Showing papers in "Iet Microwaves Antennas & Propagation in 2019"

Compact wearable MIMO antenna with improved port isolation for ultra-wideband applications

Abstract: In this communication, a compact two-element ultra-wideband (UWB) wearable multiple-input multiple-output (MIMO) antenna with high port isolation is presented. The proposed structure is composed of jeans material in which an `8' shaped stub is placed on the middle position of the antenna backside and connected to the partially suppressed ground structure to improve the port isolation characteristics. The antenna covers the frequency range from 2.74 to 12.33 GHz (about 127.27%) with the port isolation of >26 dB over the entire UWB frequency range. The envelope correlation co-efficient is found to be 9.9) throughout the complete operating band. The channel capacity loss for the proposed MIMO antenna is <;0.13 bit/s/Hz. The imprinted optimised UWB MIMO antenna covers the area size of 55 × 35 mm 2 . The performances of the proposed antenna by the simulation and experimentation equally designated it a blameless candidate for the UWB applications.

Isolation enhancement of densely packed array antennas with periodic MTM-photonic bandgap for SAR and MIMO systems

Abstract: A metamaterial photonic bandgap (MTM-PBG) periodic structure is used as a decoupling frame to improve the isolation between transmit-receive (T/R) sections of the densely packed array antenna in synthetic aperture radar (SAR) and multiple-input-multiple-output (MIMO) systems. With this technique the MTM-PBG structure is shown to effectively suppress surface wave propagations between the T/R array antennas by an average of 12 dB. MTM-PBG layer comprises a periodic arrangement of dielectric circles etched in the cross-shaped microstrip frame that is inserted between the radiating elements. Unlike other recently reported methods, the advantages of the proposed technique are: (i) simplicity; (ii) cost effectiveness as there is no need for short-circuited via-holes or three-dimensional metal walls; and (iii) can be retrofitted in existing array antennas. The proposed T/R array antennas were designed to operate over an arbitrary frequency range (9.25-11 GHz) with a fractional bandwidth of 17.28%. With this technique (i) the side-lobes are reduced; (ii) there is minimal effect on the gain performance; and (iii) the minimum edge-to-edge gap between adjacent radiating elements can be reduced to 0.15 λ at 9.25 GHz.

Machine-learning-based prediction methods for path loss and delay spread in air-to-ground millimetre-wave channels

Abstract: The unmanned aerial vehicles (UAVs) have been widely applied in various fields due to their advantages like high mobility and low cost. Reliable communication is the premise to ensure the connectivity between UAV nodes. To provide reasonable references for the design, deployment, and operation of UAV communication systems, the precise prediction of radio channel parameters are required. In this study, the authors propose prediction methods for path loss and delay spread in air-to-ground millimetre-wave channels based on machine learning. Random forest and K-nearest-neighbours are the algorithms employed in the methods. Then, a feature selection scheme is proposed to further improve the prediction accuracy and generalisation performance of the machine-learning-based methods. Generally, machine learning algorithms require massive data for training purpose. However, measuring data is time-consuming and costly, especially when the scenario or frequency changes. Therefore, transfer learning methods are introduced to predict path loss with limited data. The proposed methods for path loss prediction are compared to Okumura-Hata and COST-231 Hata models. The lognormal distribution is the contrast model in delay spread prediction. Based on the data generated by ray-tracing software, the new methods have a smaller root mean square errors than contrast models.

Reduced-cost electromagnetic-driven optimisation of antenna structures by means oftrust-region gradient-search with sparse Jacobian updates

Abstract: Numerical optimisation plays more and more important role in the antenna design. Because of lack of design-ready theoretical models, electromagnetic (EM)-simulation-driven adjustment of geometry parameters is a necessary step of the design process. At the same time, traditional parameter sweeping cannot handle complex topologies and large number of design variables. On the other hand, high computational cost of the conventional optimisation routines can be reduced using, e.g., surrogate-assisted techniques. Still, direct optimisation of EM simulation antenna models is required at certain level of fidelity. This work proposes a reduced cost trust-region algorithm with sparse updates of the antenna response Jacobian, decided based on relocation of the design variable vector between algorithm iterations and the update history. Our approach permits significant reduction of the optimisation cost (∼40% as compared to the reference algorithm) without affecting the design quality in a significant manner. Robustness of the proposed technique is validated using a set of benchmark antennas, statistical analysis of the algorithm performance over multiple initial designs, as well as investigating the effects of its control parameters that permit control efficiency vs. design quality trade-off. Selected designs were fabricated and measured to validate the computational models utilised in the optimisation process.

Beam-scanning leaky-wave antenna based on CRLH-metamaterial for millimetre-wave applications

Abstract: This paper presents empirical results of an innovative beam-scanning leaky-wave antenna (LWA), which enables scanning over a wide angle from - 35° to + 34.5° between 57 and 62 GHz, with broadside radiation centred at 60 GHz. The proposed LWA design is based on composite right/left-handed transmission-line (CRLH-TL) concept. The single-layer antenna structure includes a matrix of 3 × 9 square slots that is printed on top of the dielectric substrate; and printed on the bottom ground-plane are Π- and T-shaped slots that enhance the impedance bandwidth and radiation properties of the antenna. The proposed antenna structure exhibits metamaterial property. The slot matrix provides beam scanning as a function of frequency. Physical and electrical size of the antenna is 18.7 × 6 × 1.6 mm 3 and 3.43λ 0 × 1.1λ 0 × 0.29λ 0 , respectively, where λ 0 is free space wavelength at 55 GHz. The antenna has a measured impedance bandwidth of 10 GHz (55-65 GHz) or fractional bandwidth of 16.7%. Its optimum gain and efficiency are 7.8 dBi and 84.2% at 62 GHz.

3D non-stationary geometry-based multi-input multi-output channel model for UAV-ground communication systems

Abstract: Unmanned aerial vehicles (UAVs) with multi-input-multi-output technologies are considered as a promising platform to provide future high-speed wireless communication services. In this study, considering three-dimensional (3D) antenna arrays and 3D arbitrary trajectories of the UAV and mobile terminal, the authors propose a new 3D non-stationary geometry-based stochastic channel model for UAV-ground communication systems. Under 3D non-isotropic scattering scenarios, the computation and optimisation methods of time-variant channel parameters are investigated. In addition, the theoretical statistical properties such as the autocorrelation function, cross-correlation function, Doppler power spectrum density, level-crossing rate, and average fading duration are analysed and derived. Finally, the usefulness of the proposed model as well as theoretical derivations are verified by the comparison between theoretical, simulated, and measured results.

8-port multibeam planar UWB-MIMO antenna with pattern and polarisation diversity

Abstract: In this study, an 8-port multibeam ultrawideband multi-input–multi-output (UWB-MIMO) antenna having pattern and polarisation diversity is investigated for Wireless Personal-Area Network access points. The proposed antenna consists of two different types of slot radiators, each of them provides four directive beams. The novelty of the proposed design is the achievement of a compact 8-port UWB-MIMO antenna using a common ground plane. Eight additional grounded slits are used in this design to enhance isolation. Antenna elements are placed in such a way that two adjacent antennas contribute different patterns. Results show that all the eight antenna elements cover complete UWB (3.1 to 10.6 GHz) with port isolation better than 15 dB. Proposed design offers gain better than 2 dB with envelop correlation coefficient lower than 0.1 from all ports.

Single-fed 4G/5G multiband 2.4/5.5/28 GHz antenna

Abstract: In this study, a single-fed printed multiband antenna for 4G/5G wireless communication systems is presented. The proposed multiband antenna consists of Franklin strip monopole antenna to cover 4G, and wireless applications (WLAN and WiMAX), and a rectangular patch antenna that is designed to cover 5G band. Furthermore, a modified compact microstrip resonant cell low-pass filter is printed between the antenna parts to allow feeding the Franklin antenna at low-frequency bands while isolating the Franklin antenna from the rectangular patch at the 5G band. The proposed antenna is designed on Rogers 5880 with compact size 45 × 40 × 0.508 mm 3 . The proposed antenna is utilised to operate at triple band: 2.4, 5.5 and 28 GHz with wide impedance bandwidth (15.8, 23.5 and 11.3%) and the gain reaches (1.95, 3.76 and 7.35 dBi).

Dual-band circular polarisation generation technique with the miniaturisation of a rectangular dielectric resonator antenna

Abstract: A technique is proposed to obtain the dual-band circularly polarised (CP) response along with the miniaturisation of a singly fed rectangular dielectric resonator (DR) antenna. The miniaturised response is obtained by applying the metallic strips on the sidewalls of the DR. The specific format of the applied metallic strips excites the orthogonal degenerate modes. Thus, a dual-band CP response is obtained along with the miniaturisation of antenna. The proposed antenna is designed to obtain the small frequency ratio. The frequency ratio can also be tuned by changing the surface area of the applied metallic strips.

Compact planar reconfigurable UWB-MIMO antenna with on-demand worldwide interoperability for microwave access/wireless local area network rejection

Abstract: In this study, a compact ultrawideband (UWB) multiple-input-multiple-output (MIMO) antenna with reconfigurable band notch characteristics is presented. The key feature of the antenna is its novel dual-band notch technique which blocks worldwide interoperability for microwave access or wireless local area network band interference. The UWB-MIMO property is achieved by two square monopoles separated by grounded multi-branch T-shaped stub. The reconfigurable dual-band notch operation is performed by PIN diodes integrated in a parasitic resonant structure in the ground plane. The antenna has a compact size of 40 × 20 mm 2 , with the operating frequency band of 3-11 GHz, where port isolation is better than 15 dB. The MIMO performance is ensured by calculating envelope correlation coefficient for the isotropic, indoor and outdoor environment.

Efficient global optimisation of microwave antennas based on a parallel surrogate model-assisted evolutionary algorithm

Abstract: Computational efficiency is a major challenge for evolutionary algorithm (EA)-based antenna optimisation methods due to the computationally expensive electromagnetic simulations. Surrogate model-assisted EAs considerably improve the optimisation efficiency, but most of them are sequential methods, which cannot benefit from parallel simulation of multiple candidate designs for further speed improvement. To address this problem, a new method, called parallel surrogate model-assisted hybrid differential evolution for antenna optimisation (PSADEA), is proposed. The performance of PSADEA is demonstrated by a dielectric resonator antenna, a Yagi-Uda antenna, and three mathematical benchmark problems. Experimental results show high operational performance in a few hours using a normal desktop 4-core workstation. Comparisons show that PSADEA possesses significant advantages in efficiency compared to a state-of-the-art surrogate model-assisted EA for antenna optimisation, the standard parallel differential evolution algorithm, and parallel particle swarm optimisation. In addition, PSADEA also shows stronger optimisation ability compared to the above reference methods for challenging design cases.

Modelling of ultra-wide stop-band frequency-selective surface to enhance the gain of a UWB antenna

Abstract: Here, an ultra-wide stop-band single-layer frequency-selective surface (FSS) with high-incidence angle independence has been proposed to enhance the gain of an ultra-wideband (UWB) monopole antenna. The unit cell (0.2 λ × 0.2 λ ) of the proposed FSS consists of four asymmetric rectangular patches with circular slots embedded in it. This concept of four slotted patches is conceived to achieve ultra-wide stop-band characteristic over 4.7-14.9 GHz. An equivalent lumped circuit model for the FSS is proposed to provide insight into the working nature of the FSS. A UWB monopole antenna is also designed and integrated with the proposed FSS. Ultra-wide stop-band single-layer FSS converts the omnidirectional pattern of the monopole antenna into a unidirectional one and thereby registers a significant increase in its gain by 4.5 dBi. The design concept has been discussed and experimentally verified using simulated and measured results.

e-Textile embroidered wearable near-field communication RFID antennas

Abstract: Wearable e-textile near-field communication (NFC) radio-frequency identification (RFID) antennas fully integrated with garments using embroidery techniques, which enables everyday clothing to become connective to wireless communication systems, is presented. The e-textile wearable antennas have been designed through full electromagnetic wave simulation based on the electrical properties of conductive threads and textile substrates at the high frequency band, allocated for NFC wireless communications. The e-textile wearable NFC antenna performance under mechanical bending as well as human body effects have been experimentally studied and evaluated; the antennas can operate under significantly bending angle and body effects attributed to its broad operating bandwidth. This is highly desirable and distinguished to conventional NFC antennas; the proposed e-textile wearable NFC antennas can be placed almost any place on clothes and still capable to communicate at the desired operating frequency of 13.56 MHz. The maximum read range of the e-textile wearable NFC tags is measured to be around 5.6 cm, being compatible to typical commercially available metallic NFC tags. The e-textile wearable NFC tags can lead to numerous potential applications such as information exchange, personal security, health monitoring and Internet of Things.

Hilbert metamaterial printed antenna based on organic substrates for energy harvesting

Abstract: In this study, an investigation is conducted to realise the possibility of organic materials use in radio frequency (RF) electronics for RF-energy harvesting. Iraqi palm tree remnants mixed with nickel oxide nanoparticles hosted in polyethylene, INP substrates, is proposed for this study. Moreover, a metamaterial (MTM) antenna is printed on the created INP substrate of 0.8 mm thickness using silver nanoparticles conductive ink. The fabricated antenna performances are instigated numerically than validated experimentally in terms of S 11 spectra and radiation patterns. It is found that the proposed antenna shows an ultra-wide band matching bandwidth to cover the frequencies from 2.4 to 10 GHz with bore-sight gain variation from 2.2 to 3.43 dBi at maximum. The antenna size is compacted to a 32 mm × 24 mm using a fractal-shaped MTM when mounted on the INP substrate with a relative permittivity e r = 3.106-j0.0314 and a relative permeability μ r = 1.548-j0.0907. Finally, the maximum obtained voltage from the proposed antenna is found about 2 V at 2.45 GHz and 2.5 V at 5.8 GHz, where, the corresponding measured equivalent isotropic radiated power is about 2.35 W at 2.45 GHz and 6.12 W at 5.8 GHz.

Compact UWB FSS reflector for antenna gain enhancement

Abstract: In this study, two compact ultra-wideband (UWB) frequency selective surface (FSS) reflectors, which combine the branch loading method with the design of the traditional FSS, are proposed for antenna gain enhancement application. The working mechanism of the proposed FSS reflectors is analysed by the equivalent circuit model. The first FSS reflector, which is a 10 × 10 array with 8.25 mm × 8.25 mm unit size, not only enhances the gain of the UWB antenna, but also guarantees a constant gain with only 0.5 dB variation across the whole operation band. To further reduce the reflector size, a more compact FSS reflector with 6.25 mm × 6.25 mm unit size is designed by printing similar patterns on both sides of a single-layer dielectric slab. Compared to the first structure, the second reflector can realise a 25% size reduction and the same gain variation in the entire UWB frequency range. The only compromise is a 0.5 dB gain decrease. Finally, a good agreement between the measured and simulated results proves the feasibility of the authors' design.

Compact four-port MIMO dielectric resonator antenna with pattern diversity

Abstract: A new compact four-port multi-input-multi-output (MIMO) dielectric resonator (DR) antenna is proposed with pattern diversity. The antenna contains four DR elements, the epsilon-shaped and cylindrical. These four DR elements are placed together such that they resonate at the same frequency. The specific geometry of the DR elements allows to place them at separation which is much smaller than operating wavelength. Hence, the combination of the four DR elements acts as a single DR. This helps in maintaining the compactness of the antenna structure. The antenna operates with the hybrid of modes HEM 11δ and TM 01δ excited in epsilon-shaped and cylindrical DR elements, respectively. The excitation of these orthogonal modes helps in obtaining the pattern diversity allowing the antenna for multi-directional coverage. The antenna provides 15.73% overlapping 10 dB impedance bandwidth with the variation of gain within ranges 4-6 and 6-6.5 dBi at port 1/2 and port 3/4, respectively. In addition, the antenna provides the high radiation efficiency >93% at port 1/2 and 97% at port 3/4. Other parameters required for MIMO applications such as envelope correlation coefficient and diversity gain are within the acceptable limits.

Performance enhancement of graphene plasmonic nanoantennas for THz communication

Abstract: The concept and analysis of a graphene plasmonic nanoantenna over a SiO 2 /Si substrate at terahertz (THz) band are presented The performance enhancement of the antenna is proposed by dynamically controlling the surface conductivity of graphene using an electric field effect The controlling ability of graphene via gate voltage enables frequency reconfiguration of the antenna over a wide range of 256-498 THz The performance merits of the antenna are its high directivity, broadside radiation pattern, low reflection coefficient, stable impedance and high miniaturisation The proposed graphene plasmonic antenna has potential to be used for THz communication

Conceptual design of a compact four-element UWB MIMO slot antenna array

Abstract: In this study, an effective methodology to design a compact four-element multiple-input-multiple-output (MIMO) antenna with high performance over ultra-wide bandwidth (BW) is presented and discussed. Taking the advantages of symmetry of most planar ultra-wideband (UWB) antennas, an asymmetric feeding scheme technique is utilised to chop a previously developed UWB slot antenna with overall dimensions of 30 mm × 30 mm into two halves to yield a more compact structure while retaining its overall frequency- and time-domain performances. Benefiting from orthogonal orientation, a compact UWB MIMO antenna system is arranged using four half-sized elements. The proposed MIMO antenna has an overall size of 40 mm × 40 mm and exhibits impedance BW from 2.94 to more than 14 GHz (10 dB return loss) with port isolation better than 17 dB over the entire operating band without using any decoupling structures. This is achieved via the inherent directional radiation properties of slot antenna elements and their asymmetrical placements. Envelope correlation coefficient is computed, and it is within the acceptable limit, which validates the design concept for building a compact MIMO antenna system with good performance.

Wideband HMSIW-based slotted antenna for wireless fidelity application

Abstract: In this study, a wideband half-mode substrate integrated waveguide (HMSIW)-based cavity-backed slot antenna is proposed for Unlicensed National Information Infrastructure (U-NII, 5.180-5.825GHz) band. To reduce the size of the antenna, an HMSIW backing cavity is used. An epsilon-shaped radiating slot is etched on the top plane of the cavity to achieve a wide -10dB impedance bandwidth of 13.2% (5.092-5.817GHz). A planar exponential tapered feeding technique is used to excite the slot. A wideband is realised by coupling of hybrid mode, TM 210 mode, and modified TM 020 modes at the resonant frequencies of 5.2, 5.5, and 5.8GHz, respectively. The size of the proposed design including a feeding network is 56mm × 32mm (1.4 λ g1 × 0.8 λ g1 ). The proposed design is fabricated and experimentally tested. The experimental results show gains of 6.6, 6, and 5.5 dBi and efficiencies of 91, 89, and 87% at the frequencies of 5.2, 5.5, and 5.8GHz, respectively, with front-to-back ratio better than 15dB. Moreover, the proposed antenna shows a unidirectional radiation pattern including the advantages of low-profile and lightweight with planar integrity which makes the proposed antenna a favourable candidate for wireless fidelity application.

Miniaturised UWB antenna with dual-band rejection of WLAN/WiMAX using slitted EBG structure

Abstract: This study presents a new slitted electromagnetic bandgap (EBG) structure to filter undesired wireless local area network (WLAN) and worldwide interoperability for microwave access (WiMAX) bands in the ultra-wideband (UWB) spectrum. A simple UWB antenna which consists of a microstrip tapered feed line, a circular patch and a defected ground structure is used as a reference antenna. The antenna is then integrated with a slitted EBG structure near the feed line to notch two interfering bands. The created notched bands by the EBG structure can be independently controlled. Owing to structural symmetry of the proposed antenna, the size can be reduced by approximately half to form a miniaturised antenna. The miniaturised antenna has the same full-size antenna characteristics with little change in the notched bands specifications. Frequency domain measurement results show that the full-size and miniaturised antennas have a flat transfer function, and a group delay variation of about 0.5 ns in the entire UWB band except for the notched bands.

Wideband tunable mid-infrared cross-polarisation converter using monolayered graphene-based metasurface over a wide angle of incidence

Abstract: In this study, the authors have designed and proposed a monolayer graphene-based metasurface, performing as a broadband polarisation converter over a wide angle of incidence. The unit cell of the metasurface consists of split ring generated over a perforated single layer of graphene grown over metal-backed silicon dioxide. The structure converts incident linearly polarised wave into its cross-polarised component over a wide bandwidth with peak polarisation conversion exhibits value to unity. The plane of polarisation of the incident electromagnetic wave is rotated orthogonally on reflection from the proposed metasurface over full width at half maxima bandwidth around 72.83% compared with centre frequency of 2.16 THz, which can be maintained up to 40° incident angles under both transverse electric and transverse magnetic polarisations. The proposed structure is as thin as ~ λ /7.3 with respect to lowermost frequency and the period of the structure is about λ /10.7. The frequency band of polarisation conversion can be tuned by altering chemical doping as well as the electrostatic grating of the graphene sheet. The structure can find applications toward sensing, spectroscopy etc.

Isolation and frequency reconfigurable compact MIMO antenna for wireless local area network applications

Abstract: In this study, to design a compact four-port multiple-input-multiple-output (MIMO) antenna for dual-band wireless local area network applications, both frequency and isolation between antennas are made as reconfigurable. Both antenna element and decoupling network shown in this study are designed using meander line concept. The frequency reconfigurable capability of the antenna is achieved by incorporating positive-intrinsic-negative (PIN) diodes in the meander line resonator. By making PIN diodes ON and OFF, four antenna elements switch their operating frequency in between 2.4-2.5 and 5.1-5.8 GHz bands. Isolation between the antenna elements also made as reconfigurable by placing PIN diodes in the decoupling network. In ON state, isolation is found in 2.4-2.5 GHz band, whereas in OFF state, isolation is achieved in 5.1-5.8 GHz band. Overall, in both the bands, excellent isolation (more than 15 dB) between the antenna elements has been achieved. The proposed antenna design is printed on the FR-4 substrate and its overall area is 38 × 38 mm 2 . Each antenna element is in polarisation diversity with the adjacent antenna element which makes the proposed design suitable for achieving good MIMO performance.

New design of UWB-MIMO antenna with enhanced isolation and dual-band rejection for WiMAX and WLAN systems

Abstract: In this work, a new design of wide-band multiple-input multiple-output (MIMO) antenna with improved isolation and dual-band rejection is proposed for wireless systems operating over the entire ultra-wide-band (UWB), X-band, and Ku-band (3–18 GHz). The proposed four-element UWB-MIMO antenna is fabricated on Rogers RT/Duroid-5880 substrate with dimension of 73 × 73 × 0.79 mm 3 . It achieves high isolation (>20 dB) between antenna elements without using any decoupling structures. In order to prevent interference problems from nearby WiMAX (3.3–3.8 GHz) and WLAN (5.1–5.8 GHz) systems, a C-shaped stub is embedded in the radiating patch and a pair of U-shaped parasitic strips is inserted beside the feed line in the single element design. The effectiveness of the proposed design is demonstrated by investigating measurement and simulation results, and comparing them with other existing designs. The results show that the proposed design has an input reflection coefficient <−10 dB, a mutual coupling <−20 dB, and an omnidirectional radiation pattern across the bandwidth of interest (3–18 GHz) excluding the two rejected bands. Also, the proposed design exhibits high diversity performance in terms of envelope correlation coefficient (ECC <0.0015) and channel capacity loss (CCL < 0.3 bits/s/Hz). This reveals the effectiveness of the proposed design in wide-band wireless applications such as satellite communications and microwave medical imaging.

Compact coplanar waveguide-fed reconfigurable fractal antenna for switchable multiband systems

Abstract: In this study, a coplanar waveguide-fed reconfigurable antenna using crescent-shaped fractal geometry is presented The frequency reconfigurable approach is obtained using radio-frequency positive intrinsic negative diodes, resistor, and inductors The proposed approach has successfully allowed reconfigurable switching up to eight frequency bands between 146 and 615 GHz Good results have been obtained in terms of stable and omnidirectional radiation patterns The realised gains of the antenna system, in these frequency bands, vary from 052 to 567 dBi The designed antenna has the advantages of a multiband structure and compact size over the previously reported structures The proposed antenna is suitable for future wireless application systems

Dual-band transmission-type circular polariser based on frequency selective surfaces

Abstract: A design of dual-band transmission-type linear-to-circular polarisation converter based on frequency selective surfaces (FSSs) has been presented in this study. The proposed converter is implemented by cascading a two-dimensional periodic array of split-ring resonators bisected by metal strips and an array of rectangular patches surrounded by rectangular microstrip rings. The structure composed of metal layers and dielectric layers is designed to behave differently for field components of the two orthogonal polarisations and transmit a circularly polarised wave once illuminated by a linearly polarised plane wave within two frequency bands. Using the equivalent circuit models of the FSSs, the operating principle of the converter is presented and discussed in detail. Also, as an illustrating example, a prototype of the proposed polarisation converter operating in two frequency bands of 6.4-8.8 GHz and 12.1-13.9 GHz is simulated, fabricated and experimentally characterised. The measurement results demonstrate that the dual-band polarisation converter operates <;3 dB axial ratio in a field view of ±25° with fractional bandwidths of 31.6 and 13.8%.

Metasurface‐based radiative near‐field wireless power transfer system for implantable medical devices

Abstract: This article presents a novel approach to improve the efficiency of a radiative near-field wireless power transfer (WPT) system by using metasurface. The WPT system is designed particularly for implantable applications, which operate at the industrial, scientific, and medical (ISM) 2.40-2.48 GHz band. To construct the WPT link, a patch antenna is used as the transmitting element whereas a small planar loop antenna is considered as the receiving element. The receiving antenna is implanted under the skin tissue model operating at 2.45 GHz. The metasurface with a high refractive index is placed above the surface of the skin layer to improve the power transfer efficiency. Regarding the motion of the human body, different misalignment tolerances between the transmitter and the implant receiver are discussed. Also, the effect in the change of the skin property and placement depth of the Rx antenna inside the skin tissue model is studied. Furthermore, the study of specific absorption rate for the proposed configuration is performed. Finally, the proposed WPT system is designed and experimentally verified with and without metasurface. The experimental result confirms a significant amount of improvement in power transfer efficiency due to the integration of metasurface.

Miniature tri-wideband Sierpinski–Minkowski fractals metamaterial perfect absorber

Abstract: With rapidly growing adoption of wireless technologies, requirements for the design of a miniature wideband multi-resonators are increasing. In this study, a compact fractal-based metamaterial structure with lumped resistors is described. The structure of the authors proposed absorber is a combination of Sierpinski curve and Minkowski fractal. The new combination provides larger capacitance and inductance in the system enabling perfect absorption at lower frequencies. The final structure with dimensions of 20 × 20 × 1.6 mm 3 and an air gap of 12.5 mm provides three main resonances at frequencies of 2.1, 5.1, and 12.8 GHz with bandwidth (absorption ratio over 90%) of 840 MHz, 1.05 GHz, and 910 MHz, respectively.

High-power impedance tuner utilising substrate-integrated evanescent-mode cavity technology and external linear actuators

Abstract: A 100 W electronically controlled impedance tuner is introduced and experimentally investigated here. The proposed tuner is composed of two individually controlled coupled evanescent-mode (EVA) cavity resonators implemented on a printed circuit board (PCB) using substrate-integrated waveguide (SIW) technology. The contactless tuning mechanism relies on two high-resolution external linear actuators controlling the gap size of each resonator with submicron accuracy. A 3.3 GHz prototype tuner successfully handled up to 100 W of input power with ∼90% Smith chart coverage at the centre frequency and about 52% bandwidth demonstrating at least 50% coverage. Exhibiting low loss of 0.56 dB and very high stability as well, the proposed impedance tuner is a viable solution for high-power applications where electronic precise tuning is desired.

Compact dual-band rectenna for RF energy harvest based on a tree-like antenna

Abstract: A novel compact dual-band rectenna is presented for efficient radio frequency (RF) energy harvest at 2.45 GHz WiFi and 3.5 GHz WiMAX bands. The proposed rectenna is composed of an optimised tree-like receiving antenna and a compact rectifier based on a two-branch matching network. The tree-like antenna is found to feature a dual-band characteristic, high radiation efficiency, and improved reflection coefficient with the newly designed slotted ground plane. By introducing the matching network which consists of two different radial branches, the proposed antenna is well matched with the rectifying circuit and most part of the received power could be delivered. Besides, in order to eliminate the coupling effects, reduce the energy loss and minimise the overall dimension of the rectenna, three narrow stubs on the ground plane of the antenna are designed to perform as the ground of the rectifier. The results have demonstrated the designed rectenna exhibits relatively stable conversion efficiency for the load value varying from 0.9 to 20 kΩ under low input power and the maximum conversion efficiency is observed to be 60% at 2.45 GHz and 53% at 3.5 GHz under the input power of 0 dBm.

Ultra-wideband printed ridge gap waveguide hybrid directional coupler for millimetre wave applications

Abstract: The evolution of a wireless communication system to the fifth generation is accompanied by a huge improvement in the performance of the communication system through providing high data rate with a better coverage area. Hence, 5G technology requires access to millimetre wavebands where more spectra are available for high-speed applications. This technological progress motivates the research community to develop the essential microwave components, specially couplers, through the use of state of the art guiding structure such as printed ridge gap waveguide (PRGW). In this study, the design of ultra-wideband 3-dB planar quadrature hybrid coupler based on PRGW technology is proposed. A systematic design procedure for the proposed coupler is presented and illustrated. The proposed directional coupler is fabricated and measured, where the measured results show good agreement. The presented coupler covers over 26% bandwidth centred at 30 GHz. The presented coupler has a measured wide impedance bandwidth of a 26.5% centred at 30 GHz based on -10 dB threshold. The phase difference between output ports is 90° ± °5 over 23% relative bandwidth. On the other hand, the total bandwidth is limited by the amplitude imbalance, which has a bandwidth of 13% for± 0.75 dB variation.