Radio Frequency Identification (RFID) has become very popular in various applications such as parcel and document tracking, animal tracking, distribution logistics, and so on. Among several elements in an RFID system, a tag, which contains digital information, is one of the most critical elements. Label-type tags based on thin films such as PETs have usually a half-wavelength dipole antenna, and they are placed on various objects like papers, plastics, woods, and so on. However, when they are placed near the surface of metallic objects, the degradation of the RFID tag performance is inevitable . This degradation is mainly caused by impedance mismatch due to the high parasitic capacitance between the antenna and the metallic object . Several research groups have dealt with RFID tag antennas such as patch- or label-type antennas for metallic object applications -. Prothro, et al. described effects of metal ground plane such as input impedance and gain penalty depending on the line width of dipole antenna.