Physiological Monitoring Using Harmonic Radar and Battery-Less Nonlinear Tag

This paper introduces a portable harmonic radar system paired with an innovative tag designed for non-invasive physiological monitoring in multi-person scenarios. Operating at a fundamental frequency of 4 GHz with a second harmonic at 8 GHz, this system can detect a specific person's respiratory activity through clothing when the battery-less tag was attached to the skin. With a multi-element antenna and integrated Schottky diode, the tag in the form of a small sticker is lightweight, flexible, and adheres comfortably to the human chest, enabling seamless monitoring when placed beneath clothing. The system's effectiveness was demonstrated in experiments involving two human subjects: the primary subject wore the tag for respiratory monitoring, while a second subject walking in various directions around the monitored individual. This harmonic radar system features effective clutter and interference rejection, detecting only the tag's movement on the chest while rejecting interferences from other moving objects. By harnessing the tag's unique harmonic response and the radar system's high sensitivity and selectivity to second-order harmonic, this setup provides an efficient, unobtrusive solution for real-time patient care. This work highlights the potential of advanced harmonic radar technology in health monitoring, establishing a foundation for future applications in both clinical and home environments.