Repeater System Design For The Fresnel Zone Microwave Wireless Power Transfer

Abstract

In this article the problem of field spreading when the focal length $(z_{0}$) becomes more than the aperture size (D) in the fresnel zone wireless power transfer (WPT) is addressed at 2.45 GHz frequency of operation. One of the solution is to have the $ z_{0}\approx$D. However, the distance of WPT can be meters or Kms and building the D so large is not practically and economically feasible. This can be achieved with the use of repeater system, i.e, the lens kept at the focus of the previous lens. A comparison is made with the conventional WPT in the Fresnel zone with one lens and repeater system with more than one lens and the efficiency of power transfer is compared. The best part of this repeater system is with a very small receiver size (D <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RX</inf> = 3 cm), an efficiency of 19% (with 2 lens and always the authors are talking about the RF-RF efficiency) is achieved at 14.82 m. Also, with the increase in the repeaters, the efficiency can go upto 97% theoretically. For practical implementation the reflections from the ground and the losses from the fabricated lens have to be taken. The future work will be to design Fresnel lens with the unit cells tuned for the required amplitude and phase. In future the repeater system using Fresnel lens can help achieve higher efficiency with lower size of receiver antenna aperture, thus making practical implementation of Fresnel WPT more efficient replacing the overhead transmission cables for low duty machines and sensors.