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Abstract

This paper presents the design, analysis, and experimental investigation of magnetic self-resonating wireless power transfer (WPT) couplers based on coaxial-cable-wound spiral coils, where the intrinsic distributed capacitance of the coaxial structure is exploited to realise capacitorless resonance. Several coupler configurations are investigated, including single-core and twin-core coaxial cable flat spiral coils, as well as cylindrical designs enhanced with ferrite materials, in order to evaluate the influence of coil topology and intrinsic capacitance on WPT performance. Incorporating ferrite sheets and ferrite cylinders further improves performance by concentrating the magnetic flux, increasing mutual inductance, and boosting peak efficiency—especially at short separation distances. A theoretical framework describing the current gain, voltage transfer function, and power transmission efficiency is developed based on the equivalent circuit of magnetically coupled resonators. The analysis highlights the roles of mutual inductance, coupling coefficient, and quality factor in determining resonant behaviour and power transfer efficiency. Experimental measurements are conducted using laboratory prototypes to validate the proposed concept. The results demonstrate that the inherent capacitance of coaxial cables can effectively replace discrete compensation capacitors, resulting in stable resonance and high power transmission efficiency with a simplified topology. The characteristic features of the presented designs are as follows: maximum efficiency = 90% ± 1% - 98% ± 1%, operating frequency = 0.75 MHz ± 0.5% - 2.1 MHz ± 0.5%, separation distance = 2 - 38 mm, and intrinsic ``unloaded'' coil quality factor of the order of 100–400. The twin-core configuration provides superior performance (enhanced efficiency) over the single-core coaxial spiral couplers due to improved magnetic coupling and increased intrinsic capacitance. The proposed architecture offers reduced component count, improved electromagnetic shielding, and enhanced operational robustness, making it a promising solution for compact and high-efficiency WPT systems.

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