Multiple-input multiple-output (MIMO) antennas with four and eight elements having connected grounds are designed for ultra-wideband applications. Careful optimization of the lines connecting the grounds leads to reduced mutual coupling amongst the radiating patches. The proposed antenna has a modified substrate geometry and comprises a circular arc-shaped conductive element on the top with the modified ground plane geometry. Polarization diversity and isolation are achieved by replicating the elements orthogonally forming a plus shape antenna structure. The modified ground plane consists of an inverted L strip and semi ellipse slot over the partial ground that helps the antenna in achieving effective wide bandwidth spanning from (117.91%) 2.84–11 GHz. Both 4/8-port antenna achieves a size of 0.61 λ × 0.61 λ mm2 (lowest frequency) where 4-port antenna is printed on FR4 substrate. The 4-port UWB MIMO antenna attains wide impedance bandwidth, Omni-directional pattern, isolation >15 dB, ECC < 0.015, and average gain >4.5 dB making the MIMO antenna suitable for portable UWB applications. Four element antenna structure is further extended to 8-element configuration with the connected ground where the decent value of IBW, isolation, and ECC is achieved.

This paper initially discusses lumped equivalent models of narrowband (NB) antennas and then presents a technique to enhance isolation between UWB and NB antennas of the three antennas system. The proposed models are able to yield similar S-parameter performance as that of NB antennas at their resonant frequencies. Also, a good match is observed at other frequencies between the HFSS and AWR results. This paper reports an isolation enhancement technique using microstrip lines. The proposed approach enhances a minimum isolation of 11 dB at respective resonant frequencies. This paper also reports lumped equivalent models of dual band reject filters (DBRFs). Proposed geometry is fabricated and tested. Experimental result shows an agreement between the simulated and measured results.

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