The in-depth exploration in the future 5G technology symbolizes a revolution in technology for antenna designers to encounter the all time increasing need as well as demand for higher data rate wireless communications. The paper gives out an exhaustive review of the evolution and characteristics of the 5G spectrum allocations, the MIMO antenna design with regard to mutual coupling reduction techniques and safer user applications. It precisely covers almost all the aspects of 5G which mainly include the types of antenna designs and their performance parameters related to MIMO design. The paper also presents a brief description of massive MIMO technology for base station applications. The main aim of the paper is: (1) to emphasize the frequencies allocated for the 5G including sub-6 Ghz and mm-wave bands; (2) to underline the suitable antenna designs for MIMO applications for mobile devices and base stations; (3) to highlight the mutual coupling effects in MIMO designs and its reduction techniques; (4) to consider the gaps in the literature and the challenges for reducing SAR effects for the safety of the users. This review paper has been an attempt to explore the evolution of 5G bands and antenna designs for 5G applications, comparison based on the literature, and the techniques implemented for enhancing the MIMO antenna performances.

A novel flexible radio frequency (RF) sensor is designed to facilitate the accurate testing of various samples used in the biomedical industry at the industrial, scientific and medical (ISM) frequency band. The proposed RF biosensor comprises a liquid channel-loaded interdigitated capacitor, which is integrated on a coplanar waveguide structure. The prototype of the sensor is fabricated on a 0.13 mm thin biodegradable polyethylene terephthalate polyester film to perform the testing of various bio-graded samples by recording the corresponding resonant frequency. It is observed that there is a noticeable change between the measured resonant frequencies of these samples, which primarily occurs due to the difference in their dielectric properties. The designed sensor was used to monitor and investigate the quality of glycerol, which is the most commonly used raw ingredient in the biomedical and food industry. The determination of glucose concentration in base fluids is considered to ease the challenges faced by doctors and biochemists regarding the monitoring of glucose concentration. It is found that the proposed sensor can quantify the glycerol purity up to the minimum specified adulteration level of 2 and 1% corresponding to toxic contaminants diethylene glycol and ethylene glycol, respectively, and the glucose concentration of 0.5 mg/ml.

The ever-increasing use of electronic devices leads to a dangerous upsurge in the emission of microwave radiation; this has drawn appreciable concern in the fabrication of eco-friendly microwave absorber (MA) and it can be a prospective alternative. Present work, in the quest for possible alternatives, explores carbon-rich agricultural residues such as dry banana leaves as a microwave-absorbing material. The variation of microwave absorption efficiency with an increase in the percentage of resin has been already reported. An extensive study on the microwave absorption efficiency of dried banana leaves with sample preparation and reflectivity analysis by hardware measurement, and simulative analysis using CST microwave studio suite for different thicknesses in the frequency range of 1–20 GHz has been also explored in the present work. Single-layer MA thickness variation establishes different microwave absorption performance.

A compact multiple input multiple output (MIMO) antenna operating at 2.45 GHz industrial scientific and medical band is presented for wearable devices. Open-end slotting is used to miniaturize the antenna dimensions. Inverted U-shaped ground stub is incorporated to reduce mutual coupling. On-body performance is analyzed on a three-layered equivalent tissue phantom model. The wide bandwidth of 300 MHz and port isolation of 30 dB are obtained from measured results. The antenna shows the efficiency of 40% and directivity of 4.56 dBi when placed at a gap of “s” = 4 mm from the body. Broadside radiation pattern and low specific absorption rate make the antenna suitable for on-body communication. Further, diversity performance is measured in terms of envelope correlation coefficient (ECC), diversity gain (DG), and channel capacity loss (CCL). The value of ECC is 0.025, DG is 9.98 dB, and CCL is 0.12 bits/s/Hz at 2.45 GHz. Antenna robustness is examined by bending the structure at different radii along the x-axis and y-axis. Performance of the proposed structure is reliable with structural deformation.

By using a metasurface (MS) structure, a linearly polarized wave is converted to circularly polarized waves. Both right- and left-handed circular polarizations (RHCPs and LHCP) are obtained by a simple configuration in the proposed structure which consists of 16 unit cells arranged in a 4 × 4 layout. Each unit cell contains five horizontal and parallel strips embedded in a rectangular frame in which a single diagonal strip is placed from one corner to the opposed one. It is shown that the orientation of the diagonal line determines the handedness of the converted signal to be either LHCP or RHCP. In order to show the working conditions of the MS structure, scattering parameters are found for both co-polarized and cross-polarized responses. Axial ratio, an indicator for polarization conversion, is then obtained by dividing cross-polar response to co-polar response to demonstrate the transformation. The structure works for horizontally and vertically polarized linear waves in a wide band frequency range which is approximately 510 MHz. Since the suggested MS model is composed of a simple geometry for polarization conversion, it can be easily adjusted in any desired frequency bands for a variety of applications from the defence industry to medical, education, or communication areas.

Wideband dipole antennas are proposed using the multi-mode resonance concept. By symmetrically introducing one-pair or two-pair of stubs at the nulls of current distribution of the second odd-order mode, two radiation modes are excited in a single, center-fed dipole resonator. Using these stubs, the second odd-order mode gradually moves down to its first counterpart, resulting to achieve a wideband radiation with two resonances. Prototype antennas are then fabricated to experimentally validate the design approach. Compared with a reference dipole with a bandwidth of 17%, the proposed dipole's bandwidth can be effectively increased to 49.7%.

Metamaterial-based energy harvesting structures operating at GSM 900 and GSM 1800 frequency bands have been proposed. The basic structure is composed of four nested U-shaped elements on the front and back side of the dielectric substrate, with a 180° relationship between the front and back resonators. As energy harvesting capability is directly related to the quality of absorption, with the aim being perfect absorption, there is an air gap to allow tuning of the resonance frequency, and copper plate follows the air gap to facilitate the perfect absorption of electromagnetic waves, which is verified by means of simulation and experimental tests. A set of simulations and experimental study were further carried out to demonstrate incident and polarization angle dependency, the effects of Schottky diodes, and the optimal orientation of the U-shapes. Spectrum analysis and DC voltage measurements across the HSMS 2860 Schottky diode are also presented to test the energy harvesting capability: the spectrum analyzer shows a 40 dBm difference, while 81.7 mV maximum DC voltage is observed at 900 MHz. The simulation and experimental study results, while displaying several differences, do suggest that the proposed design can be used for running low-energy consumption devices such as sensors and switches.

A wideband and high-efficiency polarization conversion metasurface (PCM) is proposed and applied to reduce radar cross section (RCS). The proposed PCM unit is composed of two oblique asymmetry triangle split rings, which generate multiple plasmon resonances. Simulated and measured results demonstrate that it achieves polarization conversion over 90% from 9.24 to 17.64 GHz. Besides square checkerboard, the proposed PCM units and mirror units are arranged in triangle checkerboard. The mechanisms of both checkerboard PCMs are analyzed based on standard array theory, including the relationship between RCS reduction value and polarization conversion ratio value. The derived formulas provide a guideline to design checkerboard structure based on PCM. Simulated results demonstrate that both checkerboard PCMs achieve over 62% relative bandwidth of 10 dB RCS reduction under normal incidence with respect to the equal-sized metallic plate, which also means that the triangle one could be an alternative solution to reduce RCS. To verify the analyzed and simulated results, the fabricated sample and measured results of both checkerboard PCMs are presented. Good agreements are achieved between measurements, simulations and numerical analysis.

Microwave imaging is an efficient technique that can be used for the early detection of breast cancer. Therefore the current research article presents the microwave imaging of two spherical tumors (radius 4 and 5 mm) in the breast phantom by using the monostatic radar-based technique. This is carried out by using an ultra-wideband (4.9–10.9 GHz), three-layered stacked aperture coupled microstrip antenna (SACMPA) with a defected ground structure to scan the breast phantom and make near field S parameter measurements from a breast phantom. The S parameter data from different locations and at different time intervals are noted and then used in a beam-forming algorithm; Delay and Sum to process it and form a 2D image of the tumor location in the breast phantom using MATLAB.

The proposed SACMPA is a three-layered structure with overall dimensions of 37 × 43 × 4.85 mm3 that shows an impedance bandwidth of 6 GHz (4.9–10.9 GHz) and a simulated peak gain of 6.32 dB at a frequency of 9.1 GHz. The validation of S parameters and gain results are done using a Vector Network Analyzer (VNA) and an anechoic chamber. The experimental validation of the proposed microwave imaging procedure is done by allowing the SACMPA to radiate parallel to the breast phantom made from polythene (skin), petroleum jelly (fat), and wheat flour (with water as tumor) and record the S parameters on the VNA. The proposed microwave method is safe for human exposure as the antenna also shows simulated specific absorption rates of 0.271 and 1.115 W/Kg (on the breast phantom) at frequencies of 5.7 and 6.5 GHz, respectively (for 1 g of body tissue).

This paper presents the design optimization of circular antenna arrays of isotropic radiators using simulated annealing. The problem has been formulated to achieve a desired value of sidelobe level and a minimum possible value of beamwidth. This is accomplished by jointly optimizing the excitation amplitude and spacing between elements. Simulation examples have been given and comparison has been carried out with particle swarm optimization method.

In this paper, a frequency reconfigurable microstrip slot antenna for ultra-wideband applications with switchable band-notch functions is designed and manufactured. To generate a wide usable fractional bandwidth of 3 to over 13.6 GHz, square patch transforms to a fork-shaped patch, also two triangular-shaped strips have been added at above sections of rectangular slot on the ground plane. Band rejections are obtained by adding two inverted L-shaped strips in the fork-shaped patch and one L-shaped defected ground structure. Moreover, by implementation of two PIN diodes within the antenna structure, four switchable frequency responses are achieved. The designed antenna has simple structure and a small size of 20 × 20 × 0.8 mm3 while providing band rejection characteristics at WiMAX (Worldwide Interoperability for Microwave Access) and WLAN (Wireless Local Area Networks) systems.

In this paper, a band-notched compact printed monopole super wideband (SWB) antenna has been designed and fabricated. The SWB antenna composed of a radiating patch with a 50 Ω triangular tapered feed line which is connected through a feed region, and a chamfered ground plane (CGP), that covers the frequency band from 0.9–100 GHz (ratio bandwidth of 111.1:1) with a reflection coefficient |S11| < −10 dB, except in the notched band of 4.7–6 GHz for Wireless local area network IEEE 802.11a and HIPERLAN/2 WLAN band. To realize the band notch characteristics a C-shape parasitic element is employed near the CGP etched with two symmetrical L-slots and placed under the radiating patch. Proposed antenna structure occupies a relatively small space (30 × 40 × 0.787 mm3) and achieved much wider impedance bandwidth as well as higher gain compared with the existing ultra wideband and SWB antennas.

Two configurations of a modified feather-shaped antenna element are proposed for super wideband (SWB) multiple-input multiple-output (MIMO) applications. The antenna element geometry comprises of a circular slot-loaded feather-shaped radiator and rectangular notch-loaded quarter elliptical coplanar waveguide ground plane. An operating bandwidth of 4.4–51.5 GHz with inter-port isolation, S21 ≥ 15 dB for spatial diversity configuration, and 3.8–51.5 GHz with S21 ≥ 15 dB in pattern diversity configuration are achieved. The footprints of the antenna configurations are 17 × 33 and 31 × 31 mm2. Both configurations exhibited an envelope correlation coefficient of <−20 dB. The proposed MIMO configurations are fabricated and experimentally validated. The designed antenna configurations are SWB and compact.

This study presents a new design of a four-element multiple-input multiple-output (MIMO) antenna, which is suitable for X-band applications. The circular polarization (CP) operation in this work is attained by using Tai Chi-shaped patches and L-shaped feeds. The proposed four-element MIMO antenna achieves two resonant frequencies, the first one at around 7.75 GHz and the second one with CP radiation is at approximately 10.15 GHz frequency. The measured isolations between ports with 46.7 × 46.7 mm2 total dimensions are <−20 dB. Due to the experiments, the envelop correlation coefficient for the orthogonal and parallel ports of the proposed MIMO antenna is <0.003 and 0.005, respectively. Additionally, the presented antenna has a circularly polarized performance in the frequency band of 9.75–10.41 GHz and the average realized peak gain almost 2 dBic. The investigated antenna in this work is fabricated and tested in which the results are in a good agreement.

This paper discusses the design and fabrication of a low-profile Hilbert-shaped metamaterial (MTM) array-based antenna, forming a rectangular patch with partial ground plane backing; the rest is slotted with traces for RF energy harvesting. The antenna is mounted on a 28 mm × 32 mm indium nickel oxide polymerized palm fiber (INP) substrate and compared to the identical one based on FR4 substrate. The two prototypes are printed with silver nanoparticles. Numerical and experimental tests are applied to the antenna performance in terms of S11 and radiation patterns. The obtained antenna gain bandwidth product of the INP prototype is found to be significantly better than the FR4 prototype. The proposed INP antenna gain at 5.8 and 8 GHz frequencies is found to be about 4.56 and 7.38 dBi, respectively, while the FR4 antenna gain is found to be 4.56 and 6.85 dBi at 5.8 and 8 GHz, respectively. Finally, the resultant DC voltage and the efficiency of conversion from harvested RF energy are measured experimentally at 5.8 and 8 GHz for both proposed prototypes.

Millimeter-wave phase shifters are important components for a wide scope of applications. An analog-type phase shifter for W-band has been designed, analyzed, fabricated, and measured. The phase shifter consists of a reconfigurable high-impedance surface (HIS) controlled by micro-electromechanical system (MEMS) varactors and placed adjacent to a silicon dielectric rod waveguide. The analog-type phase shift in the range of 0–32° is observed at 75 GHz whereas applying bias voltage from 0 to 40 V to the MEMS varactors. The insertion loss of the MEMS tunable HIS is between 1.7 and 5 dB, depending on the frequency.

Radar sensors are used widely in modern driver assistance systems. Available sensors nowadays often operate in the 77 GHz band and can accurately provide distance, velocity, and angle information about remote objects. Increasing the operation frequency allows improving the angular resolution and accuracy. In this paper, the technical feasibility to move the operation frequency beyond 100 GHz is discussed, by investigating dielectric properties of radome materials, the attenuation of rain and atmosphere, radar cross-section behavior, active circuits technology, and frequency regulation issues. Moreover, a miniaturized antenna at 150 GHz is presented to demonstrate the possibilities of high-resolution radar for cars.

This paper presents a new technique for the enhancement of axial ratio (AR) bandwidth of a circularly polarized dielectric resonator antenna with a single feeding. To enhance the AR bandwidth, adjacent 3-dB AR passbands are merged by inserting the notches and conductive coating in the dielectric resonator. The dimensions of the notches and conductive coating are selected in such manner that impedance bandwidth remains approximately unchanged. The antenna provides the measured AR and impedance bandwidths of 55.22% and 66.45%, respectively.

In this paper, a new antenna has been proposed with dimensions of 28 × 28 × 0.8 mm3 and the ability of operating in ultra-wideband (UWB) frequency and switching three notched bands. In the structure, to achieve the removal of characteristic of three frequency bands, a rectangular patch with an H-shaped slot on its surface and two C-shaped arms on its sides have been used. In addition, the designed antennas have the ability of operating in five different modes. To achieve the performance of reconfiguring, three pin diodes are used on the slots of rectangular patch and those two C-shaped arms. Also, with the defect ground structure and creating two rectangular slots and two L-shaped slots on the ground plane we make the excite of additional resonances and thus increase antenna bandwidth. Simulation and measurement results show that the designed antenna is able to cover the range of frequencies 3–12 GHz and a controllable frequency band rejection in the three frequency bands 3.2–3.75 GHz (WiMAX), 5.05–5.9 GHz (WLAN), and 8–8.45 GHz (ITU). The proposed antenna decreases the interference of the mentioned wireless systems with the UWB frequency.

A planar rectangular slot antenna with dual-band operation and realized higher peak gain is proposed, designed, and fabricated for sub-6 GHz 5G applications. The antenna possesses a rectangular radiating slot with the inverted stub on its upper edge excited simultaneously by a micro-strip feed line having a double folded T-shaped structure. The fabricated design is of compact size with the radiating portion of 0.3 λ0 × 0.17 λ0 (λ0 represents free-space wavelength) and profile of 0.009 λ0. The measured results show the operating frequency bands of 3.29–3.63 GHz and 4.3–5.2 GHz, with a peak gain of around 7.17 dBi. The higher frequency band is generated by the feed patch and the slot whereas lower resonant frequency band is generated by the stub loaded on the slot. The measured results are in a good agreement with the simulated results. The proposed design is suitable for the International Telecommunications Union sub 6 GHz applications.