Electromagnetic Effective Coupling and High Spatial Resolution Theory in Near-Field Cardiac Sensing

Noncontact vital signs detecting in medical and healthcare applications have been widely recognized. However, due to the limited penetration depth into the skin, the microwave-based detection systems obtain the heartbeat rate and respiratory through the chest surface movements, which poses the challenges such as the overlap of multiple vital signs, dynamic information loss, restricted spatial resolution, etc. In this article, we investigate the energy coupling mechanism between the antenna and the high permittivity media by electromagnetic (EM) propagation model based on dyadic Green’s function. From the theoretical derivation and numerical calculation, the reactive near-field region of the antenna has been proven a high transmission coefficient (up to 200% in lossless media). Thus, near-field vital sign detection shows two near-field effects: enhancement and focus, which directly couples a significant amount of energy into the human body and focuses the energy on the sensing area. The in-tissue power flow is 13.2 dB higher than the far-field scenario and the spatial resolution is at the level of 75 mm in ultrahigh frequency (UHF) band. The near-field effect endows the vital sign detection with the characteristics of high signal quality and high spatial resolution. The theory and method are verified by commercial EMs simulation software, phantom model, and on-body cardiac sensing experiments.