A miniaturized tri-polarized MIMO slot antenna loaded with slit-postwall-slot step impedance resonator (SIR) is proposed. The SIR, which shortens the length of radiation slot for the miniature purpose, consist of two slit-slot lines with high impedance and a slit-postwall-slot line with low impedance, because the postwall can decrease the characteristic impedance of slit-slot line. Meanwhile, the slits can improve the polarization purity and isolation of the antenna array effectively. Compared with the conventional slot antenna array, the measured results of proposed antenna array shows that the crosspolarization ratio has improved 2 dB on H plane and 3 dB on E plane at 2.45 GHz, while the lengths of ground plane and slot are shortened 77.8% and 77%, respectively. Furthermore, the proposed antenna array operates at dual band and the isolation are −22 dB at 2.45 GHz and-27 dB at 4 GHz. The performance and structure of the proposed antenna array can be applied for the condition of high polarization purity and demanding volume.

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