**4. Conclusions**

The multi-coil WPT systems commonly use three or more coils to improve e fficiency in HF bands as well as LF bands. These systems are basically required to estimate or measure the practical mutual inductance between coils. In this paper, the unilateral approach method is proposed to estimate the mutual inductance among multiple coils. With the proposed method, it is not only simple to configure the expression of formulas but also possible to estimate the practical mutual inductance through several routes for multiple coils since the assumption that only one voltage generated with respect to the coil pair was considered. To verify the validity of the proposed method, we coaxially placed several coils along the distance and confirmed the consistent results with simulation and measurement. In reality, it is di fficult to analyze the mutual inductance in multiple coils using a public VNA that has only two ports. However, the proposed method can provide the mutual inductance using a public two-port VNA. Furthermore, it will be quite useful for the multi-coil WPT systems with a regular array of identical coils.

**Author Contributions:** Conceptualization, S.-J.J. and S.-H.L.; methodology, S.-J.J.; software, S.-J.J.; validation, S.-J.J. and S.-H.L.; formal analysis, S.-J.J. and D.-W.S.; investigation, S.-J.J. and S.-H.L.; resources, S.-H.L.; data curation, S.-J.J.; writing—original draft preparation, S.-J.J.; writing—review and editing, D.-W.S.; visualization, S.-J.J.; supervision, D.-W.S.; project administration, D.-W.S.; funding acquisition, D.-W.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Research Foundation of Korea (NRF) gran<sup>t</sup> funded by the Korea governmen<sup>t</sup> (MSIT) (No. 2018R1C1B6003854).

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