Retrodirective arrays for wireless power transfer applications



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This dissertation presents a novel one-dimensional, dual frequency, active retrodirective array for wireless power transfer applications. A retrodirective array automatically scans its transmitting antenna beam in the direction of a transmitter (interrogating signal) without prior knowledge of its location. The implementation of a retrodirective array for wireless power transfer can improve efficiency significantly over an antenna array with a fixed beam. The retrodirective array allows the use of a large transmitting antenna array to produce a narrow beam that can be scanned to provide coverage within a desired sector. The high antenna gain (narrow beam) yields much higher power densities at the receivers than an array with a fixed beam that must be wide enough to cover same sector. The retrodirective array developed in this research uses an array of microstrip circular patch antennas modified to avoid excitation of troublesome surface waves. Each microstrip antenna uses four shorting pins to suppress surface waves. The improved antenna array is used as the transmitting antenna in the retrodirective system in order to increase the efficiency and improve the overall performance. The proposed retrodirective array operates at 2.4GHz for the interrogating signal and 5.8GHz for the retransmitted signal, using up-converting mixers. The beam scanning inherent in retrodirective arrays ensures a constant power level available to the charging devices, regardless of their location within an angular sector over which retrodirectivity is achieved. Microstrip antennas have narrow impedance and axial ratio bandwidths. In order to widen the bandwidth of the circularly polarized antennas with shorting pins developed in this work, sequential rotation techniques were investigated. Specifically, two designs using sequentially rotated microstrip patches with surface wave suppression capability were investigated. One design uses one layer directly fed elements, the other design uses aperture coupled patches in a two layer configuration. A prototype of the two layer design, which has better performance, was fabricated and tested. The simulated and measured axial ratio and impedance bandwidths were improved significantly in the sequentially rotated array that also includes surface wave suppression throughout the bandwidth. To design a full system for wireless power transfer, a high conversion efficiency rectenna (rectifying antenna) using circularly polarized microstrip patch antennas with reduced surface waves is proposed. A rectenna converts a microwave signal to a DC voltage. A rectenna operating at 5.8 GHz has been designed and tested. A zero bias Schottky diode with high detection sensitivity is used as the rectifying device. In order to minimize surface waves, two microstrip circular patch antennas with four shorting pins are used. The proposed surface wave suppression technique leads to improvements in antenna gain and efficiency. Furthermore, the shorting pins inherent in the design act as return path for the DC current eliminating the need for an RF chock in the rectifier circuit. Two complete retrodirective systems using the surface wave suppressing arrays were built and tested, one system uses the narrowband antenna array and the other uses the wideband sequentially rotated array. Measurements confirm the expected improvements in power transfer capability of both systems.



Wireless Power Transfer, Microstrip Patch Antenna, Surface Wave Suppression, Retrodirective Arrays, Rectenna