Design of Miniaturized Quasi-Yagi Antenna for Portable RFID Reader Applications
A method to design a miniaturized two-element quasi-Yagi antenna (QYA) with size and gain requirements is presented for portable ultra high frequency (UHF) radio frequency identification (RFID) reader applications. The antenna consists of a driver dipole and a ground reflector, and these elements are serially connected with a coplanar strip line. The ends of both elements are folded back toward each other to reduce the lateral size of the antenna. A detailed design procedure of the proposed antenna is explained, along with a performance comparison for the input impedance, voltage standing wave ratio (VSWR), broadside gain, front-to-back (F/B) ratio, and total efficiency. A prototype antenna, covering the 860―960 MHz UHF RFID band with a gain > 4 dBi, is fabricated on an FR4 substrate with dimensions limited to 90 mm by 90 mm. The total width of the proposed antenna is reduced by approximately 41% compared to the conventional QYA without miniaturization, and an F/B ratio is improved by 1―8 dB in the band. Experiment results show that the proposed antenna has the desired impedance characteristics with a frequency band of 853―1,098 MHz for a VSWR < 2, and a stable broadside gain of 4.0―5.3 dBi in the UHF RFID band. Moreover, a measured F/B ratio > 13 dB is obtained.