Originally Posted by Incubus

I got my radar detector in 1942, so I'm guessing they have them then, possibly even earlier. In 43 they definitely do. Not sure about range, but from my experience my detector doesn't so much allow deterrence from destroyers as enough time to prepare. The destroyer will probably know about where you are, but you have enough time in advance to dive deep, slow down, silent run, and creep about until they move on.