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UHF band (860~960MHz) RFID tag strip-line antennas for non-metallic object and slotted patch RFID antenna for metallic objects have been optimized with a GA. The antennas are optimized for commercially available RFID tag IC chips. Different cell sizes of FDTD have been tried while the GA optimizes the symmetrical shape of RFID antennas. I. IntroductionAn RFID system consists of a reader, a transponder (tag) and a computer connected to the reader. A transponder consists of an antenna and an RFID IC chip. The reader has an antenna. The reader transmits modulated electromagnetic field, which powers up the tag, while the reader sends the data to the tag. The RFID IC microchip is attached to the feeding point of the tag antenna [1-2]. The passive tag receives all the required energy from the RF energy of the carrier signal of the reader. The tag sends a coded signal back to the reader using the tag antenna by backscattering method. References [3-5] introduce a method to power up the tag. The method uses a rectifying Schottky detector diode circuit that converts microwave energy into DC. The rectified or DC part of the energy is used to power up the electronics in a passive tag chip. Various shapes of UHF RFID antennas are introduced in [6]. Reference [7] introduces electromagnetic band gap (EBG) antennas for RFID tag and reader antennas. An EBG antenna is attached to metal objects. When general tag antenna attached to a metallic object, the tag cannot be powered up by the field strength emitted by the reader since the metallic object reflects RF wave. The impedance of the tag antenna, resonant frequency of the antenna and radiation efficiency will be changed due to the parasitic capacitance between the tag antenna and the metallic object. [8]. To minimize effect of the parasitic capacitor between the tag antenna and metallic object, and the effect of the reflection of the RF wave by metallic object, it is better to put a gap between the tag antenna and the metallic object, and to add dielectric material with high dielectric constant between them.A genetic algorithm has been applied to optimize an RFID antenna. The impedance of a tag antenna should be matched to the conjugate of the impedance of an RFID IC Chip. The chip impedance has real and capacitive imaginary parts due to the parasitic capacitance of the RFID chip. The different impedance values of commercially available RFID chips are matched to the impedance of antenna.

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