https://en.wikipedia.org/wiki/Radar_in_World_War_II#Germany
> In February 1943, a British bomber containing a H2S radar was shot down over the Netherlands, and the 10-cm magnetron was found intact. In short order, the secret of making successful magnetrons was discovered, and microwave radar development started.
> In June 1941, an RAF bomber, equipped with an ASV (Air-to-Surface Vessel) Mk II radar, made an emergency landing in France. Although the crew had attempted to destroy the set, the remains were sufficient for the German Laboratory for Aviation to discern the operation and its function. Tests indicated the merits of such a radar, and Wolfgang Martini also saw the value and tasked Lorenz to develop a similar system.