A Conformal Split-Ring Loop as a Self-Resonator for Wireless Power Transfer

A conventional printed spiral coil (PSC) has a self-resonant frequency and its equivalent circuit is a parallel inductor-capacitor (LC) circuit. It is desirable to use PSCs in wireless power transfer (WPT) applications; however, these are most commonly constituted by a series-primary and series- or parallel-secondary circuit. This paper proposes a printed conformal split-ring loop with the characteristic of a series LC circuit at its resonance frequency. Regarding the loop as a self-resonator, a magnetic-resonance coupled (MRC) WPT system with series-primary and secondary operating at 433 MHz in the industrial scientific and medical (ISM) band is presented. The maximum measured power transfer efficiency is 87.9% at a transfer distance of 22 mm, the best-reported result for such a configuration.