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This article presents a symmetrical reduced-size eight-element MIMO antenna array with high electromagnetic isolation among radiators. The array utilizes easy-to-build techniques to cover the n77 and n78 new radio (NR) bands. It is based on an octagonal double-negative metamaterial split-ring resonator (SRR), which enables a size reduction of over 50% for the radiators compared to a conventional disc monopole antenna by increasing the slow-wave factor. Additionally, due to the extreme proximity between the radiating elements in the array, the modal significance (MS) method was employed to identify which propagation modes had the most impact on the electromagnetic coupling among elements. This approach aimed to mitigate their effect by using an electromagnetic barrier, thereby enhancing electromagnetic isolation. The electromagnetic barriers, implemented with strip lines, achieved isolation values exceeding 20 dB for adjacent elements (MS method. The elements are arranged in axial symmetry, forming an octagon with each antenna port located on a side. The array occupies an area of 0.32 λ2 at 3.5 GHz, significantly smaller than previously published works. It exhibits excellent performance for MIMO applications, demonstrating an envelope correlation coefficient (ECC) below 0.0001, a total active reflection coefficient (TARC) lower than −10 dB for various incoming signals with random phases, and a diversity gain (DG) close to 20 dB.

In this paper, a very small 4-port MIMO antenna is designed, based on a metamaterial structure composed of embedded octagonal Split-Ring Resonators (SSRs). The antenna array shows an axial symmetry configuration with dimensions of 32 × 32 mm², corresponding to 0.157λ², approximately, related to a center frequency of 3.5 GHz, with a high electromagnetic isolation despite the radiators' closeness, reaching values bigger than 26 dB for adjacent antennas, and more than 28 dB for opposite antennas. The antenna is built on a substrate with dielectric permittivity of 2.2 and 1.27 mm thick. The Total Active Reflection Coefficient (TARC) presents a steady behavior for different random phases at the incoming signals, keeping a system bandwidth of 0.9 GHz for a -10 dB value. On the other hand, the Envelope Correlations Coefficient (ECC) reports values lower than 0.001 in all the antenna relationships, achieving a very uncorrelated performance of the electric fields in each element. The radiation pattern is quasi-omnidirectional, obtaining a low gain around -2 dBi, a trade-off that is considering the size reduction of the MIMO antenna. [ABSTRACT FROM AUTHOR]

Several papers published by prestigious Journals on Antennas and Microwave Techniques have disseminated misconceptions about total active reflection coefficient (TARC), a meaningful metric to characterize multiple‐input multiple‐output antenna arrays, and the misapprehension has been spread over the years. In this article, an extrapolation to calculate TARC for N‐antennas systems is derived and applied to published results for illustration purposes. Moreover, the inaccurate and correct TARC equations are put in context by means of its physic and mathematical insight, demonstrating the divergence of results when the interport RF isolation is comparable or higher than the port matching. The importance of assessing multiantenna systems using TARC is explained. [ABSTRACT FROM AUTHOR]

A multiple-input--multiple-output (MIMO) microstrip patch antenna array of four elements is proposed to cover the 5.8 GHz WLAN band. The characteristic mode analysis is carried out to design a defected ground structure that improves the MIMO performance without impacting negatively the radiation characteristics of the four-element array. Simulations and measurements demonstrate that the implemented prototype reaches a maximum mutual coupling of -32 dB and a gain-per-element of 5.3 dB with good MIMO attributes, surpassing many other proposals in the antenna literature: maximum envelope correlation coefficient of 0.0001, diversity gain very close to 10 dB, capacity loss (Closs) near to 0.0023 bps/Hz at the resonant frequency, multiplexing efficiency ηmuxij of 0.935, and a total active reflection coefficient weakly dependent on the excitation phases. The presented design is proposed for operation on the IEEE 802.11a/n standard. [ABSTRACT FROM AUTHOR]