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Magnetic resonant wireless power transfer (MR-WPT) has several advantages over conventional wired underground power supply methods. However, MR-WPT inevitably encounters metal conductors, which reduce the system efficiency through the eddy loss induced in the conductor by high-frequency electromagnetic waves. In this paper, the effect of resonant frequency variations caused by metal tube interference with the aim of maximizing the system efficiency is studied. According to the variation in the resonant frequency, the system efficiency is analytically derived by an equivalent circuit model. Electromagnetic simulations are carried out to further analyze the metal tube interference on the system. The results demonstrate the existence of an optimal resonant frequency that maximizes the system efficiency. Aluminum tube interference produces a lower optimal resonant frequency and higher efficiency than a 304 stainless-steel tube. When the metal tube is slotted, the optimal slot width and number of slots enhance the maximum efficiency and reduce the optimal resonant frequency and frequency drift. With slot widths of 2 and 8 mm, the system efficiency reaches ∼67% at 40.1 kHz and 55% at 48 kHz, respectively. Finally, different types of slotted tubes are fabricated, and the theoretical results are experimentally verified. [ABSTRACT FROM AUTHOR]