Resonant Near-Field Power Transfer: Revisiting the frequency-splitting phenomenon using the spherical mode theory antenna model

In this article, the resonant coupling phenomenon is revisited using the antenna theory. The methods for extracting the maximum power by the load (i.e., the frequency tracking, conjugate matching, and critical coupling adjustment) are addressed. To obtain the best performances in terms of the highest power and the greatest transmission range, the system's tuning needs to be very sharp regardless of the method applied. Conjugate matching is shown to be the best method for reaching the highest possible antenna separations and the maximum efficiency, but the critical coupling adjustment turns out to be more practical in the sense of the bandwidth when the antenna separations are not too large.