Backward Radar Cross Section Research Articles

Anisotropic hypocycloid inspired 3-bit digital coding metasurface for radar cross section reduction

Abstract Recently, researchers have realized various exotic electromagnetic (EM) control devices using the coded metasurfaces, sparking a broad investigation into the phase or amplitude-based encoding method, as well as their combination, in the field of metasurface design. In this paper, to evaluate the influence of random mutual coupling between the adjacent element on the scattering performance of metasurface, and also to minimize the backward radar cross section (RCS) of metal plate targets, a novel encoding approach combining the reflection phase and element-form has been proposed. During the implementation process, an anisotropic hypocycloid inspired 3-bit digital coding metasurface was designed. It consists of 9 different element-forms, with each capable of providing 7 phase states. Simulation results demonstrate that the random mutual coupling introduced by the proposed elements does not significantly affect the RCS performance of the metasurface. With a good polarization insensitivity property for both linearly and circularly polarized waves, the designed 3-bit digital coding metasurface can achieve more than 20 dB RCS reduction at 10 GHz, while simultaneously transmitting additional information by encoding the element forms. The good consistency between theoretical simulation and sample testing unequivocally validates the precision of the design, this paper may serve as a useful reference for expanding the design methods of metasurfaces.

Conformal frequency selective rasorber in S, C, X-band with low backward-scattering

In this paper, a polarization-insensitive high transmittance bandpass filter with low radar cross section (RCS) in both S- and X-band is proposed. This is the first study to use the partition layout loading approach for conformal structures with transmissive windows, reducing the operating band RCS. Curved structures have stronger radiation at a smaller angle to the incident wave, and that is how their scattering differs from uniform scattering from flat structures. The structure is divided by analyzing the radiative contribution of different regions. The surface was discussed in regions according to surface angles, and a new partition layout loading method was used to suppress the side currents and decreased backward scattering, achieving a backward RCS reduction of more than 10 dB at 4-8 GHz (66.7%). The bandpass layer operating at 6.9 GHz is designed through equivalent circuit theory. In combination with the lossy layer, absorption above 0.8 at 3.7-5.6 GHz and 9.1-12.5 GHz was achieved. Further, the structure was fashioned into a curved surface with varying curvature, demonstrating its effective absorption and transmission properties across different curvatures. A 15 × 15 cell structure was designed and fabricated, and there was good agreement between the test results and simulation results. The proposed structure has important applications in radomes, conformal structures, and electromagnetic shielding.

An anisotropic broadband coding metasurface based on ultralight graphene‐assembled film

AbstractAs theoretical research further develops, coding metasurfaces have been widely studied and applied due to their uniqueness in digital characterization and sequence arrangement. In this work, based on highly conductive graphene‐assembled film (GAF), an ultralight anisotropic broadband coding metasurface (ABCM) is proposed. The GAF ABCM consists of 16 units with specific phase responses and enables complete metal substitution. By arranging the unit cells in a specific order, the GAF ABCM realizes the functions of anomalous reflection, polarization conversion, and radar cross‐section (RCS) reduction. Results validate that the GAF ABCM has a measured 10 dB normal incidence backward RCS reduction within the working frequency band of 7–18 GHz, which fully covers the X‐band and Ku‐band for radar application. Furthermore, due to the lightweight property of GAF, the density of the GAF ABCM is only 0.06 g/cm3 (surface mass density of 0.038 g/cm2). All results indicate that the lightweight GAF metasurface is promising in radar stealth and communication fields.

Polarization Scattering Characteristic of Plasma-Sheath-Covered Hypersonic Vehicle

The plasma sheath covering the vehicle’s surface will be generated during the reentry of the hypersonic vehicle, which has a complex modulation effect on the electromagnetic (EM) wave and significantly affects the scattering characteristics of the hypersonic vehicle. This article analyzes the backward polarization scattering characteristics of a plasma-sheath-covered hypersonic vehicle. Based on the plasma sheath parameters at typical heights and velocities solved by numerical simulation, the current density convolution method was used to simulate the interaction between the linearly polarized plane EM wave and the hypersonic vehicle. Two backward far-field scattered components (co- and cross-polarization) are obtained using transient field extrapolation. Through studying the cross_polarization isolation (XPI) of scattering EM waves, this article analyzes the influences of flight status, EM waves’ incident angles, polarization modes (vertical and horizon), and frequency range on the backward radar cross Section (RCS) of hypersonic vehicles.

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 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.

Experimental and Numerical Investigations of Backscatter Patterns of the Blocks of Masking Digital Two-Bit Meta-covers

Introduction. The scattering patterns of non-absorbing coded checkerboard-like meta-coatings (MCs) applied for reducing the radar cross section (RCS) of metal surfaces inevitably contain side diffraction lobes. Therefore, the development of MCs with a low level of diffraction lobes is relevant. For this purpose, it is proposed to use checkerboard-like MCs in the form of a set of several basic flat blocks with the same dimensions. The paper discusses two such basic MC blocks with different coding matrices. The cells of the metasurface contain two coupled elliptical ring resonators and are distinguished by a 2-bit coding of the tilt angle of the anisotropy axis. Coding matrices of the MC blocks are built according to the block principle.Aim. To investigate experimentally and numerically backscatter patterns (BSP) for consonant (co-) and orthogonal (cross-) polarizations of the two developed flat blocks of the 2-bit digital nonabsorbing anisotropic MCs for different planes and polarizations of irradiation.Materials and methods. Full-wave simulation of the MC blocks was carried out using the HFSS software by the finite element method. BSP measurements of the fabricated MC layouts were performed in an anechoic chamber of the Center for Collective Usage “Applied Electrodynamics and Antenna Measurements” of the Southern Federal University using an automated information and computing complex.Results. The RCS reduction for the two blocks under normal irradiation is approximately the same and not less than 12 dB over the 9.8…21 GHz band. A good matching between the simulation and measurement results of backscattering patterns of the blocks in the region of the central lobes for various planes and polarizations of the irradiation is noted. In the principal planes, the blocks cancel the central lobes of the BSP by 10…25 dB; in the sector of angles of around ±40°, the backward RCS of the blocks is lower than that of the reference. In the diagonal planes, there is a cancellation of the RCS by 13…15 dB and an expansion of the central lobe of the BSP for copolarizations, as well as a bifurcation of this lobe for crosspolarizations in the sector of angles ±9°; outside of this sector the RCSs of the blocks are commensurate with the RCS of the reference.Conclusion. The developed blocks of the 2-bit digital nonabsorbing anisotropic MCs can be used for broadband cancellation of the RCS of metal surfaces.

Low‐profile circularly polarized metasurface antenna with tailored reflection phase

A low-profile metasurface antenna with circularly polarized radiation and tailored anisotropic reflection is proposed in this letter. The metasurface antenna composed of excitation structure (microstrip line with coupling slot) and a metasurface layer. The metasurface functions for two goals, generating circularly polarized wave for radiation and tailoring reflection phase for scattering, respectively. The overall structure of the metasurface antenna is compact with a dimension of 0.67λ × 0.67λ × 0.05λ at 5 GHz. Numerical and measured results confirm that the metasurface antenna exhibits 11.1% −10 dB S11 bandwidth from 4.68 to 5.23 GHz, 10.8% 3 dB axial ratio bandwidth from 4.782 to 5.37 GHz and a maximum gain of 6.83 dBic. Besides, the metasurface antenna achieves anti-phase reflection under orthogonal polarized incident waves at around 5 GHz. To demonstrate a potential application, 16 identical metasurface antennas are put in a 4 × 4 array. The metasurface antenna array achieves directive circularly polarized radiation with a maximum gain of 15 dBic. On the other hand, when under normal illumination, the scattering field of metasurface antenna array is redistributed due to phase cancelation, so that the backward radar cross section of the array is significantly reduced, which is applicable for military platform.

A Novel Switchable Absorber/Linear Converter Based on Active Metasurface and its Application

In this communication, a switchable active metasurface (SA-MS), with functions for absorption and polarization conversion, is proposed. The metasurface comprises rotated cross loops that are loaded with four sets of parallel-connected lumped resistors and p-i-n diodes on each arm. When the p-i-n diodes are in the OFF-state, the proposed structure performs broadband absorption functions from 3.7 to 10.7 GHz. When the p-i-n diodes are in the ON-state, those parallel lumped resistors are shorted and the structure transforms into a rectangular loop that converts the $x(y)$ - to a $y(x)$ -polarized one from 5.9 to 9.6 GHz. Furthermore, the proposed SA-MS and its mirror symmetry structure are used as the meta-atoms in the chessboard array. Such implementation can achieve backward radar cross section (RCS) reduction based on the phase cancellation principle in the conversion mode without any impact on the absorption function. Moreover, the scattering pattern of the checkerboard structure in the transition mode is more varied than that in the absorption mode. The two different modes provide flexibility for the RCS reduction and enable it to cope with complex electronic reconnaissance. Finally, a chessboard array and uniform array sample printed on the FR4 dielectric layer have been fabricated and measured. The experimental results are consistent with the simulated results.

Transmission–Reflection-Selective Metasurface and Its Application to RCS Reduction of High-Gain Reflector Antenna

In this article, a novel transmission–reflection-selective metasurface (MS) is introduced to realize symmetric transmission for horizontal-polarized ( $x$ -polarized) waves but asymmetric reflections for vertical-polarized ( $y$ -polarized) waves propagating along opposite directions. Based on the theoretical analysis, the proposed MS is finally engineered to transmit $x$ -polarized wave but anomalously reflect $y$ -polarized wave for forward direction. As for backward direction, the designed MS acts as a polarizer grating capable of transmitting $x$ - but specularly reflecting $y$ -polarized wave. As an example of its practical application, the designed MS is applied to form a folded reflector antenna with a reduced backward radar cross section (RCS). Both the MS and the folded reflector antenna are numerically simulated, physically implemented, and experimentally measured. The measured results of the folded reflector antenna coincide well with the simulated ones, showing a peak gain of 23.1 dB and a 3 dB gain bandwidth of 29.3% (12.8–17.2 GHz), as well as a −20 dB cross-polarized level bandwidth of 41.6% (11.4–17.4 GHz). Furthermore, the backward RCS of the antenna can be reduced from 11.2 to 18.4 GHz (48.6%) for $y$ -polarization and 14.1 to 16.2 GHz (13.9%) for $x$ -polarization, with the bandwidth defined by RCS reduction higher than 10 dB. The proposed MS and its application to backward RCS reduction of high-gain antenna may promote the uses of metamaterials to many real-world applications.

Bi-functional meta-material for vortex beam generating with low RCS characteristics

Meta-material has been proved to be an effective method to manipulate wave ranging from microwave to optic spectrum. Here, we employ a metasurface, namely two-dimensional equivalent of meta-material, to design a vortex beam generator with low radar cross section (RCS) characteristic. An anisotropic meta-atom is firstly optimized to control reflection phases independently for orthogonally polarized waves in a broad bandwidth. Then various meta-atoms with different sizes are assembled into a whole prototype, which is not only capable of reducing the backward RCS for x-polarization but also generating vortex beam for y-polarization. Simulated results further verify the theoretical prediction, indicating polarization-dependent functions for orthogonally polarized waves. This bi-functional metasurface and its application for vortex beam generating and RCS reduction may promote the application of artificial electromagnetic meta-material.

Optically transparent coding metasurfaces based on indium tin oxide films

We describe optically transparent coding metasurfaces based on indium tin oxide (ITO) thin films with high optical transparency and good electric conductivity. Four optically transparent coding metasurfaces are designed to realize desired scattering patterns. Three of them produce anomalous reflections for normally incident electromagnetic (EM) waves by encoding specific digital coding patterns. The fourth achieves diffusion-like scattering to reduce the backward radar cross section (RCS) by encoding an optimized random coding sequence. An ITO coding element based on a glass substrate is applied to reflect 0° and 180° phase responses. Based on this element, three coding schemes for beam deflection and one scheme for the RCS reduction are presented. The consistency of simulation and measurement results fully proves the powerful abilities of the optically transparent coding metasurfaces on manipulating EM waves. The center working frequency of metasurfaces is 12 GHz, in which the reflection beam angles of three schemes are 24.6°, 12°, and 36°, respectively. The diffusion-like scattering is verified by experiments, in which the RCS reduction is more than 10 dB. Considering the high-transparency property of the designed metasurfaces, we expect the proposed method to be exploited for many applications in the microwave and optical spectral ranges.

PO calculation for reduction in radar cross section of hypersonic targets using RAM

The radar cross section (RCS) reduction of hypersonic targets by radar absorbing materials (RAM) coating under different reentry cases is analyzed in the C and X bands frequency range normally used for radar detection. The physical optics method is extended to both the inhomogeneous plasma sheath and RAM layer present simultaneously. The simulation results show that the absorbing coating can reduce the RCS of the plasma cloaking system and its effectiveness is related to the maximum plasma frequency. Moreover, the amount of the RCS decrease, its maxima, and the corresponding optimal RAM thickness depend on the non-uniformity and parameters of the plasma sheath. In addition, the backward RCS of the flight vehicle shrouded by plasma shielding and man-made absorber is calculated and compared to the bare cone.

A Novel Broadband Bi-Functional Metasurface for Vortex Generation and Simultaneous RCS Reduction

In this paper, a broadband bi-functional metasurface is presented with capabilities of generating vortex beam for y-polarization and reducing the backward radar cross section (RCS) for x-polarization. With careful optimization, a meta-atom is first designed to control reflection phases for orthogonally polarized wave independently. Then, an integrated phase profile is employed to convert y-polarized spherical wavefront into a vortex spiral one, shaping a vortex high gain beam in the far-field region. By arranging a checkerboard-like phase profile onto the surface for x-polarization, the reflected wave in the backward direction will be canceled, consequently leading to the backward RCS reduction for x-polarization. Combining the above-mentioned cases into a whole, a compact bi-functional metasurface is designed, simulated, physically implemented, and finally measured. In good agreement with the simulated results, the measured ones show that the RCS reduction of 10 dB for x-polarization in comparison with a same-sized metallic plate can be realized over a 67.7 % frequency bandwidth (9–18.2 GHz). Besides, high-efficiency vortex beam conversion effect for y-polarization over a broad range of 11.2–18.2 GHz (47.6%) can also be observed from the measured results.

An ultra-thin dual-band phase-gradient metasurface using hybrid resonant structures for backward RCS reduction

We introduce and investigate, both experimentally and theoretically, a dual-band phase-gradient metasurface (PGM) to accurately facilitate dual-band beams deflection for electromagnetic waves. The designed PGM is composed of two kinds of split-ring resonators as the basic element of a super cell. These hybrid resonant structures can generate phase gradients at two distinct frequencies, which, in turn, generate appropriately artificial wave vectors that meet the requirements for anomalous reflection in terms of generalized Snell’s law. Both simulations and experiments are consistent with the theoretical predictions. Further, this PGM can work at 8.9 and 11.4 GHz frequencies providing a phenomenon of anomalous reflection, which is useful for backward radar cross section reduction.

Ultra-wideband, Wide Angle and Polarization-insensitive Specular Reflection Reduction by Metasurface based on Parameter-adjustable Meta-Atoms

In this paper, an ultra-wideband, wide angle and polarization-insensitive metasurface is designed, fabricated, and characterized for suppressing the specular electromagnetic wave reflection or backward radar cross section (RCS). Square ring structure is chosen as the basic meta-atoms. A new physical mechanism based on size adjustment of the basic meta-atoms is proposed for ultra-wideband manipulation of electromagnetic (EM) waves. Based on hybrid array pattern synthesis (APS) and particle swarm optimization (PSO) algorithm, the selection and distribution of the basic meta-atoms are optimized simultaneously to obtain the ultra-wideband diffusion scattering patterns. The metasurface can achieve an excellent RCS reduction in an ultra-wide frequency range under x- and y-polarized normal incidences. The new proposed mechanism greatly extends the bandwidth of RCS reduction. The simulation and experiment results show the metasurface can achieve ultra-wideband and polarization-insensitive specular reflection reduction for both normal and wide-angle incidences. The proposed methodology opens up a new route for realizing ultra-wideband diffusion scattering of EM wave, which is important for stealth and other microwave applications in the future.

Broadband diffuse terahertz wave scattering by flexible metasurface with randomized phase distribution

Suppressing specular electromagnetic wave reflection or backward radar cross section is important and of broad interests in practical electromagnetic engineering. Here, we present a scheme to achieve broadband backward scattering reduction through diffuse terahertz wave reflection by a flexible metasurface. The diffuse scattering of terahertz wave is caused by the randomized reflection phase distribution on the metasurface, which consists of meta-particles of differently sized metallic patches arranged on top of a grounded polyimide substrate simply through a certain computer generated pseudorandom sequence. Both numerical simulations and experimental results demonstrate the ultralow specular reflection over a broad frequency band and wide angle of incidence due to the re-distribution of the incident energy into various directions. The diffuse scattering property is also polarization insensitive and can be well preserved when the flexible metasurface is conformably wrapped on a curved reflective object. The proposed design opens up a new route for specular reflection suppression, and may be applicable in stealth and other technology in the terahertz spectrum.

Design and radar cross section reduction experimental verification of phase gradient meta-surface based on cruciform structure

A two-dimensional phase gradient meta-surface based on cross structure insensitive to polarization is designed and verified by simulation and experiment. Several periodic metal cross structures are integrated into a superstructure, and an additional component of the wave vector on the meta-surface is formed and the direction of refelction wave can be regulated. Thus the backward radar cross section (RCS) reduction can be realized by the mechanism of anomalous reflection. Experimental results indicate that in a frequency range from 3.2 to 3.4 GHz, the reduction of backward RCS of meta-surface reaches a highest value of 18.19 dB in the normal direction of meta-surface and 8 dB on the average in an angular range between -30° and +30°.

Conceptual design and RCS performance research of shipborne early warning aircraft

In order to improve the survivability of the aircraft,conceptual design and radar cross section(RCS) performance research are done. The CATIA software is used to design the 3D digital model of the shipborne early warning aircraft, and some measures are taken to reduce the RCS characteristics of the early warning aircraft at the same time. Based on the physical optics method and the equivalent electromagnetic flow method,the aircraft’s RCS characteristics and strength distribution characteristics are simulated numerically, and compared with the foreign advanced shipborne early warning aircraft. The simulation results show that under the X radar band, when the incident wave pitching angle is 0?, compared with the foreign advanced shipborne early warning aircraft, the forward RCS average value of the conceptual shipborne early warning aircraft is reduced to 24.49%, the lateral RCS average value is reduced to 5.04%, and the backward RCS average value is reduced to 39.26%. The research results of this paper are expected to provide theoretical basis and technical support for the conceptual design and the stealth design of the shipborne early warning aircraft.

Design and experimental verification of a two-dimensional phase gradient metasurface used for radar cross section reduction

Dealing with potential applications of phase gradient metasurfaces in stealth technologies, we propose to realize wide-band radar cross section (RCS) reduction by combining the two mechanisms of surface wave generation and anomalous reflection. A two-dimensional phase gradient based metasurface is designed using split-ring resonators. Around the designed central frequency f=10 GHz, the incident waves are coupled into surface waves propagating along the metasurface. While at the frequency band f>11 GHz, anomalous reflection and diffuse reflection occur. In this way, wide-band RCS reduction can be realized. A test sample with a total thickness of 2 mm is fabricated and its reflection and backward RCS are measured and compared with those of bare metallic plate with the same size. The comparison shows that the metasurface achieves more than 10 dB reduction in the measured wide range (9.5-17.0 GHz). The metasurface is a polarization independent, electrically thin, light-weight and wide-band, so it is of great application values in novel stealth technologies and materials.