Triple-Band Efficiency Improvement of Smartwatch Antennas by Sharing a Zeroth-Order Resonance Patch

This article addresses the critical challenge of radiation efficiency degradation in smartwatch antennas caused by wrist absorption effects and extreme clearance. A novel radiation efficiency enhancement methodology is proposed to simultaneously improve the efficiency of multiple antenna elements. First, the proposed approach integrates a zeroth-order resonance patch (ZORP) beneath the motherboard. This ZORP is a shorted patch that electromagnetically couples with inverted-F antennas (IFAs) embedded in the watch bezel, thereby forming a hybrid radiating structure. Then, the comprehensive analysis with an equivalent circuit model theoretically demonstrates that the radiation efficiency improvement is determined by coupling strength between the IFAs and ZORP. Finally, an optimized antenna cluster operating at GPS-L1 (1575.42 MHz), GPS-L5 (1176.45 MHz), and GSM-1800 (1750 MHz) bands is implemented to validate the methodology. Measured results reveal that peak efficiency enhancements of the on-wrist antenna exceed 3 dB across all operational bands, with maximum improvements surpassing 8 dB. The proposed design methodology is promising for developing compact wearable antennas with enhanced on-body radiation efficiency.