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Mutual coupling always seriously degrades the antenna performance in multiple-input multiple-output (MIMO) systems but this issue has been rarely investigated for antennas with circular polarization. In this paper, a planar and compact circularly polarized MIMO patch antenna with the polarization diversity is presented. Three grounded stubs and a mirrored F-shaped defected ground structure are used to achieve simultaneous matching and isolation between the two patches with offset feeding for circular polarization. The antenna elements are closely packed with the edge-to-edge distance of 0.06λ 0 at the desired frequency of 2.5 GHz. The measured results indicate that the proposed antenna achieves the impedance matching (S 11 < −10 dB), high isolation (S 12 < −20 dB), and circular polarization (axial ratio<3 dB) within the frequency band of 2.5-2.55 GHz. The radiation patterns and realized gains are measured, showing good agreement between the simulation and measurement. INDEX TERMS Circular polarization (CP), multiple-input multiple-output (MIMO), axial ratio (AR), defected ground slot (DGS), envelope correlation coefficient (ECC), left-hand circular polarization (LHCP), right-hand circular polarization (RHCP).

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