Design of external circuits for smart inductive coupling between non-self-resonant small antennas in wireless power transfer systems

Wireless power transfer systems of resonance coupling or inductive coupling utilize electrically very small antennas, which are either self-resonant or non-self-resonant. The system with non-self-resonant antennas requires the external circuit for impedance matching and high power transfer efficiency. This paper presents the design formulas for four kinds of external circuits, and discusses their performances. The power transfer efficiency is generally expressed in terms of figure of merit, the product of the inductive coupling coefficient and the system quality factor. The circuit derived from the emulation of a correct equivalent circuit of the self-resonant antenna systems can help the system of inductive coupling to act similarly as the self-resonant systems of resonance coupling.