Allied Radar Experts Opened a FuG 240 Berlin Set — The Cavity Magnetron Copy Changed Everything D
A steel drum 4 feet long sits on a workbench at the telecommunications research establishment in Malvin Worersha. It is July 1944. The drum is scorched black on one side and the paint on the casing still reads Laurens in stencileled letters that no one in the room can quite believe. Nobody in this room has held a complete functioning German centimetric radar set before. Pieces, yes.
Burned fragments pulled from crash sites, yes, but never a full set. Still wired, still recognizable, sitting in one piece on a British bench. It came off a Junker’s Jew 88G1 fighter that landed at RAF Woodbridge 4 days earlier. its crew believing they had reached friendly territory in the dark. They had not. Inside the fuselage, the technicians found a radar unit stencled FUG240, the set the Luftwaffer called Berlin.
Within 6 weeks, the men examining that drum would conclude something that none of them expected to write in a report. The radar hunting British bombers over Germany had been built in part from the wreckage of Britain’s own stolen technology. To understand why that sentence stopped a room full of engineers cold, you need to understand what this equipment was supposed to be impossible to build.
Centimetric radar, radar operating on wavelengths of 10 cm or less, depended on a device called the cavity magnetron. Britain had one. Germany, as far as anyone in this room knew in the summer of 1944, did not. The magnetron was Britain’s most closely guarded piece of hardware. more tightly held than the bomb sites, more tightly held than almost anything short of the atomic research at Chalk River.
And here, sitting on a bench in Malver, was a German radar set that clearly worked on the same short wavelengths. Wing Commander Harold Buffton, who had helped requisition the aircraft from Woodbridge, stood at the door of the workshop and asked the obvious question, how? The team leading the examination worked for the TR’s radar countermeasures section under the direction of scientists who answered ultimately to RV Jones, the Air Ministry’s assistant director of intelligence, the man responsible for tracking every piece of German radio and radar technology since 1939. Jones did not open the casing himself. That job went to a small team of engineers. Among them, a young radio physicist named flight leftenant Alan Petri, who had spent the war taking apart captured German transmitters, one
screw at a time. Petri’s first job was simple. Remove the outer casing and identify the power source. He unbolted six panel screws, lifted the cover, and found a modulator unit rated at 15 kov volts feeding a transmitter cavity roughly 4 in across. He measured the cavity by hand with calipers twice because the first measurement seemed wrong. It was 9.
5 cm, almost identical to the wavelength used by Britain’s own H2S ground mapping radar. Petri wrote three words in his notebook that night. Same band, confirm. That single number meant Germany had solved a problem German industry itself had told Berlin. Only a year earlier could not be solved. Not with the vacuum tube technology they had.
There is a photograph from that week, rarely printed, of Petri sitting on an upturned crate beside the open casing. a cold cup of tea on the floor beside him, staring at the cavity block rather than writing anything down. He would later describe that pause not as excitement, but as unease, if the Germans had genuinely solved centimetric transmission on their own, everything Britain assumed about its lead in radar was wrong.
That same week, RAF Bomber Command was flying window equipped raids against targets in the ruer, trusting that German night fighters could not see through the aluminium chaff clouds designed to blind older, longer wavelength radar. If the Germans had a magnetron of their own, window might already be obsolete. Before we go further into what they found inside that casing, if you’re the kind of person who wants the real engineering behind the war, not the version with the corners rounded off, this channel exists for exactly that. Subscribe and you’ll get the next one the day it’s out. Layer 2 began with the transmitter tube itself. Petri and his colleague, a telecommunications research establishment technician named Kenneth Dwire, pulled the tube from its housing and turned it over under a bench lamp. What they were looking for was the anode
block. The specific machine structure inside a magnetron that determines its wavelength and power. British magnetrons developed by John Randall and Harry Boot at Birmingham University in 1940 used a six cavity copper block. Resonant cavities cut in a precise radial pattern that no other nation had matched.
The German tube had eight cavities, not six. different metal, a harder alloy, not the pure copper Birmingham used, but the geometry, the radial cavity pattern itself, was close enough that Dwire set it beside a sectioned British magnetron kept in the lab for reference and photographed the two side by side.
Dwire’s note in the file read, “Independent development unlikely. Compare RRDE reference unit 4B. That same week in the skies over Germany, RAF Bomber Command lost 41 aircraft in a single raid on Stutgart. Losses that intelligence officers had been quietly attributing to improved German night fighter radar for months without being able to say why the improvement had happened so suddenly.
The men in Malvin now had a number to put against that mystery. Layer three was where the story stopped being about metal and started being about paperwork. Jones had for two years kept a standing file on a single incident. The loss on the night of February 2nd, 1943 of a British Sterling bomber near Rotterdam.
One of the first aircraft in RAF service carrying the new H2S ground mapping radar. The aircraft came down largely intact. The Germans recovered the H2S set from the wreckage almost undamaged and cenamed it Rotterdam Jerat, the Rotterdam apparatus. The examination team pulled the old file and matched serial numbering conventions on the FUG240’s internal wiring against notes taken from a partial German technical bulletin intercepted by British signals intelligence in late 1943 describing tests conducted at Telefunan on a captured Rotterdam set. The bulletin bore the name of Dr. for Wilhelm Runa, Telephunen’s chief of radar research, who had led the German analysis of the downed British equipment. The dates lined up. The
recovered H2S set reached Telefunan in February 1943. The first operational FUG 240 sets reached Luftvafa night fighter squadrons in the autumn of 1944. 18 months later, exactly the length of time an industrial redevelopment program built around a captured foreign design would be expected to take.
Petri kept a single line from that week in his private diary, not the official file. We built the thing they copied. Now we’re taking apart the copy to learn what we already knew. He did not write anything else that night. Colleagues who worked alongside him recalled him leaving the lab early for the only time that summer.
Layer 4 went further than serial numbers. Dwire traced the tuning mechanism on the FUG 240’s transmitter, a sliding plunger design that adjusted the resonant cavities to compensate for temperature drift in flight. British magnetrons of that period used a broadly similar approach developed at Birmingham specifically to solve a problem British engineers had struggled with for a year.
Frequency drift above 15,000 ft where cabin temperatures dropped fast enough to shift the metal and d-tune the cavities mid-flight. The German tuning mechanism solved the identical problem using an almost identical spring-loaded plunger. machined to a tolerance of roughly 2000 of an inch.
The kind of precision that comes from reverse engineering a working solution, not from independently discovering the same fix. Runga’s own postwar technical notes recovered by Allied intelligence teams in 1945 stated plainly that Telephunan’s magnetron program had been founded upon examination of a British apparatus recovered intact in early 1943.
The FUG240 Berlin, the radar hunting British bombers in the last full year of the war, existed because of a single downed aircraft near Rotterdam. While Petri and Dwire worked through that tuning mechanism, RAF Pathfinder crews over Germany were flying missions with instructions to jettison window bundles at precisely timed intervals.
a tactic designed against radar with characteristics nobody in bomber command yet knew their own enemy actually possessed. Layer 5 was the one that moved the report from a technical curiosity to an operational emergency. Jones’s team cross-referenced FUG 240 production dates against Luftwafa night fighter loss and kill statistics for the second half of 1944 where FUG240 equipped aircraft entered service.
RAF bomber losses to night fighters rose by a margin. The analysts could directly attribute to detection ranges of roughly four miles in good conditions, nearly double what earlier German meterwavelength sets like the Lichenstein could manage. That range mattered because British countermeasures, including the radar homing device called Serat, had been tuned to detect the older, longer wavelength Likenstein emissions against a centimetric set transmitting on a completely different band. Sirat was, in the technician’s own words in the file, presently blind. There’s a short note in the TR file, unsigned, describing a moment when three senior engineers stood around the open casing in silence for close to a minute. After that, cross reference came back. Nobody in the room
said anything the notetaker thought worth recording. What broke the silence, according to the file, was someone asking whether Farnburgh had a spare 10 cm receiver crystal they could borrow that afternoon. Layer six, the last one before the discovery that reframed the whole investigation, came from the identification plate riveted to the transmitter housing itself.
A small aluminum tag, easy to miss, stamped with a Telephunen works number and a date. March 1944, Dwer traced that works number through captured German procurement documents recovered after the fall of Arkin. The number matched a production batch manufactured at a telephan facility using tooling explicitly listed in German internal correspondence as adapted from Rotterdam sample components.
Meaning some of the actual machine tooling used to manufacture the FUG240’s magnetron cavities had been set up using measurements taken directly from the captured British H2S unit. This is where the report stopped being about a radar set and became about the war itself. The engineers in that room were not looking at a German invention.
They were looking at their own magnetron rebuilt, renamed, turned back against them. Every British bomber shot down by an FUG24 oe equipped night fighter after the autumn of 1944 had been found in some measure by a set descended from the very technology Britain had refused to share with its own allies for 2 years out of fear it might fall into enemy hands.
It had already fallen into enemy hands on a Dutch riverbank in February 1943. 18 months before this workbench in Malvin, the consequence was immediate. Within weeks, Bomber Command accelerated the roll out of H2S Mark III, shifting its operating wavelength down to 3 cm, a band the Fugu 240 could not detect.
specifically because the Fugg240 discovery proved conclusively that the Germans could build receivers for the older 10 cm band. Jones’s intelligence section also pushed forward a device called perfectos designed to trigger and track German airborne IFFF transponders, giving RAF night fighter crews a way to find German night fighters that no longer depended on radar bands the enemy might already be reading.
Crews flying the last winter of the bomber offensive over Germany did so in part protected by counter measures built directly from what Petri found inside that steel drum. Nobody could put an exact number on lives saved. Nobody ever can in this kind of work. But Bomber Command’s loss rate on nights flown with the revised H2S frequency, and the new homing equipment fell measurably against the previous winter’s figures.
A difference the official history attributes directly to the counter measures developed in response to the FUG 240 examination. The J88G1 that landed at Woodbridge was eventually broken up for parts and scrap. Its Fugji 240 set shipped between research stations before disappearing from the official record sometime in 1946.
The way so much captured wartime hardware simply stopped being tracked once the war that made it valuable had ended. A magnetron believed to be from a related batch. recovered from a separate crash site, sits today in a storage collection connected to the RAF Museum. Unlabeled beyond a generic wartime radar designation, one drawer among thousands.
Most visitors who walk past a display case holding a wartime radar set, see wires, dials, a gray metal box that means nothing to them. They do not see 18 months, a Dutch riverbank, a telephan works number, and a British physicist writing three quiet words in a notebook because he had just realized that the thing hunting his country’s bombers was, in the truest sense, something his own country had built First.