THz antennas design,developments, challenges,and applications: A review

Terahertz (0.1–10 THz) wireless communication will be the future technology to reach a top-notch data rate. THz is one of the most promising candidates for 6G systems because it provides enormous bandwidth, up to 100 GHz, and a massive data rate of up to 1 Tbps. THz antennas, antenna arrays, and MIMO antenna arrays in 6G are hot research topics for implementing 6G wireless communication systems. The 6G aims to continue to enhance the features of the 5G as it is capable of achieving the maximum high-speed data rate, excellent reliable communication, massive connectivity, and very low latency connectivity. The 6G requirements need high-gain antenna arrays and MIMO antenna arrays to combat the effect of atmospheric losses in high frequencies. An in-depth discussion of the planar THz antennas that have been extensively used in THz applications like imaging, sensing, and Internet-of-Things (IoT) has been conducted. The study of the THz antennas, antenna arrays, and MIMO antennas on different conducting materials such as copper and graphene, which are designed on different dielectric substrates such as polyimide, quartz, liquid crystalline polymer, and polytetrafluoroethylene, has been carried out in detail. Metamaterial, photoconductive, plasmonic antennas, and THz beamforming are significant parts of THz communications. This paper also provides antennas and antenna arrays based on them.     

Differential finishing of freeform surfaces (knee joint) using R-MRAFF process and negative replica of workpiece as a fixture

It is difficult and challenging to achieve uniform nanoscale surface finish in the contact zone, particularly on freeform (or sculptured) surfaces having different curvatures at different locations. Femoral (or, Knee joint component) is one of such biomedical freeform component which has complex profile along its curvature. Surface conditions of a femoral decide the life of the implant and they play a crucial role in its functionality. The variation in surface roughness of the femoral should be minimum in the contact zone. For this purpose, a special tooling is being proposed for rotational magnetorheological abrasive flow finishing (R-MRAFF) process. A negative replica of the workpiece (knee joint) as a tool (or a fixture) is used so that the medium flow velocity in the fluid flow channel is almost constant (or minimum possible variations) along the medium flow direction. It is able to do differential finishing also along the curvature. In addition, pulsating magnetic field has been used to generate vibrations in the medium in the finishing zone so that the possibility of fresh abrasive particles interacting with the surface of femoral is high. The surface finish has been achieved ranging from 26 nm to 62 nm using the proposed finishing technique and negative replica of the workpiece (femoral) as a fixture.     

Offset planar MIMO antenna for omnidirectional radiation patterns

An offset quad element multi‐band planar MIMO antenna with omnidirectional radiation patterns is proposed for nonline of site (NLOS) communication on low‐cost FR‐4 dielectric substrate for 4G and future technologies. A 1 × 2 power divider arm results in dual beam and enhances diversity parameters and omnidirectional radiation patterns. Moreover, the MIMO antenna limits the proximity/coupling effects using a T‐shaped isolator and achieves more than 12.4 dB of isolation between the radiating ports. The proposed design covers WLAN/WiMAX bands with gain and radiation efficiency of more than 2.6 dBi and 71%, respectively, in 2:1 VSWR bands of bandwidths 16.39% (2.24‐2.64 GHz) and 7.88% (3.41‐3.69 GHz). The ?10 dB impedance bandwidth is more than 280 MHz in each band. An ECC level of ≤ 0.01 has been achieved in the whole band.     

UWB antenna and MIMO antennas with bandwidth,band‐notched,and isolation properties for high‐speed data rate wireless communication: A review

The prospective of ultrawide band (UWB) technology is enormous due to its remarkable advantages such as the capability of providing high‐speed data rates at short transmission distances with low power dissipation. The swift growth in wireless communication systems has made UWB an exceptional technology to replace the conventional wireless technologies in today's use. UWB bandwidth (3.1‐10.6 GHz) covers most of the communication applications. High frequency of operation with high level of miniaturization has enhanced the interest in designing high performance antennas. There is a growing demand for small and low cost UWB antennas that are able to provide satisfactory performance in both time and frequency domains. The trend in recent wireless systems, including UWB based systems, are to build small, low‐profile integrated circuits so as to be compatible with portable wireless devices. Transformation of UWB in MIMO achieved high data rate and solved the problem of multipath propagation. The main reason for writing this review is to investigate various UWB methods and band rejection approaches on a single platform. Some of the isolation enhancement approaches are also included in article due to the requirement in MIMO antennas.     

Four‐port shared rectangular radiator with defected ground for wireless application

In this research work, a modified shared rectangular radiator is proposed for 5.7‐GHz wireless application using multiple cuts and partially stepped ground. The shared design requires no isolating structure to decouple the ports. The proposed shared geometry produces >13 dB isolation in the 4.4‐ to 6.4‐GHz operating band. The gain varies from 3.0 to 6.2 dBi, and radiation efficiency is >70 % in the band. The mutual coupling among the port is reduced by orthogonal arrangement of ports, and envelope correlation coefficient (ECC) is <0.04 in the presented band. Apart from this, isolation in the proposed shared radiator is enhanced by additional multiple cuts with stepped ground. The indoor‐outdoor suitability of multiple input multiple output (MIMO) antenna for isotropic and Gaussian medium is checked by mean effective gain (MEG) and specific absorption rate (SAR). The presented results of MEG and SAR fulfill the required safety norms as per the ITU for different environments.     

Multi‐standard,multi‐band planar multiple input multiple output antenna with diversity effects for wireless applications

Wireless generations require the miniaturized radiating elements for the portable devices. This research article presents a miniaturized multiple input multiple output (MIMO) antenna for IEEE 802.11 (WLAN) and IEEE 802.16 (WiMAX) wireless standards. The multi‐standard, multi‐band MIMO with 1 × 2 diversity arms is impedance matched with 50 Ω microstrip line on FR‐4 dielectric substrate having dielectric constant 4.4 and 1.524 mm thickness. The simple low profile design covers16.46% (2.23‐2.64 GHz) and 12.37% (3.26‐3.70 GHz) microwave frequency bands, with voltage standing wave ratio (VSWR) ≤ 2 achieves more than 12.5 dB of isolation between radiating ports. The proposed MIMO with inverted L shaped slot exhibits more than 73% efficiency, and more than 4 dBi gain at resonant frequencies. The presented MIMO is designed on FR‐4 dielectric substrate of size 45.1 × 90.2 mm². The compact size of the radiating element is 6.7 × 6.7 mm². The effect of radiations on the body has been evaluated using specific absorption rate (SAR) and found to be in safety limit.     

Multi-Element Wideband Planar Antenna for Wireless Applications

A wideband, multi-standard MIMO antenna with hexagonal geometry and slot is proposed for DCS/PCS/LTE/UMTS applications while keeping the real time application at prime to provide high data rate, low latency, high capacity, non-line-of communication, and reliability with continuity. The designed prototype covers 1.64–2.50 GHz frequency band with percentage bandwidth of 41.55% and resonates at 2.1 GHz. The isolation of more than 10 dB is achieved in the 2:1 VSWR frequency band. The total bandwidth of the MIMO antenna is 860 MHz. The designed MIMO has peak gain of 5.4 dBi, ECC?<?0.06, radiation efficiency?>?88%, and total efficiency?>?71%. The TARC active bandwidth is 600 MHz with best excitation angles of 45°, 45° at ports. The hexagonal slot is used for the control of induced current for better isolation. The proposed MIMO antenna evaluates the SAR performance at resonant frequency for listening, holding, and watching positions, and is found under the required safety norms.

     

On the effect of relative size of magnetic particles and abrasive particles in MR fluid-based finishing process

Magnetorheological fluid-based finishing (MRFF) process is widely used for fabrication of optical material such as glasses, lenses, mirrors, etc. Performance of the process is significantly affected by the properties (size, concentration, hardness, etc.) of the constituents of MR fluid. MR fluids have been prepared by varying three abrasive particles mean sizes (4 µm, 6 µm and 9 µm) with carbonyl iron particles (CIPs) having average particles size of 6 µm. Yield stress of MR fluids is measured using a rheometer. The composition of the fluid has CIPs of 25%, abrasive 10% (by volume) and rest of the base medium (liquid). The yield stress was evaluated at magnetic flux density of 0.33 Tesla. It is observed that MR fluid having the same particle size of CIPs and abrasive particles exhibits higher yield stress as compared to other combinations. The lowest yield stress is observed in case of 9 µm abrasive particles size. A set of finishing experiments is carried out to understand the effect of relative size of magnetic particles and abrasive particles on surface roughness values.