A compact uniplanar four port MIMO antenna array is presented. The antenna covers a small area of 38.3 × 38.3 mm2 and is developed on a single layer of a laminate of er=4.5 and width 0.8 mm. The MIMO array has an impedance bandwidth of 3 – 13.2 GHz with an average gain of 4.1 dBi and efficiency between 72% and 97%. The main decoupling technique used in this MIMO array was polarization diversity. Additionally, a set of slits were etched in the ground plane to reduce the ECC to 0.02 and less in the operative bandwidth. A distinctive characteristic of the proposed array is that it is uniplanar and can be easily implemented in mobile terminals where the antennas are printed in only one face of a dielectric surface that is not completely flat.

In this article, a Fibonacci circle fractal is inscribed into a circular radiator in order to provide ultra-wideband behavior as well as a 50% size reduction compared to a conventional circular monopole. The third iteration of the Fibonacci series allows the antenna to obtain a steady S11 parameter over the operation bandwidth, going from 2.7 GHz to 14 GHz, an average gain around 1 dB, with a quasi-omnidirectional radiation pattern and a group delay no bigger than 1 ns, suitable for short-pulsed communications.

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 (<0.023 λ) and approaching 40 dB for opposite ones (<0.23 λ) after analyzing the surface current distribution by the 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.