Broadband Radar Cross Section Research Articles

Multifunctional and multispectral polarization converter and checkerboard metasurface design with low infrared emissivity

With the continuous development of detection technology, single detection technologies such as radar and infrared (IR) can no longer meet the current detection needs. The design of metamaterials is gradually moving towards multispectral compatible design. In this work, a polarization converter with low IR emissivity and a checkerboard metasurface with radar-IR bi-stealth were proposed. The filling rate of indium-tin-oxide(ITO) is increased to reduce the IR emissivity without affecting the polarization conversion efficiency. In addition, based on the radar cross section (RCS) reduction theory of the checkerboard composed of polarization conversion units, a broadband RCS reduction radar-IR bi-stealth structure is designed and analyzed. A sample was fabricated and the IR and microwave characteristics of the metasurface were measured. The numerical simulation results are in good agreement with the measurement results. The results show that the polarization conversion rate (PCR) is higher than 90% in the frequency range of 7.98-12.02 GHz, and the axial ratio (AR) is between 0.5 and 2 in the frequency range of 6.92-7.56 GHz and 12.48-15.78 GHz for the proposed polarization converter. The RCS reduction of the checkerboard metasurface exceeded 10 dB in the frequency range of 7.7-11.8 GHz. Meanwhile, the IR emissivity is less than 0.3 in 8-14 µm and the structure exhibits good optical transparency. This indicates the potential application of the proposed structure in multifunctional and multispectral compatible materials.

RCS reduction of wide-band, high-gain antenna array based on asymmetric transmission metasurfaces

In this paper, an ultra-wideband stealth antenna with high gain on the basis of asymmetric transmission metasurface (ATMS) is proposed. ATMS can convert an incident y-polarised sphere wave into an x-polarised plane wave at the front side and controls the scattering of the incident y-polarised wave to the back side. Excitation of ATMS via a horn antenna, a low radar cross-section (RCS) and wideband antenna system is designed. Furthermore, through design of the meta-atoms and optimization of the macrosequencing, broadband RCS reduction is achieved. The experimental data indicated the reduction of the RCS of the antenna system by up to 10 dB and more than 20 dB in the frequency range of 10.1 GHz to 18 GHz (relative bandwidth is 56.2%) and 13.9 GHz to 18 GHz (relative bandwidth is 25.7%), respectively. In addition, a 3 dB gain relative bandwidth of 57.4% is achieved between 10 and 18 GHz, with a peak gain of 28.2 dB. It is noteworthy that the high gain and low scattering performance of the antenna are achieved in the same spectral range (10–18 GHz), and there is no interference between the scattering performance and radiation performance of the antenna, which could be controlled separately.

Absorptive metasurface with optical transparency for broadband RCS reduction

Here, we introduce an optically transparent and flexible metasurface designed for effective absorption within the microwave spectrum. Indium tin oxide (ITO) films with varying square resistances fabricate a metasurface ground layer and a lossy pattern layers. Polydimethylsiloxane (PDMS) with a low refractive index, high transparency, and high flexibility is chosen as the dielectric layer. The proposed structure exhibits a reflection band of less than -10 dB with a fractional bandwidth of 167% in the range of 3.0–33.8 GHz under vertical incident electromagnetic waves. The metasurface designed based on this unit allows for the attainment of a radar cross section (RCS) reduction bandwidth of 8.75–31.1 GHz with a 10 dB reduction and a fractional bandwidth of 112%. The metasurface can maintain a broadband RCS reduction over a range of 45° incidence angles. In addition, due to the flexibility of the structure, we analyzed its RCS reduction capability when non-planar by wrapping the structure around a cylinder. The integration of simulation and testing has demonstrated that the structure exhibits excellent performance in the field of electromagnetic stealth. It has potential practical applications in electromagnetic shielding windows and the windows or domes of airplanes or satellites.

A restrictive design strategy of diffuse metasurfaces based on fibonacci sequences for RCS reduction

The rapid and efficient design strategy represents a highly intriguing research domain within the realm of metasurfaces. It necessitates minimizing the data load in coding metasurface design, facilitating the development of a precise approach to designing metasurfaces with specific characteristics. In this paper, we proposed a restrictive design strategy of metasurface for coding phase distributions based on the Fibonacci sequences, which is efficient to optimize designated radar cross section (RCS) reduction according to application requirements. Through the given restrictive strategy, the encoding range of the metasurfaces is reduced from the entire metasurfaces to the initial phase units in the corners, greatly reducing the total amount of data in the optimization database. In order to verify the availability of the strategy, RCS reduction metasurfaces with variable phase sequences in microwave region are analyzed. For broadband and wide-angle RCS reduction, the optimized Fibonacci discrete phase gradient (FDPG) metasurface is obtained by adjusting the phase sequences. Both the simulation and measured results show that the optimized FDPG metasurfaces by the proposed designing strategy have the RCS reduction of −10 dB in the frequency ranges of 6 GHz to 18 GHz under wide incidence angles from 0° to 30°. The simulated and measured results are good consistent over the whole frequency ranges and angle regions. This method combines mathematical sequences with physics organically, making it an important field in metasurfaces research and it is expected to play an important role in future studies.

Broadband low-scattering and high-efficiency transmission radome by combining phase gradient metasurface and FSS

In this paper, a novel method to achieve high-efficiency transmission radome with broadband low radar cross section (RCS) is proposed by combining phase gradient metasurfaces (PGMs) and frequency selective surface (FSS). The PGMs-FSS is an ultra-thin single-layered structure composed of a modified Jerusalem resonator backed with an FSS. The FSS works as a high-pass filter for transmitting the in-band signals and reflecting the out-of-band signals, and the PGMs works for reducing the RCS. The insertion loss is less than −1 dB within the passband of 15.95 GHz–17.7 GHz. Moreover, the particle swarm optimization (PSO) algorithm is used to extend the RCS reduction bandwidth as well as in-band high transmission. A prototype with a thickness of 0.08 λl (λl is the wavelength of the low frequency) is designed, fabricated, and measured. The results demonstrate that an over 10 dB RCS reduction is obtained from 7.95 GHz to 18.12 GHz, i.e., the relative bandwidth reaches 78.02%. The proposed method of combining PGM-FSS can be used in designing radomes of stealth platforms for the requirements of broadband RCS reduction as well as high-efficiency signal transmission.

A low-profile coding metasurface for broadband radar cross section reduction via beam diffusion and absorption

A low-profile coding metasurface based on beam diffusion and absorption is proposed, which can be applied for broadband Radar Cross section (RCS) reduction. Through inserting resistors into the polarization conversion element, the proposed unit cell can convert a portion of the electromagnetic wave to ohmic loss as well as retain polarization conversion. Subsequently, the power loss and surface current were used to explain the mechanism of absorption and polarization conversion, and the arrangement of the elements was optimized using a simulated annealing algorithm. Furthermore, both the simulated and measured results have shown that the RCS reduction is greater than 10 dB at 6.7–19.3 GHz (the relative bandwidth is 96.9%), and the metasurface still maintains great characteristics at 45° incidence. Thanks to the proposed metasurface offer advantages such as fewer resistors, a wider bandwidth for RCS reduction, an ultra-thin profile (0.066 λL, where λL represents the longest wavelength), a stable incident angle, and a more uniform scattering of energy, it can be used in electromagnetic stealth for large targets.

Broadband RCS reduction metasurface based on vortex singularities generated by Spin-to-Orbital angular momentum conversion

A broadband radar cross-section (RCS) reduction employed by the vortex metasurface is proposed. The underlying physics is associated with the Pancharatnam-Berry geometrical phases involved in spin-to-orbital angular momentum conversion. The proposed metasurface generates spin-controlled vortex waves with different topological charges. The vortex singularity implements specular extremely low RCS reduction. The prototypes are fabricated and measured to verify the wideband performance of the RCS reduction. The simulated results illustrate that −10 dB broadband RCS reduction from 9 GHz to 22.5 GHz and the RCS reduction has peak values −37.2 dB, −40.8 dB, and −42.5 dB at 9.8 GHz, 13.1 GHz, and 20.8 GHz, respectively. The measurement results illustrate that −10 dB RCS reduction over 9.1–18 GHz is realized, and the RCS reduction peak −53 dB at 10.1 GHz. The measured electric field intensities and phase distribution of the vortex metasurface also be displayed. Due to the limitation of experimental conditions, no test was done in the range greater than 18 GHz. Far-field results achieve good agreement between simulations and experiments, and the near-field measurements are a subtle difference in the experimental results. This work reveals the potential advantages of vortex waves in the fields of multifunctional electromagnetic stealth, beamforming for communication systems, etc.

A coding based metasurface absorber with triple circular ring resonator for broadband RCS reduction and high EMI shielding effectiveness

In this paper, a novel coding-based metasurface absorber (CMA) featuring a triple circular ring resonator is introduced for broadband radar cross-section (RCS) reduction and high electromagnetic interference (EMI) shielding effectiveness. The coding element comprises a triple circular ring resonator generating a 1-bit coding scheme (‘0’ and ‘1’ element) with a reflection phase difference of 180° ± 45° across the frequency range of 7.5 GHz–15.5 GHz by employing varying dimensions for the outer circular ring. A most affordable FR-4 substrate with 1.6 mm thickness is used to design the CMA structure. Binary Particle Swarm Optimization (BPSO) algorithm combined with array theory is employed to strategically position unit cells within the subarray. Simulations demonstrate a significant RCS reduction of −10 dBsm across the entire 7.5 GHz–15.5 GHz band. Furthermore, the 2D and 3D scattering patterns observed at 8.26 GHz, 10.56 GHz, and 13.30 GHz serve as additional proof of the remarkable effectiveness of the optimized pattern in reducing bi-static RCS. The optimized CMA structure also exhibits absorption levels exceeding 80 % at different resonance frequency while maintaining high shielding effectiveness. To validate these findings, a prototype of the optimized CMA structure is fabricated, and experimental investigations are conducted. The measured results closely align with the simulated outcomes, with minor discrepancies observed outside the designated frequency band. Based on comprehensive simulation and experimental analyses, the optimized CMA structure emerges as a promising choice for achieving broadband RCS reduction and high EMI shielding effectiveness in practical applications.

Low-RCS electromagnetic metasurface antenna based on shared-aperture technique

In this paper, a novel shared-aperture method of electromagnetic metasurface and antenna is proposed to obtain low radar-cross-section (RCS) performance. In this method, the low-RCS metasurface is first designed, then this metasurface is combined with traditional antenna to obtain novel low-RCS antenna based on shared-aperture technique. Besides, the analysis and corresponding local structure modification are also conducted to ensure that the antenna has good radiation performance while reducing broadband RCS. Using this method, a dual-layer polarization rotation unit cell is first proposed and its broadband working principle is investigated by both theoretical analysis and numerical comparison. Based on this unit cell, a broadband low-RCS metasurface is constructed. Then an initial shared-aperture metasurface antenna is obtained by substituting the middle cells in the metasurface with traditional patch antenna directly. Through careful analysis of surface current in radiation mode, the gain decrease of this metasurface antenna is revealed. On this basis, a finite removal strategy is put forward and some metasurface cells in the antenna are removed by using the electric current analysis. Consequently, an improved shared-aperture metasurface antenna is proposed. This improved antenna works in a frequency range from 6.3 to 7.48 GHz, which is slightly wider than the traditional patch antenna. Its gain is also higher than that of traditional antenna, with a maximum improvement of 1 dB. Meanwhile, the apparent RCS decreases from 6 to 16 GHz for any polarized incident wave, and the reduction peak is larger than 20 dB. Finally, fabrications and measurements are conducted. The measurement results and numerical calculations are in good agreement. The well-behaved radiation performance and broadband low-RCS property of this metasurface antenna verify the effectiveness of the proposed method. Unlike most of reported design methods of low-RCS antennas directly from traditional antennas, the proposed method adopts reverse thinking to transform scattering optimization into radiation optimization, realizing the integration between metasurface and antenna, thus making low-RCS antenna design easier and faster.

Ultra-wideband and wide-angle RCS reduction of a concave structure based on a chessboard polarization conversion metasurfaces

In this paper, ultra-wideband and wide-angle radar cross section (RCS) reduction of a concave structure is designed and realized based on a chessboard polarization conversion metasurface (CPCM), employing an ultra-wideband polarization conversion metasurface (PCM) composed of a single layer of square split-ring resonators. The concave structure, which is equivalent to an octagonal-like prism, is divided into eight regions. To achieve perfect phase cancellation in the non-central region, it can be equivalent to oblique incidence when the central region is under normal incidence, and phase compensation of the unit cell of metasurfaces in the non-central region is considered. The simulated results demonstrate that the RCS reduction of the proposed concave structure is less than −10 dB in the frequency ranges of 8.8 GHz to 35.75 GHz with fractional bandwidths of 120.99% and exceeds −30 dB at numerous resonant frequencies such as 9.52 GHz, 13.89 GHz, 23.45 GHz, and 35.2 GHz under normal incidence. The experimental results are in good agreement with the simulations. Furthermore, the RCS reduction characteristics of the proposed concave structure at different azimuth angles are also evaluated. Numerical calculations and experiments show that the wide-angle RCS reduction from 0° to 34° is achieved. To the best of the information we have, this is the first time that the chessboard metasurfaces, which consist of several polarizing reflectors, have been employed to obtain broadband and wide-angle RCS reduction for the concave structure. This technique validates the novelty and effectiveness of wide-angle and ultra-wideband RCS reduction of the concave structure.

Broadband RCS reduction by means of disordered coding metasurfaces

Pattern Search (PS) optimization algorithm has been used for the design of disordered coding metasurfaces with the aim of reducing both monostatic and bistatic Radar Cross Section (RCS) in the 26–40 GHz frequency range. On this basis, high and low phase-state unit-cells have been defined with special attention to the minimization of coupling between the elementary patterns, thus allowing a quasi-constant 180° phase difference over a broad frequency range. Then, finite size metasurfaces have been designed and fabricated using the printed circuit board technological process. Finally, the experimental characterization of various disordered samples leads to −10 dB monostatic RCS reduction over a relative frequency band broader than 35% with a good agreement with both simulated and analytic theoretical predictions. This illustrates the relevance of disordered coded patterns for RCS reduction as well as the PS optimization approach as a design method of coding metasurfaces.

RCS reduction based on double parabolic phased metasurface

This paper presents a novel metasurface design strategy to realize broadband radar cross section (RCS) reduction. The phase distribution across the metasurface aperture can be regarded as applying an additional parabolic phase upon periodically arranged parabolic subarrays. Such a design fully utilizes the diffusive scattering nature of the parabolic phase distribution. Since the proposed metasurface is governed by only two focal lengths, the optimization procedure is quite easy compared to metasurface with random coding sequence. Experimental results show that the proposed metasurface can achieve more than 10 dB RCS reduction from 7.52 GHz to 19.66 GHz with a fractional bandwidth of 89.3% under both linearly and circularly polarized normal incidences, and keeping a performance of more than 7 dB RCS reduction until the incident angle increases to 40° for both x-polarized and y-polarized incidences in the frequency range of 8–19.35 GHz. When the incident angle increases to 60°, 7 dB RCS reduction can still be obtained for x-polarized incidence from 7.9 GHz to 19.35 GHz with a fractional bandwidth of 84%.

A new stitched-plain weave fabric composite structure with reduced broadband radar cross-section

In this paper, a new stitch-plain weave fabric composite (SPWF) by acupuncture technology is proposed. Two patterns are designed, one is a typical chessboard layout (SPWF-1), and the other is a layout optimized by a genetic algorithm (SPWF-2). Simulated and experimental results show that SPWF can disperse electromagnetic wave energy in all directions and effectively reduce the radar cross-section (RCS) in the frequency range of 12.5 GHz−26.5 GHz, and the maximum RCS reduction for SPWF reaches −40 dB. The tensile modulus and strength of SPWF-1 are predicted by finite element method, and the results are in good agreement with the experimental data. In addition, a new optimization objective function is proposed to design the patterns of SPWF-2.

Simultaneous Optimization of 1-bit 3-D Coding Meta-Volumes for Both Broadband and Wide Field-of-View Radar Cross-Section Reduction

Coding metasurfaces (CMSs) rely on the proper tiling of two or more kinds of complementary unit cells within an array to enable phase cancellation and corresponding destructive interference thereby reducing the radar cross-section (RCS) of a given scatterer. Typically, these unit cells are planar and inherently limited in their field of view (FoV). This work demonstrates that breaking this traditional unit cell design paradigm in favor of novel 3-D meta-volumes enables both broadband and wide-FoV RCS reduction. Further, this work introduces a modified genetic algorithm (GA) framework for simultaneously tailoring the reflection phase of multiple unit cells to achieve the best composite response. By applying these identical pair-optimization strategies, it is shown that 3-D unit cells outperform their 2-D counterparts of the same lateral size in terms of both the fractional bandwidth (FBW) and FoV. One exemplary 3-D solution purely improves the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H_{\mathrm {x}}$ </tex-math></inline-formula> response with FBW exceeding 75% across a full 70° FoV. A second 3-D solution provides an improved <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$E_{\mathrm {x}}$ </tex-math></inline-formula> polarization response with a near 70% FBW at 45° and up to 75% FBW at 70° for the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H_{\mathrm {x}}$ </tex-math></inline-formula> polarization at the expense of reduced FBW close to 50% for incidence angles less than 45°.

Polarization-Independent Ultra Wideband RCS Reduction Conformal Coding Metasurface Based on Integrated Polarization Conversion-Diffusion-Absorption Mechanism

An ultra wideband (UWB) radar cross-section (RCS) reduction metasurface has received attention in recent years. However, the majority of the research has concentrated on the physics and design of planar surfaces, which do not meet the standards of modern aerodynamics and aesthetics. In this paper, we offer a sophisticated strategy for designing a metasurface that can conform to the shape of any object, even those of moderate curvature, and can also achieve UWB RCS reduction by combining absorption, polarization conversion, and diffusion mechanisms. Firstly, an absorbing-polarization converter is designed, composed of a square patch with a truncated diagonal strip and ring. A thin Rogers RT/Duroid 5880 dielectric substrate layer is used in the structure, which is also appropriate for conformal conditions. The substrate layer and the ground plane are separated by an air gap to enhance the polarization conversion bandwidth (PCBW). For normal incident electromagnetic (EM) waves, the PCBW ranges from 10.8 to 31.3 GHz with polarization conversion ratio (PCR) values greater than 0.9 dB. Up to a 45∘ oblique incidence angle over the aforementioned band, the PCR efficiency is well maintained. Then, the optimized coding metasurface is formed by the Pancharatnam–Berry (PB) phase, consisting of meta-atoms “0” and “1” of the same size but different orientations, to realize the concept of cross-polarization diffusion. A theoretical investigation has been performed to analyze the RCS reduction performance of planar as well as conformal cylindrical surfaces. The results show that more than 10 dB of RCS reduction is experienced over UWB (10.8–31.3 GHz) for planar metasurfaces under linearly and circularly polarized incidence waves. Furthermore, the RCS reduction for cylindrical surfaces can be achieved in a similar frequency band above 10 dB up to an angle of 90∘. It can be deduced that our proposed flexible metasurface can be used as an absorber or a polarization converter and provide broadband RCS reduction, which is essential for multi-function and conformal stealth applications.

Non-resonant broadband RCS reduction based on the patch distribution of a phase gradient metasurface.

Three units consisting of five, nine, and one square patches are used to construct a phase gradient metasurface with a 2π phase distribution within 18.0-28.0GHz. The three units arranged in one-dimensional (1D) and two-dimensional (2D) directions can precisely control the reflection beam direction in the plane of phase gradient. Rotating 1D and 2D phase gradient directions can convert a single reflected wave into multi-angle low-energy beams, forming the radar cross section (RCS) reduction metasurface. Using 16×16 periodic units, a 10dB RCS reduction in 18.0-25.5GHz is achieved.

Single-layer metasurface: Optical transparency, microwave scattering reduction and infrared emissivity decrease

A single-layer metasurface with optical transmittance, microwave and infrared stealth is designed. The proposed metasurface is an ITO/dielectric/ITO sandwiched structure. By designing the ideal reflection property of two ITO unit cells in the top layer, a checkerboard consisting of the unit cells attains a reduction in radar cross section (RCS) over a wider frequency band (7.4–13.4 GHz). In contrast to typical absorbing metasurfaces, the mechanism behind the broadband RCS reduction of this metasurface is scattering cancellation, avoiding thermal accumulation. Besides, the high filling ratio of ITO in the top layer can effectively suppress infrared radiation with an infrared emissivity of 0.26 in the 3–14 μm range. Thus, the proposed metasurface integrates both microwave stealth and infrared shielding functions through single layer, reducing the complexity of the metasurface. In addition, the overall transmittance of the metasurface is 56% due to the transparent materials. The experimental results are consistent with the simulations, which indicate the good application prospects of the single-layer metasurface.

Low-profile and wide-band RCS-reduction antenna array based on the metasurface polarization converter.

Antenna elements with a low profile and high front-to-back (FB) ratio mean that no additional reflective cavity is required when forming the array, which greatly helps to simplify and lighten the entire array system. In this paper, we enhance the FB ratio of the antenna to 35 dB while maintaining an ultra-low profile of 0.05 λ0 by attaching the broadband polarization conversion metasurfaces (PCMs) as the parasitic patches to the surface of the radiating patch. Meanwhile, the array formed by the proposed antenna is arranged in a checkerboard form, which makes it have a lower radar cross section (RCS) in the X- and Ku- bands. Even with PCMs loaded, the antenna element maintains a small size of 0.58 λ0 × 0.58 λ0, which ensures the proposed array can achieve the ± 45° beam scanning, making it suitable for the phased array. For verification, we propose a low-sidelobe array composed of the proposed antenna elements, which exhibits a low profile, high FB ratio, and broadband RCS reduction through simulation and measurement.

Multifunctional Coding-Feeding Metasurface Based on Phase Manipulation.

Multiple functionalities on a shared aperture are crucial for metasurfaces (MSs) in many applications. In this paper, we propose a coding-feeding metasurface (CFMS) with the multiple functions of high-gain radiation, orbital angular momentum (OAM) generation, and radar cross-section (RCS) reduction based on phase manipulation. The unit cell of the CFMS is composed of a rectangular emission patch and two quasi-Minkowski patches for reflective phase manipulation, which are on a shared aperture. The high-gain radiation and multiple modes of ±1, ±2, and ±3 OAM generation were realized by rationally setting the elements and the phase of their excitation. The CFMS presents a broadband RCS reduction of 8 dB from 3.18 GHz to 7.56 GHz for y-polarization and dual-band RCS reduction for x-polarization based on phase interference. To validate the concept of the CFMS, a prototype was fabricated and measured. The results of the measurement agree well with the simulation. A CFMS with the advantages of light weight and low profile has potential application in detection and wireless communication systems for stealth aircraft.

A low-cost digital coding metasurface applying modified ‘crusades-like’ cell topologies for broadband RCS reduction

The popularity of metasurfaces (MSs) has been continuously grown due to their powerful ability to manipulate electromagnetic waves. One of their important application areas is radar stealth technology, in particular, the realization of radar cross section (RCS) reduction. However, the high costs of substrate material limit its large-scale applications. In this paper, a binary digital coding metasurface (DCM) with novel modified ‘crusades-like’ cell topologies is proposed and implemented using a low-cost FR4 substrate to achieve broadband RCS reduction (RCSR). To realize the 1-bit DCM, initially, two elements with rotational symmetry are chosen for polarization insensitive properties while considering an unconventional phase deviation criteria. Next, the optimal hybrid coding layout is given out by using a genetic algorithm and antenna array theory. Last, the proposed novel MS prototype composed of 40 × 40 unit cells is fabricated and measured to validate the RCSR behaviour predicted by full-wave simulation. The results show a good consistency between the theoretical simulation and experimental measurement from 7.9 to 15.8 GHz. In addition, the simulation results indicate that the designed MS features high angular stability. Our work may provide a promising approach and good reference for the low-cost MS design in radar stealth applications.