Efficiency Bound of Radiative Wireless Power Transmission Using Practical Antennas

In this communication, we provide the efficiency bound of radiative wireless power transmission (WPT) for several types of practical receiving antennas. The optimal current distribution of the transmitting surface is analytically derived. In addition, the maximum bound of the power transfer efficiency (PTE) and the optimal shape of the base station current are found in terms of the transmitting area and the transfer distance. Examples applying the proposed theory to three types of practical mobile antennas are shown and the results are compared with those of previous reports, demonstrating significant differences. The results indicate that the optimum current distribution on the transmitting surface and the maximum efficiency of radiative WPT depend on the radiating field pattern of the mobile antenna. The results further show that microwave power transmission using a compact patch antenna can achieve high power transmission efficiency, especially in the Fresnel region.

I. Introduction

Over the last few decades, wireless power transmission (WPT), which can charge the batteries of mobile devices without wires, has been developed to improve the convenience of mobile users. There are several WPT technologies, such as inductive coupling, resonant coupling, and microwave power transmission (MPT). Many recent studies on WPT have been conducted on inductive coupling and resonant coupling, which are based on the use of the near-field, or evanescent field. On the contrary, MPT, which is based on the propagating field, has not recently been studied as much, even though MPT can increase the operating range of WPT.