Optimized low-cost circularly polarized patch antenna for RFID applications

Radio-frequency identification (RFID) is a constantly evolving technology, used more and more in the most diverse applications, which demands a continuous improvement of its basic components, e.g., tags, antennas and readers. An optimized low-cost circular polarized antenna, for the US standard UHF RFID application working frequency (902-928 MHz), is proposed in this paper. An initial work was developed and obtained 5.76 dB of gain, 48 dB of return loss (S
₁₁
) and axial ratio of 1.87 dB at 915 MHz in simulation results. However, in order to achieve these results, an airgap was introduced between two plates of FR4 substrate, which caused a great sensibility in the antenna results. So, in this work the airgap was replaced for a polyethylene foam layer, which has similar proprieties to the air, with a relative permittivity (ε
_r
) of 1.2348 and a loss tangent of 0.0053. With this new substrate configuration, not only the S
₁₁
sensibility of the antenna was decreased, but also the fabrication process was simplified and the antenna structure became more stable. The antenna was optimized using the ANSYS® HFSS software and it was obtained a slightly higher gain of 6 dB, a return loss of 15 dB and an axial ratio of 0.04 dB at 915 MHz. The antenna was manufactured and its S
₁₁
was measured to compare with the simulation results.