I. Introduction
The rectifying antenna (rectenna) is one important component in wireless power transmissions and has been used to provide dc power in wireless sensors and RFID tags [1], [2]. Most rectennas are designed for a single frequency and recent developments have been summarized in [3], [4]. With the usage of multiple frequency bands in wireless communication systems, some dual-frequency rectennas have been developed for the industry, science, and medical (ISM) band [5], [6]. In these designs, the antenna elements usually have a dimension of one wavelength length or even more.