Four‐band electromagnetic energy harvesting with a dual‐layer metamaterial structure

A four‐band metamaterial harvester for harvesting 0.9 GHz, 1.8 GHz, 2.6 GHz, and 5.8 GHz signals is proposed by a dual‐layer structure. 0.9 GHz and 1.8 GHz bands are harvested by the resistors in front layer, whereas harvesting of 2.6 GHz is achieved by the resistors in second layer. All resistors in front and back layer contribute to harvesting at 5.8 GHz. Numerical calculations are verified by two different full‐wave electromagnetic solvers based on finite‐integration and finite‐element techniques. Power dissipation ratios at 0.9 GHz, 1.8 GHz, 2.6 GHz and 5.8 GHz frequencies concentrated at the resistors are found as 82.3%, 82.8%, 74.6%, and 83.6%, respectively, by the finite‐integration‐based solver. Besides, the finite‐element method‐based solver results in harvesting efficiencies of 79.6%, 93.4%, 73.7%, and 93.8%. The efficiency of the harvester is investigated for different oblique incidences. The proposed metamaterial harvester can be a good candidate for multi‐band absorption and harvesting applications.     

Compact ultrathin conformal metamaterial dual‐band absorber for curved surfaces

A compact, ultrathin conformal metamaterial dual‐band absorber for curved surfaces has been presented in this article. The absorber unit cell composed of circular and split ring resonators which are connected with plus‐shaped structure. The proposed absorber unit cell is compact in size (0.22λ_o × 0.22λ_o) and as well as ultrathin thickness (0.006λ_o), where λ_o is the wavelength at 5.8 GHz. The designed absorber gives two absorption tips at 5.8 and 7.7 GHz with more than 90% absorptivity. The full width at half maximum bandwidths are 220 MHz (5.67‐5.89 GHz) and 250 MHz (7.58‐7.83 GHz). The proposed conformal absorber is sensitive to the polarization angle and has a stable absorptivity over a wide range of incident electromagnetic wave. The parametric analysis and equivalent transmission line model have been investigated. The surface current and electric field distribution also discussed for understanding the absorption mechanism. To analyze the performance of proposed absorber on the curved surfaces, it is wrapped on the different radius of cylindrical surface and measured the absorptivity. Simulated and measured results have good agreement between them.     

Wide band fractal‐based perfect energy absorber and power harvester

In this article, a novel wide band polarization and incident angle independent metamaterial absorber (MA) and energy harvesting applications which operates at C (4GHz‐8 GHz) and X (8GHz‐12 GHz) is proposed. The unit‐cell of the proposed structure based on fractal circle loop. Four lumped resistors are mounted the structure to obtain a broad band absorption characteristics. Resistors increase the absorption characteristic of proposed MA significantly at mentioned frequency ranges. In addition, under favor of the resistors proposed MA can convert absorbed energy from incident wave to appearing power.     

Polarization independent broadband metamaterial absorber for microwave applications

A new polarization independent broadband metamaterial absorber (MA) structure based on split ring resonators which are loaded with lumped elements and via connection lines is proposed. The designed structure shows a perfect absorption between 4 and 16 GHz which is validated by simulation studies. Experimental study is only made for the structure that has no via connections and no the lumped element resistors to show the importance of these entities in the proposed metamaterial structure. Both numerical and experimental study results show that broadband MA property depends on the resistors and via connections on the proposed structure. By having high absorption in a wideband range which is numerically demonstrated, the proposed structure can be used in energy harvesting or wireless power transfer applications with higher efficiencies.     

Step impedance resonator‐based tunable perfect metamaterial absorber with polarization insensitivity for solar cell applications

In this work, a step impedance resonator (SIR)‐based structure is proposed to develop a compact tunable metamaterial (MTM)‐based perfect absorber for solar cell applications. This MTM absorber is able to improve the absorption over a wide range of visible frequency range from 550 to 650 THz. The absorption is high around the frequency 600 THz. The proposed model is designed based on SIR technique to achieve miniaturization. The parametric study of overall size of the proposed MTM absorber analyzed over the frequency range 430‐750 THz. The thickness of dielectric spacer, and top most layer (MTM Structure) illustrates the tunable characteristics of the proposed model. A complete comparative analysis of proposed model with different dielectric spacers like AlGaAs, InAs, GaAs, and AlAs are presented with the help of absorption (S₁₁) and transmission (S₁₂). The proposed model is suitable for high efficiency solar cell energy harvesting applications.     

Optically transparent and single‐band metamaterial absorber based on indium‐tin‐oxide

This article proposes and experimentally demonstrates an optically transparent and polarization‐insensitive metamaterial absorber in the terahertz (THz) frequencies. The absorber is formed by indium‐tin‐oxide (ITO) resistive films, providing efficient absorption with absorptivity of 94.1% at the peak absorption frequency of 120.8 GHz. We systematically investigate the surface current distribution and the power loss analysis, and explain the architecture of the absorber. Moreover, the absorber exhibits unique absorption properties at resonant frequencies, that is, featuring single‐band or dual‐band operation by changing the surface resistance of the ITO patterns. In addition, the experimental demonstration and measurement results are in good agreement with the simulated results. Most importantly, the fabricated absorber exhibits an optical transparency above 70% over the entire visible waveband, thereby enabling a wide range of applications such as optically transparent THz absorbers and detectors.     

A dual‐band efficient circularly polarized rectenna for RF energy harvesting systems

A multilayered circularly polarized (CP), dual‐band, stacked slit‐/slotted‐patch antenna with compact size and with compact rectifier is offered for RF energy harvesting systems. The compact dual‐band CP antenna size is able to achieve by stacking slotted‐circular‐patch (SCP) on the substrate above the tapered‐slit‐octagon patch (TSOP). Dual‐band CP radiation is realized by stacking the SCP on the TSOP and the microstrip feedline with metallic‐via to SCP. Eight‐tapered‐slit with length difference of 6.25% are embedded along the octagonal directions symmetrically on the TSOP from the patch's center and two unequal size circular slots are embedded in diagonal axis onto SCP to produce dual‐orthogonal modes with almost equal magnitude for CP waves. The designed antenna is realized measured gain of greater than 5.2 dBic across the band (0.908‐0.922 GHz) with maximum gain of 5.41 dBic at 0.918 GHz and gain of greater than 6.14 dBic across the band (2.35‐2.50 GHz) with maximum gain of 7.94 dBic at 2.485 GHz. An overall antenna volume is 0.36λ _o × 0.36λ _o × 0.026λ _o (λ _o is free space wavelength at 0.9 GHz). A compact composite right‐/left‐handed (CRLH) based rectifier with dual‐band at 0.9 and 2.45 GHz is designed, prototyped, and measured. The right‐handed (RH) part of the CRLH transmission line (TL) is formed by a microstrip line. The left‐handed (LH) part of the CRLH‐TL is formed by lumped components. The measured RF‐DC conversion efficiency is 43% at 0.9 GHz and 39% at 2.45 GHz with rectifier size of 0.18λ _o × 0.075λ _o × 0.0002λ _o at 0.9 GHz.     

Triple‐band polarization‐insensitive metamaterial absorber with low profile

In this article, a triple‐band metamaterial low‐profile absorber with polarization independence is proposed. The proposed metamaterial unit cell is composed of two modified rings with square patch at corners. In addition, the proposed absorber is consists of 10 × 10 periodic unit cells with size of 100 mm × 100 mm. To explain the mechanism, the electric field, the surface current distribution, and equivalent circuit model are present. The structure exhibiting three absorption peaks of 99.01%, 97.18%, and 99.53% under normal incidence at 8.92‐9.11 GHz, 13.78‐14.05 GHz and 14.92‐15.21 GHz which cover X and Ku‐band, respectively. In addition, the proposed structure is insensitive for the transverse magnetic/transverse electric field polarization of incident waves and also the angle of incidence. Furthermore, the three operating bands of the absorber can be adjusted independently and offers low profile which makes the design suitable for different curved surface applications. The proposed structure is fabricated and experiments are carried out to validate the design principle. Good agreements are observed between the measured and the corresponding simulated results.     

Dual‐band aperture‐coupled rectenna for radio frequency energy harvesting

The article presents a dual‐band aperture‐coupled rectenna for radio frequency (RF) energy harvesting at 2.45 and 5 GHz application. The rectenna consists of a dual‐band π‐shaped slot‐etched aperture‐coupled antenna, designed at the lower substrate of two FR4 substrate layers and a dual‐band rectifier. The proposed antenna design also introduces the harmonic suppression of third‐ and higher order harmonics, ranging from 6 up to 10 GHz from the asymmetrical stubs design at the transmission feedline. The dual‐band rectifier is designed to operate at 2.45 and 5 GHz frequency, successfully achieving high conversion efficiency at 68.83% and 49.90% with the optimum load resistor of value 700 Ω and 1.1 kΩ. The minimum DC voltage of 0.167 and 0.236 V with 0 dBm RF input power can be increased when greater RF power is being applied to it, increasing its flexibility to cater various low‐power applications.     

Broadband transparent metamaterial absorber in wireless communication band based on indium tin oxide film

In this article, a broadband optically transparent metamaterial absorber in wireless communication band is proposed. Indium tin oxide (ITO) film, a kind of resistive transparent thin film, is utilized to construct the absorber unit to realize optical transparency, and increase ohmic loss to broaden the absorption bandwidth. The proposed absorber is fabricated and measured. The measurement results are in good agreement with the simulations. It is demonstrated that the absorption rate of the proposed absorber is higher than 90% from 1.67 to 3.8 GHz. The full width at half maximum bandwidth is 5.11 GHz from 0.89 to 6 GHz. In addition, the absorption response of the absorber remains stable regardless of polarization and incidence angle of the electromagnetic waves.     

Asymmetric dual‐band linear‐to‐circular converter by bi‐layered chiral metamaterial

In this article, we present a dual‐band linear‐to‐circular transparent converter by bi‐layered chiral metamaterial (CMM) with an inverted “G” array in microwave region. The proposed metasurface consists of three layers which are the upper layer of the metasurface with a periodic regular metallic inverted “G” and wire array, the dielectric layer, and the bottom layer operating as chiral symmetric structure of the upper. The simulation results show that the transmitted right‐circular polarized wave with the axial ratio of 3 dB or less is in the range of 8.6‐10.9 GHz and the left‐circular polarized wave is within 18.1‐22.5 GHz when y‐polarized forward wave is normally incident. Specifically, the polarization conversion transmission can be maintained at over 85% at angle of incidence up to 40°. Therefore, the proposed CMM device is useful for the development of the integrated polarization manipulation devices.     

A novel meander line integrated E‐shaped rectenna for energy harvesting applications

In this article, design of a novel meander integrated E‐shaped rectenna is presented. The designed rectenna operates at ISM frequency range from 2.2 to 2.5 GHz with acceptable reflection coefficients, gain and VSWR values. The designed rectenna is simulated using HFSS 15 (High Frequency Electromagnetic Field Simulation) and FR4 epoxy material is used in rectenna design for low cost having dielectric constant of 4.4 and thickness of 1.6 mm. In the rectifying stage full wave voltage doubler circuit is designed for DC power generation with SMS7630 Schottky diode and lumped circuit elements. The impedance matching circuit between the antenna and the rectifier is designed and simulated using advanced design system (ADS) software for efficient power transmission from the antenna to the load. The simulation and measurement results with different load and input power levels prove that the designed and implemented system can be used for low power energy harvesting applications in order to feed electronic components and battery free sensor networks.     

CPW fed miniaturized dual‐band short‐ended metamaterial antenna using modified split‐ring resonator for wireless application

A miniaturized dual‐band metamaterial (MTM) antenna has been designed in this article. The designed coplanar waveguide fed antenna has composed of inner split‐ring resonator and an outer open ring resonator with rectangular stub. The series parameter of the antenna is used to determine the zeroth order resonance frequency due to short‐ended boundary condition. The whole size of proposed structure is 20 × 25.5 mm². This MTM antenna exhibits dual‐band operation at 3.17 GHz (3.1–3.22 GHz) and 5.39 GHz (5.27–5.47 GHz). The proposed MTM structure achieves measured peak gain of 0.71 and 1.89 dB at 3.17 and 5.39 GHz, respectively. The proposed antenna can be used for recent radio communication in form of S‐band application and Wi‐MAX.     

A modified Hilbert fractal resonator based rectenna design for GSM900 band RF energy harvesting applications

This article proposes a novel rectenna design based on modified Hilbert fractal shaped microstrip antenna and Villard voltage doubler rectifying circuit for RF energy harvesting applications operating at Global System for Mobile Communications (GSM) 900 MHz band. The energy harvesting antenna is numerically optimized and fabricated on a Rogers RO4003 substrate of thickness 1.52 mm with a compact overall physical sizes of 80 mm × 82 mm (λ₀/4.16 × λ₀/4.06). Various geometric parameters and circuit component values of the proposed energy harvesting system are optimized in order to achieve a matched input impedance with good radiation performance of high gain for the input power level less than ?20 dBm. The numerical and experimental results point out the technical potential of the proposed rectenna design to be utilized in DC power supply modules of low voltage, low power electronic devices.     

A compact dual‐band 2 × 1 metamaterial inspired mimo antenna system with high port isolation for LTE and WiMax applications

This article proposes a compact (43 × 26 × 0.8 mm³) dual‐band two‐element metamaterial‐inspired MIMO antenna system with high port isolation for LTE and WiMAX applications. In this structure, each antenna element consists of a square–ring slot radiator encircling a complementary split ring resonator. A tapered impedance transformer line feeds these radiating apertures and shows good impedance matching. A 2 × 3 array of two‐turn Complementary Spiral Resonator structure between the two antenna elements provides high dual‐band isolation between them. The fabricated prototype system shows two bands 2.34 – 2.47 GHz (suitable for LTE 2300) and 3.35 – 3.64 GHz (suitable for WiMAX). For spacing between two antennas of 10 mm only, the measured isolation between the two antenna elements in the lower band is around ?32 dB while that in the upper band is nearly 18 dB. The system shows a doughnut‐shaped radiation patterns. The peak measured antenna gains for the proposed MIMO system in the lower and higher bands are 3.9 and 4.2 dBi, respectively. The MIMO system figure of merits such as the envelope correlation coefficient, total efficiency are also calculated and shown to provide good diversity performance.     

Design of dual‐band microstrip patch antenna with right‐angle triangular aperture slot for energy transfer application

This work focusing on the dual‐band antenna design with rectifying circuit for energy transfer system technology for enhancement gain performance. The air gap technique is applied on this microstrip antenna design work to enhance the antenna gain. The work begins with designing and analyzing the antenna via the CST Microwave Studio software. After validation on acceptable performance in simulation side is obtained, the return loss, S₁₁ of the antenna is measured using vector network analyzer equipment. The rectifier circuit is used to convert the captured signal to DC voltage. This projected dual‐band antenna has successfully accomplished the target on return loss of ?44.707 dB and ?32.163 dB at dual resonant frequencies for 1.8 GHz and 2.4 GHz, respectively. This proposed antenna design benefits in low cost fabrication and has achieved high gain of 6.31 dBi and 7.82 dBi for dual‐band functioning frequencies.     

Design and fabrication of a new high gain multilayer negative refractive index metamaterial antenna for X‐band applications

This article presents the design of a planar high gain and wideband antenna using a negative refractive index multilayer superstrate in the X‐band. This meta‐antenna is composed of a four‐layer superstrate placed on a conventional patch antenna. The structure resonates at a frequency of 9.4 GHz. Each layer of the metamaterial superstrate consists of a 7 × 7 array of electric‐field‐coupled resonators, with a negative refractive index of 8.66 to 11.83 GHz. The number of layers and the separation of superstrate layers are simulated and optimized. This metamaterial lens has significantly increased the gain of the patch antenna to 17.1 dBi. Measurements and simulation results proved about 10 dB improvement of the gain.     

Compact circularly polarized beam‐switching wireless power transfer system for ambient energy harvesting applications

In this work, we propose a circularly polarized (CP) beam‐switching wireless power transfer system for ambient energy harvesting applications operating at 2.4 GHz. Beam‐switching is achieved using a low profile, electrically small CP antenna array with four elements and a novel miniaturized 4× 4 butler matrix. The CP antenna is designed with an e‐shaped slot and four antennas. The CP antenna measures 0.32 λ₀× 0.32 λ₀× 0.006 λ₀ at 2.4 GHz. The antenna has a gain of 3 dBic and an axial ratio less than 3‐dB at 2.4 GHz. A linear antenna array consisting of four elements is designed with the CP antenna element with an inter‐element distance of 0.29 λ₀ . A 4× 4 butler matrix with miniaturized couplers and crossovers are used to feed the four antenna array elements. Based on the input port of excitation, the main beam of the antenna array is demonstrated to be switched to four directions: ?5°, 65°, ?55°, and 20°. A CP rectenna is used to demonstrate the wireless power transfer capability of the combination of the butler matrix and the CP‐antenna array. The rectenna consists of a Teo‐shaped CP antenna and a rectifier. The open circuit voltage at the output of the rectenna is found to peak value of 30 mV at ?3°, 61°, ?53°, and 17°. Thus a complete system for CP wireless power transfer including the power transmission system as well as the RF energy harvesting sensor is designed and experimentally verified.     

A wideband rectifier array on dual‐polarized differential‐feed fractal slotted ground antenna for RF energy harvesting

This article reports a novel wideband rectenna for RF energy harvesting applications. A wideband fractal slotted ground antenna (SGA) is adopted. The operating frequency bands of the antenna are GSM, UMTS, Wi‐Fi, and LTE2600/4G bands. The antenna is fed by a dual‐polarized and differential‐feed (DP‐DF) microstrip lines disposed with an angle of 90° each relative to the other. The feed lines are etched on the bottom side of the substrate and connected to an array of four wideband RF‐to‐DC rectifiers. A nonuniform transmission lines filter ensures wideband behavior for each rectifier. The rectenna performances are simulated and measured. The experiments show an output DC voltage of 1 V at a power density of 26.6 μW/cm² over the frequency band of operation with a peak efficiency of 50%. The proposed rectenna is suitable for energy harvesting applications in urban environments.     

A metamaterial hepta‐band antenna for wireless applications with specific absorption rate reduction

Present article embodies the design and analysis of slotted circular shape metamaterial loaded multiband antenna for wireless applications with declination of SAR. The electrical dimension is 0.260 λ × 0.253 λ × 0.0059 λ (35 × 34 × 0.8 mm³) of proposed design, at lower frequency of 2.23 GHz. The antenna consists of circular shape rectangular slot as the radiation element loaded with metamaterial split ring resonator (SRR) and two parallel rectangular stubs, etched rectangular single complementary split‐ring resonator (CSRR) and reclined T‐shaped slot as ground plane. Antenna achieves hepta bands for wireless standards WLAN (2.4/5.0/5.8 GHz), WiMAX (3.5 GHz), radio frequency identification (RFID) services (3.0 GHz), Upper X band (11.8 GHz—for space communication) and Lower K_U band (13.1 GHz—for satellite communication systems operating band). Stable radiation patterns are observed for the operating bands with low cross polarization. The SRR is responsible for creating an additional resonating mode for wireless application as well as provide the declination in SAR about 13.3%. Experimental characteristic of antenna shows close agreement with those obtained by simulation of the proposed antenna.