Radar Absorbing Research Articles

Radar Absorber Fabric Design Based on Periodic Arrays of Circular Shaped Conductive Patches

In this study, the design and electrical tests of radar absorber fabric containing an array of circular shaped conductive patches positioned on a neoprene fabric are presented. In the designs, electrical performance of the radar absorber fabric is numerically studied in both planar and conformal structures. Furthermore, the two-dimensional (2D) surface current distribution at the resonant frequency is examined to better understand the operating principles of the proposed structure. Finally, the prototype of the radar absorber fabric is manufactured, and the frequency dependent reflection parameter values are measured by using the free space measurement technique to validate the numerical results. A good agreement between the measurement and numerical results is obtained.

A Review on Types of Radar Absorbing Materials

In this review paper a study about material used for radar absorption is done. Various types of materials possess the property to absorb the radar signals. The properties of material vary based on the composition, additives, manufacturing methods, and other various factors. Different materials such as carbon based, ferrites work well under different frequencies which is discussed in this paper. The absorption of the signals depends on the basic parameters such as permittivity, permeability, reflection loss, the other parameters such as transmission, reflectivity and many more factors.

The compatible method of designing the transparent ultra-broadband radar absorber with low infrared emissivity

The multi-spectrum compatible design of electromagnetic (EM) waves has captivated great interest since metasurfaces emerge as a novel structure design method in researchers’ eyesight. However, it is difficult to tailor the cross-spectrum performance simultaneously, which makes EM compatible design is still a challenging work. Here, we propose a design paradigm to endow the proposed metasurface with visible transparent, low infrared emissivity and microwave absorption. As a proof, the transparent materials indium tin oxide (ITO) films and polymethyl methacrylate (PMMA) are selected as the surface atom and substrate, respectively. To achieve the low infrared emissivity, the 6 Ω/sq ITO film with a high duty cycle serves as the frequency selective surface (FSS) on the top layer. To obtain the absorption structure, the dimension of PMMA, distribution and sheet resistivity of the ITO film are searched by genetic algorithm (GA). As a result, both simulation and experiment show that the proposed metasurface realizes more than 76% visible transmittance, less than 0.24 infrared emissivity and over 90% absorption from 4 GHz to 18 GHz. Moreover, the most of PMMA between the FSS and absorption ITO has been hollowed out by laser cutting, which will make the top structure 90% lighter.

Detection of Debonding Defects Between Radar Absorbing Material and CFRP Substrate by Microwave Thermography

Microwave thermography (MWT) is a non-destructive testing method based on the principle of microwave heating, which is widely used in the detection of debonding defects of various composite materials. Based on the principle of Microwave thermography (MWT), a resonant cavity microwave thermography detection system is proposed in this paper. The temperature variation characteristics of debonding defects in carbon Fiber Reinforced Polymer (CFRP) and radar absorbing material (RAM) are studied. This system consists of a resonator microwave source, an infrared thermal imager, a fan and two computers. This system successfully detects such defects at the pixel level by thermally loading the test piece, collecting the temperature change data of the test piece by the thermal imager, analyzing the temperature difference between the normal area and the defect area. In this paper, the advantages of microwave on the radar absorbing material (RAM) with large specific heat capacity and high thermal conductivity, high heating efficiency, large heating area and negligible damage to the radar absorbing material are verified, and the high performance of the detection system is verified. The experimental results show that microwave thermography can realize high-precision debonding detection and positioning of multilayer heterogeneous radar absorbing material (RAM).

A Flexible and Efficient Method for the Analysis of Electromagnetic Scattering by Inhomogeneous Objects With Honeycomb Structures

We present in this letter an efficient numerical method for the analysis of electromagnetic scattering by inhomogeneous objects with arbitrarily oriented honeycomb radar absorbing structures. In this method, each cellular honeycomb structure is first homogenized as a diagonal anisotropic medium in local coordinate system by using a new hybrid Hashin–Shtrikman and Mori–Tanaka formula. Then, a Euler rotation matrix is employed for describing a honeycomb structure with arbitrary orientation in a global coordinate system by transforming a diagonal tensor into a general 3 × 3 tensor. Finally, the object with homogenized honeycomb structures are simulated by using the nonconformal domain decomposition finite element-boundary integral-multilevel fast multipole algorithm. Numerical examples are presented to demonstrate the accuracy, efficiency and capability of the proposed algorithm, including the scattering by a typical sandwich panel with honeycomb core, and a complicated fighter model which includes antenna array, dielectric radome and radar-absorbing honeycomb structure.

RADAR TECHNIQUE FOR AIRCRAFT WITH AN ARTIFICIALLY REDUCED RCS UNDER CONDITIONS OF APPLICATION A RESONANT ELECTROMAGNETIC FIELD

The paper presents the results of a study of the radar technique for aircraft with an artificially reduced radar cross section (RCS) under the condition of using a resonant electromagnetic field. It is shown that the detection and tracking of air objects under conditions of artificially reduced RCS is carried out using radar information with resonant excitation of their radar absorbing coating. It is proposed to carry out tracking and detection of aircraft by a complex combination of an active radar channel with the formation of a resonant radio signal and a passive radar capable of receiving signals formed by an excited surface. It has been determined that by increasing the power level of the irradiating signal to 20-30%, it is possible to increase the area of the local conduction region of the dielectric by 10 times, which means that it will simplify the technical implementation of the method of locating aircraft. The proposed control methods make it possible to provide the necessary probability of detection accompanied by a radar target in a difficult jamming environment and thus increase the potential capabilities of the radar. In the course of the study, it was determined that the excitation and heating of the coating during enthalpy directly depends on the energy flux density of the electromagnetic microwave field per unit mass and area of the substance. It has been established that purposeful control of this effect makes it possible to substantiate the possibility of using enthalpy as a factor in excitation of air target coatings with an artificially reduced reflection area (RCS). The conditions for increasing the efficiency of radar targets with an artificially reduced RCS using a concentrated resonant electromagnetic field have been clarified. It is determined that the influence of such a field is accompanied by the concentration of the energy of the electromagnetic field in the crystal structure of the radio-absorbing coating of the aircraft due to the resonant irradiating signal, which causes temporary local conductivity and thermal radiation. Calculations of quantitative indicators characterizing the possibility of changing the electrically conductive properties of a carbon-type dielectric at a distance for the practical application of advanced radar systems are presented.

Ultrasonically assisted solvohydrothermal synthesis of nanocrystalline Zn-Ni ferrite advanced material for EMI shielding

Magnetic oxides, especially Zinc-Nickel (Zn-Ni) ferrites, are used in devices that transmit and receive electromagnetic signals. We present the synthesis of Zn-Ni ferrite, Nickel ferrite and Zinc ferrite by sonochemically assisted solvohydrothermal method. The elemental analysis revealed that the content of Zn ion in the Zn-Ni ferrite is more. The VSM results displayed superparamagnetic behaviour of Zn-Ni ferrite which could be attributed to smaller particle size. Electromagnetic interference (EMI) shielding materials which dominantly shield by absorption mechanism contain both electric and magnetic dipoles. This is an intrinsic feature of Zn-Ni ferrite materials as it has both high value of magnetic permeability and dielectric constant. The higher contribution of absorption makes it a useful material for various applications such as Radar Absorbing Materials (RAMs). Total shielding efficiency of 28.5 dB for Ni-Ferrite and 30.5 dB for Zn-Ni-Ferrite in X band were obtained, all of which was dominated by shielding efficiency due to absorption. This implies more than 99.99% incident microwaves were successfully attenuated by the materials.

Rotation included 3-axis scanning free-space measurement and curvature compensation for electromagnetic evaluation of leading-edge and curved stealth structures

Since the last decade, stealth technology has been developing at a much higher speed with much research going on to evaluate the properties of radar absorbing structures (RASs) and scattering parameters of the developed stealth structures at different phases of practical use. A vector network analyzer plays an important role in RAS performance evaluation of stealth structures. Different scientists have used closed cell measurement techniques utilizing coaxial cables and waveguides to evaluate RAS materials at high accuracy, while others have used free-space measurement techniques utilizing radiating antennas to evaluate the RAS performance of larger stealth structures without damaging the specimen. All these conventional evaluation systems can scan a single point at a time. A scanning free-space measurement (SFM) system was an initial step toward automated scanning of large stealth structures. This system was capable of scanning 1000 mm × 1000 mm flat structures using raster scanning with two linear stages. However, the two-dimensional SFM was not applicable to leading-edge and cylindrical structures, and the signal-to-noise ratio (SNR) is highly affected by the specimen curvature. In this paper, a 3-axis scanning free-space measurement (3-axis SFM) system was developed. The system has two linear stages and a rotational stage for raster scanning of the specimen. The system performance was checked by scanning cylindrical RAS specimens with a radius of 100 mm. The results were then compared to the SFM system, which showed that the SNR of the 3-axis SFM is quite high compared to the SFM system. A coordinate-based scanning algorithm was also developed for 3-axis SFM to ensure a fixed interval scan of complex geometrically shaped specimens, such as wing leading edges, at lower incident angles. The algorithm achieved a zero-degree incident angle scan with ±1 degree of accuracy for the NACA-M3 symmetrical leading-edge specimen. Different scan parameters and results are visualized in the developed graphical user interface software. Finally, the inspected results were curvature-compensated to obtain the actual RAS performance of the specimen. The accuracy of the geometry-based perfect electric conductor (PEC) reflection-loss prediction algorithm was further improved by adding a stand-off distance effect and more specimen curvature data to the algorithm. The accuracy of the algorithm was verified by comparing the predicted PEC results to the measured PEC results of the corresponding specimen.

3-D Printed Radar Absorber with Meta-material Features for X-band Application

This paper presents a structured radar absorber with metamaterial features based on graphite SLS composite. The unit cell of the proposed design was simulated on COMSOL Multiphysics to determine its frequencydependent absorption characteristics and fabricated using low-cost selective laser sintering 3-D printing technology. The measurement and simulation results showed an effective absorption bandwidth of 1.04 GHz and 2.08 GHz respectively. The optimized structure however, revealed broadband absorption in a frequency range between 8.35 to 12.20 GHz (X band) under normal incidence. Besides, the absorption performance under different polarizations and incident angles were investigated. Results indicated that the absorber exhibits polarization indifference and high absorptivity at a wide angle of incidence. The advantages of low cost, ultra-broad operating band, wide-angle feature, and polarization insensitivity made the proposed absorber a promising candidate in military and civilian applications.

Multilayer Gradient Perforated Radar Absorbing Structure for Stealth Applications

In the exploration of broadband radar absorbing structure (RAS), a new kind of multilayer gradient perforated RAS constituted of magnetic iron metal (Fe) and silicon carbide (SiC) is examined utilizing a top-down fabrication technique and metaheuristic grey wolf optimization (GWO) algorithm. The investigation of proposed structures and magneto-dielectric study of samples is carried out in 8.2 to 12.4 GHz (i.e., X-band). Distinct single and multilayered RASs are optimized using the GWO algorithm under certain restrictive conditions. In addition, the GWO algorithm is reinforced with a fitness function to acquire maximum bandwidth below -10dB threshold (RC≤-10 dB) in various oblique angles of incidence with a minimum coating thickness (t≤2.0 mm). Among all investigated RASs, triple-layer gradient perforated structure (TLGPS) is found to be effective which fulfills the lightweight, broadband, and angular stable behavior of an efficient absorber. TLGPS has considerably exhibited the concept of impedance gradient which assists in triumph broadband radar absorbing materials (RAMs) for low observable applications.

Multiwalled carbon nanotubes and zinc oxide using a high energy milling method for radar-absorbent

Radar is a technology that is always used by the military to detect an object because it can determine the shape, size, position, distance, and speed of an object. This enables the national defence system to have anti-radar technology to protect defence equipment or other important defence objects. One way that can be applied is using radar-absorbing material to coat the surface of the object. A good radar-absorbing material is made of a combination of dielectric materials with magnetic materials. MWCNT/ZnO composites were produced by a high energy milling method. The various milling times (0, 1, 3 and 5 h) using HEM on the microwave absorbing properties of the composites has an effect, which was studied. The experimental results show that the optimum microwave absorption ability is reached when the HEM process is carried out for 5 h with a thickness of 2.0 mm. The optimum return loss is −26.4 dB at a frequency of 11.2 GHz and the bandwidth correlative to the return loss is below −10 dB with a frequency greater than 1.5 GHz. When comparing MWCNT/ZnO without HEM treatment, the results show that HEM treatment can also increase microwave absorption properties.

Establishment of SAR calibration site at Antarctica: Pre-NISAR calibration activity

Upcoming Synthetic Aperture Radar (SAR) missions with larger swath like NASA-ISRO Synthetic Aperture Radar (NISAR) (~250 km) requires a large, flat and homogeneous low-background site, free from human-made structures for point targets deployment for calibration. Finding out such a large, flat, homogeneous area devoid of perceived sources of radar clutter is a challenging task. In this regard, Antarctica is a potential site for calibration as it fulfills many of the criteria required for the ideal calibration site and suitable for setting up a corner reflector (CR) network for SAR calibration. This network will help in the calibration study and aid the studies related to Interferometric SAR (InSAR) viz. ice velocity estimation. As a part of this activity, in-house designed and developed CRs were installed near Indian research base stations Maitri and Bharati at Antarctica during the 38th Indian Scientific Expedition to Antarctica (ISEA) during 2018–2019. Corner reflectors were designed keeping in mind the harsh environment of Antarctica, such as high katabatic winds, blizzards, snowfall and low temperature. Temporal and seasonal analysis of radar backscatter data using available Indian Radar Imaging Satellite (RISAT-1) and European SENTINEL-1 SAR satellite over Maitri and Bharati was carried out to find out the suitable CR deployment site. Super-hydrophobic microwave transparent cover was designed and installed over CRs to protect snow accumulation. Hydrophobic radar absorbing materials were installed over central-mount to decrease the background noise due to it. Various tests such as structural analysis of total CR system, thermal analysis of materials and Radar Cross Section (RCS) characterization of CR were done before sending it to Antarctica. Integrated Wide Swath (IW) mode data of Sentinel-1 satellite was used for analyzing time-series response of CR in SAR image.

Radar-absorbing materials damage detection through microwave images using one-stage object detectors

An improved rapid multiprobe scattering microwave imaging algorithm and a deep learning model via microwave images are proposed to detect radar-absorbing materials (RAMs) damage in real time. The imaging algorithm improves the quality of microwave images by avoiding the transmitting and receiving antennas position equivalent error. The radar-absorbing materials damage dataset (RAMDD) is constructed by combining microwave images with optical images label. The semantic segmentation method is applied to filter out cluttering caused by scattering coupling or uneven distribution of absorbents. A new target detection model is proposed by changing training process, designing a composite backbone (CB) and adding model optimization methods (K-means clustering, cosine annealing, transfer learning and label smoothing) so that the model can detect the corresponding position of RAMs damage in the optical image via the microwave image rapidly. The mean average precision (mAP) of the target detection model is 76.04% on the RAMDD.

Design method of structural microwave absorber by frequency characteristic analysis of reflectivity function

In this paper, a new design method of structural absorbers was proposed by frequency characteristic analysis of reflectivity function. A simplified Jaumann absorber is discussed, by taking derivation of the reflectivity function and monotonicity analyzing, the contribution of design parameters of the absorber to the reflectivity is determined. For a series of reflectivity targets, the value ranges of the thickness and permittivity of dielectric layer, and the square resistance of resistive layer were calculated. Physical samples were prepared for verification, with quartz plates as dielectric layers, and magnetron sputtered Ni–Cr film as resistance layer. Continuous effective bandwidths below −10 dB in 6.2∼18 GHz band and below −15 dB in 8.2∼16 GHz band were obtained respectively, and their frequency characteristics are consistent with the results of mathematical analysis.

Preparation and Thickness Optimization of Graphenic-Based Carbon Material as a Microwave Absorber

The purpose of this study is to optimize the thickness of a layered graphenic-based carbon compound, which is a non-magnetic material derived from biomass (old coconut shell). After the sample was exfoliated using HCl solution, the morphological structure showed that the material used in this study is a reduced graphene oxide (rGO), similar to carbon but with a thickness of less than 10 nm and lateral size in submicron (100 nm). The sample with a 2 mm thickness was then characterized using a vector network analyzer (VNA) to measure its reflection loss (RL). The measurement result is evaluated by converting the S-parameter values (S11 and S21) from the VNA using the Nicolsson Ross Weir (NRW) method to obtain input variables such as relative complex permeability and relative complex permittivity. Following this, the single-layer thickness of the sample was optimized using a genetic algorithm (GA), which can predict the appropriate thickness so that the optimum RL can be obtained. The optimum thickness of the sample was found to be 3.48 mm, which resulted in a much higher RL. The RL was re-measured for verification using a sample with the corresponding optimized thickness, revealing that this optimization is feasibly operational for a radar absorbing material (RAM) design.
 HIGHLIGHTS
 
 Carbon compounds containing graphenic phase derived from coconut shell are functional materials having various unique properties such as superior electrical conductivity, large surface area, and excellent structural flexibility, and microwave absorbtion
 The single-layer microwave absorber employing carbon compounds has been prepared
 The layer thickness optimized using a genetic algorithm (GA) can estimate the appropriate design with the maximum reflection loss (RL)

Ultra-thin ultra-wideband tunable radar absorber based on hybrid incorporation of active devices

Structural radar absorber has important application in stealth field for its ability to effectively absorb incoming radar wave and to bear the load at the same time. Metasurface absorbers can achieve nearly-perfect absorption of radar wave, and have characteristics of light weight and thin structure, but their bandwidth are usually narrow. To solve this problem, a new method of broadening the bandwidth of metasurface absorber is proposed in this work. With varactor and PIN diode integrated in a hybrid manner, the continuous tunning and discrete switching are combined together to broaden the effective absorption bandwidth of the absorber. Using this method, an ultra-wideband tunable metasurface absorber is designed and the absorbing mechanism is analyzed in depth. By changing the bias voltages of PIN diodes and varactors, the absorbing frequency can be continuously tuned within a wide band from 4.57 GHz to 8.51 GHz. Measured results verify the low radar cross section characteristics of the absorber and the effectiveness of the design method. The proposed method is simple and feasible, and can be extended to other broadband structure design.

Sustainable Carbon Xerogel in the Production of Electromagnetic Radar Absorbing Composites in the Ku-Band

Carbonaceous porous materials are strong candidates for producing low-cost, lightweight, and sustainable electromagnetic (EM) absorbing materials. This work investigates the production of radar-absorbing materials (RAM) by a simple method. Sustainable carbon xerogel (CX) was synthesized from tannin, a biosourced molecule. CX and commercial carbon nanotube (CNT) were embedded in a silicone matrix at proportions of 10 and 15 wt.% of CX and 0.1 wt.% of CNT to produce flexible composites. The morphology and structure of the carbonaceous materials were evaluated by field-emission scanning electron microscopy (FEG-SEM), Raman spectroscopy, X-ray diffraction (XRD), textural properties by N2 adsorption-desorption isotherms, and mercury porosimetry. The electromagnetic characterization of the composites was analyzed by a vector network analyzer (VNA) in the Ku-band. The results demonstrated that an increased concentration of CX in the composite improved reflection loss reaching -43.19 dB at 13.79 GHz.

Application of the method for obtaining radar portraits of objects by the aperture synthesis method using the SFCW radar to evaluate the effect of a radar absorbing coating on the radar significance of a cone-shaped object

Application of the method for obtaining radar portraits of objects by the aperture synthesis method using the SFCW radar to evaluate the effect of a radar absorbing coating on the radar significance of a cone-shaped object

Effective Dielectric Permeability of a Composite with Matrix Ellipsoidal Inclusions

A composite material based on ellipsoidal inclusions distributed in a bonding medium is considered. A formula is obtained for calculating the effective dielectric constant of such a material, taking into account the presence of a layer of a binder on the inclusions, which prevents their direct contact. It is shown that the calculated values of the effective dielectric constant correspond to the experimental values for a composite based on sections of conductive fibers. Keywords: composite material, radar absorbing structure, diffraction of electromagnetic waves.

Statistical modeling for the monostatic radar cross‐section characteristics of stealth aircraft using the Swerling‐chi/inverse generalized gamma composite distribution model

AbstractIn this article, a new Swerling‐chi/inverse generalized gamma composite distribution model is proposed for modeling the monostatic radar cross‐section (RCS) statistical characteristics of stealth aircraft. Under some conditions, the proposed distribution model can be reduced to the Fisher‐Snedecor distribution model and the Swerling‐chi distribution model. A typical stealth aircraft without radar absorption materials (RAM) is selected as an application for simulation using the adaptive cross approximation (ACA) technique. Based on the nonlinear least‐squares criterion, monostatic RCS statistical characteristics of the front sector of this aircraft are fitted using the proposed composite model. For the comparison purpose, monostatic RCS statistical characteristics of this aircraft are also fitted with the conventional RCS distribution models. Goodness‐of‐fit tests of the proposed distribution model and the conventional distribution models are analyzed in terms of the error‐of‐fit results and the test results. Simulation results validate the efficiency and robustness of the proposed distribution model.