With the repeated discussions about Instant-On and its undetectability when the radar gun is on standby, that plus talk about RDDs and how they work, gave me a crazy idea.
I wonder how feasible it would be to invent a device that can detect the LO emission from the police car's radio. Of course, at first blush it might make more sense to have a device that alerts to activity on police radio frequencies, but that would only work when they are transmitting. If they aren't "keying-up", it's like keeping the radar gun on standby. But, when a police radio isn't transmitting, it's receiving. And police radios are superheterodyne, and have local oscillators. And since there's no market need to cloak the LO emissions of a police radio, would it be possible to detect with enough advance warning to alert a driver of activity up ahead?
I guess it would require knowledge of the radio's LO frequency (or primary leakage frequency) and hope that other common devices don't leak on the same frequency. And different makes/models of radios might have different LO frequencies.
I wonder if the feasibility of such a device has ever been researched.