A flexible transparent wideband four-element MIMO antenna with a connected ground plane is proposed with numerical computation and experimental measurement studies. The optical transparency is obtained using flexible conductive oxide material AgHT-4 and Melinex substrate. The radiating elements are in the form of circular stub-loaded C-shaped resonators, which are positioned in a carefully structured flexible Melinex substrate with an interconnected partial ground plane structured in the form of an L-shaped resonator, attaining an overall antenna size of $0.33\lambda \times 0.48\lambda $ at the lowest operating frequency. The proposed antenna spans over a −10 dB impedance bandwidth of 2.21–6 GHz (92.32%) with an isolation level greater than 15dB among all elements. The maximum gain is 0.53dBi with a minimum efficiency of 41%, respectively which is satisfactory considering flexible structure and sheet impedance of $4\Omega $ /sq. MIMO antenna parameters in terms of the envelope correlation coefficient (ECC) and diversity gain (DG) are also extracted where all the values are satisfactory for MIMO applications. The bending analysis of the proposed transparent MIMO antenna along the X and Y axis has revealed good performance in terms of scattering parameters and radiation pattern along with MIMO diversity performance. All of these technical points make the flexible MIMO antenna suitable for smart devices using sub-6 GHz 5G and WLAN band in IoT applications where visual clutter and co-site location issues need to be mitigated with the integration ease of conformal placement on the curved component/device surfaces.

A coplanar waveguide (CPW) fed flexible interconnected 4-port MIMO antenna is proposed for UWB, X, and Ku band applications having a size $0.67\lambda \times 0.81\lambda \times 0.0028\lambda $ (at 3.58 GHz). The antenna parameters are characterized by using sets of real measurements to validate the antenna design and modelling. A precise milling machine is used for fabricating the single and 4-port antennas over a flexible FR-4 Substrate. Vector network analyser (VNA) is used for measuring scattering and transmission coefficients. As well as, a shielded anechoic chamber is utilised for measuring the far field radiations and the gain of the antenna under test (AUT). The basic antenna that is a part of the 4-port structure consists of a CPW fed octagonal structure surrounded by an octagonal-shaped slot. The MIMO ability is achieved by arranging the 4 identical elements symmetrically in a successive rotational means for achieving good polarization diversity. The connected ground between the elements is achieved by introducing a plus-shaped slotted structure separated by a slotted diamond at the center in the 4-port structure. None of the slots touches each other thus ensuring the common reference while maintaining the inter-element isolation better than 15 dB. The antenna demonstrated maximum gain and minimum efficiency of 6.8 dBi and 89%, respectively besides attaining a 10-dB impedance bandwidth of 125.83% (3.89-17.09 GHz) and envelope correlation coefficient (ECC) < 0.02. Finally, the 4-port flexible antenna is tested in terms of S parameters and ECC under bending conditions along the X and Y axis followed by a time-domain analysis that exhibited very good performance.

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.

A dual-band Multiple Input Multiple Output (MIMO) antenna with low mutual coupling is analyzed and studied for Fifth Generation (5G) Sub-6 GHz and Wireless Fidelity (Wi-Fi) 6E frequency-band applications. Each of the identical monopole radiators consists of a rectangular patch, inverted-E, and asymmetrical T-shaped strip to generate 5G Sub-6 GHz and Wi-Fi 6E frequency bands. The deployment of the radiators horizontally with end-to-end separation of 5mm makes the antenna compact with an overall dimension of $45\times 30$ mm2 ( $0.37\lambda \times 0.25\lambda $ for $\lambda $ at the lower resonating frequency of 2.5GHz). The antenna achieves measured impedance bandwidths of 2.39-2.57 GHz and 3.82-6.95 GHz, respectively, at two resonating frequencies of 2.5 GHz and 4.5 GHz. A Parasitic Dollar Shaped Structure (PDSS) introduced between the two monopole radiators to suppress the surface waves of each monopole reduces the measured mutual coupling up to -15 dB throughout the operating band. Furthermore, the measured gain, radiation efficiency, Envelope Correlation Coefficient (ECC), and Diversity Gain (DG) values remain in good agreement with simulated values and are well within the minimum limit set by industry standards. These results affirm the applicability of the MIMO antenna for 5G and Wi-Fi 6E frequency bands.

A compact quad-port MIMO antenna with dual-polarization is proposed. Each of the four antenna elements features a modified rectangular radiator (MRR) backed by a partial ground plane and fed by a T-shaped microstrip line. The four antenna elements are interconnected by an X-shaped strip and isolated by a modified plus-shaped (MPS) structure, resulting in over -18 dB of isolation. With a small footprint of only 36 mm × 27 mm, the antenna exhibits a 10-dB impedance bandwidth with a completely overlapping 3-dB axial-ratio bandwidth (ARBW) of 19.32% (7.90–9.59 GHz). Additionally, it can provide a gain of over 3.5 dBi, an efficiency of over 70%, and an envelope correlation coefficient (ECC) of less than 0.01 in both isotropic and real propagation scenarios.