Radar Absorbing Structures Research Articles

Electromagnetic Techniques and Design Strategies for FSS Structure Applications [Antenna Applications Corner

Frequency-selective surfaces (FSSs) have a wide spectrum of applications in airborne platforms, wireless communication systems, medical imaging instruments, and high-power microwave systems. To employ finite, electrically large, and conformal FSSs for such applications, the development of an efficient electromagnetic (EM) design and analysis technique is essential, which is a challenging task in terms of computational complexity and requirements of highend computing platforms. In view of assessing the efficacies of various EM techniques for the design and analysis of FSS structures employed for planar and conformal surfaces, a brief review is presented here. Furthermore, the design strategies of novel FSS structures including metamaterial (MTM)-element FSSs are discussed for specific applications such as highperformance radomes, radar absorbing structures (RASs), and antennas. The complex issues associated with the EM design and analysis of FSS structures are also outlined for radomes, RAS, and antennas.

Enhanced mechanical and microwave-absorption properties of SiCf/AlPO4 composite with PIP–SiC interphase and the MWCNTs filler

A single-layer radar-absorbing structure in the X-band (8.2GHz to 12.4GHz) was designed and fabricated by blending multi-walled carbon nanotubes (MWCNTs) with the binder matrix of SiC fiber/aluminum phosphate matrix (SiCf/AlPO4) composite. The SiC interphase was successfully prepared on SiC fibers by a precursor infiltration and pyrolysis (PIP) method. The morphology of as-received interphase was observed by SEM, and its structure was characterized by XRD and Raman spectrum. The effects of PIP–SiC interphase on the mechanical and dielectric properties of the composite were investigated. The influence of MWCNTs content on the dielectric and microwave-absorption properties of coated SiCf/AlPO4 composite was discussed. When the content of MWCNTs was between 1.5wt% and 3.5wt% and the composite thickness is in the range of 2.5–3.5mm, the SiCf/AlPO4 composite achieved excellent absorbing wave property in X-band.

Effect of delamination on the electromagnetic wave absorbing performance of radar absorbing structures

Abstract The purpose of this study is to understand a correlation between interlaminar delamination and radar absorbing performance of composite radar absorbing structures (RAS). Many researches have been conducted on composite RAS and composite delamination. No study, however, has dealt with the delamination effect on the radar absorbing performance of RAS so far. In this study, a multi-layer Dallenbach radar absorber with glass/epoxy and glass/epoxy-Multiwall Carbon Nanotubes (MWCNT) was designed. Based on the design, test specimens were fabricated and the ‘split specimen’ test method was introduced. This novel test method, for measuring the electromagnetic wave reflection and absorption, enabled quantitative analysis of the delamination effect by implementing delamination with various thicknesses and locations on the same specimen. The reflection loss of the specimen for normal incident EM waves in the X-band was measured. The measurement and analysis results verified that the changes in the thickness-wise location and thickness of the delamination altered the magnitude as well as the resonance frequency of the reflection loss.

Design and verification of a single slab RAS through mass production of glass/MWNT added epoxy composite prepreg

ABSTRACTIn this study, glass fiber composite prepreg is manufactured with multi‐walled carbon nanotube (MWNT) added epoxy using two different methods. Because MWNT agglomeration occurs, the calendering dispersion method is used to resolve this problem. The tensile and shear tests of glass/MWNT 1.8wt % added epoxy composite (CNT18) are conducted and the results are compared with the properties of a commercial glass/epoxy composite (GEP 118). The complex permittivity is measured using a network analyzer and a waveguide in the Ku‐band. A single slab radar absorbing structure (RAS) is also designed and verified. It is found that the tensile and shear properties of CNT18 are sufficient to replace GEP 118 as a structural material. Furthermore, the—10 dB bandwidth and reflection loss of the RAS using CNT18 is 12.87 to 17.78 GHz (4.91 GHz) and—29.2 dB at 14.95 GHz, respectively. The measurement results align well with the simulation results. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42019.

임피던스정합을 이용한 레이더반사면적 최소화 단층형 전파흡수구조 설계

The design of radar absorbing structures(RAS) is a discrete optimization problem and is usually processed by stochastic optimization methods. The calculation of radar cross section(RCS) should be decreased to improve the efficiency of designing RAS. In this paper, an efficient method using impedance matching is studied to design RAS for minimizing RCS. Input impedance of the minimal RCS for the specified wave incident conditions is obtained by interlocking physical optics(PO) and optimizations. Complex permittivity and thickness of RAS are designed to satisfy the calculated input impedance by a discrete optimization. The results reveal that the studied method attains the same results as stochastic optimization which have to conduct numerous RCS analysis. The efficiency of designing RAS can be enhanced by reducing the calculation of RCS.

Microwave Absorbing Properties of Lightweight Nanocomposite/Honeycomb Sandwich Structures

Thin glass-fiber/epoxy-composite sheets filled with multiwalled carbon nanotubes (MWCNTs) are manufactured to make lightweight honeycomb sandwich microwave absorbers. A multilayered sandwich structure of thin nanocomposite sheets and honeycomb spacers have been also proposed and developed to work in a wide frequency range. The nanocomposite sheets are prepared from 0.5, 1.0, 1.5, 2.0, and 2.5 wt. % of MWCNTs. A commercially available simulation software computer simulation technology (CST) microwave studio was used for the designing and development of radar absorbing structure (RAS) composed of MWCNTs/glass-fiber/epoxy-composite sheets and honeycomb cores. The measurements of return loss (RL) from sandwich structures with 5 mm and 20 mm honeycomb cores in the Ku band (11–17 GHz) show that maximum RL is achieved at 11 GHz and 16 GHz, respectively. The stacking of three nanocomposite sheets and three 5 mm-thick honeycomb spacers produced a wide band microwave absorber with −10 dB RL over 9 GHz bandwidth.

A glass coating for SiC fiber reinforced aluminum phosphate matrix (SiCf/AlPO4) composites for high-temperature absorbing wave applications

A single-layer radar-absorbing structure in the X-band (8.2GHz to 12.4GHz) was fabricated by blending conductive carbon black (CB) with the binder matrix of SiCf/AlPO4 composites. An oxidation protective glass coating was prepared on SiCf/AlPO4 composites using a simple and low-cost slurry technique. Oxidation test shows that, after oxidation in air at 1273K for 50h, the three-point bending strength of coated SiCf/AlPO4 composites keeps up to 200MPa, and the complex permittivity has little difference with the as-received sample. By simulating the reflection loss, it was found that −10dB absorbing bandwidth can be more than 3GHz for oxidation test-coated SiCf/AlPO4 composites in the range of 2.8mm to 3.2mm thickness.

Radar absorbing composite structures dispersed with nano-conductive particles

Nano-composites are typically used in radar absorbing structures (RAS) to improve their electromagnetic (EM) wave absorbing performances. The nano-composites are generally composed of E-glass/epoxy composites dispersed with carbonaceous nano-conductive particles such as carbon black and carbon nanotubes (CNT). Because the EM wave absorbing performance of the nano-composite RAS is dependent on the particle concentration of the nano-composites, investigation of its performance with respect to the different nano-composite composition is highly necessary.In this study, the nano-composite RAS were designed using the optimum design method with the various selected nano-composites. The EM wave absorbing performance of the RAS were obtained by numerical simulation with respect to the nano-composites and verified by experimental measurement. Additionally, the effects of the carbonaceous nano-conductive particles on the mechanical properties of the nano-composites were measured. Finally, the optimum nano-composite RAS was suggested considering both electromagnetic and mechanical performances.

Circuit-analog (CA) type of radar absorbing composite leading-edge for wing-shaped structure in X-band: Practical approach from design to fabrication

In this paper, a circuit analog (CA) absorber consisting of a periodic, resistive-pattern sheet and glass-fiber/epoxy composite spacer; for integration into the leading edge of a wing-shaped structure, was introduced along with practical steps to take from design to fabrication. The CA absorber was designed in a flat-plate shape for the X-band (8.2–12.4 GHz). To fabricate a periodic-resistive-pattern surface compatible with a 180 °C autoclave process, a carbon-based conducting material was used. The reflection loss of this flat-plate CA absorber was measured with an X-band, free-space measurement system. A leading edge for a wing-shaped structure incorporating the new CA absorber was designed and fabricated based on data from a flat-plate-design CA absorber. The integrated leading edge was mounted on a wing-box, and its echo-RCS was measured in an anechoic chamber. When the CA absorber was applied to the leading edge of the wing-shaped structure, the echo-RCS level measured by compact range showed a reduction of about 10 dB in most of the X-band, in both polarizations. From a practical point of view, these results confirmed that CA absorbent materials have great potential in radar absorbing structures for stealth aircraft.

Optimum design method of a nano-composite radar absorbing structure considering dielectric properties in the X-band frequency range

A radar absorbing structure (RAS) is generally constructed using nano-composites composed of an E-glass/epoxy composite dispersed with carbonaceous conductive particles to improve the electromagnetic (EM) wave absorbing performance. The EM wave absorptance of the RAS is highly dependent on the dielectric properties of the nano-composite. Therefore, an optimum design method for RAS considering the dielectric properties and the EM wave absorbing characteristics should be established.In this study, a theoretical method for the EM wave absorbing characteristics of nano-composite RAS was developed with respect to the dielectric properties of the nano-composites. Based on the theoretical investigation, an optimum design method for the RAS was developed. Then the RAS was constructed with the nano-composite composed of an E-glass/epoxy composite and carbon black and its EM wave absorbing performances were measured and compared to those obtained through theoretical calculations and numerical simulations.

Development of a damage tolerant structure for nano-composite radar absorbing structures

A radome structure, which is the protective cover for a radar antenna, is generally made of composite materials that have the high mechanical properties and low dielectric constants that are required for radome structures. Radar absorbing structures (RAS) are made of materials similar to radome structures, but include electro-conductive materials to enhance electromagnetic (EM) wave absorption characteristics.Though composite structures have both excellent electromagnetic and mechanical properties, they are vulnerable to external impact loads. When the RAS is damaged by external impact loads during operation, damages such as cracks, delamination, and dents may be generated, which can change the EM wave reflection properties of the composite structure.In this study, the surface of a RAS containing conductive nano-material was padded with ultra-high molecular weight polyethylene fabric to protect the composite structures against external impact loads. The impact characteristics of the RAS were investigated by drop weight impact tests with and without the pad, whose effect was measured by a compression test after impact (CAI). Additionally, the EM wave reflection characteristics of the padded RAS were investigated with a free space measurement. Finally, an optimum combination of UHMWPE fabric and glass composite face was suggested for the damage tolerant RAS structure.

Effects of carbon black (CB) and alumina oxide on the electromagnetic- and microwave-absorption properties of SiC fiber/aluminum phosphate matrix composites

A single-layer radar-absorbing structure in the X-band (8.2GHz to 12.4GHz) was designed and fabricated by blending conductive carbon black (CB) with the binder matrix of SiC fiber/aluminum phosphate matrix composites. The slurry comprising Al(H2PO4)3 solution, CB, and Al2O3 filler was used as matrix precursor. The complex permittivities of the composites were measured in the X-band. The reflection loss characteristics were calculated using the theory of transmission and reflection. Experimental results showed that the real and imaginary parts of the composites were proportional to the filler concentrations, and the increase rates of the real and imaginary parts were different in terms of filler concentrations. The microwave absorbing ability of the composites was further evaluated, and the addition of 4wt% CB to the composites was found to noticeably improve the absorption property of the 2.8–3.2mm-thick composites with the assistance of 5–15wt% Al2O3.

Experimental Study of the Electrical Resistivity of Glass-Carbon/Epoxy Hybrid Composites

The electrical resistivity of composite materials varies with the changing of structural parameters, and this feature could be used to improve their electromagnetic wave absorbing property. This work investigated the electrical resistivity of hybrid composites aiming at radar absorbing structures. The glass-carbon/epoxy hybrid composites with different structural parameters were prepared by filament winding machine. And the effects of hybrid style, hybrid ratio, curing pressure, fiber distribution and fiber orientation on electrical resistivity were tested. As the test results shown, firstly, the electrical resistivity of interlaminated hybrid composites is larger than that of intraply hybrid composite; secondly, the electrical resistivity of hybrid composite is inversely proportional to the content of carbon fiber; thirdly, the electrical resistivity of hybrid composite with carbon fiber homogeneous distribution is larger than that of hybrid composite with inhomogeneous distribution; finally the electrical resistivity of hybrid composite would decrease with the curing pressure increasing. Interestingly there are similar rules in the vertical and parallel direction of the fibers. These results are helpful to control and adjust the electrical resistivity of glass-carbon/epoxy hybrid composites.

A high-temperature radar absorbing structure: Design, fabrication, and characterization

Abstract In this investigation, a new sandwich structure called the SiC f /SiC sandwich radar absorbing structure (SRAS) for high-temperature radar absorption was designed using SiC f /SiC composites. Samples with the base design and optimized design were then fabricated through the PIP process, and the free space reflectivity was measured. The experimental result agrees with the calculated result, exhibiting excellent absorbing properties. The high-temperature reflectivity of the SiC f /SiC SRAS was also measured and the variation with temperature is found to be in a small range, indicating excellent absorbing properties at elevated temperatures. The factors determining the variation of reflectivity with temperature were also investigated, and the sheet resistance of the SiC fiber fabrics (particularly, the resistance characteristics of the pyrocarbon layer of the SiC fiber has a negative temperature coefficient) used as the lossy layer in the SRAS is found to be primarily responsible for the thermal variation of reflectivity.

On Current and Prospective Use of Binary Thin Multilayers in Radar Absorbing Structures

The main objective of the paper is to overview of the actual and potential capabilities of some complex multilayers to enhance, and control the microwave absorption. Going through available literature data we emphasize the role of interphase coupling, and material resonances in enhancing phenomena of importance for internal scattering, di usions, and intrinsic material absorption of propagating EM wave, especially in microwave and mm wave ranges. The theoretical background is consequently formulated in terms of transmissions line approach.

Hierarchical lattice composites for electromagnetic and mechanical energy absorptions

A new hierarchical radar absorbing structure with multifunction of ultra-light weight, anti-crushing and radar absorption was designed and made by glass fiber reinforced lattice composites filled with radar absorbing foams. Experiments were performed to reveal the electromagnetic absorption and anti-crushing behaviors. Mechanisms of the composite in electromagnetic absorption and anti-crushing were analyzed. The newly designed composite lattice displays excellent performances in absorbing both microwave and mechanical energies at ultra-light weight. To balance the anti-crushing and radar absorption behaviors, key factors of the composite lattices, including the panel thickness, the relative density of the lattice, the cell dimension and the geometry, were revealed based on the analysis and experiments.

Dielectric property of unidirectional triangle-shape carbon fiber reinforced polymeric composites

Triangle-shape carbon fiber reinforced polymeric composites is a plausible material for fabricating the radar absorbing structure. In order to design the effective electromagnetic wave absorber, the dielectric properties of its constituents are required in the target frequency band. In this paper, the unidirectional triangle-shape carbon fiber/epoxy composites were manufactured and the carbon fiber volume fraction of composite materials was measured to be about 46%. And then the permittivities of specimens were tested with the rectangle waveguide method in the vertical direction and in the parallel direction. The theoretical models and prediction equations for estimating permittivity of the unidirectional triangle-shape carbon fiber/epoxy composites were proposed with respect to the fiber volume fractions, the cross-section shape of fiber, the fiber orientations and the permittivity of components. It was found that the theoretical values agreed well with the experimental results.

항공기 날개 앞전의 레이더흡수구조 최적화

In this paper, objective functions are defined for optimization of radar absorbing structures (RAS) on the aircraft wing leading edge. RAS is regarded as a single layer structure made of dielectrics. Design variables are the real and imaginary parts of complex permittivity. Reflection coefficient(RC) and radar cross section(RCS) are used in the objective function respectively. Transmission line theory is employed to calculate the RC. The RCS is evaluated by using physical optics(PO) for a leading edge part model. Genetic algorithm(GA) is used to perform optimization procedures. The radar absorbing performance of designed RAS is assessed by the RCS of a wing which has RAS on the leading edge.

Wideband radar absorbing structure with low density material and load‐bearing MWCNT added composite material

A new design concept of a wideband radar absorbing structure consisting of load-bearing glass/MWCNT added epoxy composite and having low density/lightweight characteristics for C- to X-band and X- to Ku-band radar is presented. The proposed radar absorbing structures have triple layers which are the low density and light weight layer to reduce the total weight, covering layers to protect the foam layer from humidity absorption, and a load-bearing layer as the actual structural layer. The proposed designs satisfied −10 dB absorption in the C-band to X-band and X-band to Ku-band using only two different materials. Moreover, from a practical point of view, since thickness control of absorbing structures is an important factor for the absorbing performance, the proposed structures could minimise fabrication errors (precise thickness controls) by employing multiple layers for wideband absorbing performance.

Microwave Absorbing Nanocomposites Composed with and without Polyaniline by Use as Radar Absorbing Structure

In the past decade, new materials have been developed based on the physical and chemical properties of carbon nanotubes. The combination of polyaniline with multiwall carbon nanotubes results in a new functional material with advantageous electromagnetic properties. The objective of this study was to produce a radar absorbing structure consisting of glass fiber woven fabric impregnated with a formulation containing carbon nanotubes, polyurethane resin, with or without polyaniline. A different formulation was used for each woven sheet (multilayer structure). The electromagnetic properties of these nanocomposite materials were characterized by reflectivity measurements using Naval Research Laboratory arch method (frequency range, 8 to 12 GHz). The attenuation of both sides of each nanocomposite material was also measured and compared. The attenuation of electromagnetic energy was as high as 70 %, approximately, indicating that these materials can be used as microwave absorbers.