Understood, but it has to be an active system that tracks the phase of the impinging wave and then with knowledge of processing and re-emission time, received signal strength and phase, produces an equally strong out-of-phase return signal in the direction of the impinging radar signal. It would necessarily be very complex.
Yeah. We've had systems that could do this (Deceptive ECM)since the 70’s, maybe earlier. The term “stealth” is very broad. We couldn't use a repeater to make a platform invisible but we could move it's position (RGPO range gate pull off). Very handy in anti-ship missile defense.