A Miniaturized Quadrifilar Helix Antenna for Handheld UHF RFID Reader Applications

The paper presents a printed quadrifilar helix antenna (QHA) fed by a power divider with 0^0, 90^0, 180^0, 270^0 phases and equal amplitudes to excite circularly polarized waves for UHF RFID applications. The proposed antenna features broadband and compact size due to a small ground. The impedance bandwidth for |S11| < 10 dB ranges from 700 to 987 MHz (31% at 910 MHz). The 3-dB axial ratio bandwidth can cover 800-1000 MHz and the measured maximum gain is about 5.8 dBic at 955 MHz. The miniaturized CP antenna can be embedded to a handheld UFH RFID reader.