Compact and efficient 2.45 GHz circularly polarised shorted ring-slot rectenna

A compact and efficient rectenna based on a printed shorted annular ring-slot antenna with circular polarisation properties has been designed and evaluated at 2.45 GHz. The rectifier part is localised at the back side and centred inside the radiating element, resulting in a more compact structure in comparison with most conventional devices where rectifying circuit and antenna parts are geometrically clearly separated. In addition, the printed annular ring antenna is mismatched at the 4.9 GHz second and 7.35 GHz third harmonics, thus avoiding the use of an input lowpass filter. The proposed antenna and rectifier circuit have been first simulated and optimised separately using electromagnetic and circuit analyses, and then connected together. A maximum efficiency of 69 and an output DC voltage of 1.1 V have been measured over an optimised 2500 resistive load at a power density of 20 W/cm². This rectenna is particularly suitable for powering wireless sensors or sensor networks by recycling ambient RF energy because it exhibits a global efficiency of more than 50% for power densities more than 10 %W/cm².