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A Doppler radar system was built to detect the motion of modulated backscatter data from a passive tag, while rejecting the unmodulated backscatter from other elements. A harmonic tag was designed and fabricated to show the feasibility of sensing cardiopulmonary related Doppler shift in the received second harmonic signal. A fundamental frequency of 2.45 GHz was transmitted at a target and a quadrature homodyne receiver was used to sense the received second harmonic signal. The setup was used to first successfully isolate periodic motion from a tag on a programmable mechanical target in close proximity to other moving objects and to sense respiratory motion of a human subject. Respiratory data obtained from the tag was compared to a reference. The harmonic radar setup was then successfully used to isolate the respiration of a tagged human subject from an untagged large scattering object. The response of the harmonic radar was also compared to a 2.45 GHz Doppler radar for some experiment scenarios that clearly indicate the utility and potential for such systems.