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 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">∘</tex></formula> 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: <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">Sₓ</tex> </formula> and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">Sy</tex> </formula> . Experimental results show that the reader antenna of size 58 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">\, × \,</tex></formula> 58 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">\, × \,</tex></formula> 11 mm <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">³</tex></formula> has a 3-dB beamwidth of more than 91 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">∘</tex> </formula> . 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) <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX"><2</tex></formula> impedance bandwidth is larger than 15.1%.