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A novel coupling technique for circularly polarized square-ring patch antenna is developed and discussed. The circular polarization (CP) radiation of the square-ring patch antenna is achieved by a simple microstrip feed line through the coupling of a square patch on the same plane of the antenna. Proper positioning of the coupling square patch excites two orthogonal resonant modes with 90deg phase difference, and a pure circular polarization is obtained. The dielectric material is a square block of ceramic with a permittivity of 58 and that reduces the size of the antenna. The prototype has been designed, fabricated and found to have an impedance bandwidth of 1.1% and a 3-dB axial-ratio bandwidth of about 0.03% at GPS frequency of 1573 MHz. The characteristics of the proposed antenna have been studied by simulation software HFSS and experiments. The measured and simulated results are in good agreement.