Microwave Metamaterial Absorber Research Articles

A Graphite-Based Metamaterial Microwave Absorber

The absorption of conventional metamaterial absorbers (MMAs), originating from the electric or magnetic resonance induced ohmic loss, usually has a narrow bandwidth because of the limited surface resistance of the structured metal layers. In this letter, we experimentally demonstrate a broadband MMA by using graphite instead of copper to construct the surface pattern structure. Compared with metals, the graphite has a low electric conductivity, and its skin depth is far more than those of metals. As a result, the effective thickness of our graphite-based absorber, which determines the thickness-dependent resonance, is very sensitive to the thickness of the structured graphite layer. Due to the relatively large surface resistance and the sensitive tunable thickness-dependent resonance, our design achieves an absorption bandwidth (less than –10 dB) from 12.7 to 18 GHz. Our results may provide an effective way to tune the thickness-dependent resonance and to broaden the absorption bandwidth of MMA.

Flexible ultrawideband microwave metamaterial absorber with multiple perfect absorption peaks based on the split square ring: publisher's note.

This publisher's note amends the author listing in Appl. Opt.57, 10257 (2018)APOPAI0003-693510.1364/AO.57.010257.

High-performance dendritic metamaterial absorber for broadband and near-meter wave radar

Absorbing materials in ultra-high frequency (UHF) band has constantly been a major challenge. The size of the absorber in UHF band is large, whereas the resonant frequency band is narrow. According to Rozanov's theory, two kinds of composite metamaterial absorbers are designed to realize the requirements of low-frequency broadband metamaterial microwave absorber: the magnetic-metamaterial composite absorber1 (MA1) and the dielectric-metamaterial composite absorber 2 (MA2). In the range of approximately 300-1000MHz, both absorbers achieve absorption of over 90% and feature good adaptability to the incident angle of the incident wave. The absorbers also present good absorption rate of over 80% in the range of 0-45 degree. Processing samples of indium tin oxide (ITO) resistance film and polymethacrylimide (PMI) foam board feature simple preparation and low cost, and the most important thing is to consider the weight problem, which features certain advantages in terms of use.

Comment on “Design of a Quad-Band Wide-Angle Microwave Metamaterial Absorber”

In recent days, we read a paper called “Design of a Quad-Band Wide-Angle Microwave Metamaterial Absorber”, which was written by Ren et al. In such report, Ren et al. proposed a quad-band wide-angle absorber based on a metallic hexagonal patch layer. The absorption peaks are located at 6.8 GHz, 8.24 GHz, 11.24 GHz, and 12.7 GHz, whose absorptivity values are 97.2%, 95.2%, 97.7%, and 98.5%, respectively. However, we verified that the results which were given by Ren et al. are wrong because they ignored the polarization conversion. The proposed quad-band metamaterial absorber (QBMA) is not an absorber.

Microwave metamaterial absorber based on aqueous electrolyte solution for X-band application

Metamaterial absorber is investigated by using an aqueous electrolyte solution as a meta-“atom.” For the first time, an aqueous electrolyte solution of Aluminum Chloride (AlCl3) is used as a fluid filler for a periodic rectangular metastructure. Aqueous solutions of varying molarity (0.5, 1.0, 1.5, 2.0, and 2.5M) are encapsulated inside a flexible silicone rubber sheet to develop a meta-“atom” and realized as a resonating microwave metamaterial absorber backed with copper. The complex material permittivities of the electrolyte solutions are measured at an X-band (8.2–12.4 GHz) frequency region. A unit cell is designed and its parameters are optimized in a computer simulation software microwave studio suite to enhance a wide absorption bandwidth with 90% absorption efficiency throughout the X-band region. Additionally, the proposed absorber shows a polarization insensitive absorption along with a wide-incident angle absorption for both TM and TE waves. The resonance and absorption mechanisms are investigated with the help of electric field, magnetic field, current density and penetration depth of microwave into the electrolyte solution. The experimental absorption performance is carried out in an ideal waveguide environment.

Optically transparent and flexible broadband microwave metamaterial absorber with sandwich structure

With the aim to design broadband microwave absorbers with optically transparent, flexible and stable performances in 8–18 GHz, a sandwich structure is designed and fabricated by sandwiching the periodic arrayed ITO film into two transparent and flexible polyvinyl chloride layers. With the induced metamaterial structure to tailor the effective input impedance, the proposed sandwich absorber can realize more than 90% absorption in 8–18 GHz for both TE and TM polarization when the incident angle is less than 30°. Meanwhile, the optical transmittance of the designed absorber reaches more than 80% transmittance with the wavelength larger than 532 nm, and the average optical transmittance for the visible light (400–800 nm) is 80.2%. The proposed absorber shows broadband microwave absorption in both X and Ku band with simultaneously high transmittance in visible frequencies, indicating that the proposed sandwich metamaterial absorber has great potentials for developing optical transparent absorbing devices.

Broadband microwave metamaterial absorber with lumped resistor loading

Narrow absorption bandwidth has been a fundamental drawback hindering many metamaterial absorbers for practical applications. In this paper, by loading lumped resistors, we have successfully designed a microwave metamaterial absorber with multioctave wide absorption bandwidth covering the entire X- and Ku-bands, while keeping the thickness of the absorber less than 1/10 of the working wavelength. The polarization-insensitive absorber shows a good angular stability for both transverse electric (TE) and transverse magnetic (TM) incidences. Prototype has been fabricated and measured to validate the design principle and the simulated results, and good agreements are observed between simulated and measured results. The proposed metamaterial absorber offers an efficient way to realize broadband microwave absorption with stable angular performance, which may find potential uses in many applications, for example, electromagnetic compatibility.

Ultrathin dual-layer triple-band flexible microwave metamaterial absorber for energy harvesting applications

An ultrathin dual-layer flexible metamaterial absorber with triple-band for RF energy harvesting applications has been reported in this article. The sub-wavelength unit cell of the proposed absorber is composed of six distinct concentric annular having outer circumference of ring and octagonal inner circumference. The metallic resonators are constructed from copper foil self-adhesive tape which are affixed on flexible neoprene rubber sheet terminated by metal ground plate. The proposed absorber prototype is ultrathin and compact with the thickness less than 0.037λ0 and cell size less than 0.2λ0 at the lower absorption frequency of 1.75 GHz. Flexible dual-layer absorber exhibits triple absorption peaks of 96.91%, 96.41% and 90.12% at 1.75 GHz, 2.17 GHz and 2.6 GHz with full width at half maximum (FWHM) bandwidth of about ~6.5%. The RF performance of proposed absorber is numerically computed for different polarization and incidence angle variations. The absorption value is above 76% for the oblique incidence angle up to 45° in TE mode operation, whereas the absorption value is 94% for oblique incidence angle up to 60° in TM mode operation. The measured outcomes are in agreement with the numerically calculated results. The energy harvesting potential of the proposed absorber structure is numerically confirmed by the resulting improved RF absorption value in dependence to different resistive loading of the polarization insensitive unit cells.

A Stretchable Metamaterial Absorber With Deformation Compensation Design at Microwave Frequencies

A stretchable microwave metamaterial absorber with specific pattern design is proposed and fabricated in this paper to improve the stability of microwave absorption for deformation. Compensation mechanism is revealed by the electrical current loop for resonance absorption excitation in an equivalent circuit model. The proposed metamaterial absorber with a slotted cross pattern is compared with the conventional cross pattern absorber. A stretchable silver conductive ink validates the silk screen printing of resistive patterns on flexible polydimethylsiloxane substrate and hence realizes the stretching experiments. In a 25% stretch, the resonant frequency of the compensated absorber shifts only 0.5% for TM polarization, comparing to the 8% shifting of the conventional structure.

Flexible ultrawideband microwave metamaterial absorber with multiple perfect absorption peaks based on the split square ring.

In this paper, a flexible ultrawideband metamaterial absorber (MMA) with multiple perfect absorption peaks has been proposed and investigated. In this design, we choose the rubber as the dielectric layer to achieve flexibility and select the split square ring to reach multiple perfect absorbing peaks. For the simulation results, the three-layer absorber that reaches 90% absorptivity has achieved 3.87 to 10.84GHz. Then, we propose a five-layer absorber for easy facilitation, whose absorptivity reaching 90% has achieved 3.78 to 9.85GHz, and the absorption peak has reached 99.99%, 100%, 100%, and 99.99% at 4, 5.82, 8.46, and 9.71GHz, respectively.

Comment on “A novel ultrathin and broadband microwave metamaterial absorber” [J. Appl. Phys. 116, 094504 (2014)

In a recent article, Wang et al. [J. Appl. Phys. 116, 094504 (2014)] proposed an ultra-thin broadand microwave metamaterial absorber and claimed the absorption above 90% in the operating band of 8.85 GHz–14.17 GHz. In this comment, we argue that the authors have neglected the cross-polarized reflection in their calculation of absorption. The proclaimed anisotropic pentagon patch metamaterial is predominantly a cross polarizer for the normally incident electromagnetic wave. We demonstrate that the actual absorption of the metamaterial structure remain less than 40%. Hence, the proposed metamaterial is not a perfect broadband absorber; however, it can be used as an efficient cross-polarizer.

Performance Evaluation of Wide-Angle Ultrathin Microwave Metamaterial Absorber with Polarization Independence

A novel dual-band wide-angle ultra-thin absorber composed of dual closed-concentric ten-point stars with polarization independence is presented in this article. The ten-point stars formed resonators are engraved on low cost FR4 substrate. The EM absorber performance is studied in terms of electrical and physical parameters. The polarization independence is practically demonstrated using waveguide measurement method. The measured results are in good agreement with simulated results. The absorbance values of 99.37% and 97.18% are achieved at 2.13 GHz and 2.52 GHz, respectively. The presented absorber design is wide-angle RF absorber with high absorbance values of about 80% up to 60° under TE mode and 87% up to 75° of oblique incidence angle for TM mode excitations. The experimental results demonstrate the absorption effectiveness of the proposed design to be utilized in RF energy harvester modules for capturing ambient UMTS band signals and in RF absorber applications for S band radar systems. The presented absorber unit cell is ultrathin and compact with the thickness and size less than λ/56 mm and λ/5 mm, respectively, at lowest absorption frequency.

Refractory Metamaterial Microwave Absorber with Strong Absorption Insensitive to Temperature

AbstractThe so far developed microwave absorbers all exhibit dramatically changing absorption with temperature, hindering them from working in a broad temperature range. Here, a refractory metamaterial microwave absorber (rfMMA) with the absorption insensitive to temperature by using refractory ceramics of conductive titanium diboride (TiB2) as unit cells and dielectric alumina (Al2O3) as a spacer is designed and experimentally demonstrated. The as‐proposed rfMMA shows strong microwave absorption in a wide temperature range. A definite physical equivalent circuit model that tightly links the structural and constitutive parameters of the rfMMA to its absorption properties is established. It reveals that with a stable configuration of the as‐proposed rfMMA, the little temperature effect on the permittivity of Al2O3 spacer and the big enough conductivity of TiB2 unit cells ensure an almost constant absorption in a wide temperature range, avoiding the temperature‐sensitive problem of traditional refractory absorbers. Moreover, the excellent chemical stability of TiB2 and Al2O3 at high temperature further ensures the long‐term service of the as‐fabricated rfMMA, as confirmed by experiments. This work provides a route to achieve refractory microwave absorbers with strong absorption at a wide temperature range, and illustrates a bright prospect of the all‐ceramic metamaterial absorbers in high temperature applications.

Microwave Metamaterial Absorber for Non-Destructive Sensing Applications of Grain.

In this work, we propose a metamaterial absorber at microwave frequencies with significant sensitivity and non-destructive sensing capability for grain samples. This absorber is composed of cross-resonators periodically arranged on an ultrathin substrate, a sensing layer filled with grain samples, and a metal ground. The cross-resonator array is fabricated using the printed circuit board process on an FR-4 board. The performance of the proposed metamaterial is demonstrated with both full-wave simulation and measurement results, and the working mechanism is revealed through multi-reflection interference theory. It can serve as a non-contact sensor for food quality control such as adulteration, variety, etc. by detecting shifts in the resonant frequencies. As a direct application, it is shown that the resonant frequency displays a significant blue shift from 7.11 GHz to 7.52 GHz when the mass fraction of stale rice in the mixture of fresh and stale rice is changed from 0% to 100%. In addition, the absorber shows a distinct difference in the resonant absorption frequency for different varieties of grain, which also makes it a candidate for a grain classification sensor. The presented scheme could open up opportunities for microwave metamaterial absorbers to be applied as efficient sensors in the non-destructive evaluation of agricultural and food product quality.

A polarization-independent and broadband microwave metamaterial absorber based on three-dimensional structure

In this paper, we design a polarization-independent and broadband microwave metamaterial absorber (MMA) based on three-dimensional structure. The simulated results show that the proposed absorber has a broad absorptance band from 60.4 to 100.0 GHz with the absorptance efficiency over 90%. The effective medium theory (EMT), electric field, surface current and power loss density distributions are adopted to explain the physical mechanism of the perfect absorptance. In addition, the absorptance differences can be observed between transverse electric (TE) wave and transverse magnetic (TM) wave at oblique incidence. The proposed absorber can be utilized in many applications such as perfect absorbers and radomes.

A wideband wide‐angle ultrathin low profile metamaterial microwave absorber

AbstractIn the recent article, Sood and Tripathi (Microwave Opt Technol Lett., 58: 1131–1135. doi:10.1002/mop.29741) proposed metamaterial absorber structure composed of inclined hexagonal patch printed on grounded dielectric substrate. The reported structure was shown to have wideband absorptivity having 10‐dB bandwidth of 5.02 GHz from 8.81 GHz to 13.83 GHz. In this report, we claim that the reported structure is not a metamaterial absorber. We find that for reported structure peak absorptivity is less than 13%. The low value of absorption in our opinion is caused due to crosspolarization effect rather than absorption.

Theoretical Analysis and Design of Ultrathin Broadband Optically Transparent Microwave Metamaterial Absorbers.

Optically Transparent Microwave Metamaterial Absorber (OTMMA) is of significant use in both civil and military field. In this paper, equivalent circuit model is adopted as springboard to navigate the design of OTMMA. The physical model and absorption mechanisms of ideal lightweight ultrathin OTMMA are comprehensively researched. Both the theoretical value of equivalent resistance and the quantitative relation between the equivalent inductance and equivalent capacitance are derived for design. Frequency-dependent characteristics of theoretical equivalent resistance are also investigated. Based on these theoretical works, an effective and controllable design approach is proposed. To validate the approach, a wideband OTMMA is designed, fabricated, analyzed and tested. The results reveal that high absorption more than 90% can be achieved in the whole 6~18 GHz band. The fabricated OTMMA also has an optical transparency up to 78% at 600 nm and is much thinner and lighter than its counterparts.

Broadband metamaterial absorber on a single-layer ultrathin substrate

In this article, a broadband metamaterial microwave absorber on a low-cost FR-4 Epoxy substrate is proposed. The unit cell of the absorber consists of a staircase shape metallic patch placed on the top of the metal-backed ultrathin dielectric substrate having a thickness of 1.9 mm (0.07 λ0). The absorption of more than 90% is achieved with this proposed low profile single-layer microwave absorber throughout the operation band from 8.86 to 15.5 GHz. The performance is analyzed for different values of incident angle, polarization angle, substrate height, and dielectric constant. The surface current and the power loss density at the top and bottom planes at the two absorption peaks of 9.46 and 13.90 GHz are also analyzed to elaborate the absorption mechanism of the structure. Experimental result closely follows the simulated one. The broadband characteristics of the design with relative absorption bandwidth (RAB) of 54.51% at both TE and TM polarizations of incident wave for a wide incident angles makes it versatile for applications in the X and Ku bands of microwave frequencies. The proposed work is very compact (unit cell size: 0.22 λ0) with ultrathin substrate height (0.07 λ0) and giving RAB performance of 54.51% comparable with that of others. Thus with this single-layer low-cost substrate material a broadband absorber is achieved.

Microwave metamaterial absorber for sensing applications

Physical properties of the proposed microwave metamaterial absorber can be explained by surface current distribution at 6.46 GHz and it is presented in the figure below. This Figure shows the surface current distribution at the absorption frequency of 6.46 GHz for the proposed structure. As it is can be seen from this figure, surface currents are concentrated at the resonators. There are parallel and anti-parallel currents which prove the magnetic response at resonance frequency. A metamaterial absorber (MA) based sensor is designed and analysed for various important applications including pressure, temperature, density, and humidity sensing. Material parameters, as well as equivalent circuit model have been extracted and explained. After obtaining a perfect absorption (PA) at around 6.46 GHz and 7.68 GHz, surface current distributions at resonance points have been explained. Since bandwidth and applicability to different sensor applications are important for metamaterial sensor applications, we have realized distinctive sensor demonstrations for pressure, temperature, moisture content and density and the obtained results have been compared with the current literature. The proposed structure uses the changes on the overall system resonance frequency which is caused by the sensor layer’s dielectric constant that varies depending on the electromagnetic behaviour of the sample placed in. This model can be adapted to be used in sensor applications including industrial, medical and agricultural products.

An ultrathin microwave metamaterial absorber with enhanced bandwidth and angular stability

A very thin metamaterial absorber with enhanced bandwidth and angular stability is designed and its application in radar cross section (RCS) reduction of antenna is discussed in this paper. First, a wideband absorber element consisting of an unconnected square loop and unconnected printed cross on a metal backed FR-4 dielectric is designed. This structure has full width half maximum bandwidth of 0.41 GHz and exhibits polarization independent JPCOcharacteristics. It also shows good absorption characteristics for incident angles up to 40° for TE and TM polarized wave. Further enhancement in bandwidth up to 0.574 GHz is achieved by arranging the structures with overlapping resonances as a 2 × 2 array. This 2 × 2 array is used as the unit cell for the larger array. This structure also exhibits polarization independent characteristics and good angular stability up to 60°. The values of effective permittivity and permeability are retrieved from the reflection and transmission parameters of the structure. These parameters also validates the absorption characteristics of the structure. The performance of the fabricated structure for different polarization and angle of incidence is measured and the results are in good agreement with simulation. Finally, the use of this structure to reduce the RCS of a patch antenna is demonstrated.