Design and Evaluation of Planar Bifilar Helical Antennas for Radio Frequency Identification Tags

In this letter, planar bifilar helical antennas with varied turns for ultrahigh-frequency radio frequency identification (RFID) tags are proposed. The bifilar helix, formed using printed strips with straight-edge connections implemented by plated via-holes, is employed to match the complex impedance of the chip. The structures of the two-turn, one-turn, half-turn, and quarter-turn helical antennas are simulated and optimized. The simulation results show that the gain of the antenna is improved as the turn number increases. The gain of the quarter-turn helical antenna is 3 dB lower than that of the two-turn helical antenna, but the size of the quarter-turn helical antenna is only 3.1% of that of the two-turn helical antenna. Thereafter, the impedance, efficiency, and mutual coupling of these fabricated antennas are investigated in a reverberation chamber. The simulated and measured results show that the coupling between two stacked quarter-turn helical antennas is 15 dB lower than that of the commercial tags Alien 9662 in the operating frequency band. The proposed tags can achieve a maximum read range of over 40 cm in free space and 36 cm in dense environment. The radiation efficiencies of the tags are more than 55% at 920–925 MHz (China RFID band).