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 four-element compact wide-band optically transparent MIMO antenna with a full ground plane is proposed. The four elements transparent MIMO system has a compact size of $24\times 20$ mm ² with the undivided ground plane as most of the real-time systems demand a common reference. The complete antenna system achieves around 85% transparency due to a combination of AgHT-8 and Plexiglas which forms the transparent conductive patch/ground and substrate, respectively. The antenna geometry leads dual-band operation ranging from 24.10 - 27.18 GHz (Impedance bandwidth = 12%) and 33 - 44.13 GHz (Impedance bandwidth $=28.86$ %) targeting the mm-wave 5G applications. The 4-element antenna system achieves isolation between inter-elements >16 dB and maximum gain value of greater than 3 dBi with more than 75% efficiency. The proposed transparent MIMO antenna is evaluated in terms of diversity gain (DG), envelope correlation coefficient (ECC), total active reflection coefficient (TARC), and mean effective gain (MEG) where decent MIMO performance with isolation more than >16 dB between the adjacent and other elements is achieved. Transparent MIMO antenna achieves directional patterns for the operating band with the value of DG >9, ECC < 0.1, TARC value less than −15dB, and the ratio of MEG within the agreed limit of ±3 dB confirming acceptable MIMO/diversity performance.

This article proposes a novel concept of an optically invisible artificial magnetic conductor subarrays for bandwidth improvement of display-integrated antennas in millimeter-wave spectrums. The proposed subarrays consist of two unit-cell types that are arranged as $4 \times 4$ super cells to cover target bands of a slotted dipole antenna with triband resonances. The optical transparency is realized by using thin-metal meshes, and both solid and thin-metal mesh prototypes are fabricated to demonstrate the proposed concept from antenna characteristics standpoints. The mesh prototype exhibits the measured 10-dB matching bandwidth of 25.9% at 28.2 GHz with an average gain of 1.12 dBi, which is similar to 21.1% at 27.5 GHz and 1.25 dBi of the simulated results. These results confirm that the proposed concept is feasible to broaden bandwidths of the triband antenna, while maintaining the optical transparency of 85.1%.

This paper proposes an out-folded patch antenna with a zeroth-order resonance that is robust to a high-dielectric condition of a human body for non-invasive continuous glucose monitoring. The proposed antenna has two faces of a tapered section and a rectangular patch separated by a gap, and those faces are connected through vias at two corners near the gap. Two identical antennas are fabricated on a flexible printed circuit board, and properties of the zeroth-order resonance are compared with those of the fundamental mode to demonstrate the feasibility. The zeroth-order resonance of the proposed antenna exhibits a negligible frequency shift, and its quality factor is 2.9 times higher than the fundamental mode, when attached to the skin. The evaluation is further extended to a human experiment using the two antennas for real-time estimation of coupling strength according to the change of glucose concentration. The trend of measured coupling strength well describes that of the glucose concentration obtained from a glucose meter with a mean absolute relative difference of 12.23%. The results demonstrates that the proposed concept has a great potential to improve the accuracy of the non-invasive estimation in microwave spectrum.

This paper proposes four methods and empirical formulas of adjusting characteristic impedances for thin-metal mesh transmission lines. The characteristic impedances are discretely adjusted by changing the number and the size of unit meshes, which provides macro-tuning capability, and the discrete values can be tuned more precisely by varying the thin-metal line width and the aspect ratio of mesh geometry. The validity of proposed methods is confirmed by full-wave numerical simulations, and the simulated impedance variations are well-described by our empirical formulas. For further verifications, 26 distinguished samples of thin-metal mesh transmission lines and a 28-GHz thin-metal mesh antenna are fabricated, and their characteristics are measured in millimeter-wave spectrums. The measured results confirm that the proposed methods and empirical formulas can provide accurate and more flexible design rules for impedance adjustment, which allows potential advances in display-integrated antenna applications.