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With the introduction of the circularly symmetrical slot near the coaxial feed location of the corner truncated square microstrip antenna patch, both the circular polarization and harmonic suppression performance can be realized in this antenna. Effectiveness of the circularly symmetrical slot for harmonic suppression is studied, including its affection to circular polarization performance of the antenna. Experimental results show that application of the circularly symmetrical slot near the coaxial feed location of the microstrip antenna patch can remarkably suppress the harmonic frequencies. Measured radiation patterns indicate that the forward radiation is suppressed by more than 10~20 dB at the second and the third harmonic frequencies compared to the reference antenna, while maintaining the circular polarization performance, the cross polarization, and the backward radiation.