電磁最佳化實驗室EM Optimization LabYen-Sheng Research Group, Department of Electronic Engineering

High-efficiency rectennas for radio-frequency (RF) energy harvesting have been studied for decades, but most of the literature straightforwardly applies the rectenna aiming at dedicated RF sources to this situation, even though the level of input power is significantly different. Since previous studies address antenna design collecting more ambient RF power, the improvement of power conversion efficiency (PCE) has emerged in a scattered way because the theoretical limits of PCE have not yet been characterized, and the optimal rectenna structure approaching such maximum PCE is still uninvestigated. In this paper, we characterize the performance limits of rectennas with input power ranging from ‒20 dBm to 0 dBm, proposing optimal rectenna design demonstrating the maximum PCE. The maximum achievable PCE is cast into a mathematical programming problem; solving this optimization model clarifies the effect of design factors including operational frequencies, rectifier configurations, and parameterization. To achieve the maximum PCE, our investigation shows that the optimal rectenna design should not only optimize those design factors but also eliminate the matching circuit between an antenna and a rectifier for ultralow-power conditions. The resultant PCE at 2.4 GHz is 50 % and 63 % at –10 dBm and 0 dBm, closely approaching the theoretical bounds.