Method of Moments Analysis of Wireless Power Transfer from Air to Underwater via Magnetic Coupling

Wireless power transfer (WPT) in water, which has an extremely high relative permittivity of 81, was analyzed using the electromagnetic field simulator WIPL-D based on the higher-order basis function method. Two types of simulations were conducted: one using a program that assumes the entire structure is submerged in water (instead of air, as in conventional analyses), and the other treating water as a finite-sized dielectric. The results showed that electromagnetic characteristics such as the resonant frequency agreed well between the two methods. It was also clarified that assuming the entire structure is in water results in greater losses compared to the finite-sized dielectric model. Next, spiral coils with a pitch of 10mm were placed in both air and water, and it was confirmed that both systems resonated at a frequency corresponding to 0.35 wavelengths based on the free-space wavelength in air, and about one-ninth of that in water, considering its high permittivity. Finally, the resonant frequency was set to 15MHz for both media, and the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$S_{21}$</tex> parameter and the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$k Q$</tex> product were evaluated. As a result, it was shown that reducing the distance between the two spiral coils from 160mm to 20mm increases the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$S_{21}$</tex> value up to −6.5 dB.