Bendable over-expanded honeycomb sandwich composites for broadband RCS reduction in leading edge applications

Broadband, wide-angle, polarization-independent and lightweight low-scattering coding metamaterial based on stereo meta-atoms

In this paper, we propose a method for designing checkerboard surfaces using strategically placed artificial magnetic conductors (AMCs) for wideband radar cross section (RCS) reduction. The theoretical analysis shows that the reflection phases of the two AMCs should be linear. The cancelation conditions of checkerboard structure are analytically derived using an equivalent transmission line model, and the ideal impedance conditions of the two AMCs are obtained, which serve as guidance to select patterns of two AMCs unit. A planar checkerboard surface with two kinds of AMC elements is designed for wideband RCS reduction. A 10 dB RCS reduction is observed for approximately 91.5% of the relative bandwidth (3.77–10.14 GHz) under normal incidence. A prototype of the proposed checkerboard surface is fabricated, and the measurements results show good coincidence with the synthetic studies. The bistatic RCS of the checkerboard surface is considered for both transverse electric and transverse magnetic polarizations under oblique incidence.

In this paper, an ultra-wideband metasurface based on optimized multielement phase cancellation (OMEPC) is proposed for radar cross section (RCS) reduction. The square ring structure with adjustable size is chosen as the basic unit cells. Metasurfaces based on the traditional method of opposite phase cancellation (OPC) is designed by combining two unit cells with an approximately 180° phase difference, which has a narrow bandwidth for phase cancellation. The bandwidth and oblique incidence performance of RCS reduction are two critical factors of stealth technology. More unit cells enable the phase difference between them to be relatively stable when the frequency is changed, which extends the bandwidth of RCS reduction. Based on the array pattern synthesis theory, the PSO algorithm is used to make the search of the optimal unit cells. The designed 16-element artificial magnetic conductor (AMC) surface can realize a 10 dB RCS reduction in a superwide frequency band ranging from 5.67 to 18.72 GHz with a ratio bandwidth (f H /f L ) of 3.3 : 1 under normal incidence for both polarizations. Compared to the previous researches for planar checkboard metasurface, the proposed metasurface has a wider bandwidth. Furthermore, the performance of the proposed metasurface were studied under the oblique incidence with incident angle of θ = 20° and θ = 30° for both transverse-electric (TE) and transverse-magnetic (TM) polarizations. Significant RCS reduction is also achieved under oblique incidence, which shows the excellent performance of destructive interference. The theoretical analysis and simulation results are in good agreement and demonstrate the proposed metasurface can realize an ultra-wideband RCS reduction.

In this communication, a novel metasurface with multiple unit cells of different thicknesses based on optimized multielement phase cancellation (OMEPC) is proposed for extremely wideband, omnidirectional, and polarization-independent radar cross section (RCS) reduction under wide-angle oblique incidence. Omnidirectional characteristic refers to the capability of RCS reduction under all azimuth angles ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varphi ^{i}$ </tex-math></inline-formula> ) at certain elevation angles ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\theta ^{i}$ </tex-math></inline-formula> ). The equivalent circuit model of the square patch unit cell is analyzed under both normal and oblique incidences. Unit cells of multiple thicknesses are adopted to solve the in-phase reflection at certain frequencies. The scattered fields of the proposed multiple-element metasurface under different frequencies, polarizations, and incident angles are jointly optimized and simultaneously suppressed by OMEPC. When the range of incident angles is increased from 0° to 40°, the proposed multiple-element metasurface can achieve the 10 dB specular RCS reduction from 7.34 to 64.85 GHz with a ratio bandwidth of 8.84:1 for both TE and TM polarizations, and the 8 dB RCS reduction bandwidth is approximately 6.76–83.70 GHz. The measurement results are in agreement with the theoretical analysis and simulation results.

In this article, a chessboard metasurface is proposed to reduce the radar cross section (RCS) of the dihedral corner reflector (DCR). By loading a checkerboard metasurface on one side of the DCR, a broadband RCS reduction is achieved. The simulation results show that the proposed DCR has a significant RCS reduction of more than −10 dB in the frequency ranges of 6.5–13.53 GHz with fractional bandwidths of 70.2% under transverse electric (TE) polarization and in the frequency ranges of 6.2–13.6 GHz with fractional bandwidths of 74.75% under transverse magnetic (TM) polarization. Additionally, the RCS reduction properties of the proposed DCR at various azimuth angles are investigated, and the wide‐angle RCS reduction from 0° to 65° is realized. To obtain more wide‐angle RCS reduction, double‐sided checkerboard loading is performed on the DCR. The RCS reduction angle range increases from 65° to 90° for TE polarization and TM polarization. Namely, the RCS is reduced from 0° to 90° for TE polarization and TM polarization, as calculated by numerical simulations. The measured results are in great agreement with the simulation results. This method verifies the novelty and effectiveness of wide‐angle and wideband RCS reduction of DCR.

In this study, a broadband radar absorbing sandwich composite (BRASC) with stable absorption performance for oblique incidence is proposed to reduce radar cross-section (RCS) of an engine inlet. To design the BRASC, the objective function for broadband absorption performance is coupled with transmission line theory. The absorption bandwidth of the designed BRASC with a − 10 dB criterion is 8.2 GHz. The absorption performance of the oblique incidence in both transverse electric and transverse magnetic polarizations is excellent in maintaining the −10 dB up to 60°. The measurement results of the fabricated specimens confirm that the overall tendency of the measurement results matches well with the analysis results. The engine duct replaced with the BRASC is analyzed to confirm the effect of RCS reduction on the engine inlet. RCS simulation shows that the proposed BRASC effectively reduces the RCS over a wide frequency range. Because the S-shaped duct, which is used in low observable aircraft for RCS reduction, causes multiple scattering at oblique incidence, the absorbers applied to the engine duct should maintain a stable oblique incidence performance. Hence, the proposed BRASC may be a promising candidate for engine inlets with small RCS.

In this paper, a wide incidence angle-insensitive metamaterial absorber is proposed using eight-circular-sector (ECS). Under normal incidence, the proposed absorber shows high absorptivity at different polarizations due to its symmetric geometry. Under oblique incidence, zero-reflection conditions for transverse electric (TE) and transverse magnetic (TM) polarization are different. Nevertheless, the proposed absorber shows high absorptivity under oblique incidence of both TE and TM polarization due to ECS. The performance of the proposed absorber was demonstrated with full-wave simulation and measurements. The simulated absorptivity at the specular angles exceed 90% and the frequency variation is less than 0.7% at approximately 9.26 GHz up to a 70° incidence angle in both TM and TE polarization. We built the proposed absorber on a printed-circuit board with 20 × 20 unit cells, and we demonstrated its performance experimentally in free space. The measured absorptivity at 9.26 GHz for the specular angles is close to 98% for all polarization angles under normal incidence. As the incidence angle is varied from 0° to 70°, the measured absorptivity at 9.26 GHz for the specular angles remain above 92% in both TE and TM polarization.

A linear polarization conversion coding metasurface (MS) is proposed for the radar cross section (RCS) reduction of an object. The designed MS has both wideband RCS reduction and high efficiency of mirror reflection in different frequency bands. The cells are arranged according to different 1 bit coding sequences, and the characteristics of RCS reduction are verified by emulations and experiments. The simulation results indicate that the 01/10 coding MSs possess RCS reduction of greater than 10 dB in 9.5-13.9 and 15.2-20.4 GHz, and the 01/10 coding MS is less sensitive to polarization under normal incidence. The experiment results show that 10 dB RCS reduction is achieved in 10.2-14.0 and 15.3-20.7 GHz under normal incidence, and the relative bandwidth (BW) is 32% and 30%, while high efficiency of mirror reflection is obtained from 14.0 to 15.3 GHz. The experimental results are in good agreement with the numerical simulations. Additionally, for the 01/10 coding MS, the dual-broadband performance is also well maintained under 0°-45° oblique incidence. It is a new and practical method to suppress the scattering of metal objects by the combination of scattering and reflection, which has significant potential in the applications of antenna designs or stealth technology fields.

In this paper, we employed an efficient polarization conversion metasurface (PCM) to achieve broadband radar cross section (RCS) reduction for plane targets. Simulation results illustrate that the proposed “whale-shaped” unit achieves more than 90% polarization conversion ratio (PCR) in an ultra-broadband from 8.37 to 22.67 GHz. We combine the PCM unit into a 3 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> 3 scale lattice to form periodic structure. Subsequently, we employ the PCM lattice and its mirror lattice as “Lattice 0” and “Lattice 1” of digital coding. In order to achieve the best RCS reduction performance, we obtain the optimized combinational layout by utilizing information entropy theory and genetic algorithm. Different from the traditional checkerboard distribution, the optimized coding PCM realizes scattering beam diffusion in space. The fitting of measurements and simulations demonstrate that the proposed PCM realizes monostatic RCS reduction of −10dB in the ultra-broadband from 9.37 to 22.79 GHz. Simultaneously, the proposed optimized coding PCM operates over a wide-angle oblique incidence range from 0° to 45°. The proposed “whale-shaped” optimized coding PCM features both ultra-broadband operating bandwidth and high angular stability, which is informative for the practical application for PCMs.

An efficient and fast strategy to design and realize single layer Fourier phased metasurfaces for wideband radar cross section (RCS) reduction when illuminated by a circular polarization (CP) plane wave is proposed in this letter. The scattering phase (between 0 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> and 360 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> ) required at each unit cell of the proposed metasurfaces was computed using the Fourier phase formula in which the focal length ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</i> ) is inversely proportional to the phase distribution. Pancharatnam-Berry (PB) phase theory was applied with unit cells of subwavelength periodicity to further enhance the scattering and RCS reduction characteristics. The proposed wideband Fourier phased metasurface has a square shape and contains 30 × 30 PB unit cells with subwavelength periodicity of 5 mm ≈ 0.26λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16GHz</sub> . Both simulation and measured results show that the proposed Fourier phased metasurfaces can achieve more than 10 dB of RCS reduction under normal incidence of CP plane wave regardless of the value of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</i> . Under oblique incidence, more than 10 dB of RCS reduction was maintained for incident angles up to 60°. In addition, the single-layer Fourier phased metasurface features wideband 10 dB RCS reduction bandwidth from 10 GHz to 24 GHz with a thickness of only 2 mm. This resulted in an 82.3% fractional bandwidth (FBW) of RCS reduction which is higher than other designs reported in the literature. The proposed design strategy provides a promising way to design and realize metasurfaces for wideband and stable RCS reduction performance without the need to use a computationally complex and/or time-consuming and slow running optimization algorithm.

An efficient optimizing method of ultra-wideband radar cross-section (RCS) reduction metasurface involve unit amplitude differences and mutual coupling effects is proposed in the letter. Multiple unit phase cancellation was used to broaden RCS-reduction bandwidth. The genetic algorithm was utilized to further design and select the optimal combination from various tiles. To improve the optimization efficiency and accuracy, the space mapping (SM) technique was exploited to refine the design process, which involves the interactions of scattering fields among tiles and subarrays in wideband. Meanwhile, the constraint of wide-angle oblique incidences is also considered in wideband low RCS design. An ultra-wideband RCS reduction checkboard meatsurface can be accomplished by fast optimization of low-cost coarse model and few parameter mapping courses between fine and coarse model. A 10dB RCS reduction for HH and VV polarizations under normal and other oblique incidence angles can be achieved from 5 to 40 GHz. Good agreements between simulation and experiment results are obtained.

This article introduces the concept, theory, and design of an angle and polarization insensitive radar cross section (RCS) reduction metasurface, using a hybrid mechanism of polarization conversion and absorption. By introducing ladder‐ and rectangle‐shaped metallic patches in the vertical dimension of a 3‐D structure, polarization conversion rate (PCR) deterioration, brought by the increase of equivalent substrate thickness at oblique incidences, can be suppressed. Furthermore, lumped resistors are loaded at proper places in each polarization conversion cell, to achieve the power absorption while maintain the angular insensitivity of the PCR. With the above hybrid mechanism, a stable 10‐dB RCS reduction can be achieved regardless of the angle of incidence in a wide range and polarization directions. An equivalent circuit model is established for explaining the physical mechanism of the proposed metasurface. For validation, a prototype is fabricated and tested. Measurement results indicate that, for both monostatic RCS at the normal incidence and specular RCS of off‐normal incidences from 0° to 45°, a 10‐dB TE‐ and TM‐mode RCS reduction can be achieved in the entire X‐band (8–12 GHz) and Ku‐band (12–18 GHz).

In this work, an ultra-wideband reflective polarization conversion metasurface (PCM) is proposed at first. Because the PCM is an anisotropic structure that is symmetric with respect to both x- and y-axes, and the reflection phase difference under x- and y-polarized incidences is close to 180° in an ultra-wide frequency range, the PCM can achieve both linear polarization conversion and circular-polarization (CP) maintaining reflection in the ultra-wide frequency band from 8.3 to 41.3 GHz except for near the frequency point of 38.8 GHz. Moreover, when its unit cell structure is rotated by 90°, its cross-polarized reflection coefficient under LP incidence, together with the co-polarized reflection coefficient under CP incidence, will be changed by almost 180° in phase. Thus, based on the PCM, an ultra-wideband coding diffusion metasurface (CDM) is further proposed for radar cross section (RCS) reduction. The simulation and experiment results indicate that the CDM can achieve effective RCS reduction in the ultra-wide frequency band of 8.2–41.7 GHz under normal incidence with arbitrary polarization; in addition, an ultra-wideband RCS reduction can still be realized under oblique incidence with an incident angle less than 45°, which shows that the CDM is of good application value in radar stealth technology.

ABSTRACTIn this work, an ultra-wideband coding phase gradient metasurface (CPGM) is proposed for radar cross section (RCS) reduction. The design process is presented in detail, in which eight types of coding elements are proposed firstly by using Pancharatnam-Berry (P-B) phase. The eight types of coding elements have different reflection direction or phase response under the same EM-wave incidence for they can introduce a series of phase gradients with different directions or starting-values under both right-handed and left-handed circular-polarized incidences, so the proposed CPGM composed of these coding elements has excellent performance in RCS reduction. The simulated results show that, compared with a pure metallic plate with the same size, the RCS of the CPGM can be reduced more than 10 dB in the ultra-wide frequency band of 9.2–46.2 GHz under normal incidence with arbitrary polarization, the relative bandwidth is up to 133.6%; moreover, the RCS reduction under oblique incidence with arbitrary polarization can still be kept larger than 9.3 dB in the frequency band of 13.1–42.5 GHz when the incident angle is increased to 45°. Finally, one experiment is carried out, a reasonable agreement exists between the simulated and experimental results.

We propose an absorption–diffusion integrated metasurface that achieves high-performance stealth of electromagnetic waves with high angular stability in an ultrabroad frequency band. To this end, we designed two types of absorbing meta-atoms with reflection coefficients less than −10 dB in the broadband, which can maintain a phase difference of ∼180° in the range of 5.35–13.5 GHz. Then, the genetic algorithm is utilized to optimize the relationship between the arbitrary coding sequence of meta-atoms and their far-field patterns to obtain the optimal arrangement of the meta-atoms of the metasurface. The simulation and test results of the sample show that the polarization-independent radar cross section (RCS) reduction characteristic over −10 dB in the broadband range (4–18 GHz in simulation and 4.8–16.8 GHz in test) can be achieved. Particularly, the proposed metasurface achieves RCS reduction values over −30 dB in the 7.7–12.4 GHz range. At the same time, the RCS reduction behavior of −10 dB can be maintained to 45° oblique incidence. Experiment and simulation results demonstrate the effectiveness of the present scheme, and the proposed metasurface exhibits better RCS reduction performance than other published literature. This work is of great significance for the rapid design of high-performance absorption–diffusion integrated metasurfaces, which have important prospects in stealth, camouflage, and other related applications.