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A passive RFID tag antenna in a recessed cavity in metallic objects such as vehicles, air craft, and metallic containers, and an impedance matching technique for this configuration are proposed. The proposed tag antenna can be considered as a cavity-backed antenna and it is comprised of a bowtie antenna installed in a recessed cubic volume in a large metallic object, and the input impedance of the bowtie antenna can be easily matched by adjusting the coupling effect between the tag antenna and the cavity. Main parameters affecting the coupling are the dimensions of the cavity and the location of the bowtie antenna inside the cavity. For more practical analysis of the impedance matching process, an equivalent-circuit model consisting of several lumped elements is introduced and each element is optimized using a genetic algorithm (GA). Experimental results show that the maximum reading distance of the proposed tag antenna with optimized dimensions can be improved about 3.2 times compared to a commercial RFID tag.