security and engineering responsibilities for both. As a US Army Engineer and Counter Intelligence Corso’s (CIC) Agent in Boston he was involved with Radar and the Proximity Fuse and then he went to Iak Ridge, Tennessee and then Tinian Island with the 509 th Composite Group. Most people think General Leslie Groves was in Charge of the Manhattan Project. Groves was a front. The real records of WW II and the Cold War are being kept at places like the Hoover Institution. That’s where Edward Lansdales personal and DOD and CIA files are. Hoover brings retired US Government operatives on as fellows and they cull the documents and probably throw a bone to researcher’s once in a while. Researchers are looking at the National Archives. The real information, the private files of operatives, the ones from their government offices, like J. Edgar Hoover’s files, are at the Hoover Institution and places like it.

    1. > the Atomic Bomb ended the war

      Yes, that is the official version for the textbooks. But history is always a bit more complex. WW2 was actually over when the atomic bombs fell on Japan. But they helped the Tenno save face in the surrender – in the face of this “supernatural” power. But more important: they were a clear “STOP!” for the Soviet Union, which already occupied northern Japan. In a way, the atomic bomb was not the end of a war, but the official and globally visible start of the new – cold – war, which already took shape during WW2.

  6. AS an Army Counter Intelligence Corps Agent my father had the security responsibility for these projects. Radar, the proximity fuse.. and was the Army liaison with the construction firm Stone and Webster which was the main contractor with the Manhattan Engineer District. My father Bud Uanna was from Medford, Massachusetts a few miles from MIT. Elizabeth Short,(the Black Dahlia) was from Medford. Her mother worked at Stone and Webster. Her oldest sister worked at MIT.,She met her husband there, hew was into Radar in the Navy. They moved to Berkeley, California where he taught in the Radlab.

  7. The cavity magnetron is a tube of power
    That can produce microwaves in a shower

    It has a copper block with holes called cavities
    That can tune the waves to different frequencies

    The magnetron was invented in the war
    To make radar sets that could see afar

    It helped the Allies to detect the foes
    And win the battles with their blows

    Now the magnetron is used in many ways
    From microwave ovens to particle rays

    It is a device that significantly changed history
    And still has many applications and mystery

  8. I have read Tuxedo Park. That’s where the whole scientific electronics…. sleep studies… brainwaves… lots of stuff going on in the labs and bedrooms. Amid all the goings on of these wealthy and well educated patrons of science … Communism was not really an issue. Fascism was. Tuxedo Park is a window into when Fascism was gaining power and science and mathematics were making great strides.,Loomis – the wealthy coordinator.

  9. TUXEDO PARK Yes Alfred Lee Loomis was good friends with his cousin Secretary of War Henry Stimson. Loomis put his career on Wall Street on hold to perfect Radar. The Loomis story really is a window into a very exciting and dangerous time.,Loomis ran a salon of sorts where he brought the smartest and most powerful people together. All the talk about Oppenheimer being a Communist? That was created by two other Wall Street players, the Dulles Brothers. Who would eventually demoralize Loomis group and hijack the reigns of government. The Dulles Brothers never really left Wall Street.,They we’re in cahoots with the fascists in the 1930’s 40’s 50’s 60’s. Loomis and Oppenheimer were deep thinkers. The Dulles Brothers, C.D. Jackson and Gordon Hray were dirty tricksters. By the time of Oppenheimer’s hearing inn1954 the American Public was brainwashed with the Communust Menace. A story like Tuxedo Park would probably never be told in LIFE MAGAZINE. Compared to Loomis Eiywas a dolt. And Eisenhower stood by while our scientists were sidelined for Nazis. We have been sold down the river by people whose real loyalty lies in Europe. MATO and the European Union. This is Eisenhower and Franco in 1959 during Ike’s farewell world tour. The man smiling in the background is Vernon Walters. He would be CIA Director during Watergate which resulted in Warren Comission member Gerald Ford would become President and Rockefeller as VP.

    1. Steve,
      Now I get it. And glad to know folks out there do have great appreciation for the efforts of our great scientific community
      that ultimately saved our asses and way of life. Sadly, to few even care about the history of our men who gave everything to this country or don’t understand just how tenuous there lives really are.
      You are one lucky guy to be that close to the real history. I can only imagine how blessed you are. My only reference to the Rad Lab
      was through a guy I used to work for. But being in the RF business and having a penchant for history has enlighten me.
      My dad served in the pacific . He was mum about the war. Which made me dig deeper and deeper. My uncle Barny was in the Army airforce. He was a B17 pilot . Never met him. And my parents never told me his story..All I have is his picture and a ton or research I did on his squadron. He went down over [excuse the spelling] Swinefurt. He was only 24.

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.