Monostatic Radar Cross Section Research Articles

Coding Metamaterial Analysis Based on 1-Bit Conventional and Cuboid Design Structures for Microwave Applications.

This study aimed to investigate the compact 1-bit coding metamaterial design with various conventional and cuboid shapes by analysing the bistatic scattering patterns as well as the monostatic radar cross-section for microwave applications. The construction of this metamaterial design depends on binary elements. For example, 1-bit coding metamaterial comprises two kinds of unit cell to mimic both coding particles such as '0' and '1' with 0° and 180° phase responses. This study adopted a 1 mm × 1 mm of epoxy resin fibre (FR-4) substrate material, which possesses a dielectric constant of 4.3 and tangent loss of 0.025, to construct both elements for the 1-bit coding metamaterial. All simulations were performed using the well-known Computer Simulation Technology (CST) software. The elements were selected via a trial-and-error method based on the phase response properties of the designs. On the other hand, the phase response properties from CST software were validated through the comparison of the phase response properties of both elements with the analytical data from HFSS software. Clear closure was obtained from these findings, and it was concluded that the proposed conventional coding metamaterial manifested the lowest RCS values with an increasing number of lattices. However, the cuboid-shaped design with 20 lattices demonstrated an optimised bistatic scattering pattern of -8.49 dBm2. Additionally, the monostatic RCS values were successfully reduced within the 12 to 18 GHz frequency range with -30 to -10 dBm2 values. In short, the introduced designs were suitable for the proposed application field, and this unique phenomenon is described as the novelty of this study.

Frequency-Selective Rasorber Based on High-Q Minkowski Fractal-Shaped Resonator for Realizing a Low Radar Cross-Section Radiating System

In this article, a novel design of compact frequency-selective rasorber exhibiting absorption–transmission–absorption characteristics and having high passband selectivity has been proposed. A square-loop resonator with mounted lossy elements serves as the elemental broadband absorber from 4 to 12 GHz. The transmission frequency at 8.4 GHz is realized by printing a Minkowski fractal-shaped resonator with a high-Q factor on the resistive layer, which provides a transmission pole at the frequency corresponding to the passband of a double-cross-slot-shaped bandpass layer. The functioning of the proposed frequency-selective rasorbers (FSR) is explained by a corresponding equivalent circuit model. The proposed FSR exhibits higher selectivity at the passband, thus making it a suitable candidate for shielding the narrowband radiating system. Measurements performed on the fabricated prototype of a 17 × 17 unit cell array provides experimental validation. Further, a low radar cross-section antenna is realized by integrating a patch antenna with the proposed FSR, which achieves an average out-of-band monostatic radar cross-section (RCS) reduction of 11.92 and 5.04 dB in the lower (4–7.5 GHz) and upper (9.2–10.8 GHz) absorption bands, respectively, while maintaining the other antenna parameters. Furthermore, the bistatic total RCS reduction of 77% and 60% are achieved in the lower and upper frequency bands, respectively.

Spherical Harmonic-Based Anisotropic Point Scatterer Models for Radar Applications Using Inverse Optimization

High-performance computing-based electromagnetic (EM) emulators are used to simulate real-time complex EM wave interactions between multiple radar targets, transmitters, and receivers. The radar cross section (RCS) of the radar targets are required to be stored as a table; however, the needed storage size increases dramatically with the angle and frequency sampling density. In this article, we present an innovative approach of constructing a concise spherical harmonic-based anisotropic point scatterer model that the emulators can use as part of the computations. The point scatterer model is constructed directly from the precomputed RCS data. First, we use only the monostatic RCS data and compute the spherical harmonic-based monostatic point scatterer model by solving a linear least-squares problem which has a group sparsity constraint. Then, we further compute the spherical harmonic-based bistatic point scatterer model using the full bistatic RCS data. The problem is formulated as a bilinear least-squares problem. The problem is solved using the normalized iterative algorithm, which linearly solves two parameters in a back and forth manner. The results show that the point scatterer model can effectively represent the bistatic RCS data of a radar target.

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.

A Fast Method for SBR-Based Multiaspect Radar Cross Section Simulation of Electrically Large Targets

In this letter, a novel method is proposed to rapidly simulate the multiaspect monostatic radar cross section of electrically large complex targets based on the shooting and bouncing ray method (SBR). The core of the method is to use the inheritance of the illumination to determine the illumination of the target surface, rather than independently simulating for each required aspect angle as in the traditional SBR method. To achieve this, the illumination variation on the target surface is summarized and the genetic illumination information is defined. The error of the method is also discussed and the multiple precalculation solutions is proposed to control the error. Different simulation examples show our method has good general applicability and great efficiency. Finally, a realistic aircraft is simulated at 10 GHz using the multiple precalculation solution.

Radar Cross Section Reduction of Wideband Vivaldi Antenna Arrays With Array-Level Scattering Cancellation

Wideband phased antenna arrays always contribute large radar cross section (RCS) to low-observed platforms. Low-scattering wideband phased antenna arrays are thus highly demanded. This article proposes to reduce the RCS of wideband phased antenna arrays through array-level scattering cancellation. Two types of Vivaldi antenna elements that exhibit almost the same scanning performance and operating bandwidth are developed. However, the tapered curves are intentionally designed to be different to achieve a 180° reflection phase difference. With these two types of Vivaldi antenna elements, a low-scattering Vivaldi antenna array ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$16\times16$ </tex-math></inline-formula> ) is developed by array-level scattering cancellation. A reference Vivaldi antenna array with a single type of antenna element is also developed. Simulation and measurement results of the reference and the proposed antenna array are compared to illustrate the effectiveness of the proposed RCS reduction technique. Both antenna arrays operate over the 8–12 GHz frequency band and scan up to ±60° in the E-/H-planes. However, the proposed Vivaldi antenna array achieves an average of 10 dB monostatic RCS reduction over 6–18 GHz. It indicates that the array-level scattering cancellation technique is promising for developing low-scattering wideband phased antenna arrays.

A Low-RCS and Wideband Circularly Polarized Array Antenna Co-Designed With a High-Performance AMC-FSS Radome

In this letter, we investigate a low radar cross-section (RCS) wideband circularly polarized microstrip array antenna co-designed with a high-performance artificial magnetic conductor and frequency selective surface (AMC-FSS) radome. Compared with the absorbent frequency selective radome-based antenna, the proposed co-designed antenna can achieve a relatively low gain loss in an extremely wide frequency range with wideband in-band and out-of-band RCS reduction, due to the introduction of a novel AMC-FSS radome which can achieve low insertion loss and wideband transmission by combining a double-layer miniaturized bandpass FSS and a single-layer chessboard AMC surface. We demonstrate the effectiveness of the proposed AMC-FSS radome and co-designed antenna through simulations and measurements. The results show that the gain deterioration of the co-designed antenna is only 0.42 dB at the central frequency of 4.8 GHz and less than 1 dB in the wide frequency band, ranging from 4.23 to 5.38 GHz with a relative bandwidth of 23.9%. At the same time, the monostatic RCS for a normal incidence is greatly reduced from 4 to 18 GHz.

A Low-RCS and High-Gain Planar Circularly Polarized Cassegrain Meta-Antenna

Achieving highly directive radiation with broadband operation, low scattering, and thin profile for a circularly polarized (CP) antenna is particularly challenging and yet rarely reported. Here, we propose a strategy of a CP Cassegrain meta-antenna by combining a planar helical antenna, a metasurface main reflector, and a metamaterial subreflector. The main reflector is designed to achieve focusing for CP waves at 13 GHz. The subreflector is chessboard-configured chiral metamaterial slab composed of two different types of chiral meta-atoms, aiming to achieve spin- and direction-selective CP transmissions and reflections. The distance between two reflectors is half of focal length, which enables our antenna to be dubbed as a folded reflectarray. The low radar cross section (RCS) is achieved based on scattering cancellation technique by realizing near 180° reflection phase difference between two neighboring chessboard submeta-atoms. Thanks to the architecture of the two reflectors, the proposed antenna exhibits high gain and low profile simultaneously according to image theory. For verification, a planar CP Cassegrain antenna, excited by a left-handed CP (LCP) planar helical antenna, is numerically studied, fabricated, and experimentally measured. Numerical results are in good agreement with the experimental ones, showing a peak right-handed CP (RCP) gain of 26.6 dBi at 12.6 GHz. Furthermore, the backward monostatic RCS of the antenna is dramatically reduced over −10 dB in a broad bandwidth from 8.4 to 15.7 GHz when it is illuminated by an LCP planar wave. Our proposed Cassegrain antenna features simultaneously broadband, high gain, low profile, and low RCS, providing a new avenue to low-profile CP reflectarrays with invisibility.

Conformal and polarization adjustable cloaking metasurface utilizing graphene with low radar cross section for terahertz applications

Metamaterials with precisely chosen negative permittivity and permeability are preferable to cloak the target without scattering. In this work, a metasurface is designed by using graphene as conducting material to cloak a target cylinder under the instancing of TM and TE polarized waves in terahertz range of frequencies. The electric sheet impedance and magnetic sheet admittance played the crucial role to achieve the cloaking with good scattering reduction. Various incident angles are simulated and analyzed for obtaining the good radar cross section (RCS). The proposed metasurface resonates at three different frequencies in terahertz range of 3.8 THz, 9 THz and 13.8 THz and the bandwidth of the three resonating frequencies are 3.5–4.58 THZ, 8.8–9.5 THz and 13–14.98 THz respectively. In addition, the parametric analysis of chemical potential and relaxation time shows effective results in scattering reduction. The monostatic and bistatic RCS are simulated, which results high scattering reduction under different polarizations of different incident angles. The proposed structure is adjustable to various angle of incidence with less than 40 dB scattering reduction for various selected frequencies.

A compact transparent polarization-insensitive metasurface with broadband monostatic and bistatic radar cross-section reduction of millimeter-waves

This paper proposes an optimized optically transparent metasurface (OTM) which achieves broadband monostatic and bistatic radar cross-section (RCS) reduction with polarization and angle insensitivity. Through employing (a) theoretical formulation involving both monostatic and bistatic RCS and (b) unit cell placement optimization using the particle swarm optimization approach, we achieve monostatic and bistatic RCS reduction over a broad bandwidth with a single-layer, ultra-thin metasurface featuring only two types of unit cells. The proposed metasurface has high optical transparency and electrically small size compared to counterparts with similar performances. Simulation and experimental measurement show that the metasurface achieves more than 10 dB monostatic and bistatic RCS reduction from 18 to 34 GHz, which completely covers the 5G mm-wave spectrum. The proposed optically transparent metasurface can find use in many areas, including mm-wave applications, invisible glass technology, and vehicle windshield systems.

Radar Cross Section Prediction Using Iterative Physical Optics With Physical Theory of Diffraction

A new approach is presented to predict radar cross section (RCS) of electrically large conducting geometries using iterative physical optics (IPO) in conjunction with physical theory of diffraction (PTD). IPO is based on iterative refinement of physical optics (PO) currents to include multiple reflections. However, IPO does not yield sufficient accuracy in diffraction dominant regions since it does not properly include effects of edge diffractions. Hence, the proposed approach includes diffraction effects using PTD, which introduces edge currents. Diffracted fields due to these edge currents are radiated on triangular facets to modify the PO currents before IPO iterations. Scattered fields are computed from converged IPO surface currents and PTD edge currents. A new set of PTD edge currents excited by the fields due to the IPO currents are also found and the fields radiated by these PTD edge currents are added to the scattered fields. Therefore, interactions between the surface and edge currents are included. Computations are accelerated by parallel processing using message passing interface (MPI). Monostatic and bistatic RCS are predicted using the proposed approach for tail-wing and trihedral corner reflector geometries. The IPO-PTD results are compared with method of moments (MoM) and multilevel fast multipole method results obtained using field calculations involving bodies of arbitrary shape (FEKO) commercial electromagnetic simulation software for validation.

A Wide-Angle and Fully Polarimetric Retrodirective Array Based on Tri-Polarized Antennas With Pattern Complementation

A wide-angle and fully polarimetric Van Atta retrodirective array (RDA) based on tri-polarized antennas is proposed in this article. By leveraging the pattern complementary and polarization orthogonality of three resonant modes including a TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub> monopole-like mode and two orthogonal TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> microstrip patch modes, the RDA could better sense and retrodirect the incident wave with arbitrary polarization over a large field of view. A prototype of the RDA consisting of four tri-polarized antenna elements is designed and fabricated, which works at 5.8 GHz and has a low-profile planar structure with a compact volume of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.93\times 0.56\times 0.059\,\,\lambda _{0}^{3}$ </tex-math></inline-formula> . The simulated and measured monostatic radar cross-section (RCS) results show that, compared with the RDA composed of dual-polarized microstrip patches, this tri-polarized RDA has a significant improvement of the retrodirective angle range for the transverse-magnetic (TM) incident wave. Specifically, for the tri-polarized RDA, the −3 dB monostatic RCS beamwidth is 132°, and the −7 dB monostatic RCS beamwidth can reach 180°; while for the dual-polarized RDA, the −3 and −7 dB monostatic RCS beamwidths are only 52° and 73°, respectively.

Salt-templated synthesis of CuO/Carbon nanosheets for efficient microwave absorption

With the development of technology, microwave absorption technology shows its important position. In this work, A simple salt-template method was used to prepare copper phthalocyanine as N-doped graphene intercalated with CuO (CuO/NC). And a method to controllably regulate the dielectric constant by varying the ratio of salt templates and sintering temperature was proposed. When the treatment temperature is 900 °C and the ratio of salt template to raw material is 100:1, The minimum reflection loss (RLmin) of CuO/NC composite can reach −42.05 dB and the effective absorption bandwidth can reach 6.88 GHz with a thickness of 2.1 mm and a raw material filling of only 2 %, covering the entire Ku band. And the superior performance of the material was confirmed by simulation of the monostatic radar cross section. This research provides a new research direction for the synthesis of lightweight broadband microwave absorbing materials.

Design of soft and hard composite patterns for electromagnetic scattering controlling at both normal and grazing incidence

AbstractIn this paper, the soft and hard composite patterns for electromagnetic scattering control at both normal and grazing incident angle is proposed. The soft and hard parts are composed of dielectric substrate sandwiched by transverse or longitude metal strips with metal via and the perfect electrical conductor ground, and these two parts are combined at a certain inclination angled to form the soft and hard composite patterns. The simulation results demonstrate that the proposed composite patterns have the monostatic radar cross‐section (RCS) reduction of 20.79 and 18.36 dB at 11.24 GHz under normal incidence for transverse electric polarization and transverse magnetic polarization, respectively. Also, this structure achieves 2.78 and 5.43 dB monostatic RCS reduction under grazing incidence from 60° to 90°. The simulation results are in good agreement with the experimental ones. The physical mechanisms of the proposed composite patterns can be elucidated by the equivalent impedance method based on its near‐field distribution. The proposed composite patterns can be functionalized by changing the electromagnetic parameters of the soft and hard surfaces, which has a great potential to be used as the traveling wave antenna enclosure for scattering control.

Complementary Multi-Band Dual Polarization Conversion Metasurface and Its RCS Reduction Application

In this paper, we present a metasurface composed of complementary units that can realize orthogonal linear and linear-to-circular polarization conversion in multi-band. Linear polarization conversion has seven high-conversion frequency bands: 9.1–9.7 GHz, 15.6–17.6 GHz, 19.4–19.7 GHz, 21.2–23.1 GHz, 23.5–23.8 GHz, 26.2 GHz, and 27.9 GHz. Linear-to-circular polarization conversion also has seven frequency bands with axial ratios (ARs) less than 3 dB: 8.9–9.0 GHz, 9.9–14.7 GHz, 19.1–19.3 GHz, 23.2–23.35 GHz, 23.4 GHz, 24.1–25.4 GHz, and 27.2–27.8 GHz, with the generation of multiple bands extended by the combination of complementary units. Then, we utilize the combined polarization conversion unit’s mirror placement to form a 4 × 4 array to realize the phase difference cancellation of the reflective field, giving the metasurface the radar cross section (RCS) reduction function and the dual-band 10-dB monostatic RCS reduction bandwidth: 8.9–9.7 GHz and 15.5–26.1 GHz. The measured and simulated results were essentially identical. Because the design uses the complementary units to form an array to expand the polarization conversion frequency bands, it provides a novel idea for future designs and can be applied to multiple microwave frequency bands.

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.

Dynamic RCS Reduction Performances of Antenna Array with Coding Metasurface

On the basis of combining the two concepts of digital coding metasurface and metasurface antenna, low radar cross-section (RCS) coding metasurface antenna array with dynamic scattering performances is presented in this paper. Extending scattering factor theory to coding metasurface antenna, by controlling the states of PIN diodes randomly, the scattering performances of coding metasurface antenna array can be tuned dynamically without degrading its radiation property. Based on phase cancellation principle, a 8 × 8 antenna array was finally simulated and fabricated. By comparing several different layouts, taking checkerboard layout, new chessboard layout, and “0101” square ring nested layout as examples, both monostatic and bistatic RCS of the antenna mentioned above can be reduced 10 dB or more in the frequency range of 9–10.5 GHz under the illumination of x-polarized incident wave. The measured data are consistent with the simulation data, which proves the effectiveness of the proposed method.

Design of a frequency selective surface‐backed microstrip reflectarray antenna using Minkowski ring elements

A novel frequency selective surface (FSS)-backed subwavelength reflectarray element is proposed in this article. The element consists of an inner square ring and an outer Minkowski ring. The proposed fractal element could achieve smaller interelement spacing and better reflection phase performance due to its space-filling property and self-similar geometry. An 11 × 11-element reflectarray with reduced element spacing of 0.38 λ 0 ( λ 0 -the free space wavelength at 5.8 GHz) is designed. A FSS is used to replace the perfect electric conductor (PEC) ground. In the working frequency, the electromagnetic waves are reflected and result in a smoother and more linear reflection phase curve. Moreover, the radar cross section (RCS) of the reflectarray antenna is also reduced by using the FSS ground. A prototype antenna is then fabricated and measured. The measured results show that the antenna has a peak gain of 18.6 dBi, and an aperture efficiency of about 31.9%. A measured 1-dB gain bandwidth of 14.3% is obtained. Meanwhile, the measured cross-polarization and side-lobe levels are −20 dB and −13 dB, respectively. The simulated results demonstrate that the monostatic RCS can be significantly reduced with a fractional bandwidth of 32.76% and 44.83% for the lower and upper bands with reduced levels of 5 dB, respectively.

Sleep Posture Recognition With a Dual-Frequency Microwave Doppler Radar and Machine Learning Classifiers

An automated, robust, noncontact sleep posture recognition technique is proposed in this letter, which uses optimizable (Bayesian hyperparameter tuning) machine learning (ML) classifiers applied to dual-frequency (2.4 GHz, 5.8 GHz) monostatic continuous-wave radar-measured effective radar cross section and chest displacement. The technique is demonstrated to accurately recognize three different key sleep postures categories for 20 participants, with greater accuracy and computational efficiency than prior published research involving either a custom ML model or threshold-based assessment. Three ML classifiers (K-nearest neighbor, support vector machine (SVM), and decision tree) were assessed, with an SVM using a quadratic kernel achieving an accuracy of 85 and 80%, at 2.4 and 5.8 GHz, respectively, and the decision tree classifier recognizing sleep postures in less than 2 min with 98.4% accuracy for dual-frequency combined measurements.

Design of a Dielectric Dartboard Surface for RCS Reduction

This letter proposes a dielectric dartboard metasurface, which achieves broadband monostatic radar cross section (RCS) reduction and evenly bistatic scattering. Two types of dielectric elements with different permittivity are alternately arranged on a metallic plate as a dartboard structure. By choosing the thickness of the dielectric, the phase difference of 180° ± 37° between elements is obtained to meet the condition of phase cancellation for RCS reduction. Simulation results show that the design achieves 84% (4.5–11 GHz) fractional bandwidth of 10 dB monostatic RCS reduction and evenly distributed bistatic RCS pattern. We fabricated a prototype to verify this design, the measurement and simulation results are in good agreement.