Excellent Angular Stability Research Articles

Enhanced broadband perfect absorption in visible and near-infrared regimes

ABSTRACT It is still a challenge to achieve perfect ((near-unity)) absorption across a wide wavelength range with a three-layer structure. This paper presents a metamaterial absorber (MA) based on MXene that can achieve broadband and perfect absorption in the near-infrared and visible regions. The MA with a simple three-layer structure consists of a top Ti3C2T x layer, an intermediate dielectric layer and a bottom gold layer. It can achieve perfect absorption (the absorptivity over 99%) in the wavelength range of 730–900 nm. The electric field distribution reveals that the localized surface plasmon resonance occurring at the interface of Ti3C2T x and SiO2 leads to strong absorption over the wide wavelength range. Furthermore, the MA exhibits excellent angular stability. This work provides a method to construct MAs with broadband perfect absorption in the near-infrared and visible band.

Ultra Miniaturization and Transparency Frequency Selective Surface for Dual Band ISM Shielding

This article presents an ultraminiaturized frequency selective surface (FSS) for shielding 2.4 GHz and 5.8 GHz (ISM) signals. The unit cell size is 0.056λ × 0.056λ (λ is the free space wavelength at the first resonant frequency). It has smaller cell size and good transparency compared to previous ISM band studies, and transparency at 54%. The proposed FSS design consists of a wall structure and a compound square loop with 2.4 GHz and 5.8 GHz resonance frequencies for ISM band coverage. This FSS structure was fabricated on float glass with a dielectric constant of 8 by a photolithographic process. The fabricated FSS structure has excellent angular stability and polarization stability, which is further verified by experimentation. An easy optimization method is proposed to tune the independent resonant frequencies by optimizing the geometries individually. This FSS is interpreted by filtering through surface induced currents and equivalent circuit models. A prototype of the proposed FSS is fabricated and measured. The simulation results are in good agreement with the measured results. The proposed FSS has polarization insensitivity and 80∘ angle stability and is suitable for solving the EMI problem in ISM band.

A Novel Ultra-Miniaturized Angularly Stable Frequency Selective Surface for L-Band Shielding Applications

This paper presents a novel ultraminiaturized frequency selective surface (FSS) for L-band electromagnetic shielding applications. A basic square loop incorporated with folded arms in the top layer constructs the single-layer FSS design. Band-stop characteristics for the entire L-band operating frequency ranging from 1 GHz to 2 GHz are offered by the proposed design. An ultraminiaturized profile of 0.028λο × 0.028λο is achieved by the designed FSS, where λο corresponds to the lowest operating frequency, achieving at least 32.1% miniaturization when compared to the state-of-the-art FSS designs in the literature. Polarization insensitivity with a peak shielding effectiveness of 45.4 dB is manifested by the rotational symmetry of the proposed design. The proposed FSS exhibits excellent angular stability performance up to 80° with a frequency deviation of less than 1%. The equivalent circuit model helps to understand the working principle of the proposed FSS. The experimental results of the designed FSS prototype agree well with the simulation results. Hence, the proposed spatial filter is a suitable candidate for enhancing security in wireless communication at the carrier frequencies of GPS systems.

Receiving and Transmitting Antenna Pairs Based Rasorber With High Angular Stability

Frequency selective rasorbers (FSRs) with out-of-band absorption and in-band transmission performance have been widely reported for stealth radome applications over the past few years. However, developing an FSR with high angular stability in the transmission band remains challenging. Therefore, to improve the transmission angular stability, we propose an innovative architecture in this work for constructing the FSR with independently explorable absorption and transmission paths. First, a band-stop frequency selective surface (FSS) and an absorber are employed in the absorption path to obtain out-of-band absorption. Second, the receiving (RX) and transmitting (TX) antenna pairs connected by the corresponding enclosed transmission lines (TLs) are utilized to comprise the transmission path. Note that a wide-beam RX antenna is employed to capture incident electromagnetic (EM) waves at large angles and achieve in-band transmission with high angular stability. Besides, another second-order FSR with a sharp roll-off transmission filtering response is proposed by using the second-order band-stop FSS as the ground (GND) of the RX antenna. Furthermore, two prototypes of the FSRs (one narrowband and the other second-order) are fabricated and measured as demonstrations of the proposed architecture. Experimental results agree with simulation ones, indicating the two devices exhibit the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$-$</tex-math> </inline-formula> 3-dB transmission bands with excellent angular stability of 60 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^\circ$</tex-math> </inline-formula> , and both transmission windows are located on a wide <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$-$</tex-math> </inline-formula> 10-dB low reflection band.

Highly selective, fractal-configured UWB-FSS for sub-6 GHz 5G, GSM, WLAN, and C band electromagnetic stealth application

AbstractA miniaturized and flexible frequency-selective surface (FSS) has been presented in this article with a unit cell size of 0.049 λc × 0.049 λc where λc is the free space wavelength at the lower cut-off frequency. In order to achieve an ultra-wide (−3 dB) second order pass band of 151.3% with enhanced selectivity factor of 0.887, a cascaded triple layered hybrid resonating structure has been proposed with symmetrical Minkowski island-shaped fractal geometry pair and spiral-shaped middle layer in optimized air gap coupling. Furthermore, 149.8% ultra-wide pass band also ascertains the conformal feature of the proposed structure. In addition to this, the proposed FSS provides the stable angular response for both TE and TM polarization. An equivalent circuit model has been synthesized for accurate frequency response. Finally, a sample prototype has been fabricated to verify the experimental validation. Excellent angular stability under large oblique incident and significant conformal characteristics ensure the compatibility of the proposed structure for electromagnetic stealth in 0.9–1.8 GHz GSM band, 2.10–2.14 GHz wireless medical telemetry band, 2.4–2.5 and 4.9–5.8 GHz WLAN band, 3.4–3.7 and 4.4–4.9 GHz sub-6 GHz 5 G band, and 3.7–4.2 GHz C band.

Design of a Compact Single-Layer Frequency Selective Surface With High Oblique Stability

In this article, a compact single layer band stop frequency selective surface (FSS) with superior resonant stability is presented. The proposed FSS unit comprises of a symmetrically modified square loop. The square loop has been modified in order to increase the perimeter. Moreover, modified square loop is arranged in C4 rotational symmetry to ensure the polarization independence. The designed FSS exhibits highly stable response for the incident angle variation from 0° to 86°. At the resonant frequency of 5.54 GHz, its miniaturized unit cell size is 0.09 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ₀</i> × 0.09 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ₀</i> and the thickness is only 0.007 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ₀</i> . The operating phenomenon of this FSS is explicated by analyzing the equivalent circuit model, electric field and surface current distributions at resonant frequency. A test sample of the designed FSS has been constructed for the validation purpose. The transmission coefficients response of the proposed FSS has been measured and found to be congruent with the simulated responses. The salient features that distinguish the proposed FSS design over others are low complexity, low thickness, compact unit cell and excellent angular stability.

A Miniaturized Wireless Passive Frequency Selective Surface Sensor for High-Temperature Applications

A passive and wireless sensor based on frequency selective surface (FSS) is presented for high-temperature applications. The sensor consists of a bent square loop array deposited in an alumina dielectric substrate. By inserting four symmetrical slots in the square loop structure, the dimension of the FSS unit cell is as compact as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.13 \lambda _{{0}}$ </tex-math></inline-formula> . The results show that the resonant frequency of the sensor decreases from 13.96 to 13.172 GHz with an average sensitivity of 0.81 MHz/°C when the temperature rises from 25 to 1000 °C. It was found that the resonant frequency shift is mainly attributed to the variations in permittivity of the substrate material. The sensor has a stable resonant frequency for both TE and TM polarizations under incident angles with a range of 0°–30°. The proposed sensor, which has the advantages of miniaturization, low profile, high sensitivity, and excellent angular stability, can be potentially applied in various harsh environments.

A Flexible Multifunctional Active Frequency Selective Surface With Angular and Polarization Stability for Curved Surface Conformal

This article demonstrates a miniaturized flexible multifunctional active frequency selective surface (MAFSS) with excellent angular stability and polarization insensitivity. The proposed structure consists of two metal layers printed on a thin, flexible substrate to achieve curved surface conformal capability. The PIN diodes are welded on two metal layers. By independently switching the bias voltage of PIN diodes, four working states and two functions, including electromagnetic (EM) switching and polarization selection, are realized. The unit cell takes itself as a bias feeder to excite the PIN diodes, avoiding the negative impact of the additional feeding network on the overall performance. A spiral wire-based miniaturization technique is utilized to achieve excellent angular stability in the planar and conformal states. On this base, a double-layer symmetrical structure based on the via-connected metal strips is designed to realize polarization insensitivity. The miniaturized unit size can reach <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.082\lambda _{0} \times 0.082\lambda _{0} \times 0.005\lambda _{0}$ </tex-math></inline-formula> at the center resonance frequency. The flexible MAFSS prototype is measured in planar and conformal states. The measured operating bandwidth with a transmission coefficient difference greater than 10 dB is 2.8 GHz (5.2–8 GHz). The measured and simulated results have good consistency, demonstrating the effectiveness of the proposed structure.

Wideband and Tunable Reflective Cross-Polarization Conversion Metasurface for TeraHertz Applications

An ultrathin and wideband reflective cross-polarization converter is presented in this paper. The design consists of a pair of modified dumbbell shaped resonators which are connected together with a conductive strip. The proposed single-layer THz metasurface achieved a high polarization conversion ratio (PCR) of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\ge$</tex-math></inline-formula> 95% in the operating frequency ranging from 1.76–4.26 THz. This wide-bandwidth cross-polarization conversion is obtained by exciting multiple closely spaced resonant frequencies at 1.98 THz, 2.61 THz, 2.95 THz, and 3.93 THz. The working mechanism of the proposed structure is analyzed and the polarizer exhibits an excellent angular stability upto 40 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{0}$</tex-math></inline-formula> of incidence. Further, the proposed cross-polarizer exhibits continuous tunability of PCR over the operating frequencies when photoconductive silicon (Si) is filled in the gap between pair of dumbbell-resonators. The wideband characteristics and the switchable property show that the proposed reflective cross polarizer is an excellent candidate for manipulation and control of polarization in THz communication systems.

Wideband Radar Cross Section Reduction of Microstrip Patch Antenna using Polarization Converter Metasurface

Antenna is the key device of communication on stealth platform. It is the greatest contributor to the overall RCS. So, it is desired to reduce the radar signature of the antenna. In this paper, a novel antenna is designed using polarization converter metasurface. The polarization converter metasurface converts the polarization of the incident wave into its orthogonal component. This PCM structure is loaded around patch antenna to reduce the RCS of the antenna over wide frequency range. With this novel design, the overall RCS reduction bandwidth of 140.57% is obtained for the frequency range of 3.49 GHz – 20 GHz. In addition to this, the 10 dB RCS reduction bandwidth is obtained for the frequency ranges of 8.84 GHz – 10.42 GHz (16.41%) and 16.99 GHz – 17.81 GHz (4.71%). The design shows excellent angular stability for both TE and TM polarization. Discernible Bistatic Radar Cross Section reduction is obtained over a wide frequency range. Simulation and experimental results show that there is no degradation in antenna radiation pattern and other parameters. So, with this novel design the problem of obtaining wideband RCS reduction of a microstrip patch antenna is addressed without degradation in radiation pattern along with excellent angular stability for both the polarization modes.

Tuning metallic stub loaded symmetrical resonator based dual band metamaterial absorber for wave shielding from Wi-Fi frequencies

In this article, a simple metamaterial absorber (MMA) is presented aiming absorption of microwave signals from Wi-Fi frequencies of 2.4 GHz, and 5 GHz. The MMA unit cell constructed on 20 × 20 mm2 FR4 substrate provides maximum absorption of 99.2% and 99.9% at 2.4 GHz and 5 GHz with the flexibility of on-design tuning of maximum absorption frequencies. The tuning metallic stubs connected with the outer ring provide the provision of frequency adjustment from 2.34 GHz to 2.45 GHz, whereas middle ring stubs along with the innermost ring assists for frequency tuning from 4.68 GHz to 5.1 GHz. Due to symmetrical structure, excellent angular stability up to 60° is achieved with near zero cross-polarization effect for both transverse electric (TE) and transverse magnetic (TM) modes of excitation. The measured result of the proposed MMA is analyzed that shows good similarity with the simulated one. Due to its simplistic design within nominal dimension, high absorption, flexibility to frequency tuning, wide angular stability, and no cross-polarization effect, this MMA can be a good candidate for absorber applications in wireless communication systems, especially for wave shielding from Wi-Fi frequencies.

A dual‐stopband FSS using knitted and strong‐coupled structures with excellent angular stability and polarisation insensitivity

In this paper, a novel dual-stopband frequency selective surface (FSS) based on the knitted and strong-coupled structures is proposed. The proposed FSS works stably at frequencies of 2.51 and 5.06 GHz for both TE and TM mode, even when the incident angles vary from 0° to 75°. The element size and thickness of the proposed structure are λ0/16 and λ0/170, respectively, where λ0 represents the free-space wavelength. To clarify the operating principle, a general equivalent circuit model (ECM) is developed. A prototype of the proposed FSS is fabricated and measured. It is demonstrated that the proposed structure has good angular stability, polarisation insensitivity, and independent operation in the dual-stopband. Importantly, the proposed structure has good prospects for application in dual-band electromagnetic shielding in complex environments.

Optical transparent metasurface for dual-band Wi-Fi shielding.

An optical transparent metasurface for dual-band Wi-Fi shielding is presented in this paper. The unit cell of the proposed metasurface is composed of a hexagonal ring and a three-petal oval flower which resonate at 2.4 and 5.5 GHz, respectively. The corresponding equivalent circuit is modelled to better understand the physical phenomena of electromagnetic shielding. Based on transmission line theory and curve fitting technique, a convenient and efficient method for extracting permittivity of substrate is presented. Simulation results show that the proposed metasurface is insensitive to the polarization of incoming wave under normal incidence and offers excellent angular stability. For verifying the design, two prototypes are fabricated using different manufacturing technologies, flexible printed circuit and ink-jet printing of silver nano-particles. The measured results are in good agreement with the simulated ones. The proposed metasurface has potential applications of electromagnetic wave suppression and information security in indoor environments.

Flexible frequency-selective rasorber based on metal-graphene hybrid metamaterial.

In this work, we demonstrate a flexible frequency-selective rasorber (FSR) with great angular stability and conformal superiority using metal-graphene hybrid metamaterial. The metamaterial is fabricated by filling the designed metal patterns with graphene-based ink. It can replace the traditional resistors to introduce loss for the absorption purpose. Therefore, the FSR does not need any lumped components soldered onto its surface, enabling a high flexibility and conformal capability. The metal-graphene FSR is fabricated and measured on flexible substrates that have a 60° angular stability and a 128° conformal angle. The simulations and experiments confirmed that the proposed metal-graphene FSR exhibits excellent angular stability and considerable flexibility, making it ideal for stealth systems.

A 3-D Wide Passband Frequency Selective Surface With Sharp Roll-Off Sidebands and Angular Stability

In this letter, a three-dimensional (3-D) wide passband frequency selective surface (FSS) is proposed. Compared with planar wide passband FSSs, 3-D structures usually present superior roll-off characteristics, especially under a lower frequency. However, 3-D wideband FSSs typically have greater insertion loss in the passband, and the transmission characteristic is sensitive to the angle of incident electromagnetic wave. The proposed FSS, by concurrently applying the design methods of planar FSSs and 3-D FSSs, provides sharp double roll-off sidebands while maintaining excellent angular stability and low insertion loss in a wide passband. The simulation results show that the designed FSS can achieve a passband range from 5.86 to 13.4 GHz with an insertion loss of less than 1 dB, and it can retain a stable transmission characteristic with the incident angle ranging from 0° to 45°. The proposed FSS is fabricated and measured in an anechoic chamber, and the measured results show that the fabricated FSS achieves a −3 dB passband from 5.33 to 14.5 GHz. Under different incident angles ranging from 0° to 45°, the FSS also features the lower and upper roll-off transition bandwidths of 6.4% and 4.7%, respectively.

An Ultraminiaturized Frequency Selective Surface With Angular and Polarization Stability

This letter presents the design and testing of a novel triple-layer frequency selective surface (FSS) with ultraminiaturization for electromagnetic shielding applications. The proposed unit cell is constructed using meandered lines running along the three layers and they are interconnected using metalized vias to increase the electrical length of FSS. The FSS is designed to offer band stop resonance at L-band frequency with a foot-print of 0.022 λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> × 0.022 λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> with a thickness of 0.006 λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> and it provides at least 38% of miniaturization in comparison with similar literature. To validate the transmission performance, a prototype FSS with a periodicity of 6 mm is fabricated. The FSS consists of 2500 elements arranged in a 50 × 50 square lattice. The transmission performance of the proposed FSS is verified using simulation and experiment. The proposed FSS has rotational symmetry and hence it is polarization insensitive. Furthermore, the FSS offers excellent angular stability up to the maximum incidence angle of 60° under both transverse electric and transverse magnetic modes of operation. The proposed FSS can be used for shielding applications in global positioning systems tracking and aircraft surveillance systems.

A Novel Fractal Inspired Iterated Four-Legged Loaded Loop Elements Based 2.5-D Miniaturized Frequency Selective Surface

This letter presents a 2.5-D frequency selective surface (FSS) based on four-legged loaded loop elements of third-order iteration inspired by fractal geometries. This FSS design strives to harness the efficacy of the space-filling property of fractals and an additional degree of freedom associated with the use of metal vias in planar structures to obtain a miniaturized unit cell by enhancing the electrical length altogether. The proposed FSS provides a band-stop response with resonance at 1.056 GHz and demonstrates identical transmission curves for transverse-electric and transverse-magnetic modes of polarization of the incident wave. The unit cell of the proposed FSS achieves a figure-of-merit <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$({{{\rm{\lambda }}_{0}}/{\rm{P}}{\boldsymbol{\ }}}){\rm{\ = \ 29}}$</tex-math></inline-formula> , where <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${{\rm{\lambda }}_{0}}$</tex-math></inline-formula> is the free space resonant wavelength and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</i> is the periodicity exhibited by the unit cell, and offers an excellent angular stability up to 75° for oblique incidence. The measured results advocate the real-time shielding effectiveness of the fabricated prototype.

Miniaturized frequency selective surface with high angular stability

Abstract In this paper, a miniaturized bandstop frequency selective surface (FSS) with high angular stability is presented. Each FSS element consists of four sets each consisting eight octagonal concentric interconnected loops. The four sets are connected with each other through outermost octagonal loop. The unit size is miniaturized to 0.066 λ0 at the resonant frequency of 1.79 GHz. The proposed configuration achieves excellent angular stability (only 0.025 GHz resonant frequency deviation is observed upto 83° oblique angles). The working mechanism of FSS is explained with the help of equivalent circuit model (ECM), electric field distribution, and corresponding surface current distribution. A prototype of the designed bandstop FSS is fabricated to verify the simulated frequency response. The experimental results are consistent with the simulation results. Simple geometry, low profile, high angular stability, and compact cell size are prominent features of the proposed structure.

C-Band and X-Band Switchable Frequency-Selective Surface

This paper presents a highly compact frequency-selective surface (FSS) that has the potential to switch between the X-band (8 GHz–12 GHz) and C-band (4 GHz–8 GHz) for RF shielding applications. The proposed FSS is composed of a square conducting loop with inward-extended arms loaded with curved extensions. The symmetric geometry allows the RF shield to perform equally for transverse electric (TE), transverse magnetic (TM), and 45° polarizations. The unit cell has a dimension of 0.176 λ0 and has excellent angular stability up to 60°. The resonance mechanism was investigated using equivalent circuit models of the shield. The design of the unit element allowed incorporation of PIN diodes between adjacent elements for switching to a lower C-band spectrum at 6.6 GHz. The biasing network is on the bottom layer of the substrate to avoid effects on the shielding performance. A PIN diode configuration for the switching operation was also proposed. In simulations, the PIN diode model was incorporated to observe the switchable operation. Two prototypes were fabricated, and the switchable operation was demonstrated by etching copper strips on one fabricated prototype between adjacent unit cells (in lieu of PIN diodes) as a proof of the design prototypes. Comparisons among the results confirmed that the design offers high angular stability and excellent performance in both bands.

A Novel Miniaturized Strong-Coupled FSS Structure With Excellent Angular Stability

In this article, a novel miniaturized concept named strong-coupled frequency selective surface (SC-FSS) with excellent angular stability is presented. First, a conceptual design of SC-FSS is introduced, where the resonant frequencies are insensitive to almost all incident angles. A corresponding equivalent circuit model is developed to interpret the operating principle and formulate relevant design equations with an error of 0.2%. Furthermore, an ultraminiaturized and ultrathin band-stop structure with a unit cell size of λ 0/35 and thickness of 0.26 mm ( λ0 /578) is designed and fabricated to verify this concept. Analysis and experiment show that the proposed band-stop SC-FSS structure can work stably at 2 GHz for both TE and TM modes, even when the incident angle varies from 0° to 84°. The measurement results are consistent with those obtained by full-wave simulations and the equivalent circuit model, which fully demonstrates that the concept of SC-FSS can be widely used in designing antenna reflectors, electromagnetic interference shielding, and angle-insensitive absorbers.