Radar Absorbing Structures Research Articles

3D-printed broadband electromagnetic wave-absorbing basalt/CF/PLA composites with a high current path for lightning strike protection

In this study, we used the three-dimensional (3D) printing method to design and fabricate a radar-absorbing structure (RAS) to protect against damage caused by lightning strikes. A carbon black-dispersed 3D filament was used as reinforcement in the composite, and metalized fibers served as the feedstock for 3D-printing. The return losses measured in the X-band and Ku-band (8.2–18 GHz) indicated a maximum return loss of −28.98 dB at 10.68 GHz, with a broad bandwidth of 6.56 GHz in the frequency range of 8.2-18 GHz. A coupled electrical–thermal analysis confirmed that the proposed carbon black-dispersed filament provided conductive paths, which contributed to direct current, energy dissipation, and lightning strike protection. The 3D-printing method ensured a sufficient level of electrical conductivity, limiting the area and depth of damage caused by lightning strikes. Therefore, 3D-printed-RASs, reinforced by electrically conductive materials, can effectively protect against lightning strikes.

Multi-scale design of MWCNT/glass fiber/balsa wood composite multilayer stealth structure with wide broadband absorption and excellent mechanical properties

In unmanned aircraft applications, electromagnetic wave (EMW) absorbers suffer from defects in narrow absorption bands and poor mechanical properties. To solve the problems, a lightweight multilayer stealth structure with wide broadband absorption performance and excellent mechanical properties was designed and prepared by adjusting microscopically the number of multi-walled carbon nanotubes (MWCNT) and modulating macroscopically the thickness-matching relationship of the structure to promote the absorption of EMW synergistically. Under the MWCNT of 30 wt% and the depletion layer with the thickness of 0.2 mm, the effective absorption bandwidth (EAB) covers the entire Ku-band while maintaining a minimum reflection loss (RL) of −15 dB. Besides, the radar cross-sectional area attenuation is as high as 23.1 dBm2, as well as the mechanical properties of the radar absorbing structures (RAS) were improved significantly due to the reducing structural density from balsa wood and the enhancement effect of glass fiber mats (GFM). The study constructed balsa-based RAS with excellent EMW absorbing and mechanical properties from both micro-nano scale and macro-structure, providing a research route for designing high-performance and lightweight stealth structures.

Screen-Printed Metamaterial Absorber Using Fractal Metal Mesh for Optical Transparency and Flexibility

In stealth applications, there is a growing emphasis on the development of radar-absorbing structures that are efficient, flexible, and optically transparent. This study proposes a screen-printed metamaterial absorber (MMA) on polyethylene terephthalate (PET) substrates using indium tin oxide (ITO) as the grounding layer, which achieves both optical transparency and flexibility. These materials and methods enhance the overall flexibility and transparency of MMA. To address the limited transparency caused by the silver nanoparticle ink for the top pattern, a metal mesh was incorporated to reduce the area ratio of the printed patterns, thereby enhancing transparency. By incrementing the fractal order of the structure, we optimized the operating frequency to target the X-band, which is most commonly used in radar detection. The proposed MMA demonstrates remarkable performance, with a measured absorption of 91.99% at 8.85 GHz and an average optical transmittance of 46.70% across the visible light spectrum (450 to 700 nm), indicating its potential for applications in transparent windows or drone stealth.

Design and measurement of a tunable metasurface low-frequency radar absorber

At the turn of the millennium, developments on metasurfaces lead to a significant step forward in the design of radar absorbing structures, especially at lower frequencies, thanks to a reduced thickness. Still, as with other passive impedance matching systems, they remain constrained in their bandwidth-to-thickness ratio. Fortunately, the recent integration of active electronic components onto metasurfaces has multiplied their functionalities, unlocking new ways to overcome these limitations. For instance, using varactor diodes, which act as variable capacitors, an absorbing metasurface can be rendered tunable to cover a frequency range wider than would any passive system of similar thickness. This is a valuable feature when facing modern frequency-hopping radars. However, the bias-dependent resistance introduced by the varactor diodes can compromise the impedance matching throughout the tunable frequency range. This work, therefore, focuses on the design and measurement of a tunable radar absorbing metasurface using varactor diodes, highlighting the necessity for a joint tuning of capacitance and resistance to grant near-perfect absorption over a wide frequency range.

Fusedly Deposited Frequency-Selective Composites Fabricated by a Dual-Nozzle 3D Printing as Microwave Filter.

We report a fusedly deposited frequency-selective composite (FD-FSCs), fabricated with a dual-nozzle 3D printer using a conductive carbon black (CB) polylactic acid (PLA) composite filament and a pure PLA polymer filament. The square frequency-selective pattern was constructed by the conductive CB/PLA nanocomposite, and the apertures of the pattern were filled with the pure dielectric PLA material, which allows the FD-FSC to maintain one single plane, even under bending, and also affects the resonating frequency due to the characteristic impedance of PLA (εr' ≈ 2.0). The number of the deposition layer and the printing direction were observed to affect electrical conductivity, complex permittivity, and the frequency selectivity of the FD-FSCs. In addition, the FD-FSCs designed for an X-band showed partial transmission around the resonant frequency and was observed to, quite uniformly, transmit microwaves in the decibel level of -2.17~-2.83 dB in the whole X-band, unlike a metallic frequency selective surface with full transmission at the resonance frequency. FD-FSCs embedded radar absorbing structure (RAS) demonstrates an excellent microwave absorption and a wide effective bandwidth. At a thickness of 4.3 mm, the 10 dB bandwidth covered the entire X-band (8.2~12.4 GHz) range of 4.2 GHz. Therefore, the proposed FD-FSCs fabricated by dual-nozzle 3D printing can be an impedance modifier to expand the design space and the application of radar absorbing materials and structures.

Evaluation of electromagnetic characteristics of 3D-printed radar absorbing structures based on three-dimensional period pattern surface

This paper aims to assess the electromagnetic (EM) characteristics of 3D-printed radar absorbing structures (RASs) based on three-dimensional period pattern surface (3D PPS) within the X-band frequency range (8.2–12.4 GHz). The assessment is validated through HFSS software, a 3D printer (Markforged, X7), and a scanning free-space measurement (SFM) system. Firstly, the analysis indicates that the EM characteristics of RASs based on 3D PPS are impacted by impedance matching in the thickness direction and the scattering of EM waves at the edges. The utilization of Onyx (Chopped carbon fiber/Nylon) reveals variations in resonance frequencies and EM wave absorption performance contingent upon the surface shape. Furthermore, significant differences in the variations of EM characteristics are observed even for RASs with the same surface shape but different materials. Secondly, the measurement results of 3D-printed RAS specimens based on PPS using Onyx exhibit similar trends to the analysis results obtained with the SFM system. Ensuring precise measurement results requires specific measurement conditions tailored to each surface shape, along with an examination of the EM characteristic distribution across the entire specimen area. This is crucial due to variations in the size, number, and range of reflected EM waves in the time domain. The analysis and measurement techniques presented in this study hold applicability in designing 3D-printed RASs that demand high EM wave absorption performance, whether under normal or oblique incident EM waves.

Confined pyrolysis-driven one-dimensional carbon structure evolution from polyacrylonitrile fiber and its microwave absorption performance

The evolution of one-dimensional carbon structures from polyacrylonitrile (PAN) fiber precursors is investigated based on confined pyrolysis strategy, which is employed to convert electrospun PAN fibers into carbon in a closed system. Carbon tube with ultra-large diameter derived from silica-coated organic PAN precursor fiber is observed and its low-performance of microwave absorption is revealed. To enhance microwave absorption, cobalt acetylacetonate is introduced as a magnetic resource into PAN, facilitating one-dimensional carbon structure evolution and yielding magnetic/carbon nanofibers. The mechanism resulting in such one-dimensional carbon structure evolution is discussed. The microwave absorption performances of the prepared magnetic carbon nanofibers are analyzed based on their complex permittivity and permeability. Remarkably, the magnetic carbon nanofibers exhibit strong microwave absorption intensity of −42 dB at only 20 wt% filler content. Further simulations are conducted based on radar-absorbing structure with prepared magnetic carbon nanofibers-coated laminate and wing models by using CST software. The laminated composite structure appears the maximum RCS reduction value of 18.70 dBsm at 6 GHz with 3.0 mm thickness in the frequency range of 2–18 GHz. In the corresponding wing structure, highly efficient electromagnetic wave absorption is observed and the RCS reduction can achieve −14.12 dBsm at 10 GHz with 3.5 mm thickness.

Fabrication of multi-layer radar absorbing structures based on continuous fiber 3D printing and thickness correction method

Three-dimensional (3D) printing technology has revolutionized the fabrication of complex geometries, including electromagnetic wave absorbers. In this study, the multilayer radar absorbing structure (RAS) was designed and fabricated using a continuous fiber 3D printing process based on material extrusion (ME). However, a large thickness error occurred during the 3D printing process. The thickness error in the multilayer RAS, which is a resonant electromagnetic wave absorber, causes a large error in the electromagnetic wave absorption performance. To address this issue, the thickness correction method based on unit thickness was proposed. The unit thickness, which is the basic thickness of the multilayer RAS, was calculated to account for the thickness increase during printing. The redesigned RAS based on the calculated unit thickness effectively corrected the thickness error. The total thickness and thickness of each layer of the corrected RAS were measured and found to be similar to the design value. The measured electromagnetic wave absorption performance closely matched the analyzed value, confirming the effectiveness of the unit thickness-based correction method in improving the ME-based 3D printing process for fabricating RAS.

Global rigorous coupled wave analysis for design of multilayer metasurface absorbers.

In this work, a novel Global NV-ETM RCWA method is proposed to accelerate the optimization of the periodic stepped radar absorbing structure. This method is based on the rigorous coupled-wave analysis (RCWA) utilizing the normal vector field (NV) and enhanced transmittance matrix (ETM) approach. The NV field dramatically improves the convergence rate for both dielectric and magnetic metasurfaces. The Global NV-ETM RCWA algorithm is developed to further accelerate the complete search calculations. Using the proposed method, the periodic stepped radar absorbing structures are efficiently optimized to realize the entire band absorption in 2-18 GHz. The optimization results demonstrate the Global NV-ETM RCWA method significantly increase the computational efficiency, with a 38-fold improvement over direct NV-ETM RCWA calculations when the truncation order N=3. This method provides a powerful tool for designing metasurface absorbers with various desired functionalities.

Numerical simulation to investigate the stealth behaviour of polymer composite

Radar signals expose objects that are present in their vicinity. To avoid being detected by these signals, stealth technology provides an aid. This work focuses on the simulation of the radar-absorbing structure (RAS). A study on the effect of polyvinylidene difluoride (PVDF) and carbon nanotubes (CNT) in glass fiber mats has been done. Spin coating is utilized to increase the β phase in that material. Simulation results showed that at a particular frequency, the composite mat containing PVDF and CNT has the ability to reflect the minimum rays that come from radar, thus providing stealth behavior.

Ultra-Thin Fss Based Radar Absorbing Structure For X-Band Applications

Radar Absorbing Structures (RAS) are widely used in defense applications in order to reduce the electromagnetic reflections from predominant hotspots. With majority of military radars operating in X-band, thin frequency selective absorbers catering to this frequency range are the need of the hour especially for airborne platforms. In this regard, a novel ultra-thin Frequency Selective Surface (FSS) based RAS with superior absorption performance in the frequency range of 8GHz to 12GHz is presented. The absorption characteristics and radar cross-section (RCS) of the RAS has been analysed using commercially available EM simulation software. Further, the ultra-thin RAS, with a thickness of 0.053l at the centre frequency, has been fabricated and the performance parameters have been measured. The measurement results show that the percentage of power absorbed by the proposed RAS is greater than 90% over majority of X-band. In addition, it provides atleast 8dB RCS reduction (RCSR) in monostatic as well as bistatic modes of operation in comparison with its metallic counterpart of identical dimensions.

Broadband, Polarization-Insensitive and Ultra-Thin Metasurface-Based Radar-Absorbing Structure for Radar Cross-Section Reduction of Planar/Conformal Hotspots

Broadband, Polarization-Insensitive and Ultra-Thin Metasurface-Based Radar-Absorbing Structure for Radar Cross-Section Reduction of Planar/Conformal Hotspots

Properties and Defence Applications of Carbon Nanotubes

Properties and defence applications of carbon nanotubes are discussed in the paper. Carbon nanotubes are important emerging materials, showing diverse unique and interesting properties, such as optical, electronic, magnetic and mechanical properties, at the same time high chemical reactivity as well, due to their quantum confinement effects, more important, these properties are tunable in some extend. Lots of applications such as strength-enhanced structure materials, radar-absorbing structure, armor, body armor, wearable thermoelectric devices, smart textiles, water purification, warfare agent sensors and flat panel display, and so on, can be found using carbon nanotubes.

A Novel Hybrid Approach for Computing Electromagnetic Scattering from Objects with Honeycomb Structures

We propose in this paper a novel hybrid numerical modeling method for computing electromagnetic scattering from inhomogeneous targets containing honeycomb structures. In the proposed approach, the whole honeycomb structure is divided into the inner and outer two subregions. Each thin wall of a unit cell in the outer subregion is replaced by a zero-thickness surface, with the aid of a resistive sheet boundary condition (RSBC) to describe the electric and magnetic field discontinuities across the surface. Each unit cell in the inner subregion is homogenized by using the Hashin–Shtrikman and the Mori–Tanaka formulae. The two subregions are further divided into smaller subdomains by introducing the Robin-type transmission condition to couple subregion interfaces, as well as subdomain interfaces. The whole solution region is then discretized and solved using the nonconformal domain decomposition-based hybrid finite element–boundary integral–multilevel fast multipole algorithm (FE-BI-MLFMA). The numerical results demonstrate that the proposed approach exhibits a high accuracy, efficiency, and flexibility. Solutions of scattering by a wing-like object and a practical unmanned aerial vehicle (UAV) model with honeycomb radar-absorbing structures are presented, showing the superior performance of the proposed algorithm.

Low-Profile Polymer Composite Radar Absorber Embedded With Frequency Selective Surface

This paper proposed a novel design for a thin, lightweight polymer composite Radar Absorber (RA) with broadband bandwidth in the frequency range 2.8-9.6 GHz. A 3-mm-thick, thin polymer composite structure comprised of glass fiber, epoxy resin, and FSS films was then fabricated using the hand layup process. The β-phase behaviour enhanced polyvinylidene fluoride (PVDF) through electrospinning was deposited on a frequency selective surface (FSS). The PVDF phase structure was evaluated using X-Ray diffraction (XRD). The formation of nanofibers was evaluated by scanning electron microscopy (SEM). A significant increase in the absorption of electromagnetic (EM) waves with a broad operational bandwidth is produced by the addition of FSS to the glass fiber. Experiments were carried out and assessed by comparing the results to numerical models in CST Microwave Studio, indicating that the RA integrated with PVDF coated FSS has an operational bandwidth of 2.8-9.6 GHz and a reflectivity of less than -10dB. Finally, the absorption and mechanical characteristics of the fabricated Radar Absorbing Structure (RAS) was also investigated.

Reconfigurable Plasma Composite Absorber Coupled With Pixelated Frequency Selective Surface Generated by FD-CGAN

Conventional plasma absorbers are challenging to obtain high electron density and sizeable spatial scale for effective absorption while meeting the applied requirements of low profile and low power consumption. Although the frequency selective surface (FSS) has proved to realize a lower profile of plasma absorber with some empirical patterns adequately, the issue of the FSS design matching the dispersion distribution of complicated plasmas is still in suspense. A reverse prediction method referenced as the forecast and design Conditional Generative Adversarial Network (FD-CGAN) is proposed to generate a pixelated FSS between double-layer plasma periodic arrays. The reflection attenuation characteristics examined by experiments show that the addition of the FSS makes the coupling absorption effect surpass that of either pixelated FSS or plasma solely. Measurements in reconfigurable working modes and array arrangements demonstrate that the proposed configuration maximizes absorption effectiveness in the same profile, accompanied by the simulation. An interfacial void model is proposed to assist the design of the composite absorbing structure, together with an equivalent circuit for the hybrid absorber including periodic patterns with stochastic distribution characteristics, which analyze the absorption effect of the composite structure. The study provides a new approach for various microwave applications, including multilayer radar-absorbing structures, plasma-based stealth technology, and reconfigurable filters.

A space stealth and cosmic radiation shielding composite: Polydopamine-coating and multi-walled carbon nanotube grafting onto an ultra-high-molecular-weight polyethylene/hydrogen-rich benzoxazine composite

The application of stealth design to satellites provides an advantage in reconnaissance activities by evading ground observation and increasing survivability by preventing attacks from anti-satellite weapons. A shield that absorbs microwave irradiation was developed for incorporation into an ultra-high-molecular-weight polyethylene (UHMWPE)/hydrogen-rich benzoxazine composite for cosmic radiation shielding and to prevent electronic malfunction. Through the application of a polydopamine coating to UHMWPE followed by the grafting of carbon nanotubes, microwave absorption was achieved and the mechanical properties were improved. Moreover, radiation dose analysis confirmed that the proposed material exhibited a higher radiation shielding performance than conventional radar absorbing structures.

Robotic scanning free-space measurement system for electromagnetic performance evaluation of curved radar absorbing structures

Radar absorbing structures are manufactured for stealth missions and total quality inspection applies to evaluate their performances before assembly to stealth weapon systems. This study adopted a six-axis robot arm to move a target specimen in a scanning free-space measurement system for electromagnetic performance evaluation. The six-axis robot arm completely enables the system to maintain the specimen at the focal point of the antenna and solves the issue of curvature effect on the return loss results of the curved specimens, faced in the two- and three-axis scanning free-space measurement systems. The six-axis robotic scanning free-space measurement system uses a RobotStudio to extract the position and orientation of each target point on the specimen to be evaluated and uses a robotic scanning algorithm to transform all the points into a scan path. The system was verified by variable and constant curvature radar absorbing structure specimens with frequency selective surfaces. Return losses ( S 11s) of the nonsymmetrical cambered airfoil specimen and S-shaped double curvature specimen that could not be accurately evaluated with a three-axis stage-based scanning free-space measurement system were measured by the six-axis robotic scanning free-space measurement system. All the specimen results adhere to the theoretical parameters of the specimen. The transition region results of the S-shaped specimen were studied using effective radius of curvature. Finally, the inspected results of the S-shaped specimen were curvature compensated to check the effect of negative and positive curvature on the specimen resonance frequency performance.

Optimal Design of Multilayered Radar Absorbing Structures (RAS) using Swarm Intelligence based Algorithm


 
 
 The steady progress in the fields of material science and processing technologies has made multi-layered radar absorbing structures (RAS) an attractive option w.r.t. stealth technologies. They possess the ability to reduce radar cross-section with minimum thickness and is therefore most preferred in airborne applications. As far as their electromagnetic performance is concerned, the sequence of material layers and thickness profile plays a pivotal role. Optimization of these two factors becomes complex in case of availability of large number of potential materials. Commonly used EM simulation software can be employed for the optimization of thickness profile. However, selection of suitable material layer sequence is out of their scope. In this context, a particle swarm optimization (PSO) based algorithm is presented for sequencing of material layers and optimization of thickness profile of multi-layered RAS configurations. The fitness function has been appropriately formulated to achieve maximum power absorption over broad band of frequencies and wide range of incident angles. Further, the efficacy of the algorithm has been demonstrated using a suitable case study.
 
 

Recent developments in RAM based MWCNT composite materials: a short review

The need to develop radar absorbing materials (RAMs) that meet the structural requirement of defense applications is growing swiftly, due to the extensive use of electromagnetic waves in Radar. With the major developments and extensive use of Radar in military applications, the role of Radar absorbing, light weight and high strength structures has become a prime objective for researcher working on the development of RAMs. Various composites have been developed by using reinforcements which are dielectric, such as carbon nanotubes (CNT), magnetic materials like ferrites, that have been extensively used for reinforcement when developing RAMs. Ferromagnetic materials have a high density and can only achieve a narrow bandwidth of absorption. CNT have very good electrical, mechanical and thermal properties and can achieve high dielectric losses but their complex synthesis process is a barrier to commercial applications. Research on reinforcing multi-walled carbon nanotubes (MWCNT) with ferrites and metal oxides can also be extended to the study of thermoplastic polymers like polyether ether ketone and polyaryletherketone, which are gaining prominence in aeronautical applications, owing to their superior mechanical and low moisture absorbing properties. Further research can be carried out on reinforcing metal oxides, as metal oxides, when reinforced with MWCNT enhance the dielectric properties which improve the reflection losses as reported in a few studies.