The French Navy began to be concerned about rumors of radio detection systems by 1939, and the French firm of SFR in Paris began experiments. SFR employed noted researcher H Gutton.
Using a 12 segment magnetron, the first devices produced power outputs of 6 watts at 10 cm wave length, and 10 watts at 16 cm wave length. Using a pulsing technique, the power was up to 50 watts operating on a wave length of 16cm, just before the the fall of France in 1940. This set demonstrated an ability to detect a battleship to a max range of 10,000 meters. Gutton tried using an oxide cathode in place of a thoriated tungsten, and that increased the power ten fold to 0.5 kw. As France fell, these test results were transfered to Megaw in England, resulting in Megaw insisting on modifying the British cavity magnetron to include a large oxide cathode.
The Vichy Navy attempted continued clandestine development of the SFR 16cm equipment. Little is known, but some claim that the equipment was eventually able to detect a battleship to a max range of 25,000 meters by 1944. All prototypes were destroyed, along with the documentation, when the Vichy fleet was scuttled.
Entering a night sea battle is an awesome business.The enveloping darkness, hiding the enemy's.. seems a living thing, malignant and oppressive.Swishing water at the bow and stern mark an inexorable advance toward an unknown destiny.