The review focuses on the emergence of 5G wireless communication and the need for multiple-input multiple-output antennas to support high-speed communication systems. The article discusses the advantages of MIMO antennas, including increased channel capacity and the ability to focus radio frequency energy on specific users. However, the challenges of creating compact MIMO antennas with ideal isolation are addressed, including short wavelengths, connection losses, constrained bandwidth, and path losses in the millimeter-wave range. Design techniques and methods to enhance the performance of conventional antennas for 5G applications are discussed, along with potential solutions for upcoming challenges. The article provides an overview of MIMO antennas for 5G applications, covering frequency bands, system architecture, advantages, challenges, advancements, performance enhancement techniques, design techniques, and state-of-the-art developments.

An investigation of the use of Aerosol jet 3D Printing of frequency selective surface for millimetre and low-THz applications is presented in this article. This 3D printing technique allows the fabrication of intricate details of the designs with high resolution. Band-stop and band-pass FSS are designed and tested. The band stop FSS consisted of a Square loop array that operated in the 26-28 GHz sub-millimetre band. This design is printed on glass substrate and can be used for deployment in windows. The bandpass FSS arrays consisted of simple slot elements arranged in a square lattice and operated at 125 GHz and 280 GHz. The slot arrays were printed on Kapton. Surface profiles demonstrated the uniformity and precision of this printing technique. Simulated and measured results compared well and offered good performances at both the millimetre wave and low-THz bands. The designs find applications in 5G and imminent 6G communications. This printing technique also provides environmentally friendly, rapid, and sustainable alternative for development of highly customised FSS which can be deployed to improve communications in buildings and in future Terahertz applications.

An approach is presented to enhance the isolation of a two-port Multiple Input Multiple Output (MIMO) antenna using a decoupling structure and a common defected ground structure (DGS) that physically separates the antennas from each other. The antenna operates in the 24 to 40 GHz frequency range. The innovation in the presented MIMO antenna design involves the novel integration of two arc-shaped symmetrical elements with dimensions of 35 × 35 × 1.6 mm³ placed perpendicular to each other. The benefits of employing an antenna with elements arranged perpendicularly are exemplified by the enhancement of its overall performance metrics. These elements incorporate a microstrip feed featuring a quarter-wave transformer (QWT). This concept synergizes with decoupling techniques and a defected ground structure to significantly enhance isolation in a millimeter wave (mm wave) MIMO antenna. These methods collectively achieve an impressively wide bandwidth. Efficient decoupling methodologies have been implemented, yielding a notable increase of 5 dB in isolation performance. The antenna exhibits 10 dB impedance matching, with a 15 GHz (46.87%) wide bandwidth, excellent isolation of more than 28 dB, and a desirable gain of 4.6 dB. Antennas have been analyzed to improve their performance in mm wave applications by evaluating diversity parameters such as envelope correlation coefficient (ECC) and diversity gain (DG), with achieved values of 0.0016 and 9.992 dB, respectively. The simulation is conducted using CST software. To validate the findings, experimental investigations have been conducted, affirming the accuracy of the simulations.

This article presents a simple planar millimeter-wave (mm-wave) multiple input multiple output (MIMO) antenna with four closely spaced radiating elements. The antenna is intended to enhance the bandwidth for 5G new radio (NR) applications and also cover the industrial Ka-band (26.5-40 GHz), Q-band (30-50 GHz), and U-band (40-60 GHz) frequency ranges. The antenna is designed using Rogers RT/duroid 5880 substrate with a size of mm³. The proposed antenna consists of four identical radiating elements with two of them positioned in a linear configuration facing each other, while the other two elements are placed opposite to the first two, which improves isolation between the elements. Across the entire operating frequency range (26-60 GHz), the antenna demonstrated excellent performance, including 34 GHz wide bandwidth, high isolation of >25 dB, nearly omnidirectional radiation patterns, high gain of 11.1 dB with a high radiation efficiency of 94%, low envelope correlation coefficient (ECC) of 0.0068, and high diversity gain (DG) of 9.967 dB. The simulated findings exhibit a strong positive correlation with the experimental results. The proposed MIMO antenna design is a promising solution for 5G NR applications.

A comprehensive study of developing a novel printing system using a low-cost open-source printer for fully 3D printing frequency selective surface is presented in this paper. The novel printing setup employs a low-cost printer to print a plastic based filament and a conductive silver ink paste simultaneously. As there were no printers available in the market for this application, the open-source Fused Filament Fabrication (FFF) printer was modified to accommodate two extruders mounted on the same extruder carriage. Techcon TS250 air pressure dispenser was employed for the extrusion of silver ink. Extension pieces for the extruder carriage were also 3D printed using a Fused Deposition Modelling (FDM) printer to reduce the production costs. A bandstop FSS comprising of square loop elements was designed to demonstrate the full fabrication. The FSS operated at a central frequency of 2.55 GHz and provided a good angle of response with wide bandwidths. Surface profiles of the printed FSS and substrate demonstrate the reliable fabrication of the FSS design. This full 3D printing method provides an economical, eco-friendly, swift, reliable, and viable substitute for the fabrication of FSS designs that could be highly customised in terms of fabricating three-dimensional FSS designs with reliable performances. The designs can be printed and deployed to reduce the drop in signal within an enclosed environment.