A Dual Circularly Polarized RFID Reader Antenna With Wideband Isolation

This letter presents a dual circularly polarized microstrip antenna with wideband isolation for radio frequency identification (RFID) reader application. The proposed antenna consists of a radiating patch and ground plane with a branch-line coupler (BLC) fed by separate Tx/Rx ports. The BLC is located underneath ground and can provide dual circularly polarized radiation coupled to the radiating patch through H-shaped slots. The prototype antenna was fabricated and shows <formula formulatype="inline"><tex Notation="TeX">${ S}_{11} \leq -18~$</tex></formula>dB and axial ratio <formula formulatype="inline"><tex Notation="TeX">${\rm AR} \leq 2~$</tex></formula>dB from 860 to 940 MHz. The <formula formulatype="inline"><tex Notation="TeX">${-}$</tex></formula>25-dB isolation characteristic between Tx and Rx ports achieves wide bandwidth of 65 MHz (864–929 MHz), which covers both Europe (865–869 MHz) and USA (902–928 MHz) RFID bands.

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