**5. Conclusions**

A frequency-controlled dual-impedance operation which prevents excessive variation of inverter load is proposed. The low-impedance coils or the high-impedance coils are selected depending on coupling condition simply by frequency selection. This contrasts with the conventional impedance matching which relies on mechanical relay switch arrays to select capacitor bank or coil snippet.

The increment of coil volume is kept minimized by packing one coil within the other coil. The proposed method obviates the necessity of any impedance tuning switches—such as mechanical relays and back-to-back series semiconductor switches—which cause losses and high volumes. In summary, a compact, high efficiency, and mechanical-part-free method of impedance reconfiguration in response to coupling variation is realized.

Increasing the number of coils and frequency bands is not feasible at this moment because of the eddy current. For two coils in this paper, it is easy to prevent the eddy current following the scheme of Figure 3c. However, it is not directly applicable to more than 3 coils. Triple-band systems with eddy current blocking capability will be a topic for further research.

**Author Contributions:** Conceptualization, W.L.; Methodology, D.A.; Validation, D.A. and W.L.; Formal Analysis, W.L.; Investigation, W.L.; Resources, D.A.; Data Curation, D.A.; Writing—Original Draft Preparation, W.L.; Writing—Review & Editing, D.A.; Visualization, D.A.; Supervision, D.A.; Project Administration, D.A.; Funding Acquisition, D.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work is supported by Incheon National University Grant (#2018-0482).

**Conflicts of Interest:** The authors declare no conflict of interest.
