Abstract
This paper introduces a novel piecewise empirical framework for modelling near-field specific absorption rate (SAR) behaviour in wearable devices, explicitly distinguishing between reactive and radiative coupling regimes. Comprehensive full-wave simulations of planar inverted-F antenna (PIFA) and patch antenna at 2.45 GHz reveal a critical transition in near-field coupling at an 8 mm separation, defining a distinct empirical transition region. The patch antenna demonstrates markedly superior near-field performance, achieving up to 44% lower SAR in typical wearable configurations. Extending the framework to 5G frequencies shows that the compressed reactive near-field at 3.5 GHz intensifies electromagnetic coupling constraints while further emphasizing the advantages of the patch antenna. The proposed model achieves a 32% reduction in RMSE compared with conventional single-region approaches and provides practical, quantifiable guidelines for the design of next-generation wireless and wearable antenna systems.