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Radar gun emits a continuous stream of microwaves at a preset frequency. According to the Doppler effect, when the waves reflect off an object moving toward the source, their frequency is shifted higher (objects moving away shift the frequency lower). A stationary gun measures the degree of shift, which determines velocity.
Batteries power a Gunn diode, part of a microwave oscillator. It emits microwaves that are focused into a beam by a horn and lens (together, the "antenna"). The antenna receives waves reflected by objects in the beam's path. A mixing diode compares the outgoing and incoming signals and sends the difference through an analog-to-digital converter to a digital-signal-processing (DSP) chip. The DSP analyzes the incoming signal frequencies and sends the speed value of the strongest return (typically the closest vehicle) to the gun's display.