A multimode collocated microstrip patch antenna with reduced mutual coupling is proposed in this paper. The antenna is designed to achieve polarization and pattern diversity for use in multiple-input-multiple-output (MIMO) terminals. The four-port antenna resonates at 2.45 GHz and have total dimension of 1.03λ with reduced mutual coupling (< -20 dB) between its ports. It consists of a simple square patch and a square ring antenna, with a novel square ring slot defected ground structure (DGS). Square ring slot on ground improves isolation by 7 dB by reducing surface waves in both E and H planes. With defected ground structure (DGS), coupling between patch and ring antennas is about -25 dB and correlation factor is less than 0.1. Pattern diversity, mutual coupling and correlation coefficient between signals for a four-port antenna fabricated using FR4 substrate is discussed in this paper.
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