“I recently got some cheap microwave motion sensors, mostly because I was wondering how China managed to make a radar for under a dollar”
The whole radar is a single transistor oscillator at 3.18 GHz with an S shaped copper track serving as resonator and antenna. The brains are a BISS0001, a chip sold for infrared motion sensors. The oscillator drifts several MHz from a nearby hand, and it doesn’t matter, because detecting motion only needs millivolt changes. Good enough beats precise when the bill of materials is under a dollar.