Flexible Frequency Selective Surface Research Articles

3D-printed polarization-independent low-cost flexible frequency selective surface based dual-band microwave absorber

Abstract A 3D-printed polarization-independent low-cost lightweight and flexible frequency selective surface based dual-band microwave absorber is presented in this paper. Two concentric square loops fabricated at different heights using 3D printing technology are responsible for exhibiting dual-band responses at 3.32 GHz (S-band) and 5.46 GHz (C-band) with more than 97% absorptivities. The corresponding full widths at half maximum bandwidths are observed as 230 MHz (3.21–3.44 GHz) and 450 MHz (5.27–5.72 GHz). The proposed topology is polarization-insensitive owing to the four-fold symmetry. The absorption phenomenon is explained with the analysis of current distributions at the surface and impedance curves at the frequencies of resonance. Further, the performance has been evaluated for both planar and curved surfaces with different angles of curvature, and the good agreement between the measured and simulated responses confirms the flexible behavior of the proposed structure.

Design, simulation and manufacture of a flexible frequency selective surface based on aramid/carbon fiber woven fabric

Frequency selective surfaces (FSSs) are significant for the efficient and accurate transmission of microwave signals due to its ability to selectively shield electromagnetic (EM) waves of different frequencies. In some special scenarios, mechanical properties are vitally required. Herein, a flexible frequency selective fabric (FSF) is proposed. Different from traditional FSSs or reported FSFs, aramid/carbon fiber woven fabric serve as the flexible substrate in which carbon fiber provides functionality instead of metallic materials, and patches made of carbon fiber prepreg are periodically applied for property reinforcement. Equivalent circuit model is used to guide the basic determination of patch’s geometry, and it is further optimized via full-wave simulation software HFSS. A simulation model that reasonably reflect the correlation between structure and frequency-selection characteristic is provided, and structural factors affecting the characteristic are analyzed and discussed. Further, an FSF sample is prepared and its frequency response is measured. Measurement revealed the FSF selectively shields EM waves in the frequency range of 8.9 GHz to 11.4 GHz, and is of the bandpass-bandstop-bandpass characteristic. Benefit from EM property and flexibility, the proposed FSF has advantages in applications pursue lightweight, high strength, and require excellent EM functionality, such as aerospace and structure-EM function integrated products.

Monopole antenna design with flexible frequency selective surface

Monopole antenna design with flexible frequency selective surface

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.

Low specific absorption rate quad-port multiple-input-multiple-output limber antenna integrated with flexible frequency selective surface for WBAN applications

This paper presents the design and analysis of a multiple-input-multiple-output (MIMO) textile antenna for wireless body area network (WBAN) applications. The MIMO antenna is comprised of four identical modified rhombus-shaped monopole antenna elements of size of 0.57, where is the wavelength calculated at the lowest operating frequency. The antenna is backed by a 6 frequency selective surface (FSS) of dimensions of 0.84 to improve gain and to reduce specific absorption rate (SAR). The antenna has an impedance bandwidth (S 11 ⩽ −10 dB) of 8.8 GHz (2.8–11.6 GHz) and isolation of >19 dB between the resonating elements. In order to assess the MIMO antenna’s flexibility, the bending analysis is performed for various bending radii. The obtained diversity metrics are: envelope correlation coefficient <0.5 dB, diversity gain <10 dB, channel capacity loss <0.4 bits s−1 Hz−1, and total active reflection coefficient <−10 dB. The performance of the antenna with and without FSS is investigated for gain enhancement and SAR reduction. With the help of FSS, the antenna gain is increased to 8.44 dBi, and the SAR reduced from 6.99 Watt kg−1 to 0.0273 Watt kg−1. The FSS achieves the highest efficiency of 96%. The designed antenna is suitable for smart textile applications due to its low SAR, high gain, and wider impedance bandwidth.

Design of miniaturize flexible wideband frequency selective surface for electromagnetic shielding application

In this paper, a flexible, ultra-thin, miniaturized broadband frequency selective surface (FSS) is demonstrated for wideband electromagnetic shielding application. The unit cell of the proposed FSS structure consists of cross dipoles loaded with the metallic circular strip. The proposed miniaturized FSS structure has an extremely low profile with a thickness of and a unit cell size of . This FSS structure provides wideband shielding of more than 20 dB from 1.1 to 2.6 GHz frequency region. Besides this wide stopband, the FSS structure acts as a transparent window in the frequency band which can be used for the 5G communication purpose. Moreover, by comprehending the interaction between the incident electromagnetic wave and unit cell, a conceptual equivalent circuit model is developed that helps to interpret the mechanism of the miniaturization as well as the broadband response of FSS structure. We also compared the proposed FSS with other recently reported structures and observed more than 55% improvement in size reduction with several other miniaturized flexible FSS structure. Furthermore, this FSS structure exhibits 20 dB attenuation over a wide frequency region with a fractional bandwidth of 111%. Moreover, excellent resonant stability is achieved in regards to different polarization and incident angle up to .

3D Flexible Frequency Selective Surface with Stable Electromagnetic Transmission Properties (Adv. Mater. Technol. 7/2022)

3D Frequency Selective Surfaces In article number 2101316, Yuhang Li, Taisong Pan, and co-workers present a 3D flexible frequency selective surface fabricated by releasing pre-strains in the flexible substrate and thus buckling the 2D Jerusalem metal precursors locally bonded on it, capable of being conformally adhered to complex surfaces, which can maintain stable frequency-selective properties as well as transmission performance when subjected to biaxial stretching strains.

Polarization-insensitive ultrathin fractal shaped frequency selective surface for ultra wide band shielding

An ultrathin and flexible Frequency Selective Surface (FSS) for ultra-wideband electromagnetic interference (EMI) shielding has been proposed in this article. The novelty of the suggested design lies in its capacitive coupled Greek cross-shaped fractal structure that is printed on a single and flexible layer dielectric substrate having an extremely thin thickness of 0.0013λ 0 , where λ 0 refers to the free space wavelength of the lower cutoff frequency. The introduction of capacitive coupling approach between the modified Greek cross fractal metallic structure and four symmetrical triangularly slotted conventional right-angled triangular metallic geometry results in an excellent ultra-wide stopband (UWSB) of more than 154% (2.07–15.76 GHz) and 105% (4.38–14.05) for < −10 dB and < −20 dB fractional bandwidth, respectively. The conformal configuration of the proposed geometry also exhibits a significantly better bandwidth more than 148.89% (−10 dB) and 109.5% (−20 dB) for UWSB. To accomplish the physical interpretation of the proposed FSS, the equivalent circuit model optimization has been illustrated also. Furthermore, the proposed structure demonstrates strong polarization independence and angular stability under both TE and TM polarization. Finally, the ultra-wideband shielding characteristics of both the planer and conformal designs have been confirmed experimentally in a free space measuring system.

3D Flexible Frequency Selective Surface with Stable Electromagnetic Transmission Properties

AbstractFrequency selective surfaces (FSS) that can be tightly laminated to complex nondevelopable surfaces have a wide range of applications in many engineering areas. This paper presents a 3D flexible FSS prepared by mechanically guided 3D assembly, capable of being conformally adhered to complex surfaces, while maintaining stable frequency selective properties as well as transmission performance. The stretching strain applied to the prestrained ecoflex substrate can precisely tune and control the buckled 3D metal structures, which leads to the stretchability of the flexible FSS, introduces the increase of the inductance inside the metal cells, and expands the metal gap resulting in a reduction of the capacitance between the unit cells. The mutual correlation of capacitance and inductance ensures in the mechanism that the FSS transmission characteristics will not be significantly affected by the applied strain, as verified by calculations using the equivalent circuit method. Both the results obtained by the experiments and simulations indicate that the resonant frequency of the 3D flexible FSS is 5.7 GHz and the bandwidth at −10 dB is 0.94 GHz, while the resonant frequency shift will not exceed 0.45 Hz, and −10 dB bandwidth change will not exceed 0.32 GHz with applied strain of 17.8% in the ecoflex substrate. At the same time, the 3D flexible FSS maintains stable transmission performance for oblique incidence, with only 0.5 GHz frequency shift at 45° incidence. This stretchable flexible FSS with stable electromagnetic (EM) properties may find potential applications in EM shielding and spatial filtering due to its high flexibility, ready applicability, and cost‐effectiveness.

Single-Layered Flexible Dual Transmissive Rasorbers With Dual/Triple Absorption Bands for Conformal Applications

This article presents flexible frequency selective surface (FSS) based rasorbers exhibiting dual/triple absorption bands along with dual-band transmission. The proposed rasorbers are designed by combining the individual designs of dual-/triple-band resonant absorber and dual bandpass FSS on the two sides of a flexible substrate. At the absorption frequencies, the bandpass FSS acts as reflective metallic ground. The proposed FSS based rasorbers are flexible, single-layer structures having a thickness of $0.005\lambda _{0}$ at the lowest absorption frequency and can serve as suitable candidates for the much practical conformal applications. The working of the proposed rasorbers is analyzed using an equivalent circuit model. Further, an optimum range at each absorption frequency is determined by studying the parametric effects on the proposed design. Prototypes consisting of $17\times25$ unit cells are fabricated and experimental validation is achieved. The performance of the rasorbers is also studied by bending their fabricated prototypes with 120°, 150°, and 180° degrees of curvatures.

Design and fabrication of a reconfigurable and flexible frequency selective surface with a buckling dipole via mechanical stretching

Design and fabrication of a reconfigurable and flexible frequency selective surface with a buckling dipole via mechanical stretching

A miniaturized flexible frequency selective surface for dual band response

AbstractIn this paper, a novel miniaturized and flexible dual band frequency selective surface (FSS) is presented. This FSS provides effective shielding in X-band and Ku- band, with a frequency response of 9.4 and 16.7 GHz, respectively. The proposed FSS provides 924 MHz bandwidth at X-band and 1.34 GHz bandwidth at Ku-band with an insertion loss of 20 dB. Moreover, the proposed design is polarization-independent and it provides stable frequency response at various angles of incidences for both transverse electric and transverse magnetic modes. More significantly, the proposed FSS analyzed the bandstop response of the selective frequency and also is suitable for conformal applications. A prototype of the proposed FSS is fabricated. The measured results and simulated results are good in agreement.

Flexible and Reconfigurable Frequency Selective Surface With Wide Angular Stability Fabricated With Additive Manufacturing Procedure

In this letter, a flexible and frequency-reconfigurable bandstop frequency selective surface (FSS) is studied theoretically and experimentally. The FSS patterns are fabricated on an ultrathin flexible substrate through screen printing, a low-cost, high-efficiency additive manufacturing methodology. With the embedded varactors biased from 0 to 20 V, resonant frequency tuning from 3.5 to 5.7 GHz was measured. Outstanding wide angular stability under different bias voltages has also been examined in theory and measurement by changing the incidence angle from 0° to 60°. As a demonstration of the flexible FSS in the conformal application, the FSS is tightly covered on a half-cylinder with a diameter of 10 cm, which agrees well with the case when placed on a flat surface. Furthermore, the unit cell has a size of λ/15 at 3.5 GHz, featured with miniaturization. The mechanical flexibility, frequency reconfigurability, and angular stability make it meaningful in applications of flexible/conformal covering for aiming objects, including electromagnetic scattering control, electromagnetic compatibility (EMC), etc.

Flexible Serpentinelike Frequency Selective Surface for Conformal Applications With Stable Frequency Response

In this letter, a flexible stopband frequency selective surface (FSS) with serpentine stripe unit cell is presented. Compared with the FSS with straight stripe and zigzag stripe unit cell, the proposed FSS on planar thin polyimide (PI) substrate shows very small offset of resonant frequency with electromagnetic waves having incident angles from 0 $^\circ$ to 60 $^\circ$ , according to the simulation results. The stable transmission performance of flexible FSS with serpentine stripe unit cell on PI substrate is further demonstrated by the computational and experimental results. When the FSS with serpentine stripe unit cell is bent with bending angles from $ \pi /3$ to $ \pi$ , the offset of resonant frequency is negligible, whereas the resonant frequency of FSS with straight stripe and zigzag stripe unit cell shifts about 15.4% and 5.8% when bending angle is $ \pi$ , respectively.

Flexible Convoluted Ring Shaped FSS for X-Band Screening Application

A flexible and low profile frequency selective surface (FSS) with bandstop behavior for $X$ -band signals is demonstrated in this paper to support the demand of the flexible wireless communication devices in the today’s market. The resonating element of the FSS is realized using the convoluted ring loop element that is inspired by the conventional ring loop element. An additional of four stubs are added at each 90° of the conventional ring loop to miniaturize its overall dimension. The convoluted ring loop FSS has reduced the overall dimension of the conventional ring shape FSS to 0.25 $\lambda _{0}$ with a reduction of 33%. The proposed FSS is developing by utilizing the flexible polyethylene terephthalate substrate with the measured thickness of 0.125 mm and dielectric constant of 2.7. From the simulated transmission response of the FSS, it shows that the proposed convoluted ring loop FSS with planar feature manages to provide a signal attenuation up to 37 dB at 10 GHz for both normal and oblique angle of incidence up to 45° at TE and TM polarization. For validation purpose, the prototype of the FSS is manufactured using the inkjet printing technique. The FSS is bent in a different manner, such as bending the array FSS with different radius and bending the unit cell convoluted ring FSS row by row, to examine the bending effect toward the transmission response performance. All the simulated results are compared with the measured results, and these results are in concurrence with each other. By having the conformal features, the employability of the FSS is enhanced whereby the proposed flexible FSS can be employed as the sub-reflector for wearable antenna and conformal shields for electromagnetic compatibility applications.

Flexible and Portable Textile-Reflectarray Backed by Frequency Selective Surface

A center fed C-band portable and rollable textile-reflectarray (TRA) using frequency selective surface (FSS) is presented. The radiating elements are embroidered on the textile samples using conductive thread. The TRA is made of $15 \times 15$ elements ( $8.25\times 8.25\lambda ^{2}$ at 5.8 GHz) for broadside radiations. The embroidered flexible FSS aims to replace the solid ground plane. The element size variation backed by the FSS provides a $\text{330}^{\circ }$ phase variation, which is adequate to design the TRA. Wideband multilayer stacked circular patches are utilized as a feed for the TRA. The antenna provides a measured 7.3% of 0.5 dB gain bandwidth, $-$ 15 dB sidelobe level, $-$ 25 dB cross polarization, and 29% maximum aperture efficiency.

Fully Conformal Square-Patch Frequency-Selective Surface Toward Wearable Electromagnetic Shielding

This letter reports on a fully conformal square-patch frequency-selective surface (FSS) consisting of liquid metal alloy embedded within a flexible silicone elastomer. The flexible and elastic nature of the FSS provides high conformity to doubly curved surfaces. The transmission properties of the FSS on both singly and doubly curved surfaces are studied. This FSS can facilitate electromagnetic interference reduction in the X-band frequency range when wrapped on cylindrical or spherical radomes over dipole-like antennas, thus surpassing many existing flexible FSS technologies that usually allow conformable contact only with singly curved surfaces.

A dual-band flexible frequency selective surface with miniaturized elements and maximally flat (Butterworth) response

A dual-band flexible frequency selective surface (FSS) with miniaturized elements and maximally flat (Butterworth) response is presented in this paper. It is composed of three metallic layers, which are fabricated on thin flexible polyimide substrates and bonded together using thin bonding films. The overall thickness of the proposed structure is only about 0.3 mm, making it an attractive choice for conformal FSS applications. All the three layers can constitute a miniaturized-element FSS (MEFSS) and produce the first pass-band with miniaturization property, while the up and bottom layers can constitute a symmetric biplanar FSS and produce the second pass-band with maximally flat (Butterworth) response. The two pass-bands are independent and there is a wide band spacing up to 30 GHz between them. The principles of operation, the simulated results by using the vector modal matching method, and the experimental values of the fabricated prototype are also presented and discussed.