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A Compact Multiple-Input Multiple-Output (MIMO) antenna of size 16 × 26 mm2 (0.144 λ0 × 0.234 λ0) with improved isolation is presented in this paper. It covers the bandwidth of 2.7–14.9 GHz (FBW is 138.2%), which includes the entire UWB, X band, and Ku band applications. The elements in the structure are placed at a distance of 0.018 λ0 (2 mm). Optimized values of two parallel strips are placed between the elements for isolation, giving│S21│ > 15 dB in most of the band. For further enhancement, four small rectangular strips with different dimensions are protruded to the ground plane, resulting in improved isolation│S21│ > 20 dB in most of the band. The radiation efficiency and peak gain values are 86–96% and 0.8–6.6 dBi respectively. Diversity performance is evaluated with the Envelope Correlation Coefficient (ECC 9.975 dB) and Total Active Reflection Coefficient (TARC

The Theory of Characteristic Modes (TCM) enables for the systematic design of conducting radiators. TCM gives a deep insight into the device physics of the conductor so that its radiation mechanism and resonant frequencies can be estimated in a systematic manner, when the conductor is used in antennas. In the proposed work, the resonant behavior and radiation properties of curved conducting planes of different radii of curvature are studied. The important Characteristic Mode parameters like characteristic angle, modal significance and surface currents are obtained and analyzed for the flat and curved conducting plane. This analysis is very helpful in the design of conformal antennas, where antenna shape is required to be bent.

A compact 2 × 2 MIMO diversity antenna of size 32 × 32 × 1.6 mm3 is presented in this paper for portable devices. The proposed structure uses polarization and pattern diversities for isolation enhancement. The orthogonal arrangement of radiating elements, pattern diversity, and defected ground structure is achieved isolation. The whole configuration is designed and fabricated over the FR4 substrate with er 4.4, and fed by a 50Ω microstrip line. The proposed structure operates from 7.9 GHz to 13 GHz, and it resonates at 9 GHz, 12.3 GHz. The isolation between orthogonal elements is above 20 dB and the anti-parallel element is above 15 dB. The radiation efficiency and peak gain values are above 86% and 3.1–5.2 dB, respectively. The diversity metric value of ECC is less than 0.07, DG is above 9.985 dB, and TARC is below −10 dB for the entire desired band. Simulated and measured values are in good concord.

A novel compact structure of wrench-shaped MIMO antenna for ultra-wideband applications is presented in this paper. The acceptable return loss ( $\mathrm{S}11 \leq -10$ dB) values of the proposed layout ranging from 3.18 GHz to 11.15 GHz. Half circular slotted T shaped stub is used for isolation enhancement. The isolation ( $\vert \mathrm{S}21\vert$ ) between elements is higher than the 15 dB for the entire operating band and it is above 20 dB in mystifying the band The realized peak gain and efficiency values are 1.97-5.61 dB and 85-95% respectively. MIMO performance metrics checked with MEG (Mean effective gain), MEG ratio (MEG1/MEG2), ECC (Envelope correlation coefficient), DG (Diversity gain) and TARC (Total active reflection coefficient) values. The structure is designed and simulated on a low-cost FR4 substrate of size 24 × 32 × 1.6 mm3, constant 4.4 (€r) with loss tangent of 0.002.