The thing is, every country knew about radar. They had working radar setups the day the war officially started. The German radars were even more effective than the British ones, but Hitler and his cronies weren’t very interested in developing defensive weapons.

The Americans even had a radar beam detect the attack on Pearl Harbor, but because the technology was so new the report went ignored because the people in charge didn’t know what the result meant.

Every country pretended not to know about radar to their enemies just in case their enemies didn’t know about it either. Everyone was real secret about it, so much so that radar wasn’t used optimally for much longer than necessary.