An analytical expression of AC resistance of a single-layer air-core Helmholtz coil is presented. Proximity effects between both bundles and strands of litz-wire as well as skin effect are considered. To obtain an accurate analytical expression of proximity effect on bundle level and strand level, a precise distribution of magnetic field is discussed. The analytical expression of AC resistance and quality factor is verified with the experimental results, and the theoretical predictions are in agreement with the measured results.
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