Wideband RCS Reduction Using Coding Diffusion Metasurface.

In this paper, a 4-bit reflective coding metasurface with the polarization-insensitive unit cell is designed for wideband radar cross section (RCS) reduction. The metasurface unit has rotational symmetry; therefore, it produces the same electromagnetic scattering response for both xand y-polarizations. To attain 4-bit phase response, the dimensions of the unit cell are optimized so that a phase difference of 22.5° is realized between respective digital elements and the magnitude of reflection is more than 0.95 from 15 GHz to 40 GHz. Therefore, 16 digital elements have phases of θ, θ + 22.5°, θ + 45°, θ + 67.5°, θ + 90°, θ + 112.5°, θ + 135°, θ + 157.5°, θ + 180°, θ +202.5°, θ +225°, θ +247.5°, θ +270°, θ +292.5°, θ +315°, and θ + 337.5°. Discrete water cycle algorithm (DWCA) is applied to the array factor to get the optimal coding sequence matrix for better RCS reduction. The coding metasurface can achieve more than 10 dB RCS reduction from 15 GHz to 40 GHz as compared with the same size of a copper sheet. The simulation and experiment results validate the ability of proposed coding metasurface for robust control of EM-wave and wideband RCS reduction.

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

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.

This paper proposes a circularly-polarized (CP) metasurface antenna with ultra-wideband radar cross section (RCS) reduction. Initially, the relationship between RCS reduction and phase difference with different reflection amplitudes is analyzed theoretically. Inspired by the shared-aperture principle, a high-frequency resistive layer is introduced into the antenna element to further fulfill ultra-wideband RCS reduction and low-profile requirements. Numerical simulations demonstrate that the inserted layer has no effect on the radiation performance of the antenna array. Then, the proposed element and its orthogonal unit are arranged in a checkerboard pattern for good CP radiation. In this configuration, the proposed metasurface array antenna has a maximum gain of 16.7 dBi with 72.1% aperture efficiency. The bandwidth of 5 dB RCS reduction with x- and y-polarized incidences is 7.10–17.95 GHz and 7.34–17.85 GHz, respectively. The proposed design is verified by the fabrication of prototype antennas and measurements.

In this communication, wide-angle monostatic radar cross section (RCS) reduction of a metal plate is reported. The whole angular range (0°–90°) is divided into three parts according to the different dominant scattering mechanisms. Each dominant scattering mechanism is accurately identified and suppressed effectively. Frequency selective surface (FSS) absorber is employed to reduce the specular echo intensity within the first angular part. The tapered resistive sheets (TRSs) are loading as an edge termination to the metal layer and FSS layer, respectively, providing a slowly varying surface current to taper the transition between the free space and the targets. Significant RCS reduction in the rest two angular parts is achieved. Simulation results clearly show that more than 10 dB monostatic RCS reduction is realized within a wide angular range (0°–10° and 30°–85°) for the frequency band ranging from 1.3 to 2 GHz. Samples have been fabricated and measured in the anechoic chamber, and a remarkable agreement between the simulation and measurement is obtained. The effectiveness of the design is fully verified.

This article presents an efficient and effective design approach of the phase distribution calculation across metasurface for significant radar cross section (RCS) reduction of a circular polarization (CP) and liner polarization (LP) radar waves. The RCS reduction using the proposed design approach is achieved by imposing a novel cusp phase mask (which is usually used to generate 3D self-accelerating and self-healing cusp beams) at each geometric phased anisotropic unit cell composing the proposed cusp phased metasurface. By solving the cusp phase formula using MATLAB, it is found that the cusp phase mask required to achieve more than 10 dB of RCS reduction over a wide frequency band can be calculated without the need of significant lengthy optimizations or huge computer resources. The ability of such cusp phase mask metasurfaces to achieve significant backward scattering and RCS reduction has been rigorously investigated by means of simulations and measurements. When illuminated by a far-field radar CP or LP plane wave, the proposed cusp phased metasurface realizes more than 10 dB of RCS reduction from 10.9 to 26 GHz, corresponding to a fractional bandwidth of FBW = 81.8%. The 10 dB RCS reduction bandwidth of the cusp metasurface is maintained under both normal and wide angular incidence up to 75∘. The proposed cusp metasurfaces have potential applications to make objects stealthy where the incidence radar signal has an unknown frequency, polarization, or angle of incidence.

In this article, a multi-layer metallic structure based on phase cancellation for wideband radar cross-section (RCS) reduction is proposed. Firstly, the process of perfect electric conductors (PECs) positioned at different heights to generate reflected waves with different phases, ultimately achieving phase cancellation and RCS reduction, is investigated. Secondly, the accumulative nature of phase cancellation is studied, which advances understanding on phase cancellation and provides new a way for wideband RCS reduction. The proposed multi-layer metallic structure is simulated, which offers at least a fractional bandwidth of 122.2% (2.1 GHz-8.7 GHz) with 10 dB RCS reduction. A good agreement between the calculated and simulated results validates the proposed design.

Metasurface has attracted much attentions due to its ability of flexible manipulation of electromagnetic wave. However, the metasurface is limited to the single operating band and poor design generality when it is used for radar cross section (RCS) reduction. In this paper, a stacked metasurface design method has been developed to flexibly achieve dual wide band RCS reduction. Two subarrays with polarization conversion characteristic are firstly designed to operate in two wide bands, respectively. Then a low-pass frequency selective surface (FSS) is developed and sandwiched between two metasurface subarrays to achieve a subarray with the dual-band polarization conversion performance. With the arrangement of the dual-band subarray into a 2×2 array in a rotation way according to the polarization cancellation, the resultant metasurface structure has two −10 dB RCS reduction bands covering 6.6-12.7 GHz and 27.8-38.1 GHz. The designed prototype is fabricated and measured. The simulation and measurement results are given to demonstrate the good dual-band RCS reduction performance.

In this paper, a switchable radar cross section (RCS) reduction between the two broadbands' (1-7 GHz and 4-10 GHz) has been achieved by just considering the integration of hybrid frequency selective surface (FSS) based phase cancelation structure (PCS) with a modified ground square patch antenna operating at 2.4 GHz. The full metallic ground plane of the conventional microstrip antenna (MSA) is replaced by a 2 × 2 array of band-stop FSS having the same frequency as that of the MSA. This ground plane modification helps to increase the maximum out-of-band RCS reduction from 16.4 dB to 20 dB. The ground modified MSA without any PCS exhibits both in-band and out-of-band maximum RCS reduction of 20 dB over a wideband (1-7 GHz). A new approach is adopted to form the PCS. It is formed by placing two non-overlapping hybrid FSSs at different heights above the MSA. The MSA with FSS based ground plane along with the hybrid FSS based PCS exhibits good monostatic and bistatic RCS reduction across 4 to 10 GHz without affecting the radiation characteristics of the conventional MSA. The measured results show a close resemblance to the simulated ones.

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%.

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.

To achieve radar stealth and address the issue of narrow bandwidth in radar cross section (RCS) reduction, this paper proposes a chessboard metasurface structure with broadband RCS reduction performance. The designed cell exhibits effective polarization conversion performance in the frequency band from 7.5 GHz to 14.1 GHz. These unit cells, along with their mirror counterparts, are arranged in a classic checkerboard array to form a metasurface, achieving RCS reduction over a broad bandwidth. The simulation results obtained by CST show that under the condition of vertical incidence of plane waves, the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10 \times 10 metasurface array achieves more than 10 dB$</tex> reduction in RCS within the frequency band of 7.5 GHz to 13.6 GHz, with a reduction of more than 30 dB at 7.9 GHz. Based on this metasurface, an antenna design is proposed, where the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\left\vert\mathbf{S}_{\mathbf{11}}\right\vert&lt;-10 \text{dB}$</tex> bandwidth spans two intervals: 8.95 GHz to 9.5 GHz and 9.57 GHz to 11.3 GHz. This design achieves RCS reduction both within and outside the operational band.

A metamaterial approach to wideband radar cross section reduction that is thin, low-cost and doesn't dictate object shaping is presented in work from Iran. The approach uses principles potentially applicable to infrared and visible light as well as radar. The wideband RCS reducing surface is produced on a commercial high-frequency circuit substrate using conventional printed circuit manufacturing techniques Reducing an object's detectability to radar systems is now a well established priority for military applications, most famously in aircraft applications. ‘Stealth’ aircraft that were once well kept secrets are now a familiar concept to the general public and even fly in air shows. The radar cross section (RCS) of an object is essentially a measure of how detectable an object is to radar systems and so reducing an object's RCS is the basic goal of radar focused ‘stealth’ technologies. There are three main approaches to reducing RCS; radar absorbing material (RAM), object shaping and reflected wave cancelling by object coating. RAM materials, as the name suggests, are designed to reduce radar reflections back to the detector by absorbing the electromagnetic energy transmitted by the radar system, converting it into heat. However, they can be bulky and high maintenance. Object shaping works by shaping the surface of the object to minimise reflections of the radar waves back to the radar system, instead reflecting them in other directions. The obvious disadvantage to this approach is that it dictates the shape of the object, which can compromise the performance of the object for its primary purpose. For example, above all an aircraft must be shaped to enable it to fly, an aircraft that is very hard to detect but cannot take off is of no use to anyone. The work reported in this issue, by researchers at Iran University of Science and Technology (IUST) uses the third approach, in which a surface coating reflects the radar waves in such a way as to reduce the amount of radar energy that returns to the detector. This approach is low profile, low bulk and doesn't dictate the shape of the object. In the IUST team's work it can also be cheaply and simply manufactured using printed circuit board manufacturing techniques. Their proposed structure consist of a chessboard arrangement of two different artificial magnetic conductor (AMC) unit cell patterns, each of which reflects the radar waves differently. The key difference in the reflections is a 180° phase shift, which causes the reflections from the two different AMC cell types to interfere destructively and so significantly reduces the useful energy returned to the radar system's detector. “Since radar systems usually send out a short pulse signal in the time domain, wide bandwidth of the RCS reducer is necessary. The proposed AMC structure achieves a RCS reduction over a very wide bandwidth, about 85%,” said Dr Seid Hasan Sedighy, one of the IUST researchers on this project. The wide bandwidth achieved is possible because the critical 180° phase difference in the reflections is produced by the AMC unit cells over a wide frequency range, from 9.4 GHz to 23.28 GHz, providing 10 dB RCS reduction in that range. The peak RCS reduction is in excess of 40 dB, at 15 GHz. Printed circuit manufacturing techniques were used to create the AMC patterns on a commercial high-frequency circuit substrate, providing a low-cost manufacturing method. As flexible circuit board technologies are now available and continue to develop, this AMC-based, effective and relatively cheap approach to RCS reductions could be applied to arbitrary shapes. AMCs are metamaterial structures that behave as perfect magnetic conductors in a limited range of frequencies. The two unit cell AMC patterns used in this work are an arrangement of ‘E’ shapes and saltire arrows. The patterns were optimised to achieve the desired 180° phase difference in their reflections by simulation before being confirmed by experiment. The RCS reducer consists of a chessboard pattern of AMC patches using two different AMC unit cell designs, which produce reflections 180° out of phase with each other. The resulting destructive interference provides the intended RCS reduction The principles of AMCs and using destructive interference to reduce reflected energy are of course applicable to other frequencies of the EM spectrum, not just the band used for radar. This means that similar effects could be achievable in very different frequencies if the structures can be manufactured. Dr Sedighy expects this to lead to developments closer to photonic frequencies “It seems that this design approach will develop in the other frequency bands such as visible and infrared frequencies, which have many civil and military applications.”

A phase gradient metasurface (PGM) with Jerusalem crosses is presented for radar cross section (RCS) reduction of curved surfaces. The PGM consists of supercells of which 8 different Jerusalem crosses are printed on a grounded F4B substrate. Using gradient reflection phases, the PGM can redirect the specular reflection into a predetermined direction and realize monostatic RCS reduction. Simulated results show that the PGM on the curved surface achieves an average RCS reduction of 6.7 dB. The presented PGM can be a potential solution of missile stealth.

This paper explores the mechanism of radar cross section (RCS) reduction under plasma. The Runge–Kutta exponential time differencing–finite difference time domain method is used to compute the radar cross section of a target covered by plasma. Then, a method for analyzing the electromagnetic coupling effect between electromagnetic waves and plasma-covering targets based on field intensity visualization images and a probe is proposed. This method can elucidate the interaction process between electromagnetic waves and plasma-covering targets, ultimately revealing the key finding that the complex multiple reflections between the air–plasma interface and the plasma–metal interface are the fundamental mechanism of radar cross section reduction. Furthermore, the influence of the electron density, collision frequency, plasma thickness, electromagnetic wave frequency, and non-uniform distribution characteristics on the radar cross section reduction is discussed. A radar cross section reduction ridge is discovered in a two-dimensional diagram of radar cross section variation with electron density and electromagnetic wave frequency. It can be observed that the places where radar cross section reduction is more significant all satisfy the condition that the frequency of the electromagnetic wave is slightly higher than the cutoff frequency of the plasma, which provides guidance for the design of plasma stealth parameters.