A Spiral Electrically Small Magnetic Antenna With High Radiation Efficiency for Wireless Power Transfer

An innovative design of spiral electrically small magnetic antenna (ESMA) with high radiation efficiency is proposed for wireless power transfer (WPT). The achievable wireless power transfer efficiency (PTE) by the proposed ESMA is far beyond than that of the traditional spiral shape with the same size. The operation frequency for the proposed ESMA is 13.56 MHz, and ka = 0.04, where k and a are wavenumber in free space and radius of a minimum sphere enclosing the antenna, respectively. The ESMA consists of an embedded multiloop feed and an innovative spiral resonator. In order to increase the radiation efficiency, conductor strip is used instead of the wire to reduce loss, and the internal and external turns stagger in the axial direction in order to confine the electric current to the outermost surface of the occupied volume. The input impedance is made closer to $50 \Omega$ by the embedded multiloop feed. The radiation efficiency for the single proposed ESMA is 93.5%, and the PTE between two symmetric proposed ESMAs are 92.4%, 77.4%, and 61% at distances 60 cm (3 times the maximum side length), 70 cm (3.5 times the maximum side length), and 80 cm (4 times the maximum side length), respectively.