Modified Helical Coils Structure for Uniform Magnetic Flux Density

In this paper, we propose a coil structure with uniform magnetic field intensity and efficiency in the near field by using four helical coils with different turns ratio and diameter. In order to maintain a constant efficiency irrespective of the positional change of the receiving coil, the optimum number of required helical coils, turns ratio, and diameter can be determined by calculation through the proposed multiple helical coil modeling. Experimental results show that the conventional single helical coil generates a shadow region which cannot be charged near the center of the coil, and the magnetic field intensity at the coil edge is maximized, resulting in an uneven charging efficiency. However, the proposed structure has a very uniform charging efficiency.