Change of read range for UHF passive RFID tags in close proximity

In this paper, we discuss change of read range between a UHF passive RFID tag and reader when a tag is close to another tag. The read range is determined by the strength of forward link and reverse link, which are represented by received power at a tag and reader, respectively. Approaching of tags causes electromagnetic interference between antennas of tags and results in change in impedance matching condition between an antenna and IC of a tag. This change in impedance matching condition decreases in read range. Therefore, we measure the change in received power of a tag and reader experimentally so as to identify which link limits read range and to restrict the existence range of IC impedance. In addition, a technique for extending read range when tags are close is proposed.