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The authors propose a compact single-feed stack antenna consisting of a square loop radiator with perturbation, an aperture-coupled structure, and a straight-strip feed line for circular polarization (CP) and unidirectional radiation applications. This perturbation applies both dual-mode and orthogonal-mode effects simultaneously in the square loop resonator to present bandwidth and CP characteristics. The stack antenna presents the desired axial ratio (AR) bands of 2.42 GHz with 3-dB bandwidth (BW) = 62 MHz (2.5%). The circular polarization is demonstrated with AR spectrum and orthogonal modes distributions. The proposed antenna is successfully simulated and measured with frequency responses, radiation patterns, and current distributions.