Lossy Frequency Selective Surface Research Articles

Polarization-insensitive graphene-based band-notched frequency selective absorber at terahertz

This paper introduces a new polarization-insensitive graphene-based frequency selective absorber (FSA) with a reflective notch designed for terahertz applications. The proposed structure features two absorption bands on either side of a central reflection band. The design composes a lossy frequency selective surface (FSS), a bandstop FSS with a metal backing, and an air spacer between. A wideband absorber structure is developed in the first step, leveraging graphene as an absorbent material in the lossy layer to achieve wideband absorptive characteristics. Subsequently, a reflection band is introduced by integrating a bandstop, lossless FSS layer into the absorber structure. The overall structure demonstrates two distinct absorption bands, characterized by absorptivity exceeding 80% within the frequency ranges of 0.30 to 0.57 and 0.67 to 0.90 THz. Simultaneously, a reflection notch is achieved at 0.60 THz. Extensive simulations assessed the performance of the designed FSA. The proposed structure exhibits stability under oblique incidence up to 40 deg and allows tunable absorption specifications by adjusting the chemical potential of graphene. It is noteworthy that the FSA reflector offers advantages such as eliminating the need for complicated, high-cost 3-D structures and welding of the lumped resistors.

An Angle-Stable Ultra-Wideband Single-Layer Frequency Selective Surface Absorber

An ultra-wideband polarization-insensitive frequency selective surface (FSS) absorber is proposed for S to K-band applications. The absorber comprises two compensation slabs, a lossy FSS layer and a grounded dielectric plate. The FSS unit cell is a combination of a second-order Chinese knot and a cross. To enhance the bandwidth and angular stability of the single-layer FSS absorber, a compensation layer composed of FR4 and polymethyl methacrylate (PMMA) slabs is incorporated. The proposed FSS absorber demonstrates a remarkable absorption rate of over 90% within the frequency range of 3.1–22.1 GHz, exhibiting a fractional bandwidth of 150.8%. Even when subjected to an oblique incidence of 45°, the absorber maintains an 80% absorption rate in the frequency range of 4.4–19.1 GHz for both TE and TM polarizations. The total thickness of the FSS absorber is 0.0848 λL (the wavelength at the lowest cutoff frequency), and only 1.08 times the Rozanov limit. To validate the design, a prototype of the proposed absorber was fabricated and measured. Good agreements were observed between the simulations and measurements.

Wide-angle, dual-polarized frequency selective rasorber based on the electric field coupled resonator using characteristic mode analysis

A wide-angle, dual-polarized frequency selective rasorber (FSR) with two absorption bands located at both sides of a passband is proposed. The structure comprises a lossy frequency selective surface (FSS), a bandpass FSS, and an air spacer located in between. A modified electric field coupled (ELC) resonator is used as a parallel resonance at the lossy layer to achieve a passband within the absorption band. The characteristic mode theory is utilized to investigate the absorption behavior of the lossy layer. Extensive simulations were carried out to assess the performance of the presented structure. Under the normal incidence, the proposed structure provides an operating bandwidth (|S11| < −10 dB) from 1.94 to 7.16 GHz, corresponding to a fractional bandwidth (FBW) of 114.7%. The achieved passband is around 4.3 GHz with a minimum insertion loss of 0.81 dB. The absorption bands with an absorption rate higher than 80% are 1.81–3.69 GHz (FBW of 68.4%) in the lower band and 4.95–7.43 GHz (FBW of 40%) in the upper band, respectively. It exhibits quite stable characteristics up to 50° angle of incidence. Furthermore, a prototype was fabricated and measured, which confirms that a good agreement exists between the experimental and simulation results. The proposed FSR is a suitable candidate for lowering the radar cross section (RCS) of the communication equipment or making them stealthy.

Design of a Frequency Selective Rasorber Based on a Band-Patterned Octagonal Ring

In this study, a dual-polarization and low-profile frequency-selective rasorber (FSR) constructed from a novel band-patterned octagonal ring and dipole slot-type elements is investigated. We show the process of designing from a full octagonal ring to realize a lossy frequency selective surface of our proposed FSR, and it has a passband with low insertion loss between the two absorptive bands. An equivalent circuit for our designed FSR is modeled to explain the introduction of the parallel resonance. Surface current, electric energy, and magnetic energy of the FSR are further investigated to illustrate the working mechanism. Simulated results indicate that S11 < -10 dB bandwidth within 5.2-14.8 GHz, S21 > -3 dB passband within 9.62-11.72 GHz, lower absorptive bandwidth within 5.02-8.80 GHz, and upper absorptive bandwidth within 12.94-14.89 GHz are obtained under normal incidence. Meanwhile, our proposed FSR possesses the properties of dual-polarization and angular stability. To verify the simulated results, a sample with thickness of 0.097 λL is manufactured, and the results are experimentally verified.

A Miniaturized Frequency-Selective Rasorber With Absorption Bands on Two Sides of Passband for Antenna Dome

A miniaturized frequency-selective rasorber (FSR) with absorption bands on two sides of the passband is proposed in this letter. The FSR consists of one lossy frequency-selective surface (FSS) layer and one lossless FSS layer, and the equivalent circuit model of FSR is also analyzed. The lossy layer is composed of four electric-field-coupled (ELC) resonators loaded with four resistors, and the lossless layer comprises a square slot structure. By embedding the ELC resonators into the square ring, a passband can be added betweven the absorption bands, and the size of the overall structure can be reduced. Under normal incidence, the −10 dB absorption band is from 4.28 to 8.98 GHz and from 11.19 to 13.77 GHz. The transmission operates at 10 GHz and has a small insertion loss with 0.25 dB. Furthermore, an FSR prototype was fabricated and measured. The measured results are consistent with the simulation ones. The proposed FSR can be applied to radar radome to enhance the anti-interference and stealth performances.

A Low-Profile Ultra-Wideband Absorber Using Lumped Resistor-Loaded Cross Dipoles With Resonant Nodes

This article presents the design and analysis of a dual-polarized low-profile circuit analog absorber with wideband absorption characteristics. A lossy frequency selective surface consisting of cross dipole with central resonant node element is used as the top layer, while there is a continuous copper ground as the bottom layer. The novelty of the design lies in the use of the resonant node as it improves the absorption bandwidth by providing an additional resonance without compromising the low-profile nature of the design. It has a thickness of 0.077 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lambda_{L}$</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">$\lambda {\_}L$</tex-math></inline-formula> is the wavelength corresponding to the lowest operating frequency). The absorber also exhibits a stable absorption characteristics up to 30 <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> angle of incidence and can avoid grating lobes within the operating bandwidth up to 40.79 <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> angle of incidence. An equivalent circuit model is proposed to discuss the operating principles of the design. The validation of the design is done experimentally using the time-gating technique, and a good agreement between the measured and the simulated results is observed. The 10-dB reflection reduction bandwidth and the thickness-to-bandwidth ratio of the absorber are obtained as 108.67% (from 3.58 to 12.10 GHz) and 0.104 from full-wave simulation and 115.67% (from 3.5 to 13 GHz) and 0.098 from measurement, respectively.

A Dual-Polarized Frequency-Selective Rasorber With a Switchable Wide Passband Based on Characteristic Mode Analysis

This study proposes a dual-polarized frequency-selective rasorber (FSR) with a switchable wide passband. The FSR is made up of two layers: the first is a lossy frequency-selective surface (FSS), which is made up of meander lines and lumped resistors that absorb energy. The position of the lumped resistors can be determined via characteristic mode analysis. Another layer is a lossless FSS with PIN diodes for the reconfigurable function. The lossless FSS is made up of three layers for a wide passband. When PIN diodes are turned off, the absorption bands in the lower band vary from 2.73 to 3.76GHz, while the upper band ranges from 6.07 to 7.94 GHz. Meanwhile, the reflect band with a value greater than −3 dB spans the frequency range from 4.21 to 5.88 GHz. When the PIN diodes are turned on, the passband with insertion loss less than −1 dB extends from 3.82 to 5.3 GHz. At the same time, the absorption band ranges from 5.9 to 7 GHz. Due to the nonlinearity of PIN diodes, there is a certain difference between the simulated and the measured results. Also, we analyzed the influence of the inductance value of PIN diodes on the whole structure.

Characteristic mode analysis of resistor-loaded frequency selective surfaces: Theoretical research and experimental verification

In this paper, the modal behaviors and the frequency properties of resistor-loaded frequency selective surfaces have been systematically studied based on the periodic moment method and characteristic mode analysis. Through the observation of the phase curves of the reflection coefficients, we found that as the resistance value increases, the resonant frequency gradually moves toward higher frequencies. By calculating and analyzing the modal currents and the modal reflection coefficients of each mode, we know that the phenomenon is caused by higher-order modes. In order to reduce the influence of higher-order modes on the resonant frequency, a method of using more load resistors is proposed. To verify the proposed method, an absorber with high absorptivity is designed and measured. The measurement shows that the absorptivity of the designed absorber is more than −18dB in the range of 1.92–3.99GHz. The experiment proves that the proposed loading method can be used to flexibly regularize the equivalent impedance of the lossy frequency selective surface without worrying about the negative effects of load resistors in the design of a circuit analog absorber.

Approaching the lowest operating frequency thickness limits with complex surface impedance of ultrathin absorbers.

In this paper, the physical model of electrically thin weakly conductive film with intrinsic surface impedance is established, indicating that the imaginary part of high surface impedance is non-negligible at microwave frequencies. In the design of lossy frequency selective surface absorbers, we introduce the imaginary part of intrinsic surface impedance for the first time. With the experimentally established relationship between the complex surface impedance and the DC square resistance, this complex surface impedance allows us to accurately predict the electromagnetic response of high surface impedance film at microwave frequencies and provides an advantage in reducing the thickness of absorber. The proposed ultra-thin absorber can provide -10 dB reduction over the frequency range of 4.5-13.3 GHz. Total thickness of microwave absorber is only 0.06λ at lowest operating frequency, which is close to the theoretical limitation. The measurement is provided to verify the validity of the equivalent relationship and the reliability of the full-wave model. This study provides a new way to reduce the thickness of absorber, exhibiting promising potential for stealth technique.

Additive Manufacturing of PLA-Based Microwave Circuit-Analog Absorbers

Fused filament fabrication (FFF) was used to additively manufacture (3-D print) two-prototype circuit-analog (CA) absorbers. The CA absorbers consist of a lossy frequency selective surface (FSS), substrate, and ground plane. In this article, the FSS and substrate were manufactured using two different FFF materials to make a cohesive structure manufactured during a single printing procedure. To design the CA absorbers, the complex permittivity of FFF printed polylactic acid (PLA), bronze-, brass-, copper-, and iron-powder infused PLAs, and a graphite-PLA composite was measured using a free-space materials measurement system. Out of the candidate materials measured, graphite PLA and standalone PLA were selected as the FSSs and substrates, respectively. Complex permittivity data from the selected materials were input to Computer Simulation Technology Microwave Studio so that a genetic algorithm could optimize absorber dimensions. Reflectivity of the printed absorbers was measured using a free-space measurement setup. Measured reflectivity data were compared to that from simulations. A simulated-geometric tolerance study corroborated differences noted between ideal models and measured data. The results showed that FFF techniques can be used for CA-absorber designs and that 3-D printing settings can ultimately affect absorber performance.

Miniaturized-Element Frequency-Selective Rasorber Design Using Characteristic Modes Analysis

A dual-polarization frequency-selective rasorber (FSR) with two absorptive bands at both sides of a passband is presented. Based on the characteristic mode analysis, a circuit analog absorber is designed using a lossy frequency-selective surface (FSS) that consists of miniaturized meander lines and lumped resistors. The positions and values of resistors are determined according to the analysis of modal significances and modal current. After that, the presented rasorber is designed by cascading of the lossy FSS and a lossless bandpass FSS. Equivalent circuits of the FSR are modeled, and surface current distributions of both FSSs are illustrated to explain the operation mechanism. Measurement results show that, under the normal incidence, a minimum insertion loss of 0.27 dB is achieved at a passband around 6 GHz, and the absorption bands with an absorption rate higher than 80% are 2.5–4.6 GHz in the lower band and 7.7–12 GHz in the higher band, respectively. Our results exhibit good agreements between measurements and simulations.

A novel wideband absorptive frequency selective surface based on circuit analog absorber

In this paper, a single polarized absorptive frequency selective surface (AFSS) with ultra-wide transmission band and broad absorption band has been proposed numerically and experimentally. Based on the principle of circuit analog absorber (CAA), the proposed wideband AFSS is realized via replacing the ground plane with a band-stop frequency selective sturcture. The proposed AFSS is finally a hybrid structure composed of a wideband lossy frequency selective surface (FSS) in the top and a 3D band-stop frequency selective structure in the bottom. The wideband lossy FSS with thinner profile in the top is realized by utilizing a low pass capasitive elements to compensate the equivalent load inductance arising from the air space that is less than quarter wavelength installing between FSS and ground plane, and the low pass capactive lossy FSS is synthesized using “Gong”-shaped patterns with lumped resistors. Meanwhile, the unit cell of 3D frequency selective structure consists of two electrical coupling C-shaped patterns is investigated to achieve a wide stop band with high selectivity, which can be equivalent to the ground plane in the absorption band. By cascading the wideband lossy FSS and the 3D band-stop frequency selective structure, a thinner hybrid AFSS is presented with ultra-wide transmission band and broad absorption band. The results obtained from simulation shown that an absorption band with absorptivity above 90% is obtained from 6.6 to 11.6 GHz, and the 1 dB transmisson band is from 1 to 3.5 GHz with minimum insertion loss of 0.21 dB. Moreover, the performance can be guaranteed for TE-polarized oblique incidence up to 30°. Finally, a prototype is fabricated to validate its performance, the measured results shown that the 1 dB transmission band ranges from 1 to 3.5 GHz, and the absorption band is operating from 6.3 to 11 GHz at normal TE-polarized incidence. A good agreement between the experiment and simulation results is achieved, which verifies the effectiveness of the design.

Absorptive/Transmissive Frequency Selective Surface With Wide Absorption Band

An absorptive/transmissive frequency selective surface (ATFSS) with absorption bands at both sides of a passband is presented. Equivalent circuits of the ATFSS that consists of a lossy frequency selective surface (FSS) and a lossless FSS were modeled. To improve the rejection at an undesired band, a transmission zero was introduced and controlled by loading the lossless FSS with four-legged loaded slots. The parasitic passband was suppressed when the cross structure in the lossless FSS was loaded with resistance. In order to expand the absorption band, loaded dipoles were utilized for the lossy FSS design. An ATFSS with wide absorption bands was realized after three ATFSSs with different performances were investigated. The proposed ATFSS was fabricated and measured. The measurement results showed that a passband at around 5 GHz with a minimum insertion loss of 0.92 dB was obtained. Within an ultrawide band from 2.8 to 11.4 GHz with |S 11 | <; -10 dB, the absorption rates of higher than 80% were obtained at a lower absorption band, from 2.7 to 3.8 GHz, and at a higher absorption band, from 6.2 to 11.7 GHz. Our results showed good agreements between the measurements and simulations.

&lt;italic&gt;X&lt;/italic&gt;-Band Circuit-Analog Absorbers Using Unidirectional Carbon Fiber

This letter explores single-ply pre-impregnated (pre-preg) unidirectional carbon-fiber-reinforced polymer (CFRP) laminas for use in X -band (8.2–12.4 GHz) absorbers. Specifically, the unidirectional CFRP sheets were used as lossy frequency selective surfaces in single- and two-layer circuit-analog absorbers (CAAs). The aim of this letter was to assess the feasibility of using only the fiber orientation and areal weight of the material as a tuning aid. A free-space materials characterization system was used to characterize the anisotropic complex permittivity and anisotropic surface impedance of two different areal weight unidirectional CFRP samples. The absorbers were prototyped in a mechanically reconfigurable fixture with the aid of a vacuum-bagging system. The reconfigurable approach allowed reflectivity measurement performance to be measured without the need to fabricate cured sandwich constructions. Reflectivity measurements were compared to transmission-line models as well as full-wave models developed using commercial simulation software. Regions of validity for the transmission line models were identified. The results show that single-ply pre-preg unidirectional CFRP laminas have the potential to be used in single- and two-layer CAA configurations for use in X -band absorbers.

Design of thin microwave absorbers using lossy frequency selective surfaces

ABSTRACTAn alternative procedure for the optimal design of both narrow and wide band electromagnetic absorbers is presented. The method, based on equivalent‐circuit model in conjunction with the full‐wave simulation, is used for computing lumped parameters of the frequency selective surface (FSS) elements. Thus, the procedure is applied to high‐impedance surfaces (HIS) to derivate the desired absorbers. The design presented here use simplified modeling to achieve numerical dimensioning of the FSS which results on the designed absorbers with performance very close to the desired attenuation and bandwidth. The resulting absorbers are implemented by placing lossy FSS over a dielectric layer backed by a metal ground plane. The experimental results show good agreements with theoretical results and the validity of the modeling method for the proposed structure. Characteristic attenuation for fabricated narrow band absorber show almost 20‐dB in 5.5 GHz. The attenuation bandwidth below −10 dB of the wideband absorber can reach 10–16 GHz with 3 mm thick only. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:928–933, 2015

Analysis and design of triple-band high-impedance surface absorber with periodic diversified impedance

In this paper, a triple-band planar absorber with high-impedance surface (HIS) is designed and fabricated. The absorber structure is composed of polyurethane foam sandwiched between a lossy sheet of frequency selective surfaces (FSS) and a perfect electric conductor. The lossy FSS possesses different resistances in a periodic composite unit as compared with typical HIS absorber. Losses in the FSS are introduced by printing the periodic composite square ring pattern on blank stickers using various resistive inks. Physical mechanism of the HIS absorbers is analyzed by equivalent circuit model and electric field distribution studies. The proposed absorber with periodic composite units offers superimposed triple-band absorption as compared with that of the single units having single- or dual-band absorption characteristics. The reflection loss measurements show that the 90% absorption bandwidth of the HIS absorber is increased by 42% by the proposed composite periodic units.

Improvement on the wave absorbing property of a lossy frequency selective surface absorber using a magnetic substrate

An equivalent-circuit model is used to analyse the improvement of the wave absorbing performance of the lossy frequency selective surface (FSS) absorber by using a magnetic substrate, showing that it is possible to widen the wave absorbing bandwidth. Three pieces of magnetic substrates are prepared. According to the complex permittivity and permeability, the reflectivity of the corresponding absorber is calculated by the finite difference time-domain (FDTD) method, and the bandwidth of the reflectivity below −10 dB is optimized by genetic algorithm. The calculated results indicate that the wave absorbing performance is significantly improved by increasing the complex permeability of the substrate; the reflectivity bandwidth below −10 dB of the single layer FSS absorber can reach 3.6–18 GHz with a thickness of 5 mm, which is wider than that with a dielectric substrate. The density of the FSS absorber is only 0.92 g/cm3. Additionally, the absorption band can be further widened by inserting a second lossy FSS. Finally, a double layer lossy FSS absorber with a magnetic substrate is fabricated based on the design result. The experimental result is consistent with the design one.

Broadband metamaterial absorber based on coupling resistive frequency selective surface

We report the design, fabrication, and measurement of a broadband metamaterial absorber, which consists of lossy frequency selective surface (FSS) and a metallic ground plane separated by a dielectric layer. The compact single unit cell of the FSS contains crisscross and fractal square patch which couple with each other. Both qualitative analysis by equivalent circuit and accurate numeric calculation show that the coupling between the crisscross and the fractal square patch can enhance the bandwidth with the reflectivity below -10dB in the frequency range of 2-18GHz by producing a third absorption null. In the end, the designed absorber was realized by experiment.

Design of a Lightweight Magnetic Radar Absorber Embedded With Resistive FSS

In this letter, the design of a lightweight magnetic radar absorber (RA) having broadband bandwidth in frequency range of 1-18 GHz is demonstrated. A 5-mm-thick magnetic RA with weak wave-absorbing performance is obtained on the basis of polyurethane foam filled with flake ferrous microwave absorbent. The use of frequency selective surfaces (FSSs) results in a significant increase of the absorbing intensity and operating bandwidth of the RA. By selecting the unit dimensions and modulating the location and square resistance of FSS, a maximal operating bandwidth with the reflectivity below -10 dB can be obtained. Experimental results are presented and compared to numerical simulations, and they demonstrated that the RA embedded with lossy FSS has a super broad bandwidth of 3.19-18 GHz with the reflectivity below - 10 dB for the double-layer case. The density of the absorber is only 0.62 g/cm3 , which is far more less than that of the conventional magnetic coating radar absorbing materials (RAMs).

Equivalent circuit method analysis of the influence of frequency selective surface resistance on the frequency response of metamaterial absorbers

A simple equivalent circuit method has been proposed to provide understanding of the absorption mechanism of a class of metamaterial absorbers. The absorbers are composed of lossy frequency selective surfaces (FSSs) over the grounded dielectric substrates. Based on the analytical method, the influence of FSS surface resistance on the frequency responses of metamaterial absorbers is discussed in detail. It is shown that wideband or narrowband absorbers can be obtained by changing the surface resistance of the FSS. The narrowband absorbers show a similar surface resistance dependence of the absorbing performance to Salisbury absorbers and the maximum bandwidth of metamaterial absorbers can be obtained with the proper surface resistance. This can serve as a guideline for the fast and efficient design of metamaterial absorbers. Finally, experimental results are presented for verifying the validity of the analytical method.