A circularly-polarized-reconfigurable patch antenna based on fluidic control method is proposed in this paper. A liquid dielectric, ethyl acetate, is introduced to change the substrate of a patch antenna from homogeneous to inhomogeneous. Two orthogonal fundamental transverse magnetic (TM) modes are excited with a 90° phase difference in order to produce circularly polarized (CP) radiation. Then, the polarization of the proposed patch antenna is switchable between left-hand circular polarization (LHCP) and right-hand circular polarization (RHCP) through the injection of the liquid dielectric to different channels in the substrate. For demonstration, the proposed antenna is designed at 2.4 GHz for radio frequency identification (RFID) application. According to the measured result, the proposed circular-polarization-reconfigurable patch antenna has a stable circularly polarized (CP) gain of 8 dBic and an efficiency of 90 % across the effective frequency band. Additionally, the measured overlapped impedance bandwidth of two polarization state is from 2.24 to 2.52 GHz which can fully cover the measured overlapped 3-dB axial ratio (AR) bandwidth from 2.38 to 2.43 GHz.

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