Broadband Radar Cross Section Research Articles

A Broadband Metasurface Antenna Array With Ultrawideband RCS Reduction

In this communication, a metasurface-based antenna array with wideband radiation and broadband radar cross section (RCS) reduction is constructed. Without introducing additional radiators, the broadband radiation performance of the antenna array is achieved by inspiring the metasurface itself. In addition, the characteristic mode theory is introduced to analyze the radiation mechanism of the radiator elements and the proposed antenna array. Both simulations and measurements are carried out to verify the performance of the proposed antenna array. The results show that the impedance bandwidth and 3 dB gain bandwidth of the proposed antenna array are 5.1–9.0 and 5.1–8.0 GHz, respectively, with a peak gain of 12.1 dBi. Monostatic RCS reduction more than 10 dB is achieved within the frequency band of 6.0–26.2 GHz.

Analysis and Design of Wideband Low-RCS Fabry–Perot Antennas With Reduced Profile

In this article, two incomplete partially reflective surface (PRS)-based Fabry–Perot (F-P) antennas each with a high gain, wideband low radar cross section (RCS), and reduced profile are proposed. The reductions of the antenna profiles and RCS are realized based on artificial magnetic conductor (AMC) surfaces and the phase cancellation, respectively. In the first design, two AMC surfaces are placed around the radiation patch and above the PRS. The thickness of the F-P cavity is reduced by half due to the zero-phase reflection of the lower AMC surface around the radiation patch. The income waves are partly reflected by the upper AMC surface above the PRS with zero reflection phase and partly penetrate the PRS and then reflected by the lower AMC surface also with zero reflection phase. The reflected EM waves from the bottom AMC surface travel another distance of quarter wavelength in the upward direction making the total electrical path traveled be half wavelength, thereby causing a phase shift of 180° with the reflected waves from the upper AMC surface, and thus resulting in a wideband RCS reduction. For the second design, another AMC surface is placed below the PRS. On one hand, the middle and lower AMC surfaces have the same reflection phases and then an F-P cavity with a subwavelength thickness is generated. On the other hand, the phase difference between the upper and lower AMC surfaces is also designed as 180°, which is used to suppress the total backward RCS. The two F-P antennas are both fabricated and measured. Measured results validate their reflection, radiation, and scattering performance well. To the best of our knowledge, this work is the first design, which integrates the physical (thin F-P cavity), radiation (gain enhancement), and scattering (broadband RCS reduction) performance of the F-P antenna.

A Wideband Low-RCS Metasurface-Inspired Circularly Polarized Slot Array Based on AI-Driven Antenna Design Optimization Algorithm

A metasurface (MS)-inspired low-profile circularly polarized (CP) slot array with a wide CP band and broadband low radar cross section (RCS) is proposed in this communication. The slot array consists of four element antennas, four grounded substrates, and a sequential-rotated feeding network. In terms of radiation performance, the array yields a wide CP band resulting from the CP element antenna and the sequential-rotated feeding network. The CP element antenna is achieved due to the polarization conversion property of the MS-based superstrate. The feeding network with multiple related design parameters is optimized by an artificial intelligence (AI)-driven antenna design method to find the widest bandwidth. In terms of scattering performance, broadband RCS reduction is achieved by using a hybrid RCS reduction technique that combines two destructive interference principles. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\vert \text{S}_{11}\vert &lt; -10$ </tex-math></inline-formula> dB bandwidth reaches 53.2%, the AR < 3 dB bandwidth reaches 50%, and the RCS reduction bandwidth reaches 147.8% for a low-profile structure with a relatively low number of MS unit cells. A prototype was fabricated and measured. The measured and simulated results are in good agreement.

Broadband Radar Cross Section Reduction Binary Metasurface With a High-Efficiency Intraband Transmission Window

In this letter, a broadband radar cross section (RCS) reduction binary metasurface with a high-efficiency intraband transmission window is proposed and implemented based on multilayered elements combining the Jerusalem cross and cross-slot patterns. Electromagnetic responses of such structures are investigated by the equivalent circuit method. The results indicate that the metasurface consisting of elements with two types of Jerusalem crosses can invoke diffused scattering to reduce the RCS of metallic targets. Meanwhile, the metasurface has a high-efficiency transmission window since the bandpass cross-slot pattern is deployed here. The simulation results indicate that the metasurface can achieve a −10 dB RCS reduction from 3.5 to 11.5 GHz and transmission with an insertion loss of 0.72 dB at 8.4 GHz. The performances of the proposed metasurface are verified experimentally. Our design may find potential applications in the low-RCS antenna or the stealth radome for wireless facilities.

Dual-function flexible metasurface for absorption and polarization conversion and its application for radar cross section reduction

In this paper, a flexible metasurface with dual functions of absorption and polarization conversion is proposed and applied for radar cross section (RCS) reduction. The metasurface unit adopts a metallic-backed structure, and its width and thickness are approximately 0.62 and 0.24 times the free-space wavelength at the center working frequency, respectively. Different from the traditional metasurface, the resonators with low-frequency absorption and high-frequency polarization conversion are horizontally combined to achieve dual-function integration without the use of tunable elements. The structure adopts a flexible substrate, which is also suitable for conformal conditions. From 3.74 to 14.84 GHz, the metasurface has good impedance matching characteristics. The metasurface performs effective absorption and polarization conversion in the frequency bands of 3.78–6.34 GHz and 7.90–14.80 GHz, respectively. The absorption is mainly achieved through the ohmic loss of the lumped resistance, while the polarization conversion is performed through the electromagnetic resonance of the metallic structure. Then, the sample prototype is fabricated for demonstration, and the measurement result is well consistent with the simulation one. Furthermore, the checkerboard-arrangement array of the metasurface and its mirror unit can efficiently reduce the RCS over 7 dB in the range of 3.52–15.28 GHz. As expected, the proposed flexible metasurface can not only be used as an absorber/polarization converter but also be combined to realize broadband RCS reduction, which is of great significance for multi-function and conformal stealth applications.

Ultrabroadband RCS Reduction Design by Exploiting Characteristic Complementary Polarization Conversion Metasurfaces

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.

Design of low RCS high gain CP slot antenna using polarization conversion metasurface

ABSTRACT In this work, a circularly polarised (CP) slot antenna based on linear polarisation conversion metasurface (PCM) is proposed for broadband radar cross-section (RCS) reduction and gain improvement. The proposed metasurface is designed using the chequerboard alignment of PCM unit cells and its mirrored ones to meet the phase cancellation principle required for RCS reduction. An improved loading technique of the PCM consisting of rearrangement of the PCM unit cells across the radiating slot is employed to enhance the gain of the proposed antenna. The measured results show that the operating bandwidth of the proposed antenna is in the range of 9.8–10.8 GHz. The maximum gain achieved is 2.4 dB more than the reference antenna while the 3-dB axial ratio bandwidth (ARBW) obtained is 9.8–10.6 GHz. The proposed antenna achieves an average RCS reduction of 11 dB in the frequency region of 6–18 GHz in comparison to the reference antenna.

Ultra-wideband RCS reduction based on coupling effects between beam diffuse and absorptive structures.

A hybrid design method for broadband radar cross section (RCS) reduction is proposed and successfully demonstrated based on the coupling effects between diffuse and absorptive structures. The reflection energy is distributed into more directions away from the source direction by the one-bit diffuse coding metasurface (CM). The two-layer resistive frequency selective surface (RFSS) is employed in the one-bit CM structure, reducing the amplitude of the co- and cross-polarized reflected waves under circularly polarized wave incidence by converting it into ohmic loss. In addition, the bandwidth of RCS reduction is further broadened through the coupling effects between the metallic patterns and the two-layer RFSS. The coupling effect shows that the absorption rate of the composite structure is significantly improved compared to the only RFSS structure. A lightweight CM loaded with RFSS (the area density is 597 g/m2) was fabricated, analyzed, simulated, and measured. The results show that the proposed mechanism can effectively break the bandwidth constraints of traditional diffusion and absorption methods. Furthermore, the proposed mechanism significantly expands the bandwidth of RCS reduction. The proposed metasurface can achieve a 10 dB RCS reduction in an ultra-wideband from 7.3 to 44.2 GHz with about 143.3% fractional bandwidth. Moreover, the metasurface also has good performances under wide-angle oblique incidences. Under the condition of maintaining lightweight, the design provides an idea for broadening the frequency band.

Ultra-Broadband RCS Reduction Based on Optimized Coding “Whale-Shaped” Polarization Conversion Metasurface With Angular Stability

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.

Supervised-Learning-Based Development of Multibit RCS-Reduced Coding Metasurfaces

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.

A tunable metasurface based on Vanadium dioxide for Broadband RCS reduction

In this paper, a novel chessboard-like of metasurface based on two kinds of unit cell, using Vanadium dioxide (VO2) is proposed, designed, and demonstrated for radar cross-section (RCS) reduction. By accurately designing the geometric parameters of two units, the reflection phase difference between the reflected waves can be obtained over a broadband region, thereby causing the interference cancellation and leading to the RCS reduction. In addition, the utilization of VO2 enables this metasurface to realize the reconfiguration of the working band while under different temperature conditions. Based on the above points, simulation results show that the proposed metasurface is provided with a −10 dB RCS reduction in 6.40–7.23 GHz and 14.97–23.97 GHz at a low-temperature state (300 K), and can be shifted to 7.10–14.13 GHz and 24.10–25.87 GHz at a high-temperature state (350 K), actualizing full coverage of the working frequency band from 6.40–25.87 GHz. Besides, the influences brought by different periodic arrangements of the unit cell and dielectric layer thickness on the reduction of RCS are also discussed in detail.

Broadband radar cross section reduction by an absorptive metasurface based on a magnetic absorbing material.

A highly feasible approach to achieve a broadband radar cross section (RCS) reduction using a simple magnetic metasurface is presented. A magnetic absorbing material (MAM) with high permittivity and magnetic loss is introduced into the metasurface design instead of the more common dielectric material to considerably reduce its thickness. The metasurface is composed of an optimized two-dimensional array of MAM meta-atoms and a metal plate in back. The meta-atoms share a simple square ring shape but with variable geometrical parameters, forming strong absorption in different frequency bands with large reflection phase differences. By hybridizing the absorption and phase-cancelation technique, a 10-dB RCS reduction from 3.4 to 18 GHz is achieved at a thickness of only 4 mm. Further experimental measurements are provided to evaluate the performance. Our work provides a promising way to broaden the bandwidth of RCS reduction with low density, reduced thickness, and stable performance, which can be utilized in harsh physical and chemical environments.

Broadband RCS Reduction by a Quaternionic Metasurface.

A quaternionic metasurface consisting of two pairs of units with destructive phase difference is proposed to extend the bandwidth of radar cross section (RCS) reduction. The two pairs of units are designed to have complementary phase-different bandwidth, which extends the bandwidth of RCS reduction. The overlaps of their bandwidth enhance the RCS reduction, resulting in a metasurface having broadband and strong RCS reduction. This design and the wideband RCS reduction of the quaternionic metasurface were verified by analytical calculation with superposition principle of electric field, numerical simulation with commercial software package CST Microwave Studio and experiment in microwave anechoic chamber. The scattering mechanism and the angular performance of the quaternionic metasurface were also investigated.

Machine-Learning-Powered EM-Based Framework for Efficient and Reliable Design of Low Scattering Metasurfaces

Popularity of metasurfaces has been continuously growing due to their attractive properties including the ability to effectively manipulate electromagnetic (EM) waves. Metasurfaces comprise optimized geometries of unit cells arranged as a periodic lattice to obtain a desired EM response. One of their emerging application areas is the stealth technology, in particular, realization of radar cross section (RCS) reduction. Despite potential benefits, a practical obstacle hindering widespread metasurface utilization is the lack of systematic design procedures. Conventional approaches are largely intuition-inspired and demand heavy designer's interaction while exploring the parameter space and pursuing optimum unit cell geometries. Not surprisingly, these are unable to identify truly optimum solutions. In this article, we introduce a novel machine-learning-based framework for automated and computationally efficient design of metasurfaces realizing broadband RCS reduction. Our methodology is a three-stage procedure that involves global surrogate-assisted optimization of the unit cells, followed by their local refinement. The last stage is direct EM-driven maximization of the RCS reduction bandwidth, facilitated by appropriate formulation of the objective function involving regularization terms. The appealing feature of the proposed framework is that it optimizes the RCS reduction bandwidth directly at the level of the entire metasurface as opposed to merely optimizing unit cell geometries. Computational feasibility of the optimization process, especially its last stage, is ensured by high-quality initial designs rendered during the first two stages. To corroborate the utility of our procedure, it has been applied to several metasurface designs reported in the literature, leading to the RCS reduction bandwidth improvement by 15%-25% when compared with the original designs. Furthermore, it was used to design a novel metasurface featuring over 100% of relative bandwidth. Although the procedure has been used in the context of RCS design, it can be generalized to handle metasurface development for other application areas.

Resonance phase and geometric phase integrated diffusion metasurface for broadband scattering control

In this paper, we demonstrate a binary diffusion metasurface with a broadband radar cross section (RCS) reduction performance based on the integration of resonance and geometric phase manipulation. The working principle of such a metasurface was investigated and the general criterion for the electromagnetic (EM) responses of the meta-atom was derived. In particular, we deployed a split metallic ellipse ring with a beam along the short axis and its counterpart with a 90° inclined angle as the binary elements to construct the diffusion metasurface. EM responses of the proposed anisotropic meta-atom were explored in the framework of the equivalent circuit model. The inherent resonance phases of the meta-atom along its two symmetric axes were adjusted by changing its structural parameters. Then, we integrated the manipulation of the resonance phase and geometric phase together and designed a metasurface that can achieve broadband specular RCS reduction (more than −10 dB) from 4.3 GHz to 16.5 GHz. The RCS reduction performance of the metasurface is polarization-independent and shows a certain degree of stability under oblique incidence. For verification, a prototype with optimized structural parameters was fabricated and the measured results coincide well with the simulated results. Our work provides a convenient approach to realize a binary diffusion metasurface for the purpose of broadband RCS reduction.

플라즈마의 특성 분석을 위한 수정된 로렌츠 분산 모형 기반 FDTD 알고리즘

In this paper, to analyze the dispersive characteristics of plasma, a dispersive model suitable for the finite-difference time-domain (FDTD) method is proposed. The dispersive characteristics of plasma with varying electrical properties according to frequency were accurately and efficiently expressed using the modified Lorentz dispersive model. The coefficient of the modified Lorentz dispersive model was extracted by applying complex curve fitting based on the weighted least squares method. The method of applying the modified Lorentz dispersion model to the FDTD is discussed. The plasma model based on the modified Lorentz dispersion model proposed in this paper was verified to match the data of the dielectric barrier discharge (DBD) plasma generator from dielectric slab equivalent modeling. The accuracy of the proposed FDTD algorithm based on the modified Lorentz dispersive model was verified by comparison with the results of the broadband radar cross-section (RCS) measurement experiment.

Design of Nonresonant Metasurfaces for Broadband RCS Reduction

A design method using simple patches with nonresonant mode to achieve broadband phase-gradient metasurfaces of reflective type is proposed and an example to apply it for radar cross section (RCS) reduction is given. The method is based on treating the patch in an element as a pixel and the phase differences are obtained by setting the pixels to be empty or full. As only two states can be guaranteed for a single element, super cell concept and multilayer configuration are introduced to improve the performance. A design example of reflective RCS reduction structure with the −10 dB bandwidth of 5.3–18 GHz is given to prove the efficiency of the method.

Time-Domain Scattering Characteristics and Jamming Effectiveness in Corner Reflectors

The outboard corner reflectors are widely used as a countermeasure to radar tracking and detection. Therefore, it is essential to determine the composite time-domain scattering characteristics of the corner reflector and ship against the sea background and the jamming effectiveness of the corner reflector array. In this paper, the composite electromagnetic scattering characteristics of ship target, corner reflector, and the sea surface are studied based on the time-domain bouncing ray method (TDSBR). The radar echo and broadband radar cross section (RCS) results for the time domain pulse incident are also obtained and the high-resolution range profile (HRRP) is calculated. We then devise an evaluation system based on centroid jamming intensity and range profile similarity to determine the jamming characteristics of passive jamming. We further propose an array composing of three sizes of corner reflectors. Finally, the centroid jamming effectiveness is analyzed by comparing the RCS of the corner reflector array and the ship, and the deception jamming effectiveness is measured by calculating the range profile similarity based on the cosine similarity method and the range profile peak envelope overlap. Simulation results show that the reflector array can achieve a good performance both for centroid and range profile deception. The simulation results given and the evaluation method proposed in this paper provides significant practical insights into the design and analysis of the passive jamming systems.

Fabry-Perot Antenna Employing Artificial Magnetic Conductors and Phase Gradient Metasurface for Wideband Monostatic RCS Reduction and High Gain Tilted Beam Radiation

Platforms with low radar signatures require compatible antennas (low scattering) for communication purposes. Under this context, the article presents two Fabry-Perot (FP) Cavity antennas (Ant-1 and Ant-2). Ant-1’s aperture is an integrated design of artificial magnetic conductor (AMC) surface and a phase gradient metasurface (PGM), incorporating three unit-cell structures. The antenna achieves reduced monostatic scattering over broad bandwidth, and simultaneously obtains high gain as well as tilted beam peak radiation. Patch radiating at 7 GHz forms excitation source of the cavity, and cavity height is $0.33\lambda $ , smaller than the heights of conventional cavity designs ( $\lambda $ /2). RCS reduction is obtained over a bandwidth of 105.8% (4-13 GHz) for arbitrary polarizations, covering C/X bands. Antenna gain is 12 dB, and peak radiation tilt is −60° in elevation plane. Simulations have been verified through fabricated prototype. Ant-2 is a modification of Ant-1. Ant-2’s aperture is constructed from two unit-cell structures, and its aperture size is 41% smaller than Ant-1. Its cavity height is $0.3\lambda $ . Ant-2 also achieves wideband backscattering reduction; however, due to asymmetric layout of unit-cell elements over the aperture, radar cross section (RCS) reduction bandwidth is different for the two polarizations. For vertically polarized (VP) incident wave, RCS reduction bandwidth is 105.8% (4-13 GHz), but for horizontally polarized (HP) wave, it is 85.7% (5.2-13 GHz). Nevertheless, broadband RCS reduction is still achieved for both polarizations. Peak gain is 11.73 dB, achieved at −37° tilt angle. Simulations are presented for Ant-2. Compared to literature, both antennas show various improved results.

Surrogate-Assisted Design of Checkerboard Metasurface for Broadband Radar Cross-Section Reduction

Metasurfaces have been extensively exploited in stealth applications to reduce radar cross section (RCS). They rely on the manipulation of backward scattering of electromagnetic (EM) waves into various oblique angles. However, arbitrary control of the scattering properties poses a significant challenge as a design task. Yet it is a principal requirement for making RCS reduction possible. This article introduces a surrogate-based approach for rapid design optimization of checkerboard metasurfaces. Our methodology involves fast metamodels, and a combination of surrogate-assisted global optimization with local, gradient-based tuning. It permits an efficient control of the EM wave reflection characteristics, and ensures arriving at that the globally optimum solution within the assumed parameter space. The design procedure is fully automated. The framework is employed to develop a novel broadband checkerboard metasurface, where the RCS reduction is fundamentally based on the backward scattering manipulation carefully controlled by simultaneous adjustment of the unit cell dimensions. The properties of the structure are demonstrated using simulated monostatic and bistatic RCSs. The proposed metasurface exhibits 6 dB RCS reduction within the frequency range from 16 to 37 GHz. The numerical results are validated using physical measurements of the fabricated prototype. Experimental data indicates that the relative RCS reduction bandwidth is 83 percent, which makes the proposed structure outperforming the designs reported in the literature.