Recently reconfigurable intelligent surface (RIS) has attracted great attention because it can create a smart wireless environment. Hence it can enhance the capacity and coverage of the wireless network significantly. A thorough review of RISs has been presented in this paper focusing on the hardware aspect of the RIS. Beyond-5G/6G communication will have a smart propagation environment, where RIS can be used for such communications. RIS consists of various small unit cells. The unit cells should have some tunning mechanism so that the incoming waves can be reflected or transmitted in the desired direction. It is possible to tune the impedance of the unit cells using PIN diodes, varactor didoes, microelectromechanical (MEMS), thermal, and other ways. In this paper, initially, the background of RIS has been discussed where RIS is going to play a significant role in beyond-5G/6G communications. We have also added the theoretical background of RIS and motivations to writing this paper. After that several published papers in the literature have been presented so that the readers can get an overall idea about the RIS and its hardware. Hence, this paper will be very useful for practitioner engineers and researchers. RISs have been presented in various tables and various parameters have been presented. We have discussed challenges and solutions for the hardware of the RIS design. We have also discussed potential research and research gap that can be explored in the future. Lastly, we have added a conclusion for this review paper.

The brain-machine interface (BMI) is a new area of research and it is still in the development stage. The key component for the brain-machine interface is internal and external antennas. In this paper, we propose an implantable antenna for wireless brain signal sensing and monitoring using an inhomogeneous multi-layer model of the human head. A model with seven layers composed of skin, bone, fat, dura, cerebro spinal fluid (CSF), grey matter and white matter was adopted for our multi-layer model. The antenna was embedded below the bone and above the dura of the head phantom. Artificial tissue emulating (ATE) materials were fabricated in semi-solid form and measurement was carried out to check the permittivity and loss tangent of each semi-solid ATE. Implanted antennas for wireless brain signal monitoring data must be compact, lightweight and biocompatible. The proposed antenna was designed with Taconic RF-35 as the substrate with an overall size of 10 mm ×10 mm × 0.5 mm3. The proposed antenna has a -10 dB reflection bandwidth of 2.42-2.50 GHz and a gain of -25 dBi at the broadside direction. The top and bottom insulators with a thickness of 0.5 mm each were also designed with a Taconic RF-35 substrate. A good agreement between simulated and measured results was achieved for the proposed antenna for both in free space and inside of head phantom.

In this paper, a substrate integrated waveguide (SIW)-based equilateral compact and dual band triangular antennas have been proposed. A SIW-based coaxial fed equilateral triangular antenna has been initially considered. The antenna is found to resonate at 5.28 GHz corresponding to U-NII-2 WLAN band. Two sides of the said antenna are made perfect electrical conductor wall with array of metallic vias and the third side is kept open which acts like a perfect magnetic conductor (PMC) wall. A single slit has been introduced on the PMC wall of the initial antenna which causes a decrease in resonating frequency from 5.28 to 4.29 GHz. The antenna has been further miniaturized by introducing another parallel extra slit on the PMC wall which leads to a decrease in the resonating frequency to 3.3 GHz. The former antenna with single slit is found to exhibit 33% miniaturization while the latter with double slits reflects 56% miniaturization. By changing the coaxial feed location of the initial antenna at off-axis position, a dual band antenna has been realized. The dual band antenna is found to excite both TE101 and TE202 modes efficiently with corresponding resonating frequencies at 4.18 and 8.85 GHz. The prototypes have been fabricated on Arlon AD270 substrate and simulated results are compared with the measured ones. A good agreement has been observed between the simulated and measured results.

In this work, half-mode substrate integrated waveguide (HMSIW) based low profile, rectangular slit(s) loaded compact semi-circular antennas are presented as sensors to operate in the S- and C-band. The parent sensor is a HMSIW antenna initially designed at the centre frequency of 5.22 GHz with a gain of 6.4 dBi for IEEE 802.11a WLAN U-NII low band application. The semi-circular antenna has metallic vias all along the semi-circular edge thereby making it perfect electrical conductor (PEC) wall while the other side devoid of vias is a perfect magnetic conductor (PMC) wall. It is miniaturized upon placing rectangular slit(s) along the PMC edge which reduces the fundamental resonant frequency to 4.68 GHz and further to 3.85 GHz with single and double slits respectively. Both antennas radiate linearly polarized wave corresponding to TM010 mode with respective gain of 5.2 dBi and 4.6 dBi. The antenna prototypes are fabricated using Arlon AD270 substrate and good agreement between simulated and measured results are obtained.

In this paper, a novel digitally reconfigurable unit-cell and $4\times 4$ transmitarray operating at X-band is proposed that allows the implementation of beamforming and polarization conversion operations in a programmable way. The lower layer of transmitarray responsible for beam steering consists of two PIN diodes and a circular type patch which has two states of operations with 0 and $\pi $ phase responses which we name “State1” and “State2”. “State1” and “State2” can be represented by a digital notation as “1” and “0” elements. Similarly, the upper layer of the transmitarray responsible for polarization conversion operation consists of two states of operation which we name as “State3” and “State4”, respectively. “State3” and “State4” also can be represented by digital notations as “1” or “0” elements. By coding “0” and “1” elements for the lower side of the transmitarray in a 1-bit sequence, we can steer the beam in our desired direction. Similarly, by coding “0” and “1” elements in a 1-bit sequence element for the upper layer, we can get X-polarized and Y-polarized beams. Combining the upper layer 1-bit sequence and lower layer 1-bit sequence, we can get both polarization conversion and beam steering simultaneously. A $4\times 4$ array with our proposed unit cell was designed and fabricated. A good agreement between simulated and measured results was observed. The proposed design is a promising concept because it can be fabricated using the conventional fabrication process, it is cost-effective compared to conventional phased array antenna for satellite and radar applications. A $4\times 4$ array was taken just to check the transmission and reflection coefficients of the proposed unit cells. A very small transmitarray was manufactured and experimentally characterized just as a proof of concept, to verify that the unit cells work properly.