Position Independent Wearable 6.78 MHz Near-Field Radiative Wireless Power Transfer using Electrically-Small Embroidered Textile Coils

Coupled wireless power transfer (WPT) has been widely used for near-field high-efficiency WPT applications. However, the efficiency of the WPT link is highly sensitive to separation and alignment, and is prone to over-coupling, making it unsuitable for mobile systems with unknown or loose coupling such as wearables. While ultra-high frequency (UHF) and microwave radiative WPT (0.3-3 GHz) enables meters-long separation between the transmitter and the receiver, free space propagation losses, and rectification inefficiencies, adversely limit the end-to-end efficiency of the WPT link. This work proposes radiative WPT, in the 6.78 MHz license-free band, based on resonant electrically small antennas fabricated using embroidered textile coils, tuned using L-matching networks. The proposed WPT system achieves a stable forward transmission of $\mathrm{S}_{21} \gt$ – 17 dB and $\mathrm{S}_{21} \gt$ – 28 dB, independent of coil, separation on the XZ and XY planes respectively, in a $27 \mathrm{m}^{3}$ volume space. The presented approach demonstrates the highest WPT-link efficiency, and promises higher end-to-end efficiency, compared to UHF WPT.