A Compact and Broadband Microstrip Stacked Patch Antenna With Circular Polarization for 2.45-GHz Mobile RFID Reader

A compact and broadband microstrip stacked patch antenna with circular polarization (CP) for a mobile 2.45-GHz passive radio frequency identification (RFID) reader is presented. The proposed antenna consists of two stacked patches fed with an S-shaped impedance-matching network (IMN), which is connected by a probe. With the proposed S-shaped IMN feed technique, a good impedance matching and symmetrical broadside radiation patterns could be obtained. The CP operation could be attained simply by the upper parasitic patch rotating 90° sequentially based on the lower patch and truncating corners. Additionally, with four symmetric-L narrow slots on the patches, the compact operation could be obtained. The proposed reader antenna matching is optimized by regulating the sizes of the S-shaped IMN: S_x and S_y. Experimental results show that the reader antenna of size 58 × 58 × 11 mm³ has a 3-dB beamwidth of more than 91°. The peak gain is optimized in the frequency range from 2.31 to 2.56 GHz of larger than 6 dBi, and it achieves 6.32 dBi in the center frequency 2.45 GHz. The voltage standing wave ratio (VSWR) <; 2 impedance bandwidth is larger than 15.1%.