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A circularly polarized (CP) stack antenna with tri-band operation is presented. The proposed antenna has a square loop resonator with the perturbation to present dual-mode effects, and it is excited by a strip with the tuning stub through the coupling of an aperture in the ground plane. The ability to enhance the bandwidth and reconfigure the CP performances is achieved by the perturbation and the tuning stub. The proposed antenna presents the desired bands of 1.87 GHz with bandwidth (BW) = 210 MHz (12%), 3.75 GHz with BW = 200 MHz (5.3%) and 5.32 GHz with BW = 1320 MHz (25.4%). The circular polarizations for tri-band are demonstrated with AR spectrum and orthogonal modes. The design antenna is successfully simulated and measured with frequency responses, radiation patterns and current distributions.