Feasibility of Retro-Reflective Beamforming for Microwave Power Transmission Utilizing Harmonic Reradiation From Rectifiers

Retro-reflective technology is recognized as an efficient method for controlling microwave power transmission beams. In a standard retro-reflective beamforming setup, the transmitter analyzes a pilot signal emitted by the receiver and then utilizes the phase-conjugation principle to direct a power beam toward the receiver. However, in many practical scenarios, generating the necessary pilot signal can consume power that receivers cannot afford, especially when the receiver's battery is depleted. To address this challenge, a novel approach for generating the pilot signal by harmonic reradiation of an interrogating signal from the microwave power transmitter is proposed. Specifically, the transmitter emits an interrogating signal at half the frequency of the desired microwave power beam. The rectenna in the receiver captures this interrogating signal, and due to the nonlinear characteristics of the rectifying diode in the rectenna, a second harmonic is generated. This second harmonic is then reradiated from the rectenna, serving as the pilot signal, which is in the same frequency as the power beam. In this letter, a rectifier that converts a 2.9 GHz interrogating signal into a 5.8 GHz pilot signal is designed and tested, which achieves a minimum conversion loss of 12 dB. The rectifying efficiency of this rectifier at 5.8 GHz is measured to be 60%. An experiment utilizing this rectifier has been implemented and tested. Results indicate that the rectifier successfully transforms the 2.9 GHz interrogating signal into the 5.8 GHz pilot signal, enabling the power transmitter to accurately direct a 5.8 GHz power beam toward the receiver based on the phase-conjugation principle.