Compact 0.92/2.45-GHz Dual-Band Directional Circularly Polarized Microstrip Antenna for Handheld RFID Reader Applications

This paper presents a compact 0.92/2.45-GHz dual-band directional circularly polarized (CP) microstrip antenna for handheld radio-frequency identification (RFID) reader applications. The proposed antenna comprises a wideband dual-feed network and two stacked concentric patches assembled by two orthogonally placed vertical probes. The dual-feed network feeds quadrature signals across the RFID bands between 0.92 and 2.45 GHz. The two stacked concentric patches provide resonance frequencies in fundamental mode for lower band and higher band, respectively. Additionally, the antenna features only one connection port and same sense CP radiation at two bands, beneficial to cost and complexity reductions of the dual-band front end of RFID readers. The measurement results show the performances of return loss (RL) > 10 dB, 3-dB gain variation, and axial ratio (AR)< 3 dB are achieved on the bands 0.911-0.933 GHz and 2.40-2.57 GHz. The measured peak gains are 3.8 dBic at 0.926 GHz and 8.9 dBic at 2.48 GHz. In addition, the antenna provides symmetrical patterns with wide-angle half-power beamwidths and wide-angle 3-dB AR beamwidths. The size of the antenna 110 x 110 x 6.6 mm(3) is much smaller than reported dual-band one port RFID directional CP reader antennas. The antenna appropriates to both ultra-high frequency (UHF) and industrial scientific and medical (ISM) bands in handheld RFID reader applications.