Comprehensive Analysis and Measurement of Frequency-Tuned and   Impedance-Tuned Wireless Non-Radiative Power Transfer Systems
  This paper theoretically and experimentally investigates frequency-tuned and impedance-tuned wireless non-radiative power transfer (WNPT) systems. Closed-form expressions for the efficiencies of both systems, as a function of frequency and system (circuit) parameters, are presented. In the frequency-tuned system, the operating frequency is adjusted to compensate for changes in mutual inductance that occur for variations of transmitter and receiver loop positions. Frequency-tuning is employed for a range of distances over which the loops are strongly coupled. In contrast, the impedance-tuned system employs varactor-based matching networks to compensate for changes in mutual inductance and achieve a simultaneous conjugate impedance match over a range of distances. The frequency-tuned system is simpler to implement, while the impedance-tuned system is more complex but can achieve higher efficiencies. Both of the experimental WNPT systems studied employ resonant shielded loops as transmitting and receiving devices. 
Jason D. Heebl, Erin M. Thomas, Robert P. Penno, Anthony Grbic
cs.OH