Reconfigurable Metasurface Based on Shape Memory Alloy Actuators for Ultra‐Wideband RCS Reduction

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.

A new design method for ultrabroadband radar cross section (RCS) reduction by exploiting characteristic complementary polarization conversion metasurfaces (PCMs) is proposed and validated. The proposed method lifts the conventional bandwidth limitation enforced by the performance of a single PCM and expands the RCS reduction bandwidth. A systematic strategy for ultrabroadband RCS reduction design is developed and an effective reflection coefficient amplitude of the composite surface ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Gamma _{ {eff}}$ </tex-math></inline-formula> ) is derived as a new RCS reduction indicator. Based on this indicator, complete polarization conversion (CPC) frequency points of PCM are identified as important characteristics, and PCM pairs with interleaved CPC points, termed as initial PCM (I-PCM) and complementary PCM (C-PCM), are designed to compensate each other for ultrabroadband RCS reduction. A scaling-and-tuning two-step method for designing the C-PCM of a specific I-PCM is developed and validated. To further improve RCS reduction performance, taking the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\Gamma _{ {eff}}}$ </tex-math></inline-formula> as an indicator, the particle swarm optimization (PSO) algorithm is used to select the optimal I-PCMs, C-PCMs, and their area ratios. Finally, an ultrabroadband RCS reduction surface is designed, fabricated, measured, and validated. It realizes 10-dB monostatic RCS reduction ranging from 7.6 to 26.2 GHz (110.7% relative bandwidth) which exceeds the polarization conversion bandwidth of each individual PCM. Besides, good polarization insensitivity and bistatic RCS reduction performance are also obtained. The proposed method provides a new route for designing broadband RCS reduction surface based on PCMs with unsatisfactory characteristics and greatly alleviates the performance requirement for PCMs.

In this paper, a checkerboard metasurface based on a novel physical mechanism, optimized multielement phase cancellation, is proposed for greatly expanding the bandwidth of radar cross section (RCS) reduction. More basic metaparticles and, in particular, the variable phase difference between them, greatly increase the ability to control electromagnetic waves. Interactions between multiple local waves produced by the basic metaparticles at multiple frequencies sampled in a superwide frequency band are manipulated and optimized simultaneously to achieve phase cancellation. The proposed metasurface can achieve a 10 dB RCS reduction in a superwide frequency band from 5.5 to 32.3 GHz with a ratio bandwidth (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</sub> /f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sub> of 5.87:1 under normal incidence for both polarizations. Furthermore, the RCS reduction is larger than 8 dB from 5.4 to 40 GHz with a ratio bandwidth of 7.4:1. The metasurface also has a good performance under wide-angle oblique incidences. The optimal metaparticle distribution is found to obtain the superwideband bistatic RCS reduction. The theoretical analysis, simulation, and experimental results are in good agreement and verify the ability and capability of the proposed mechanism.

Phase modulation of metasurfaces for polarization conversion and RCS reduction

Multilayer metasurface-based sandwich composites for wideband radar cross section reduction

A novel metasurface based on uneven layered fractal elements is designed and fabricated for ultra-wideband radar cross section (RCS) reduction in this paper. The proposed metasurface consists of two fractal subwavelength elements with different layer thickness. The reflection phase difference of 180° (±37°) between two unit cells covers an ultra-wide frequency range. Ultra-wideband RCS reduction results from the phase cancellation between two local waves produced by these two unit cells. The diffuse scattering of electromagnetic (EM) waves is caused by the randomized phase distribution, leading to a low monostatic and bistatic RCS simultaneously. This metasurface can achieve -10dB RCS reduction in an ultra-wide frequency range from 6.6 to 23.9 GHz with a ratio bandwidth (fH/fL) of 3.62:1 under normal incidences for both x- and y-polarized waves. Both the simulation and the measurement results are consistent to verify this excellent RCS reduction performance of the proposed metasurface.

In this communication, a high-gain partially reflecting surface (PRS) antenna with both in-band and out-of-band radar cross section (RCS) reduction (RCSR) is presented. A microstrip patch antenna acts as the source and is surrounded by the metamaterial ground plane (MGP), which is to reduce the in-band RCS of the proposed antenna. A PRS is located approximately one-half of the wavelength above the MGP to enhance the antenna directivity. In addition, an absorbing surface is adopted to reduce the out-of-band RCS. The design procedure of the antenna is illustrated, and the simulated results reveal that a significant RCSR has been accomplished in the frequency ranges of 8–17 GHz. Compared with the reference antenna, the average out-of-band RCSR is about 13 dB for both TE and TM polarization incident waves. What is more, the in-band RCSR is about 17 dB for the TM polarization incident wave. The simulated results match the measured results well, and the peak gain of the proposed antenna is 18.4 dBi at the operating frequency of 10 GHz.

An active reconfigurable radar absorbing structure (RAS) with the pin diode was proposed to reduce the radar cross section (RCS) of antenna. The operating states of the RAS reflector can be switched by using the pin diode. For ON-state and OFF-state diodes, the reflection coefficients of the RAS reflector were less than −25 dB and more than −0.8 dB around 8.3 GHz, respectively. The RAS reflector with ON-state diodes can be used as a dipole antenna reflector and has the same radiation performance as a dipole with a metal reflector, while the RAS reflector with OFF-state diodes can be used as a radar absorber for RCS reduction. Meanwhile, chessboard-like geometry RAS reflector was proposed to achieve wideband RCS reduction. The RCS reduction band covers the working band and is extended to 5-18 GHz. The results show that the reconfigurable RAS reflector can contribute to the antenna RCS reduction at working frequency without loss of radiation performance of dipole antenna. With the detection and stealth technology developing rapidly, radar cross section (RCS) reduction (1-4) has been a serious problem in military applications and has attracted significant attention recently. Since the scattering of the antennas has contributed a lot to the total RCS for low observable platforms, more and more attention has been paid to antenna scattering. Shaping, coating with radar absorbing materials (RAM) and using passive and active cancellation technology are basic methods in stealth technology (5-7). Furthermore, a number of new structures and materials are applied to the RCS reduction of antenna, such as artificial magnetic conductor (AMC), radar absorbing structure (RAS), frequency selective surface (FSS), etc. (8-11). However, these structures and materials cannot be used for stealth in the working band. When RAS is used, the gain of the antenna has a large degeneration (12), and the application of AMC will increase the profile of the antenna. All of them greatly deteriorate the stealth performance and application of radar system or other platform. In early years, antennas did not need to work in airborne or missile borne systems continuously. To realize the stealth of antenna, the radar was opened in close proximity to the target and shut down through the dangerous area. This method has the advantages of simple, feasible, and good stealth characteristic. In addition, the radiation performance of the antenna is preserved. However, this method can only reduce the antenna mode scattering, and the structural mode scattering still exists. Meanwhile, general metamaterials are completely passive structures, and the properties of these metamaterials cannot be changed. Therefore, in the complex electromagnetic environment, these metamaterials cannot be able to quickly adapt to the changes in the external electromagnetic environment and reduce its effect. In order to overcome these problems, reconfigurable RAS is put forward as a new concept. Reconfigurable technology has been successfully applied to antenna design. Early research focused on changing the current distribution by adjusting the radiator structure, which made the frequency, pattern, polarization or impedance of antenna changed. Currently, the reconfigurable metamaterial is

The recent advances in the development of coding metasurfaces created new opportunities in realization of radar cross section (RCS) reduction. Metasurfaces, composed of optimized geometries of meta-atoms arranged as periodic lattices, are devised to obtain desired electromagnetic (EM) scattering characteristics. Despite potential benefits, their rigorous design methodologies are still lacking, especially in the context of controlling the EM wavefront through parameter tuning of meta-atoms. One of the practical obstacles hindering efficient design of metasurfaces is implicit handling of RCS performance. To achieve essential RCS reduction, the design task is normally formulated in terms of phase reflection characteristics of the meta-atoms, whereas their reflection amplitudes—although contributing to the overall performance of the structure—is largely ignored. As a result, the conventional approaches are unable to determine truly optimum solutions. This article proposes a novel formulation of the metasurface design task with explicit handling of RCS reduction at the level of meta-atoms. Our methodology accounts for both the phase and reflection amplitudes of the unit cells. The design objective is defined to directly optimize the RCS reduction bandwidth at the specified level (e.g., 10 dB) w.r.t. the metallic surface. The benefits of the presented scheme are twofold: (i) it provides a reliable insight into the metasurface properties even though the design process is carried out at the level of meta-atoms, (ii) the obtained design requires minimum amount of tuning at the level of the entire metasurface. None of these is possible for phase-response-based approach fostered in the literature. For practical purposes, the design is conducted using a surrogate-assisted procedure involving kriging metamodels, which enables global optimization at a low computational cost. To corroborate the utility of our formulation, a high-performance metasurface incorporating crusader-cross-shaped meta-atoms has been developed. The obtained results indicate that the system characteristics predicted at the design stage are well aligned with those of the EM-simulated structure (which is not the case for the traditional design approach). The metasurface features 10-dB RCS reduction in the frequency range of 16.5 GHz to 34.6 GHz, as validated both numerically and experimentally.

ABSTRACTThis paper presents a single‐layer multi‐bit coding polarization conversion metasurface (MCPCM) designed for the attainment of broadband and wide‐angle radar cross section (RCS) reduction. The metasurface unit possesses a symmetrical structure composed of a bidirectional arrow and a defective rectangular ring, making it polarization‐insensitive to electromagnetic (EM) waves. The designed metasurface unit exhibits broadband polarization conversion efficiency across the frequency spectrum of 11.2–25.6 GHz, achieving a polarization conversion rate (PCR) beyond 90% and displaying four resonance points. We obtained multi‐bit phase responses with phase differences of 180°, 90°, and 45° through the optimized design of the unit's structure. A discrete artificial bee colony (DABC) algorithm optimizes the coding sequence, enhancing RCS reduction performance. Compared with an equivalently sized perfect electric conductor (PEC), the optimized MCPCM achieves over 10 dB RCS reduction within the 11–25 GHz frequency range. Additionally, the optimized MCPCM achieves a broader RCS reduction bandwidth compared to traditional chessboard‐patterned metasurfaces. Furthermore, we investigated the proposed MCPCM's bistatic RCS performance, achieving substantial RCS reduction over a broad spectrum of incident angles. Simulation and measurement results validate the proposed metasurface capability for effective EM wave manipulation, demonstrating its potential for broadband and wide‐angle RCS reduction.

Coding metasurfaces have been introduced as efficient tools allowing meticulous control over the electromagnetic (EM) scattering. One of their relevant application areas is radar cross section (RCS) reduction, which principally relies on the diffusion of impinging EM waves. Despite its significance, careful control of the scattering properties poses a serious challenge at the level of practical realization. This article is concerned with (global) design optimization of coding metasurfaces featuring broadband RCS reduction. We adopt a two-stage optimization procedure involving data-driven supervised-learning, sequential-search strategy, and direct EM-based design closure of the entire metasurface oriented toward maximizing the RCS reduction bandwidth. Our framework is then used to develop a two-bit coding metasurface. To handle the combinatorial explosion at the concurrent meta-atom optimization stage, a sequential-search strategy has been developed that enables global search capability at low computational cost. Finally, EM-based optimization is executed to maximize RCS reduction bandwidth at the level of entire metasurface. The properties of the coding metasurface are demonstrated using monostatic and bistatic RCS performance. The 10-dB RCS reduction can be obtained in the frequency range of 14.8–37.2 GHz, in a monostatic configuration. Also, 15-dB RCS reduction can be maintained in the frequency range of 16.7–37 GHz. Simulations are validated using physical measurements of the fabricated prototypes. Finally, the performance of the structure is benchmarked against recently reported designs.

From the begining of military warfare, it has always been extremely important to know the enemy position and hide oneself to capitalize on elements of surprise and initiative, and same is true for naval warfare. Radar is the primary instrument used for detecting enemy platforms today.Radar detects a target by clocking time taken by a known pulse of electromagnetic energy to get to the target and return. Radar cross section (RCS) is the measure of reflective strength of a target. Reducing the RCS of a platform implies its late detection, used to capitalize on surprise and initiative. RCS is also important for survivability evaluation since most modern weapons use installed radars during final engagement phase. As a result, RCS of a warship has transformed into a very important design factor for stealth to achieve surprise, initiative and survivability. Thus accurate RCS determination and RCS reduction are matters of extreme importance. The purpose of this study is to provide an understanding RCS reduction and RCS determination methods used on warships today. In doing so, this study will discuss importance of RCS, radar fundamentals and RCS basics, RCS reduction and RCS determination methods. It will also present hindrances in optimizing RCSR on warships, impact of these hindrances on navies around the world, and comment on possible remedies to these hindrances.

An approach for wideband radar cross section (RCS) reduction of a microstrip array antenna is presented and discussed. The scheme is based on the microstrip resonators and absorptive frequency selective surface (AFSS) with a wideband absorptive property over the low band 1.9–7.5 GHz and a transmission characteristic at high frequency 11.05 GHz. The AFSS is designed to realize the out-of-band RCS reduction and preserve the radiation performance simultaneously, and it is placed above the antenna with the operating frequency of 11.05 GHz. Moreover, the microstrip resonators are loaded to obtain the in-band RCS reduction. As a result, a significant RCS reduction from 1.5 GHz to 13 GHz for both types of polarization has been accomplished. Compared with the reference antenna, the simulated results exhibit that the monostatic RCS of the proposed array antenna in x- and y-polarization can be reduced as much as 17.6 dB and 21.5 dB, respectively. And the measured results agree well with the simulated ones.

Absorption-transmission-diffusion-type radar cross section (RCS) reduction metasurfaces based on the frequency selective absorber and the polarization conversion chessboard structure are proposed in this paper. The RCS reduction metasurfaces consist of three parts, i.e. the top layer realized by a cross-shaped meander line loaded with lumped resistors for low-frequency absorption and high-frequency transmission, the mid-layer constructed by angular ring vias frequency selective surface (FSS) sandwiched with two square patches FSS for broadband transmission in mid-frequency zone, and the bottom layer employed chessboard metasurfaces composed of dual-arrow type polarization conversion units for scattering cancellation in high frequency zone. The physical mechanisms of the RCS reduction metasurfaces with multifunctional electromagnetic controlling including wave absorption, bandpass transmission, polarization reversal, and phase cancellation are also discussed and analyzed. Simulation and experimental results demonstrate a reflection of lower than −10 dB for normal incident linearly polarized waves over a frequency range of 2.5 GHz–18 GHz, with a relative bandwidth of 151.22%; transmission passband properties with insertion loss better to −3 dB in the frequency range of 5.62 GHz–11.0 GHz, with a relative bandwidth of 64.74%. Due to the resonance-coupling-resonance principle, the broadband transmission with low insertion loss was obtained. The RCS properties of the proposed multi-functional metasurfaces were further analyzed by numerical simulation and experimental measurement, and the results showed that RCS reduction of over 10 dB was achieved for normal incident different polarized waves at the frequency range of 2.5 GHz to 17 GHz. Even in the case of normal incidence, there was a deviation between the reflection characteristics and RCS reduction. The RCS reduction metasurfaces are expected to be employed in applications where multi mechanism collaborative control of electromagnetic waves is of extreme importance.

In this paper, a checkerboard metasurface is designed to achieve the radar cross section (RCS) reduction within a wide frequency band. The basic meta-atoms are printed on a substrate plane with two choices of layer thickness, and destructive interference are obtained by adjusting the geometry parameters of the meta-atoms. The combination of multiple layer thicknesses increases the range of the phase cancellation and improves the ability to control the electromagnetic waves of the planar target. Based on the optimized multielement phase cancellation (OMEPC), the designed metasurface can realize a 10 dB RCS reduction in a wideband from 4.8 to 19.6 GHz with a ratio bandwidth ( fH/fL) of 4.08:1 under normal incidence for both polarizations. In addition, a 10 dB RCS reduction performance can be preserved under wide-angle oblique incidences under TM wave. The estimated and simulated results are in good agreement and prove that the proposed metasurface can realize wideband cancellation interference in its style sheet. This work is a verification to prove that the increase of layer thickness can expand the bandwidth for RCS reduction apparently compared with a single layer.