Respiratory monitoring using a doppler radar with passive harmonic tags to reduce interference from environmental clutter

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 used to sense the received signal from a tag on the target. The setup was used to successfully isolate periodic motion from a tag on a programmable mechanical target and to sense respiratory motion of a human subject, even when the targets were in close proximity to other moving objects.