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 degrees 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 x 58 x 11 mm(3) has a 3-dB beamwidth of more than 91 degrees. 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%.