Circularly Polarized Stack Yagi RFID Reader Antenna

A structure and power robust, circularly polarized (CP) stack Yagi antenna for radio frequency identification (RFID) reader application is proposed in this letter. To increase the bandwidth, a parasitic patch is added to an air-filled microstrip patch antenna to generate another resonance on the axial ratio (AR) curve. To increase its gain, a guiding patch is placed at a distance slightly below λ/4 away from the parasitic patch, which serves as the director of a common Yagi antenna. It increases the boresight gain by more than 1 dB. To fix the position and rotation angle of these three patches, a center column with a three-layer structure is applied. Also, to achieve better impedance matching, a shorting screw is added to the basic patch. The proposed antenna achieves a measured -10 dB impedance bandwidth of 5.3% (884-932 MHz), a 3 dB AR bandwidth of 4.8% (890-934 MHz), a peak gain of 9.8 dBic, and an efficiency of -0.42 dB at 915 MHz, which makes it very suitable for the 902-928 MHz ultra high frequency (UHF) radio frequency identification application. Its size is 0.73 × 0.73 × 0.33 λ915MHz (λ915MHz is the free-space wavelength at 915 MHz). The proposed antenna has an all-metal structure, which gives rise the characteristics of high mechanical strength, high power-handling, high radiation efficiency, easy to assemble and low fabrication cost.