To enhance axial ratio (AR) bandwidth, this research proposes a circularly polarized (CP) single-fed microstrip patch antenna using a circular artificial ground structure (AGS) and meandering probe. To achieve broader AR bandwidth, the circular AGS is populated with rectangular unit cells and partially cut unit cells along the circular contour, while the meandering probe is used to improve the impedance bandwidth. Simulations are performed and results compared with that of conventional rectangular-AGS antenna. The simulation results show that the circular-AGS antenna, given 62 mm circular ground plane, achieves broader impedance (5.12-9.00 GHz and 54%), AR (5.21-8.27 GHz and 45%) and gain bandwidths (3.85-7.00 GHz and 58.06%), in comparison with the rectangular-AGS antenna (4.50-7.45 GHz and 49%; 4.52-7.42 GHz and 21%; and 4.00-6.80 GHz and 51.58% for impedance, AR and gain bandwidths). The circular-AGS antenna is capable of converting linear polarization in the off-axial ratio band into circular polarization. To verify, a circular-AGS antenna prototype is fabricated and experiments undertaken. The experimental impedance, AR and gain bandwidths of the circular-AGS antenna are 47.82% (5.17-8.42 GHz), 43.81% (5.24-8.17 GHz) and 60.74% (3.75-7.00 GHz). The proposed circular-AGS antenna can achieve broader AR bandwidth and is thus ideal for broadband CP applications. The novelty of this research lies in the use of circular AGS to effectively enhance AR bandwidth, as opposed to rectangular AGS which is conventionally used in CP polarizers. INDEX TERMS Artificial ground structure (AGS), artificial magnetic conductor, broadband antenna, circular polarization, microstrip patch antenna.
This research proposes a single-fed broadband circularly polarized (CP) bidirectional antenna operable in 840–960 MHz frequency band for readers of universal UHF-RFID applications. The proposed antenna is comprised of upper-layer conductor, lower-layer conductor, and wall patches. The upper-layer conductor consists of two diagonally aligned rectangular copper plates with a feeding gap at the center, and the lower-layer conductor is of two diagonally adjoined rectangular plates. The upper- and lower-layer conductors are adjoined with the wall patches. The diagonal alignment technique of the upper- and lower-layer plates was used to realize circular polarization and improve 3-dB axial ratio (AR) bandwidth. The double layers were deployed to improve impedance bandwidth (|S11| < −10 dB) and achieve bidirectional radiation pattern. The simulated impedance bandwidth and 3-dB AR bandwidth were 772.19–1014.6 MHz (27.13%) and 675–1000 MHz (38.80%), and the corresponding measured results were 759–1011 MHz (28.47%) and 648–1110 MHz (52.55%). The simulated LHCP/RHCP half-power beamwidth (HPBW), 3-dB AR beamwidth, and gain were 56° – 90°/54° – 92°, 60° – 104°, and 4.94 – 5.89 dBic, while the corresponding measured results were 52° – 98°/62° – 97°, 96° – 126°, and 4.28 – 5.72 dBic. As a result, the single-fed broadband CP bidirectional antenna is applicable to universal UHF-RFID readers. Besides, the novelty of this research lies in the use of diagonal alignment of conducting plates to achieve circular polarization and wider AR bandwidth.
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