Ultra-wideband Radar Cross Section Reduction Research Articles

An absorptive coding metasurface for ultra-wideband radar cross-section reduction

In this paper, an absorptive coding metasurface (ACM) is proposed for ultra-wideband radar cross section (RCS) reduction, the design process is presented in detail, in which a lossy polarization conversion metasurface (PCM) is proposed at first. The lossy PCM is an anisotropic resistive structure with both polarization conversion and absorption performances, so that its co-polarization reflection coefficients under u- and v-polarized incidences can be kept at less than − 10 dB in magnitude in the frequency range from 7.5 to 45.2 GHz. Though the magnitude of the cross-polarization reflection coefficient cannot be very small only due to the absorption, its phase will be changed by nearly 180° when the unit-cell structure of the lossy PCM is rotated by 90°. Thus, the lossy PCM can be used as one of the two types of lossy coding elements for an ACM when its unit-cell structure is rotated by 90° or not. Based on the lossy PCM, an ACM is proposed. The simulation and experimental results show that the ACM has an excellent RCS reduction performance under arbitrary polarized incidence, it can achieve effective RCS reduction under normal incidence in the ultra-wide frequency band from 7.4 to 45.5 GHz with a ratio bandwidth (fH/fL) of 6.15:1; moreover, an ultra-wideband RCS reduction can still be achieved when the incident angle is increased to 45°, which indicates that the ACM has good stealth performance under the detection of various radars working in X, Ku, K and Ka bands, it is very practical.

Ultra-wideband radar cross section reduction achieved by an absorptive coding metasurface

An absorptive coding metasurface (ACM) is proposed to achieve radar cross section (RCS) reduction in this paper. In the design progress of the ACM, two different lossy coding elements are proposed at first, which can both be regarded as a composite composed of a two-layer resistive frequency selective surface (RFSS) and a polarization conversion metasurface (PCM). The two-layer RFSS has a certain wave-absorbing property due to ohmic loss. In addition, the PCM can achieve ultra-wideband linear polarization conversion, and the polarization-converted reflected waves in the two coding elements under the same incidence will differ by nearly 180° in phase because the sub-unit-cell structures in them are perpendicular to each other. Thus, based on the two lossy coding elements, the ACM is proposed, which can achieve ultra-wideband RCS reduction due to absorption and phase cancelation. Numerical simulations demonstrate that the RCS of the ACM under normal incidence can be reduced more than 10 dB in the ultra-wide frequency band from 6.9 to 41.2 GHz with a relative bandwidth of 142.6%. Moreover, it has the advantages of polarization-insensitivity and wide incident angle. Finally, one effective experimental verification is carried out, and a reasonable agreement is observed between the simulation and experimental results.

Ultra-wideband RCS reduction realized by a coding diffusion metasurface

In this study, a coding diffusion metasurface (CDMS) based on a circular-polarization (CP)-maintaining metasurface is proposed for radar cross-section (RCS) reduction. The CP-maintaining metasurface can achieve ultra-wideband CP-maintaining reflection, and its main reflection phase under CP incidence can be adjusted at will by the Pancharatnam-Berry phase generated by the rotation of its unit cell, so it can be used to design CDMS. Thus, based on the CP-maintaining metasurface, the 2-bit CDMS is proposed. The simulated and measured results demonstrate that the CDMS can achieve highly effective and ultra-wideband RCS reduction under arbitrarily polarized incidences, compared with a pure metallic plate of the same size, its RCS can be reduced by more than 10 dB under normal incidence in the ultra-wide frequency band of 7.1-33.6 GHz with a relative bandwidth of 130.2%; moreover, when the incident angle is increased to 40°, an ultra-wideband RCS reduction can still be achieved.

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.

Low-profile shared-aperture metasurface array antenna with ultra-wideband radar cross section reduction and circularly polarization

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.

Design of Ultrawideband RCS Reduction Metasurface Using Space Mapping and Phase Cancellation

An efficient optimizing method of ultra-wideband radar cross-section (RCS) reduction metasurface involve unit amplitude differences and mutual coupling effects is proposed in the letter. Multiple unit phase cancellation was used to broaden RCS-reduction bandwidth. The genetic algorithm was utilized to further design and select the optimal combination from various tiles. To improve the optimization efficiency and accuracy, the space mapping (SM) technique was exploited to refine the design process, which involves the interactions of scattering fields among tiles and subarrays in wideband. Meanwhile, the constraint of wide-angle oblique incidences is also considered in wideband low RCS design. An ultra-wideband RCS reduction checkboard meatsurface can be accomplished by fast optimization of low-cost coarse model and few parameter mapping courses between fine and coarse model. A 10dB RCS reduction for HH and VV polarizations under normal and other oblique incidence angles can be achieved from 5 to 40 GHz. Good agreements between simulation and experiment results are obtained.

Ultra-wideband hybrid polarization conversion-absorption metasurface with a transmission window and narrow transition bands

This paper proposes an ultra-wideband hybrid polarization conversion-absorption (PCA) metasurface with a transmission band for radar cross section (RCS) reduction application. To achieve the in-band transmission of the incident wave and the out-of-band suppression of the scattering wave, the proposed metasurface is composed of a low-frequency PCA array, a high-frequency polarization conversion (PC) array, and a second-order frequency selective surface (FSS). The lumped resistors are used in the PCA array to generate the absorption characteristic, thus giving rise to the reduction of the co-polarization component of the scattering field and meanwhile the elimination of the high-frequency resonances. An interdigital structure is introduced into the PCA array to not only achieve a good transmission for a co-polarization component of the incoming wave but also result in an extra PC band for narrowing the transition band between the transmission window and the high-frequency PCA band. The orthogonal layout of the PCA array and the PC array is used to decrease the interaction between them, thus resulting in an ultra-wideband PCA band. The second-order FSS consisting of a stacked structure is designed to generate a good passband with the steep roll-off and the good out-of-band suppression. The proposed metasurface has an ultra-wideband PCA band, a good transmission band, and narrow transition bands. When the proposed metasurface is arranged into an array in a rotation way, the ultra-wideband RCS reduction performance can be obtained. Simulated and measured results are given to demonstrate a 10 dB RCS reduction band of 3.4–24.6 GHz with a −1 dB transmission band of 8.45–10 GHz.

Low RCS, Broadband Polarization Conversion Metasurface Antenna Based on Reactance Loading

A reactance-loaded polarization conversion metasurface (PCMS) is presented and exploited to realize a metasurface antenna (MTA) with both low radar cross section (RCS) and broadband radiation. Distinct from the conventional low-RCS PCMS, a metallic surface with etched open and closed slits is integrated into a PCMS consisting of double L-shaped particle pairs. Due to the edge capacitance induced by open slits and the surface inductance imposed by closed slits, the PCMS is endowed with reactance features that act as a perturbation to modulate the near-field interactions, thereby expanding the polarization conversion bandwidth. Besides, the reactance-loaded metallic surface, which has the same period as L-shaped particle pairs, combined with the grounded substrate, constitutes a broadband metasurface radiator. With the help of characteristic mode analysis, a linearly polarized aperture-coupled PCMS antenna is proposed, which effectively stimulates two resonant modes with broadside radiation to help bring about a broad operating bandwidth of 45.52%. Subsequently, the checkerboard-layout PCMS array is coupled to a sequentially feeding network through apertures on the ground plane, implementing a circularly polarized MTA with a broad 3 dB axial ratio bandwidth of 20.11%. Relying on phase cancellation and outstanding polarization conversion behaviors, an ultrawideband RCS reduction bandwidth of 7–39 GHz (139.13%) is also available.

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

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

Ultrawideband RCS Reduction of Planar and Conformal Surfaces Using Ultrathin Polarization Conversion Metasurface

A metasurface-based polarization converter for ultrawideband radar cross-section (RCS) reduction of planar and conformal surfaces is presented in this paper. Initially, a polarization converter is designed which consists of a modified concentric double (MCD) square ring resonator-based unit cell along with an air gap in between the substrate and the ground plane. This air gap enhances the polarization conversion bandwidth (PCBW) of the converter. An ultrawide PCBW ranging from 6.3 – 20.5 GHz (106%) is obtained with a high polarization conversion ratio (PCR) of 0.9 for normal incident EM waves. The conversion efficiency is also found to be stable for oblique incidences over the aforementioned band with a PCR of above 0.8 upto ±35° angles of incidence. Later on, a checkerboard configuration of the polarization conversion metasurface (PCMS) array and its mirror array is applied on a planar as well as on a cylindrical conformal surface for reducing the radar cross section (RCS). It is observed that a consistent 10 - dB co-polarized monostatic RCS reduction is achieved with the planar PCMS surface over an ultrawideband (5.5 – 20.5 GHz). Even with the cylindrical surface, nearly 10 dB RCS reduction is obtained upto central angles of 60° over a wideband. Both the structures are simulated and measured after fabrication, and a reasonably good agreement is obtained between them.

An ultra-wideband polarization conversion metasurface for RCS reduction

In this paper, a polarization conversion metasurface is proposed, which can realized ultra-wideband circular polarization (CP) maintaining reflection in the frequency range 7.3–28.7 GHz; in addition, when its unit cell structure is rotated by an angle ψ, almost ±2ψ Pancharatnam-Berry (PB) phase will be generated in its co-polarized reflection coefficient under CP incidence. Thus, base on the polarization conversion metasurface, an ultra-wideband PB 2-dit coding metasurface is further proposed for Radar Cross Section (RCS) reduction. The simulation and experiment results show that the coding metasurface can achieve more than 10 dB RCS reduction in the ultra-wide frequency band 7.2–28.6 GHz under normal incidence with arbitrary polarizations; in addition, when the incident angle is increased to 45°, the ultra-wideband RCS reduction can still be realized, which shows that the coding metasurface has good stealth performance in X, Ku and K bands, and it is very practical for most airborne radar, fire control radar and imaging radar working in these bands.

Breaking the High-Frequency Limit and Bandwidth Expansion for Radar Cross-Section Reduction: A low-observable technology

This article investigates the bandwidth limit of radar cross-section (RCS) reduction (RCSR) and proposes a new strategy for realizing multiband and ultrawideband RCSR. The high-frequency bound is broken while guaranteeing the stability of the low frequency and is extended to the grating-lobe appearance region. The short circuit blind zone of RCSR was first discovered in structures, such as the metasurface, frequency-selective surface (FSS), artificial magnetic conductor (AMC), and electromagnetic (EM) bandgap (EBG), through analyzing the equivalent circuit model and the reflection phase of the unit cell.

Ultra Wideband Radar Cross Section Reduction by using non-Resonant Unit Cells

A general approach is proposed to design ultra-wideband radar cross section reduction (RCSR) metasurface by using non-resonant unit cells in chessboard arrangement. The proposed miniaturized artificial magnetic conductor unit cell is composed of two stacked non-resonant patches separated from one another by thin dielectric substrates. The genetic optimization algorithm is used to optimize the unit cell design parameters and obtain wide 10-dB RCSR bandwidth. The proposed approach is performed to design three different RCSR metasurfaces, ideal, ROGERS and low cost. The low cost RCSR metasurface composed of low cost commercially available FR-4 substrate is fabricated and tested which reduces RCS more than 10-dB from 5.22 GHz to 30.85 GHz, 142% fractional bandwidth. This metasurface has significantly wider RCSR bandwidth rather than the state of the art references as well as low cost, simple and light weight structure which facilities its practical applications.

Multi-octave radar cross section reduction via integrated dispersion engineering of polarization-conversion metasurface and metamaterial absorber

In this paper, we propose a method of achieving multi-octave radar cross section (RCS) reduction by simultaneously engineering the dispersion of polarization-conversion metasurface (PCM) and metamaterial absorber (MA). To this end, a three-layer sandwich structure is devised, with two PCMs as the lower and upper panels and one MA as the lattice core structure. By engineering dispersions of the two PCMs, high-efficiency cross-polarization reflection can be achieved in two separate bands with a wide gap in between them. To bridge this gap, the absorption band of the MA is tailored by engineering the dispersion of spoof surface plasmon polariton (SSPP) modes excited on the MA. In this way, the three operating bands of the two PCMs and the MA can meet end-to-end, spanning over a multi-octave band. To demonstrate this method, we design a MA operating in 7.2–16.8 GHz, two PCMs operating in 16.0–35.0 GHz and 2.0–8.0 GHz. The combined three-layer structure can reduce RCS by more than 10.0 dB in 2.0–35.0 GHz, with a bandwidth more than 1:17. Both the simulation and experiment results verify the design. This work provides an alternative method of designing ultra-wideband RCS reduction structures and the method can also be extended to designing multi-functional planar or conformal structures.

Ultra‐wideband Low‐Detectable Coding Metasurface

A coding metasurface is designed based on hybrid Array pattern synthesis (APS) and Particle swarm optimization (PSO) method for ultra-wideband low-detectable application. The metasurface is composed of Electromagnetic band-gap (EBG) structures of two Minkowski fractal elements with reflection phase difference of 180° (±37°) over a wide frequency range. Two different types of EBG unit cell printed on a thin grounded dielectric substrate produce reflection phase difference of about 180 degrees over a wide frequency range. Ultra-wideband Radar cross section (RCS) reduction results from the phase cancellation between two local waves produced by these two unit cells. The diffuse scattering of EM waves is caused by the optimization of phase distribution, leading to a low monostatic and bistatic RCS simultaneously. The proposed metasurface can achieve 10 dB monostatic and bistatic RCS reduction in a wide frequency band from 5.8 to 18.0 GHz with a ratio bandwidth (fH/fL) of 3.10:1 under normal incidence for both polarizations. The theoretical analysis, simulation and experiment results are in good agreement and validate the proposed metasurface can achieve ultra-wideband RCS reduction and diffuse scattering.

Ultra-wideband RCS reduction and gain enhancement of patterned-surface-based aperture coupling patch antenna with optimized arrangement method

A patterned-surface-based aperture coupling patch antenna (ACPA) with ultra-wideband radar cross section reduction (RCSR) and gain enhancement is presented in this paper. Due to the air-filled gap structure of the ACPA, an air spacer is introduced to design double-layer patterned surface (PS), which broadens the RCSR bandwidth of ACPA. Moreover, to obtain better gain of antenna, the arrangement of PS is optimized based on its current distributions. Experimental results show that impedance matching of PS-based ACPA is preserved well and operation band is broadened slightly from 8.0 to 11.3 GHz. The boresight gain is enhanced from 7.5 to 11.5 GHz, with a peak gain enhancement of 4.1 dB at 9.7 GHz. Meanwhile, the PS-based ACPA possesses the ability of specular RCSR over the band 3.0-20.0 GHz for both polarizations. The 10-dB RCS reduction of the antenna is achieved almost from 5.5 to 17.0 GHz for normal incidence.

Ultrawideband Monostatic and Bistatic RCS Reductions for Both Copolarization and Cross Polarization Based on Polarization Conversion and Destructive Interference

Based on polarization conversion and destructive interference, a novel monolayered metasurface is proposed for ultrawideband monostatic and bistatic radar cross section (RCS) reductions for both copolarization and cross polarization. Unit cells share a simple asymmetric double arrow shape, but with variable geometry parameters, correspondingly with different polarization conversion capabilities and different reflection coefficients. The particle swarm optimization (PSO) algorithm and array theory are combined to optimize and select appropriate parameters for unit cells. The optimized unit cells (O-cells) simultaneously realize the polarization conversion and destructive interference to reduce copolarized RCS. In addition, mirrors of these O-cells (M-cells) are adopted to achieve the complete destructive interference to reduce cross-polarized RCS. Therefore, the total monostatic RCS can be greatly reduced. To improve the bistatic RCS reduction, random distribution of unit cells is carefully designed to realize diffusion patterns. In both the simulation and measurement results, a 10 dB monostatic RCS reduction at the normal incidence is achieved from 7.5 to 22.5 GHz (100% bandwidth). Furthermore, good specular and bistatic RCS reduction performances under oblique incident waves are also obtained. The proposed methodology opens up a new route for ultrawideband RCS reduction.

Ultrawideband Radar Cross-Section Reduction by a Metasurface Based on Defect Lattices and Multiwave Destructive Interference

Wide-band stealth technology is important for many wave-based applications, particularly involving radar, but is held back because expanding bandwidth for radar cross-section reduction is extremely difficult for traditional phase-cancellation methods. This study develops a physical mechanism for multiwave destructive interference (MWDI), and presents a metasurface with variable layer thickness and engineered defect configuration. This unconventional metamaterial harnesses second destructive interference to break the current bandwidth constraints in radar stealth applications.

Ultra Wideband Radar Cross Section Reduction by Using Polarization Conversion Metasurfaces

In this paper, a polarization conversion metasurface (PCM) is designed for ultra wideband radar cross section (RCS) reduction. The proposed polarization conversion metasurface consists of double-heads arrow unit cell with its 90°, 180° and 270° rotated ones to create the destructive interferences cancellation and radar cross section (RCS) reduction, consequently. The proposed metasurface demonstrates ultra-wide band 10-dB RCS reduction from 9 to 40 GHz (126.5%) for normally TM- and TE- polarized incident waves. The good agreement between the simulation and measurement results at 0°, 20° and 40° incident angles prove the idea, also. The ultra wideband RCS reduction of the proposed metasurface as well as its low profile, light weight and low cost prove its high capability compared with the state of the art references.

Lightweight ultra-wideband radar cross section reduction structure using double-layer metasurfaces

In this paper, we propose the design of a lightweight RCS reduction structure using double-layer metasurfaces. The structure is composed of two layers of metasurfaces spaced by PMI foams. To extend the bandwidth, two unit structures are firstly designed to operating in two respective bands for circularly polarized (CP) waves, which are then combined to act as the unit structure of the double-layer metasurface. The metasurface is expected to operate in two continuous bands. Nevertheless, due to the couplings between the two unit structures, there is Fano resonance that prohibits merging of the two bands. To solve this problem, we tune the resonant frequency of the bottom unit structure to suppress the Fano resonance and obtain a merged band. The combined unit structure is employed as the element of chessboard configuration, which can reduce RCS by more than 10 dB in an ultra-wide band (4.5–16.5 GHz). We designed, fabricated and measured a lightweight prototype (the aerial density is 726 g m−2). Both the simulation and experiment result verify the design. This work provides an effective method of extending the operation band of RCS reduction without sacrificing the efficiency.