Fig. 1. Block diagram of rectenna. nant resonance frequency if a circular microstrip antenna (CMSA) is chosen. This paper also experimentally clarifies wide frequency band characteristics of the CMSA. It is especially shown that the CMSA with slits possesses an excellent higher harmonic suppression characteristic (1). The block diagram of the rectenna in this paper is shown in Fig. 1. The CMSA with slits is used as a receiving antenna, a higher harmonic rejection filter as an input filter, a diode bridge using four Toshiba 1SS154 Si Schottky diodes as a rectifier, a by-pass chip condenser as an output filter in the rectenna. The rectenna of Fig. 1 was built as a trial, and its RF to DC conversion efficiency was measured. In the experimental results, the conversion efficiency is not high. However, the rectenna obtained in this paper has many unique and attractive properties — low in profile, light in weight, compact and conformable in structure, easy to fabricate, and easy adaptation to the photoetching technique in fabrication. CIRCULAR MICROSTRIP ANTENNA (CMSA) The general geometry of the microstrip antenna without feed is shown in Fig. 2. The microstrip antenna is often called a “patch” antenna because of its shape. The microstrip antenna has received much attention because of its many unique and attractive properties. It is low in profile, light in weight, compact and conformable in structure, easy to fabricate, easily integrated with solid-state devices, and easy to adapt to the photoetching technique in fabrication (2,3). All of these features are desirable for the receiving antenna of the rectenna. Moreover, eigenvalues which correspond to the resonance frequencies of the antenna can be controlled if a proper patch is chosen. For example, if a circular microstrip antenna (CMSA) is chosen, its eigenvalues are given by the following equation:
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