Radio frequency identification (RFID) technology provides wireless identification and tracking capability that, is more convenient than the use of bar codes and optical scanners [1]. Antenna is one of the key factors for RFID systems. There have been intensive researches on RFID tag antennas in the ultra high frequency (UHF) band over the years. Various tag antennas have been reported for different RFID applications [2-4]. Generally, most of these.tag antennas are dipole-like antenna; their performances are similar to those of a simple dipole antenna in spite of varied, structures. The performance of these tags, usually evaluated with a reading range, is relatively satisfactory when they are mounted on objects with small dielectric constants. However, 'when the tags are mounted on objects with high conductivity such as metallic cans or water bottles, they do not work properly. To. overcome this problem, a number of solutions have been proposed, such as using planar inverted-F antenna (PIFA) or patch antenna to form the RFID tag [5]; Using artificial, magnetic. conductor (AMC) to configure the platform tolerant- tag [6]; or co-design the RFID tag by taking into account the property of the platform [7].
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