Double-layered folded spiral resonator for coupled-resonant wireless power transfer

In this paper, we propose a double-layered folded spiral resonator for near field wireless power transfer (WPT). To realize maximum available efficiency, impedance matching of real part and imaginary part is necessary. It is known that a folded dipole antenna, which is used in far-field communication systems, has a capability of adjusting real part of input impedance by changing radius of the resonator wire. In the proposed structure, folded structure is applied to spiral resonator for near-field WPT. Furthermore, to decrease proximity effect and increase capability of impedance matching, folded structure is formed in double layer. Numerical simulation demonstrates that by using the double-layered folded spiral resonator, transfer distance over 80% eta(max) extends 1.78 times compared with the conventional single-layered folded spiral resonator.