Design of Novel Multiband Radar Absorbers Using Cylindrical Frequency Selective Surfaces

We present a general method to design multiband absorber by replacing the ground plane in a conventional metamaterial absorber with an artificial magnetic conductor. Due to its unique property of in-phase reflection at some specific frequency, the artificial magnetic conductor is used to introduce new absorption in the operation band. Meanwhile, out of the in-phase reflection band, the original absorbing capability of the absorber is reserved. To demonstrate it, we design a metamaterial absorber comprising three layers which are grids patterned resistive frequency selective surface, dielectric layer and the ground plane respectively. With an appropriate design, the absorber performs an absorbing peak at about 10 GHz. Then, we utilize a single band artificial magnetic conductor at 6.25 GHz and a dual-band one at 6.27 GHz and 8.17 GHz, which are both lossy and comprised of patches array varying in periodic size with a thickness of 0.6 mm, to replace the ground plane in the metamaterial absorber separately. The reflectivity of these multiband absorbers are simulated, and experiments are carried out later. Experimental results agree well with the simulations. All results verified that the method presented at the beginning is effective. The results show that additional absorptions exist at the frequencies where microwaves are nearly reflected in phase on the artificial magnetic conductor. Meanwhile the original absorbing capability of the metamaterial absorber has been preserved mostly. Based on the artificial magnetic conductor, the multiband absorber performs better with an increasing absorption bandwidth from 8.5 GHz to 10 GHz compared to the metamaterial absorber.

In this study, a compact and cost-effective Frequency Selective Surface (FSS) unit cell absorber is proposed to mitigate electromagnetic interference in the sub-6 GHz 5G bands, specifically targeting the n77 (3.3–4.2 GHz), n78 (3.3–3.8 GHz), and n79 (4.4–5.0 GHz) frequency ranges. The absorber structure, fabricated on an FR-4 substrate, was designed through a systematic geometric optimization of patch dimensions and substrate thickness to enhance absorption performance via inductive-capacitive resonance tuning. Simulation and experimental results show strong agreement, with a maximum absorption rate of 98% at 4.2 GHz and over 80% absorption between 3.5 GHz and 4.7 GHz. The structure also demonstrates robust angular stability up to 60°, making it suitable for practical applications requiring directional reliability. Compared to existing designs, the proposed FSS absorber offers superior absorption efficiency, simpler fabrication, and enhanced angular resilience. This work contributes a practical and validated solution for targeted EMI suppression in 5G-enabled electronic systems.

In this paper, we present three ultra-thin frequency selective surface (FSS) microwave absorbers on a single layer, that have three remarkable features. First, the various absorbing bandwidths can be obtained in our absorber by adjusting the two absorbing peaks, which are produced by two different resonance effects on the FSS. By manipulating the dimension parameters, we design the dual-band, 3-dB and 10-dB band absorbers. We achieved the absorber's thickness of $0.039\lambda_{\rm g}$ with the single layer due to high loss of inter-digital capacitor patterns on the FSS. Additionally, the proposed absorbers are insensitive to polarized electromagnetic (EM) waves because of the symmetric structure of the FSS. In this study, the absorber mechanism and transmission line (TL) modeling for the absorber design are studied. Each of the proposed absorbers is analyzed by using the TL equivalent circuit model and full-wave analysis. The loss factor is studied in order to demonstrate the origin of losses, and a parametric study is conducted to explain how to vary the absorbing bandwidths. The performances of each proposed absorber are also experimentally demonstrated.

Frequency selective surface (FSS) is a spatial filter that has frequency selection characteristics for electromagnetic waves with different operating frequencies, polarization states and incident angles. At the same time, frequency selective surface has the advantages of simple structure and easy processing compared with the traditional filters. FSS have been investigated for many decades and extensively utilized in various applications. It can used in radar radomes to reduce the radar cross section (RCS) of an array antenna, as a reflector to transmit and reflect electromagnetic waves in different frequency bands. Normally, electromagnetic waves are incident at an oblique Angle onto the target surface. Therefore, the research of FSS absorbers with stable absorbing performance under oblique incidence has tremendous practical application value. This paper proposes a new design scheme for designing a FSS absorber with stable oblique incident response range from 0° to 70°. A single-square-loop array with lumped resistors and square patch printed on the dielectric substrate is proposed, and it exhibits a characteristic impedance matching for wide incident angles from 0° to 70°. The design scheme of the FSS layer under the condition of oblique incidence is introduced in detail, and the equivalent circuit is adopted to make the design clearer and more direct. The results show the absorption bands with oblique incident angles from 0° to 30°, 30° to 50°, and 50° to 70° reflection coefficients below –10 dB are 3.44 to 9.03 GHz, 4.49 to 9.06 GHz, and 4.87 to 8.96 GHz for TE polarization, respectively. The absorber proposed in this paper will contribute much to the development of ultra-wide-angle oblique incidence.

In this paper, a thin, broadband and low frequency with high absorption (more than 90% absorption) radar absorber (RA) consisting of magnetic-type material substrate and double period frequency selective surface (FSS) is demonstrated. The geometric parameters were investigated for its contribution towards absorption by the finite-difference time domain (FDTD) method. The effects of incidence angles of electromagnetic radiation were also discussed. The simulation indicates that the use of the double period FSS makes a remarkable increase of operational bandwidth and moves the resonant frequency to the lower frequency comparison with single magnetic radar absorption materials (SMRA). Due to the excellent symmetric nature of the proposed structure, the reflectivity is insensitive to incident angle from 0° to 30°. The reflectance of RA lower than -10 dB (over 90% absorption) is obtained in the whole frequency range of L-band which is 1.5 times wider than the bandwidth of the same thickness SMRA when the structure thickness is only 2.62 mm. The simulation method is verified by comparison with the experiment in the literature. Thus, the presented RA breaks the limitation of the thickness (λ/4) for the traditional low-frequency band absorbing material and has wide application prospect in electromagnetic shielding and low-frequency stealth.

A technique is proposed for the design of multiband frequency selective surfaces (FSSs) that is based on the use of fractal screen elements. The authors demonstrate the design methodology by exploiting the self-similarity of the crossbar fractal to implement a tri-band FSS. Another benefit of this design approach, in addition to its multiband performance, is that the frequency response of both the TE and TM modes are exactly the same, owing to the symmetry in the tree-like fractal structure of the screen elements. Dielectric loading effects have been considered for practical implementation of designs with coverage exceeding two bands. Finally, it is noted that the locations of the individual bands can be controlled by the scaling or similarity factor used in the construction of the fractal screen elements.

This paper presents the design, simulation, manufacturing and testing of a simple invisibility cloak based on a frequency selective surface (FSS). The work is focused on cloaking an electrically thin dielectric cylinder with an easy to manufacture FSS made of copper strips glued to the cylinder surface. In contrast to many papers in the literature, the full procedure from formulation to measurement results is presented
\nhere. An original approach to obtain the effective surface
\nimpedance of the cylindrical FSS from either simulated or
\nmeasured far fields is introduced. The measurement results show
\nexcellent and relatively wide band performance of the cloak
\nprototype.

In this paper we report on a resistively loaded Frequency Selective Surface (FSS) absorber design which is insensitive to the polarization of microwave signals incident at angles of 45 o ± 5 o . The metal backed periodic structure is composed of an array of conductive rectangular loops, each loaded with a resistor at the center of the four sides. The geometry of the absorber and the resistance value of the vertical and horizontal resistor pairs are carefully chosen so that the structure presents a real impedance of 377 Ω at the center operating frequency for both TE and TM polarized waves incident at 45 o . Numerical predictions of the electromagnetic scattering from three different absorbers, designed to work at X-band, are used to investigate the effect of thickness and resistance value on the reflectivity bandwidth and angular sensitivity.

This paper presents a design of thin and broadband non-magnetic metamaterial absorbers using circular loop shaped resistive frequency selective surfaces (FSS). A straightforward visual graphic representation analysis which extends the microwave impedance matching theory into wideband metamaterial absorbing structure design is carried out on the key parameters analysis of the absorbers. The single layer absorber is analyzed. Numerical computing results are in good agreement with the analyzed ones. The study shows that visual graphic representation analysis could greatly decrease the range of variable. The visualization method is simple and practicable to introduce the working principles of the wideband non-magnetic metamaterial absorbing structure and to derive the best-suited element for wideband absorption.

This paper presents the theory, design, simulation, fabrication, and performance of a flexible dual-band MM absorber, which is resonant at microwave frequencies. The sandwich structure of the MM absorber is composed of the periodic array of the T-shaped metallic patches and a continuous metallic plane, which are separated by a middle flexible dielectric layer. The optimized geometric parameters were obtained by numerous simulations using the full wave finite integration technology of CST 2015. The simulated results indicate that the proposed MM absorber has two distinct absorption peaks at 16.77 and 30.92 GHz with the absorption ratio of 98.7% and 99.3%, respectively. The absorber has a thickness of 0.2403 mm, which is only 1/74 and 1/40 of the wavelength for the resonance frequency of 16.77 and 30.92 GHz. The influence of the material's properties and structural curvature on the absorption performance was investigated by numerous simulations. The proposed MM absorber is highly sensitive to the polarization of the incidence EM wave and has good absorption properties over a large range of the incidence angle for the incidence EM wave. The electric field and surface current distributions at two independent resonance frequencies were analyzed for providing insight into the EM wave absorption mechanism. Simulated results show that two different resonance modes are introduced into the single patterned metallic resonance structure to realize the dual-band performance. The laser ablation process was adopted to fabricate the sample of the proposed absorber. Measured results for the normally incident EM wave show an agreement with the simulated results. The fabricated MM absorber shows significant mechanical flexibility and can easily be conformed to the unusual surfaces such as cylindrical, pyramid, and spherical. Furthermore, this design concept can be extended to the other absorber structure and the other frequency bands, therefore, which can greatly enrich the applications in antenna, sensing, thermal image, and detection. For instance, in the design of projectile-borne conformal antenna array, a flexible ultrathin MM absorber can easily be loaded between the antenna and the projectile body to reduce the radiation interference, weaken the coupling loss, and reduce the RCS.

A multi-band reflectarray structure that transmits and receives at both Ka- and X-bands is introduced in this paper. A separate reflectarray is designed for each band and a frequency selective surface (FSS) is used as the ground plane for the Ka-band reflectarray. Additional FSS's are implemented to further enhance the isolation between bands. The reflectarrays also implement a circular to linear polarized transformation which results in a simplified feed geometry.

This study proposes an ultra‐wideband multilayer absorber by combining frequency selective surface (FSS) and absorbing material. The structure is composed by three layers which are FSS loaded with resistors, absorbing material at the bottom and each layer is separated by air. The FSS in bottom has good absorption performance at high frequency while the upper layer is used to achieve low‐band absorption. To extend the absorption band to the low frequency, it further add a layer of absorbing material at the bottom. Equivalent circuit is employed to analyze the principle of the designed absorber. The simulation results show that an ultra‐wide absorption band from 0.61 to 12.33 GHz with a fractional bandwidth of 181% is realized for both TE and TM waves under the normal incidence. Furthermore, the thickness of the structure is only 0.07λL, where λL represents the wavelength of the minimum operating frequency. The designed absorber is fabricated and measured, and the measurement is in good agreement with the simulation.

A unique strategy for the design of reconfigurable microwave absorbers based on the frequency selective surface (FSS) concept is presented in this paper. Three different topologies are considered (namely, Structure A, B, and C), and their resonance frequencies are varied by increasing the height and subsequently changing the capacitance value, with the help of a modular interlocking block system. This reconfigurability further reduces the electrical size of the absorber structures, thereby leading to miniaturization. Structure A has an absorption frequency switching from 4.89 GHz to 2.26 GHz (with a 53.7% miniaturization). Similarly, Structure B has a frequency range between 2.98 and 1.89 GHz, leading to a size reduction of 36.5%, whereas the frequency shift is observed in Structure C from 2.25 GHz to 1.79 GHz (with a shift of 20.4%). The absorptivity curves in L, S, and C bands for various structures are unaltered by variation in polarization and incident angle (up to 30 degrees), indicating polarization-insensitive and angularly stable responses. Impedance responses (real and imaginary) are also evaluated to justify the absorption characteristics. The proposed structures are fabricated using the three-dimensional (3D) printing technique, and their test results are consistent with the simulated responses.

Design and measurement of a planar radome absorber under oblique incidence are presented. The structure is composed of a bandpass multilayers frequency selective surface (FSS) combined with periodic cylindrical-shaped absorbers. This configuration made the resulting structure transparent at the intended antenna operational frequencies (C-band) and absorptive at higher frequencies (X-band). The prototype has been fabricated with a dimension of 24.5 cm × 24.5 cm × 0.65 cm. Performance under oblique incidence (without the antenna) is investigated using NRL arch technique in the absorption band. Reflection less than −10 dB is maintained in the range of 8.5 GHz–11 GHz up to 40° incidence angle for incoming waves with perpendicular polarization. Under parallel polarization, the −10 dB reflection is achieved up to 50° in the range of 8 GHz–12 GHz.

This paper presents the design, fabrication, and electromagnetic testing of a frequency selective surface (FSS)-based novel hybrid nanocomposite absorber for enhanced electromagnetic absorption. The proposed hybrid absorber consists of an FSS pattern printed over a dielectric substrate backed by aluminum foil, which is integrated together with a customized dielectric nanocomposite sheet. The dielectric sheet comprises the conducting carbon black (CB) nanopowders embedded into the epoxy matrix. The fabricated nanocomposite sheet is first characterized for the effective complex permittivity using the free-space measurement technique, and the measured electromagnetic properties are then used for the design of a hybrid absorber. The proposed hybrid absorber having overall thickness of 2.6 mm shows enhanced −10-dB absorption bandwidth of 3.5 GHz, which is much higher than that of the FSS and CB nanocomposite samples measured individually. The proposed work provides an effective way to combine the absorption properties of FSS with that of a customized dielectric absorber sheet in order to design an efficient hybrid absorber having reduced thickness with enhanced absorption bandwidth for stealth technology and various strategic applications.