Circuit Analog Absorber Research Articles

Accelerating multilayer frequency selective surface absorber design by combining equivalent circuit models with machine learning

Designing high-performance electromagnetic functional materials and artificial structures is of great significance for electromagnetic wave modulation applications. Optimizing performance in the high-dimensional parameter space of electromagnetic functional materials has posed a challenging problem. This paper proposes the use of reinforcement and transfer learning methods to facilitate the quick and accurate design of wideband circuit analog absorbers (CAA). A trained reinforcement learning network is utilized to comprehend the relationship between reflectivity performance and changes in impedance parameter. Furthermore, the transfer learning method is applied to accelerate the training process, leading in a 20-fold reduction in the number of simulations. To validate the effectiveness of this approach, a double-layer absorber aiming for reflectivity less than −20 dB at 3–10 GHz is designed, requiring only 50 simulations. This demonstrates that the proposed method provides a flexible strategy for the interactive design of equivalent circuit and electromagnetic structural parameters. Moreover, it presents a novel and efficient solution for microwave absorber design, with potential applications across various microwave design fields.

Active low frequency band expansion mechanism of electromagnetic wave absorber composite structure

Bandwidth expansion of modern antennas and communication systems raises growing demand for electromagnetic wave absorber in compatibility equipment. Single absorption mechanism is hard to reach well-balanced characteristics including wide bandwidth, low frequency absorption and small thickness. Based on the composite structure concept, composite structure for electromagnetic wave absorber’s active low frequency band expansion is proposed, by combining circuit-analog absorber and active absorber to form a superimposed structure. The characteristic of composite structure is better than the single absorber structure, which improves enhancement mechanism brought by the composite structure. Sample fabrication and experiments indicate reflectivity curve envelope reaches −10 dB in 2.15–18 GHz frequency range (relative bandwidth 157.3%), and the tunable absorption frequency range in low-frequency domain is greater than 1.3 GHz. The structure has good adaptation on TE and TM polarizations, as well as good angle insensitivity. This composite structure has application potential in intelligent response system related with electromagnetic compatibility, energy absorption and object security management.

Design of Broadband Circuit Analog Absorber With Optimal Thickness for Stable Angular Response in C, X, Ku, and K Bands

Design of Broadband Circuit Analog Absorber With Optimal Thickness for Stable Angular Response in C, X, Ku, and K Bands

Design of a multilayer ultra‐wideband absorber based on hybrid absorption structure

AbstractA multilayer, ultra‐wideband absorber is proposed in this paper, which consists of a hybrid circuit analog absorber and a metal resonant absorber. The circuit analog absorber is located at the upper part of the absorber to absorb low‐frequency electromagnetic waves, and the metal resonant absorber is located at the bottom of the absorber to absorb high‐frequency electromagnetic waves and provide a reflecting plate for the absorber. Through simulation, the absorption rate of the designed absorber can be greater than 90% in the range of 2.33–18.43 GHz, and an equivalent circuit is established for verification. In addition, the absorber has good angular stability and polarization insensitivity. The prototype of the absorber was manufactured and measured, and the test results are basically consistent with the analysis results.

Design of dual-band planar thin absorber for vehicular radar test scenario

ABSTRACT Aiming at the requirement of radar wave absorption in automotive radar tests for both the 24 GHz and 77 GHz vehicle radar bands, two ultra-thin dual-band circuit analog absorber designs based on frequency-selective conductive film have been proposed. They are obtained by first optimising the equivalent circuit parameters to achieve the desired reflection response, then determining the physical parameters (geometrical size and square resistance) of unit cells from the equivalent circuit parameters, and finally fine-tuning these physical parameters. Both designs employ unit cell based on the nested structure of conductive square ring and a conductive square patch, which has the advantages of simple structure, ease of processing, high design freedom, and good polarisation and angular stability. The 1st design employs different square resistance values for the conductive ring and patch, while the 2nd design employs a single square resistance value for both conductive shapes. Simulation results show that, for each of the two designs, the lower and upper 70% absorption or −10 dB reflection bands simultaneously cover the 24 GHz and 77 GHz vehicle radar bands, respectively, with good angular stability. The dual-band coverage keeps stable until the elevation incident angle reaches 20° and 50° for the 1st and 2nd designs, respectively. A realistic sample is processed regarding the 2nd design and its dual-band absorption performance has been verified by experimental results, which have good agreement with the simulated counterparts.

An Ultrawideband and High-Absorption Circuit-Analog Absorber With Incident Angle-Insensitive Performance

This work presents an ultrawideband and high-absorption structure (UHAS) obtained by a fast design method. The proposed analytic approach can simply design single-FSS-layer absorber with ideal performance stability under oblique incidence. With this method, a composite material reinforced circuit-analog (CA) absorber is designed, which unit cell of CA sheet is a square ring loaded with chip resistors. At normal incidence, both TE and TM polarization simulation results demonstrate that the reflection coefficient below −10 dB is from 5.8 to 22.2 GHz; meanwhile, the reflection coefficient less than −20 dB covers a bandwidth of 7.4–19.1 GHz; also, there is a −30 dB absorption band in 8.6–17.2 GHz. Furthermore, under TE polarization, experimental results indicate that UHAS can maintain a −20 dB absorption band from 7.4 to 18.0 GHz within 40° oblique angle of incidence; −10 dB absorption is from 6.1 to 18.0 GHz within 50° oblique angle of incidence. The good agreement between simulation and measurement validates the proposed method and UHAS. Finally, the UHAS, with a protective layer, will be more stable in practical applications.

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.

Wide-Angle, Ultra-Wideband, Polarization-Independent Circuit Analog Absorbers

In the analysis of most circuit analog (CA) absorbers, it is only the absorption performance under normal incidence that is taken seriously, leading to a much worse absorption with the increase of incident angle, especially when it is larger than 30°. In this communication, a novel absorber, which consists of a conductive square-loop (SL) array embedded with lumped resistors and a well-designed wide-angle impedance matching (WAIM) layer, is proposed. Equivalent circuit (EC) and strict formula derivations are introduced to present a great insight into the deterioration of absorption performance under oblique incidence and the effect of WAIM layer. An absorber sample is further fabricated and measured for verification. It is shown through measurement that the structure has a fractional bandwidth of 137.1% for at least 10 dB reflection reduction under normal incidence. When the incidence angle is increased to 45°, the overlapped bandwidth for both transverse electrical (TE) and transverse magnetic (TM) polarizations is still measured to be 110.5%.

A novel modified circular ring-based broadband polarization-insensitive angular stable circuit analog absorber (CAA) for RCS applications

AbstractIn this study, a compact modified circular ring-based circuit analog absorber (CAA) is proposed. It comprises a resistor-loaded frequency selective surface (FSS) underneath an FR-4 substrate, and the other end is terminated with a perfect electric conductor (PEC) separated by an air-spacer. The proposed design shows a minimum −10 dB reflectance from 10.55 to 36.03 GHz (25.48 GHz bandwidth and 109.4% fractional bandwidth) with two dips at 13.5 GHz and 35.23 GHz having reflectivity −17.18 dB and −24.9 dB, respectively. The existing four-fold circular symmetry architecture shows insensitivity for different polarization angles under normal incidences and up to 45° stability for Transverse Electric (TE) and Transverse Magnetic (TM) modes under oblique incidences. The proposed cell is compact with a periodicity of 0.193λL, and a thickness of 0.09λL, where λL is the free-space wavelength corresponding to the lowest frequency of the absorption band. The novelty of the design lies in the topology of using an underneath FSS to take advantage of effective permittivity control for both broader bandwidth and better angular stability. To demonstrate the ability of the proposed design, a 28 GHz microstrip patch antenna prototype is fabricated, and its radar cross section (RCS) is measured after loading with the absorber. A 15.15 dBsm RCS reduction is observed in comparison with the PEC. The authors strongly believe that the proposed design has potential applications such as RCS reduction and radio-frequency energy harvesting.

Frequency-Selective Rasorbers: A View of Frequency-Selective Rasorbers and Their Application in Reducing the Radar Cross Sections of Antennas

A frequency-selective surface (FSS) can be employed as a spatial filter for incident electromagnetic (EM) waves, which are most often used in radome design to protect the antennas and to reduce the radar cross section (RCS) of the antenna system <xref ref-type="bibr" rid="ref1" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[1]</xref> – <xref ref-type="bibr" rid="ref2" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"/> <xref ref-type="bibr" rid="ref3" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[3]</xref> . In-band signals transmit through the FSS with little power loss, while out-of-band incoming waves are reflected strongly, making them detectable by radar. Thus, to further reduce the biostatic/multistate RCS of the radome, circuit analog absorbers or absorbing materials are introduced into the radome design. As a result, the out-of-band signals are absorbed and reflection is reduced.

Broadband polarization insensitive wide angular stable dual‐split square ring circuit analog absorber for radar cross section and electromagnetic interference shielding applications

This work presents a simple broadband polarization-insensitive circuit analog absorber (CAA) with wide angular stability. It comprises of dual-cut square loop resonator loaded with eight lumped resistors as top patch and an inductive grid. The normal incidence reveals a 90% absorption bandwidth of 20.51 GHz (110% fractional bandwidth) covering the entire X to K-band with a minimum flat absorption rate of 92.82%. Equivalent circuit analysis is carried out, and responses are validated with full-wave simulations. The proposed prototype is insensitive to different polarization angles under normal incidence and stable with 45° in transverse electric (TE) and transverse magnetic (TM) incidences. The design's novelty lies in using two resistors on each side of the square ring, which improves the absorptivity performance. The inductive grid enhances TM mode's angular stability. The proposed absorber is fabricated, and the measured results are in good agreement. In-band bistatic radar cross section reduction of 12.97 dBsm is obtained with the absorber using an X-band antenna. The temperature profile of the prototype is illustrated using near-in-lock infrared thermography, and 2.1°F of temperature rise is noticed in the absorption band. The proposed structure possesses a low profile, broad bandwidth, polarization insensitivity, and wide angular stability to the best of the author's knowledge.

Circuit-analog radar absorbing structures using a periodic pattern etched on Ni-coated glass fabric

A circuit-analog broadband radar-absorbing structure with improved mechanical properties was proposed based on a periodic pattern etched on the Ni-coated fabric. Using the conventional polymer-based substrate as the resistive sheet of a conventional circuit-analog absorber, a decrease in the mechanical properties of the composite material was witnessed. Thus, we fabricated a periodic pattern fabric with a loop pattern using a nickel-coated glass fiber fabric and photolithography etching. Radar-absorbing structures exhibiting a >90% electromagnetic wave absorption performance in the broadband of the C- and X-bands (6–12.4 GHz) were designed using a genetic algorithm optimization. Three-point bending and tensile tests were then performed to evaluate their mechanical properties. Overall, the proposed radar-absorbing structures showed >90% absorption band of 6.7 GHz (6.3–13 GHz). In addition, these structures exhibited improved mechanical properties without deterioration in the interlaminar shear strength or tensile modulus compared to conventional circuit-analog radar-absorbing structures.

Broadband and wide-angle metamaterial absorber based on the hybrid of spoof surface plasmonic polariton structure and resistive metasurface.

Electromagnetic (EM) wave absorber with broad and robust absorption performance over wide incident angle range is persistently desired in specific applications. In this work, we propose and demonstrate a broadband and wide-angle metamaterial absorber (MA) based on a hybrid of stereo spoof surface plasmonic polariton (SSPP) structure and planar resistive metasurface. At first, we design a broadband SSPP absorber by adjusting the dispersion and loss of the artificial plasmonic structure (PS) simultaneously. Furthermore, owing to utilize its spatial phase manipulation ability, we integrate a resistive metasurface on top of the PS to construct a modified circuit analog (CA) absorber with a dispersive metamaterial spacer. The absorption mechanism of the hybrid structure is analyzed theoretically. The results indicate that the hybrid MA is equipped with broad and robust absorption performance over a wide incident angle range due to the synergistic absorption of the PS and metasurface. Finally, a prototype of the hybrid MA is fabricated by silk-printing technic and its absorption performances are measured. The experimental results can verify the theoretic ones and indicate that proposed hybrid MA can achieve 90% absorptivity from 3.9 GHz to 10.6 GHz with thickness of 7.0 mm, which is only 106% times of the ultimate thickness corresponding to the absorption performance of MA. In general, the concept and design offer a distinct approach of utilizing SSPP to design absorbers with excellent performances from radio frequency to optic band, which are promising for extensive applications.

An ultra-broadband microwave absorber based on hybrid structure of stereo metamaterial and planar metasurface for the S, C, X and Ku bands

Broadband electromagnetic (EM) wave absorbers are persistently desired in the past decades due to their extensive applications in different fields. In this paper, we proposed and demonstrated a metamaterial absorber (MA) based on a hybrid structure of stereo and planar resistive metamaterial elements. In principle, this hybrid MA is a combination of the Dällenbach layer absorber and circuit analog (CA) absorber. By adjusting its dispersion feature, the stereo metamaterial based on standing resistive patch can work as a broadband artificial Dällenbach layer absorber for C, X and Ku bands. Such stereo metamaterial was also utilized as a dispersive dielectric spacer and a metasurface consisting of resistive circle rings was deployed on the top of it to implement an improved CA absorber for S and C bands. By combing them together, the proposed hybrid MA can achieve absorption over 90% in an ultra-broad bandwidth from S to Ku band (2.59–16.80 GHz). Since our design takes full advantage of the physical space, the hybrid MA achieves an actual thickness as only 1.075 times of the ultimate thickness corresponding to its absorption performance. Furthermore, the hybrid MA has comprehensive advantages in terms of stability against oblique incidence, easy processing and light-weight. The MA prototype was fabricated assisted by the silk screen printing process and its ultra-broadband absorption performance was experimentally validated by the measured results. The new concept of hybrid MA and specific design scheme proposed here provide a promising approach for broadband, low-profile and light weight EM absorber.

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.

Compact, broadband, wide angular stable circuit analog absorber at sub‐6 GHz for radar cross section reduction

AbstractThis work presents a compact broadband wide angular stable polarization‐insensitive circuit analog absorber at sub 6 GHz. It shows almost 95% absorption from 2.26 to 5.76 GHz over the entire operating range. A transmission line equivalent circuit is modeled, and the response is validated. The design is obliquely stable up to 45° for both transverse electric and transverse magnetic incidences. The analysis of surface current distributions at two resonant frequencies confirming magnetic resonances' role in near‐unity absorption. It is compact with a periodicity of 0.1 λL and thickness of 0.08 λL. Finally, bistatic radar cross section reduction of the proposed absorber attached with an antenna is measured.

Broadbanding of low-scattering coding metasurface by combining wave absorption and anti-phase cancellation mechanisms

Traditional circuit analog absorber (CAA) composed of identical resistive patch elements suffers from narrow bandwidth. A novel absorptive coding metasurface is constructed by introducing another kind of resistive patch element into the CAA. The newly introduced elements resonate at higher frequency than the original patch elements do so that a reflection phase difference of about 180° is produced above the original absorption band between the two kinds of elements. Hence the metasurface achieves a much wider low-RCS band of which the lower and higher sub-bands attribute mainly to electromagnetic absorption and anti-phase cancellation, respectively. A preliminary and an advanced designs of the proposed metasurface are presented. Both of these two designs employ the same element configuration, but adopt different element layout schemes, for one is partitioned distribution and the other is random coding. Full-wave simulation and experimental results of the two designs confirm the characteristic of the proposed metasurface in broadband and polarization-insensitive monostatic RCS reduction under normal incidence. As for the preliminary design, in particular, the low-RCS bandwidth ratio versus electrical thickness reaches to 40.792, which is comparable with or even superior to those for most of the existing metasurfaces using compound RCS reduction mechanism. Meanwhile, the advanced design also exhibits broadband bistatic low-RCS property, and keeps a stable specular RCS reduction performance with regard to incident elevation angle up to 30°. The proposed metasurface features simple structure, light weight, low cost and easy fabrication, making it a competitive candidate in applications such as radar stealth and electromagnetic compatibility.

Wide-angle, Ultra-wideband, and Polarization-insensitive Circuit Analog Absorbers

Most previous circuit analog absorbers only considered absorption performance under normal incidences, leading to bad absorption for large incident angles, particularly those > 30°. With the advancement in modern bistatic radar detection technology, radar electromagnetic waves may come from different spatial directions, thereby necessitating radar absorbers with high absorption performance under normal and oblique incidences. Thus, in this paper, we present a novel wideband absorber comprising a conductive square-loop array embedded with lumped resistors and a well-designed Wide-Angle Impedance Matching (WAIM) layer. Results show that the WAIM layer can significantly improve absorption under oblique incidences. To make the absorber design clear and simple, an Equivalent Circuit (EC) and strict calculating formulas are proposed under normal and oblique incidences. Fractional bandwidth is increased into 137.1% through measurement under normal incidence, and the structure has a common fractional bandwidth of at least 110.5% for at least 10 dB reflection reduction when the incidence angle < 45°. The similarity among EC calculated, simulated, and measured results proves the validity of the designed absorber.

Wideband radar cross section reduction based on absorptive coding metasurface with compound stealth mechanism* *Project supported by the National Natural Science Foundation of China (Grant Nos. 61871280, 61372012, and 61671315).

A scheme of combing wave absorption and phase cancellation mechanisms for widening radar cross section (RCS) reduction band is proposed. An absorptive coding metasurface implementing this scheme is derived from traditional circuit analog absorber (CAA) composed of resistive ring elements which characterize dual resonances behavior. It is constructed by replacing some of the CAA elements by another kind of resistive ring elements which is singly resonant in between the original two resonant bands and has reflection phase opposite to that of the original elements at resonance. Hence the developed metasurface achieves an improved low-RCS band over which the lower and higher sub-bands are mainly contributed by wave absorption mainly while the middle sub-band is formed by joint effect of wave absorption and anti-phase cancellation mechanisms. The polarization-independent wideband RCS reduction property of the metasurface is validated by full-wave simulation results of a preliminary and an advanced design examples which employ the same element configuration but different element layout schemes as partitioned distribution and random coding. The advanced design also exhibits broadband bistatic low-RCS property and keeps a stable specular RCS reduction performance with regard to incident elevation angle up to 35°. The advanced design is fabricated and the experimental results of the sample agrees qualitatively well with their simulated counterparts. The measured figure of merit (i.e., low-RCS bandwidth ratio versus electrical thickness) of the sample is 40.572, which is superior to or comparable with those for most of other existing metasurface with compound RCS reduction mechanism. The proposed compound metasurface technique also features simple structure, light weight, low cost and easy fabrication compared with other techniques. This makes it promising in applications such as radar stealth and electromagnetic compatibility.

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.