In recent years, applications for radio frequency
identification (RFID) systems in the ultra-high frequency
(UHF) band has been receiving much interest in several
service industries, purchasing, distribution logistics, and
animal tracking applications [1]. For a commercial RFID
system, it usually consists of a reader and many tags. The
reader antenna generally has a circular polarization radiation
pattern to increase the orientation diversity, however, most of
RFID tags are linear polarization (LP) in the UHF frequency
band. Therefore, the polarization mismatch between them
would result in that only half of the transmission power is
received by tags. However, if the tag antenna is also
circularly polarized, the power received by the CP tag
antenna can be improved by 3 dB, and the maximum reading
range can thus be increased by 41%, theoretically [2].