Microwave Power Transfer With Optimal Number of Rectenna Arrays for Midrange Applications

In this letter, the microwave power transfer (MPT) system with the optimal number of rectenna arrays for midrange applications is proposed, theoretically analyzed, and verified. A retrodirective power transmitter is designed to overcome the degradation by the near-field effect and enhance the power transfer efficiency. The implemented transmitter is composed of 16 $\times$ 1 patch antennas with a $\lambda /2$ spacing, and each of the antennas has a circuit for simultaneously detecting and shifting the phase, and operates at a frequency of 2.45 GHz. To further improve the power transfer efficiency, the parallel dc combining rectenna array is designed since the conventional RF combining rectenna has several problems in the case of midrange. The fabricated rectenna array consists of 8 $\times$ 1 rectennas with a $\lambda /2$ spacing and a dc power management circuit. In addition, the optimal number of rectenna arrays is proposed, considering the size of the receiver. In the experiments, the optimal number of rectennas is 3 when the transfer distance is 1 m. The achieved power transfer efficiency from three-arrayed rectennas is 4.47%, while the efficiency of 5.01% is achieved from eight-arrayed rectennas. Experimental results from the implemented MPT system show good agreement with the proposed theory.