A capacitively fed rectangular patch antenna for anti-metal RFID tag was studied. It was found that by using capacitively feeding and a thin micro-strip line, the input impedance could be stabilized with respect to ground plane size. As a result, the reading range degradation was avoided, and the presented antennas have almost the same reading range when used on metal or non-metal objects. The mechanism was explained by a basic antenna design and a transmission line model. Then a more practical compact design having much smaller size was presented. Prototypes of both the basic design and the compact design were fabricated and tested. The testing results validated the effectiveness of the designing method.
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