As a time traveler it would be easy for me to take a short hike up that hill where the RADAR operators were and point out to them that indeed the equipment was working just fine and they should probably call it in. Assuming they believed me, it is arguable that my lone single action could start a chain of events that would allow the US to meet the Japanese planes and stop them from attacking the battleships. As a result, the US people would still be angry but not motivated to enter the war fully since the Japanese were not a perceived threat. Thus, you don't begin research on the atomic bomb until well after Hitler has already dropped a couple on London