In 1929 Rudolf Kuhnhold of the German Navy Signals Research Division began work on an air 'echo sounder' and in 1933 was suggesting research on centimetric waves to detect aircraft and ships. He was using Phillips magnetrons and Yagi directional antenna with a 13cm wavelength. The experiments were promising so he went to Telefunken in 1934. He spoke to mid-level manager Wilhelm Runge who was working on decimeter wavelengths. Knowing no one at Telefunken was working on centimetric waves Runge told Kuhnhold that he had neither the funds nor staff to spare to collaborate with the Navy. Kuhnhold took this as a rejection by the whole company and so left the navy and went off to form GEMA, a company to continue radar work for the navy. This took some years to set up and produced the early Seetakt radars but an early opportunity to collaborate on powerful radar systems was missed by a misunderstanding.
Lots of people were playing with centrimetric magnetrons in the 30's.
The problem is, they were all very low power - I believe the USN one, in 1940, could make about 500 watts