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A Null Zone Reduction for HF-band RFID by Diversity Combining of Loop Antennas

Nowadays, the market of an RFID tag is rapidly growing. Particularly, the RFID tag using HF band (13.56MHz) is widely equipped in cellular phones or credit cards. They are being commodities as electronic wallets or electronic tickets for public transportation systems. However, it is known that there exists a null zone in which the RFID tag does not respond although the RFID tag is in the closed vicinity to the reader/writer (R/W). Since consumers suffer inconvenience from such a null zone, it is desired to avoid an occurrence of the null zone. Many investigations to improve an RFID performance are reported [1]-[3]. On the other hands, null zone has been kept off by expanding an antenna size, which occupies larger space. In this paper, we propose a method to overcome null zone by splitting antennas and combining antenna outputs, without enlarging antenna size. As combining circuit, three circuits are considered and compared. We have found by an FDTD simulation that the proposed method increases received power by 13.9 dB at the position of deepest null.