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.

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.

Modern wireless communication systems demand high bandwidth and effective interference mitigation. This research introduces a compact ultra-wideband (UWB) reconfigurable antenna utilizing a Radio Frequency Micro-Electro-Mechanical Systems (RF-MEMS) switch to address these challenges. The study evaluates three antenna designs: a partially ground-fed antenna, a Co-Planar Waveguide (CPW)-fed antenna with a rectangular ground-plane slot, and a C-shaped slot UWB antenna. Reconfigurability is achieved by incorporating an RF-MEMS switch, connected to the ground-fed antenna through an open stub. The proposed antennas employ a reduced electrical size and an innovative virtual ground feed technique to provide outstanding frequency reconfigurability, along with a single-band notch function controlled by ON and OFF states of the RF-MEMS switch. The Fixed–Fixed RF-MEMS switch provides low insertion loss and high isolation, ensuring excellent RF performance. The proposed antenna design integrates an RF-MEMS switch to enable frequency reconfigurability, dynamically switching between UWB (3.1–11 GHz) and a notched band (5–6 GHz) for interference suppression. The simulated and fabricated prototype demonstrates a peak gain of 9 dBi in UWB mode, with a notch band attenuation of − 5 dBi, ensuring reliable performance across the operating range. The antenna maintains VSWR < 2 across the band except for the notched region, with stable omnidirectional radiation patterns. Compared to existing designs, the proposed structure offers compact size (28 × 23 × 1.6) mm³, superior reconfigurability, and improved gain stability, making it an excellent candidate for next-generation UWB communication systems.

Folded waveguide traveling wave tubes (FW-TWTs) exhibit maximum potential in sub-THz to THz regime than its counterparts with respect to power and bandwidth, thereby facilitating its application in high data rate 6G communication systems. However, at high frequency, for synchronous and efficient interaction between electron beam and RF wave, FW-TWTs have to be operated at considerably high beam voltages. Furthermore, it has been reported that the interaction efficiency of FW-TWT is very poor; thus, most of the beam energy remains unspent. The efficiency of such devices may be increased by recovering the unspent beam energy in the collector. A corrugated single-stage collector has been designed with multiple periodic corrugations for an FW-TWT considering the size constraints. This geometry modification has resulted in self-trapping of secondary electrons and increased surface area yielding in >84% collector efficiency (~6% improvement in collector efficiency as compared with conventional collector). Detailed thermal and structural analyses of this collector have been carried out for different electrode materials, namely, copper and graphite. Various thermal management techniques have been employed to improve the thermal dissipation which includes forced air cooling, axial grooves on the outer envelope, and radiator fins. Simulation results reveal that a percentage reduction in the maximum temperature of 32.68%, 22.63%, and 72.59% has been achieved using forced air cooling, axial grooves, and radiator fins respectively. Further simulations also show a percentage decrease of 57.14% and 81.41% in the maximum collector temperature by using forced air cooling in combination with axial grooves and radiator fins, respectively. Based on the thermal and structural results, recommendations for the optimum thermal management technique have also been provided.