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https://en.wikipedia.org/wiki/Cavity_magnetron#History
Despite patenting the device in 1935, the German authorities opted against developing Hans Hollman's version of the Cavity Magnetron; though it lacked the sophistication of the later British 1940 version, with further funding it could have reached it within a couple/few years. This prevented the Germans from developing the much more powerful Gigahertz band radar later in the war, despite their honing of the tube based radars to a fine art. This meant their ground based gun laying was more vulnerable to jamming and were less effective at targeting; it also meant that the aircraft mounted radar was much heavier and shorter ranged, which reduced performance due the large weight and drag of the antennae of the sets; when the Germans IOTL captured British airborne radar sets they produced the superb Berlin radar set, though far too late to matter.
https://en.wikipedia.org/wiki/Berlin_(Radar)

So what if the Germans had ran with Hollman's invention and had a working multi-cavity magnetron operational in 1938? I figure three years is enough time to improve the device and get it operation with some radar set. By 1940 the radar sets would be very powerful and exact, much more accurate and longer ranged than even the Wurzburg sets that were the world gold standard of accuracy at the time. That means German FLAK would be more effective than even IOTL, as magnetron gun laying radar enabled the British to pretty much shutdown German night attacks on Britain beyond mid-1943 and were instrumental at neutralizing the V-1 missile threat. By 1941-42 there would be a FuG 240 airborne radar set that would keep the Bf110 a serious threat to bombers into 1944. It would also enable the Germans to potentially tackle the Mosquito bomber, as they wouldn't have the draggy antennae holding them back. Also German radar would be pretty immune to 'Window' the Allied radar jamming chaff (British magnetron based sets were immune to it), which would prevent the shutting down of German night defenses in mid-1943 than enabled Operation Gemorrah and the Hamburg Firestorm. We could also potentially see a German version of H2S for night bombing, which would impact British jamming attempts at shutting down German night bombing raids in 1941. It would also enable an early airborne early warning system to get even earlier warning of incoming raids (the Germans were working on it IOTL, but they developed magnetronic radar too late to get it into service in time).

How would this impact the air war over Europe? It seems to me that had the early version of the magnetron been developed the Germans could have had some very powerful radar even earlier than the Allies IOTL, which would have conceivably made the British night bombing attempts very difficult in 1942-44 and made German air defenses much more dangerous and accurate, seriously reducing the number of shells be shoot down of bombers, even during the day. This may well have altered the Allied ability to hit German industrial targets throughout the war.
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