Due to the rapid development of wireless communication technologies, the number of wireless users are radically increasing. Currently, ~23 billion wireless devices are connected to the internet, and these numbers are expected to increase manifolds in the years to come. The technology growth of the fifth-generation (5G) wireless systems will be needed to meet this high demand of the network. 5G wireless systems offer data-rates of up to 10Gbps, 1-ms latency, and reduced power consumption. It is a known fact that 5G wireless systems will be exploiting beyond the presently used 3 GHz microwave and millimetre-wave (mm-wave) frequency bands. This is the primary driver in the development of the 5G wireless system. Multi-beam Phased array antenna (PAA) systems are typically used in the deployment of 5G systems for high-gain and directionality. In current 5G and future Beyond 5G (B5G) antenna array systems, beamforming networks (BFNs) such as the Butler Matrix (BM) will play a key role in achieving multi-beam characteristics. So, this paper presents an extensive review of the BM based BFNs, and discusses which type of BM will be suitable for the phased array antenna (PAA) systems in the upcoming 5G and next-generation of B5G wireless systems. Moreover, this paper also summarizes the different types of BM designs based on the number of layers. The BMs are classified into the bi-layer, tri-layer, and four-layer structures. It includes different techniques that have been used to solve the problem of crossing, narrow bandwidth, and size reduction of the BM. From the previous studies, it is found that most of the past research work was performed using the bi-layer BM system, whereas the difficult geometries like tri- and four-layer BM are avoided due to their complex fabrication process. It is also found in this paper that the metamaterial (MTM) based bi-layer BM achieves low insertion-loss and phase-error, excellent bandwidth and compact size, and good S-parameter performance, which makes them an ideal BFN candidate for the upcoming 5G and next-generation B5G systems.

This paper proposes a novel compact 4x4 butler matrix (BM) with improved bandwidth based on open-circuit coupled-lines and interdigital capacitor unit-cell to develop composite right/left handed (CRLH) transmission-line (TL) metamaterial structure. The BM is implemented by the combination of compact 3dB quadrature hybrid couplers, 0dB crossover and 45° phase shifter on a single FR4 substrate (εr = 4.3 and h = 1.66 mm). The simulated and measured result shows that the return loss and isolation loss are better than 14 dB at all the ports, good insertion loss of -7 ± 2dB, which cover the frequency range of 3.2 GHz to 3.75 GHz. The phase difference of -45°,135°, -135° and +45° are achieved with a maximum average phase tolerance of 5°. The overall dimension of the BM is 70mm x 73.7mm, which shows the compactness of the proposed design that is 75% size reduction and 8.2 times improvement in the bandwidth (550MHz) as compared to conventional BM. The CST microwave studio is used to design and perform the simulations. Additionally, the simulated and measured scattering parameters and phase differences show that they are in good agreement. This compact and improved bandwidth of the proposed BM is suitable for 5G beamforming network.

In this article, two distinct kinds of metamaterial (MTM) antennas are proposed for fifth-generation (5G) indoor distributed antenna systems (IDAS). Both antennas operate in the sub-6 GHz 5G band, i.e., 3.5 GHz. The antenna’s radiating structure is based on a combination of triangular and rectangular patches, as well as two complementary split-ring resonators (CSRR) unit-cells etched on the top layer. The bottom layer of the first MTM antenna is a complete ground plane, while the bottom layer of the second MTM antenna is etched by a 3 × 3 cross-slot MTM structure on the ground plane. The use of these structures on the ground plane improves the antenna bandwidth. The proposed antennas are designed using two different substrates i.e., a high-end Rogers thermoset microwave materials (TMM4) substrate (h = 1.524 mm/ε_r = 4.5/tan δ = 0.002) and a low-end flame-resistant (FR4) epoxy glass substrate (h = 1.6 mm/ε_r = 4.3/tan δ = 0.025), respectively. The antenna designs are simulated using CST microwave studio, and in the end, the antenna fabrication is performed using FR4 substrate, and the results are compared. Furthermore, parametric analysis and comparative studies are carried out to investigate the performance of the designed antennas. The simulated and measured results are presented for various parameters such as return-loss, gain, and radiation pattern. The two MTM antennas have an overall dimension of 18 × 34 mm², demonstrating that the proposed design is 60 percent smaller than a standard microstrip patch antenna (MPA). The two proposed MTM antenna designs with complete ground plane and 3 × 3 cross-slot MTM on the bottom layer using FR4 substrate have a measured gain/bandwidth characteristic of 100 MHz/2.6 dBi and 700 MHz/2.3 dBi, respectively.

In this research study, a metamaterial (MTM) based $1\times4$ linear antenna array is designed with an integrated butler matrix (BM) beamforming network (BFN) to execute the switched-beam antenna array (SAA) operation, which can be employed for sub-6 GHz 5G applications. The proposed SAA is designed and simulated using CST microwave studio and then fabricated using FR4 epoxy glass substrate with the thickness ( $h$ ) = 1.6mm and dielectric constant ( $\varepsilon _{r}$ ) = 4.3. The MTM-BM BFN has shown excellent performance in both simulated and measured results, having return-loss of below 14 dB and insertion-loss at ports 5–8 to be 7 ± 2dB between the frequency range of 3.2-3.75GHz, respectively. Moreover, the phase difference between the output ports of the BM is simulated and measured. The $0.5\lambda $ spaced complementary split-ring resonator (CSRR) based antenna array integrated with MTM-BM BFN achieved a measured gain of 4.8-6.1 dB when each input port of BM was excited. The SAA analysis has been performed to identify the variation of gain and grating lobes; the distance between MTM antennas was altered by $0.3\lambda $ , $0.5\lambda $ , and $0.7\lambda $ , respectively. Depending on the BM input ports excitation, the main beam of the SAA is steered in four different directions −40°, −15°, +15°, and +45°, respectively. These radiation pattern studies have shown that the MTM-BM connected with four linear MTM antennas has successfully generated four beams in four different directions, which can be utilized for 5G applications to improve the channel capacity and enhance the transmission quality.

This paper proposes two different types of metamaterial (MTM) antennas for fifth-generation (5G) mid-band frequency operation. The radiating patch of the proposed antenna is based on the combined Minkowski-Sierpinski carpet fractal structure. On the ground plane, two different types of MTM structures: Complementary split-ring resonator (CSRR), and Strip-gap are etched to enhance the overall bandwidth and gain of the antenna. The introduction of these structures on the ground plane also leads to antenna miniaturization. Both the proposed MTM structure based antennas resonate at 3.5GHz. The overall size of the two fabricated prototypes is 28×29.5 mm² / 28×28.5 mm², which shows the compactness of the proposed design by 43% / 42% as compared to the conventional microstrip patch antenna (MPA). The measured bandwidth/gain achieved by the proposed combined Minkowski-Sierpinski carpet fractal antenna (CMSFA) having CSRR MTM and strip-gap etched on the ground plane are 370 MHz / 2.42dBi and 440 MHz / 3.75dBi, respectively.

Partial discharge (PD) is a localized electrical phenomenon occurring within degraded insulation of high-voltage (HV) equipment, where its discharge mechanisms and radiated electromagnetic characteristics vary significantly across air, oil, and gas-insulated systems. Undetected PD can gradually damage insulation and eventually lead to equipment failure and power outages. Therefore, researchers are urged to develop reliable detection methods that are effective across different insulation media to ensure the safe operation of power systems. In this study, ultra-high frequency electromagnetic signals generated from PD activity are analyzed using an external log-periodic dipole array (LPDA) antenna. This research focuses on broadband sensing and considers the PD activity in various insulation materials, including air, oil, and gas. Moreover, lumped inductors are added to achieve a more compact antenna design without compromising the performance. The proposed LPDA antenna is fabricated on an FR-4 substrate and evaluated using a vector network analyzer. As a result, a reflection coefficient below −10 dB from 0.4 GHz to 2 GHz with a directional gain of 4–6 dBi is obtained, which satisfies the IEC 62478 standard. The results demonstrated that the proposed miniaturized LPDA antenna is effective for PD detection in different insulation environments.