Composite sandwich constructions for absorbing the electromagnetic waves

RAS (radar absorbing structures) is a key component for weapon systems such as aircrafts, warships, and missiles to achieve both the stealth performance by absorbing EM (Electromagnetic) waves incident on and load bearing capability. In this work, the RAS was fabricated as sandwich constructions composed of nanocomposite, carbon fabric/epoxy composite, and PVC foam. The nanocomposite composed of E-glass fabric, epoxy resin, and CNT (carbon nanotube) was adhesively bonded to the outside of the sandwich construction in order to absorb EM waves. The carbon fabric/epoxy composite had the dual roles as the reflection layer of incident EM waves and load bearing face material of sandwich constructions. Using the fabricated sandwich constructions, the EM absorbing characteristics were measured by the free space measurement system and the bonding characteristics between nanocomposites and carbon fabric/epoxy composites also were investigated.     

Development of the composite RAS (radar absorbing structure) for the X-band frequency range

Since the EM properties of fiber reinforced polymeric composites can be tailored effectively by adjusting its composition, they are plausible materials for fabricating the radar absorbing structure (RAS) of desired performance. In this study, the composite RAS which has superior load bearing capacity and EM absorption characteristics has been developed by blending the conductive carbon black with the binder matrix of the E-glass/polyester composite, and its EM absorption characteristics has been measured by the free space method in the X-band frequency range (8.2–12.4 GHz). The composite RAS was designed so as to have the optimal performance for the X-band centered at 10 GHz. From the investigation, it was found that the composite RAS of 2.93 mm thickness with the conductive carbon black absorbed more than 90% of incident EM wave throughout the entire X-band frequency range.     

Novel optimization method of single square FSS impinged and cascaded radar absorbing composites

It is well known that radar absorbing potentiality of existing magneto-dielectric composites can be significantly enhanced by the application of frequency selective surface (FSS) and cascaded electromagnetic (EM) structures. But the optimization of such complex EM structures and validation of the adopted optimization strategy is still a very challenging task for the researchers. Therefore, in this study, an effective effort has been made for the optimization and the corresponding validation for Single Square FSS (SS-FSS) impinged and cascaded radar wave absorbers using advanced computational EM software’s like FEldberechnung fur Korper mit beliebiger Oberflache – a German acronym (FEKO) and high frequency structure simulator (HFSS). In addition, a critical analysis of dielectric constant (ε′) has been carried out to select the best combination of composites for the development of efficient radar wave absorbers. A comparison between optimized and simulated results have been carried out to examine the effect of advanced EM approaches over reflection loss (RL) characteristics of composite radar absorbing materials (CRAMs). A rapid change in radar absorption properties of composites has been observed after the application of SSFSS and cascading. A SS-FSS impinged composite has been found to provide a wide absorption bandwidth of 3.6 GHz at X-band. A cascaded absorber having layer thickness 1.8 mm provides a peak RL of ?42.6 dB at 10.6 GHz with an absorption bandwidth of 2.5 GHz. The strong agreement between mathematical model, HFSS and FEKO results clearly reflects the efficiency of adopted approach for distinct practical EM applications.     

Radar absorbing sandwich construction composed of CNT,PMI foam and carbon/epoxy composite

RAS (radar absorbing structure) is effective for both load bearing and EM (electromagnetic) wave absorbing capability of the stealth technology. Although the RAS is usually designed to absorb the EM waves in broadband range of wave frequencies, it may be more effective to absorb the EM waves in certain frequency range as a narrow band stop filter for the specific applications.     

Fabrication of radar absorbing structure (RAS) using GFR-nano composite and spring-back compensation of hybrid composite RAS shells

The fiber-reinforced composite materials have been advanced to provide excellent mechanical and electromagnetic properties. The radar absorbing structure (RAS) is such an example that satisfies both radar absorbing property and structural characteristics. The absorbing efficiency of RAS can be obtained from selected materials having special absorptive properties and structural characteristics such as multi-layer and stacking sequence.

In this research, to develop a RAS, three-phase composites consisted of {glass fiber}/{epoxy}/{nano size carbon materials} were fabricated, and their radar absorbing efficiency was measured on the X-band frequency range (8–12 GHz). Although some of GFR (Glass Fiber–Reinforced)-nano composites showed outstanding absorbing efficiency, during their manufacturing process, undesired thermal deformation (so called spring-back) was produced. The main cause of spring-back is thought to be temperature drop from the cure temperature to the room temperature. In order to reduce spring-back, two types of hybrid composite shells were fabricated with {carbon/epoxy} and {glass/epoxy} composites. Their spring-back was measured by experiment and predicted by finite element analysis (ANSYS). To fabricate desired final geometry, a spring-back compensated mold was designed and manufactured. Using the mold, hybrid composite shells with good dimensional tolerance were fabricated.     

Critical analysis of frequency selective surfaces embedded composite microwave absorber for frequency range 2–8 GHz

Radar wave absorbers are important for the reduction of radar cross section of the target for stealth applications. Earlier the radars were available in the frequency range 8–12 GHz (X-band) and 12–18 GHz (Ku-Band). Due to recent advancement in radar technology, radars are now available from 2 to 18 GHz frequency range. So there is an urgent need to develop such a material that can work as radar wave absorber in the lower frequency band of the microwave spectrum i.e., 2–8 GHz. For this purpose the selection of material is an important criterion as the radar wave absorption depends primarily upon the material characteristics i.e., complex permittivity and complex permeability. For lower frequency radar wave absorption, the material must also possess the conducting property along with dielectric and magnetic properties. Therefore, an attempt has been made to develop a radar wave absorbing nano-composite material by selecting constituent materials with such inherent properties that can work for the absorption of radar wave in the lower frequency range. It is observed that the developed composite give good absorption in the lower frequency range but with narrow radar wave absorption bandwidth (4–7 GHz). So we have explored the possibility of the efficient use of an advanced electromagnetic technique like frequency selective surface to enhance the radar wave absorption bandwidth in the lower frequency region of the microwave frequency spectrum and precaution has been taken such that complexity due to FSS can be avoided. It has been observed that the synthesised single layer absorber with single square loop, cross dipole and Jerusalem cross FSSs provides radar wave absorption bandwidth in the frequency range 2–8 GHz.     

Fabrication and electromagnetic characteristics of electromagnetic wave absorbing sandwich structures

The radar absorbing structures (RAS) having sandwich structures in the X-band (8.2–12.4 GHz) frequencies were designed and fabricated. We added conductive fillers such as carbon black and multi-walled carbon nanotube (MWNT) to composite prepregs and polyurethane foams so as to efficiently increase the absorbing capacity of RAS. In order to improve the mechanical stiffness of RAS, we adopted the sandwich structures made of composite face sheets and foam cores. Glass fabric/epoxy composites containing conductive carbon black and carbon fabric/epoxy composites were used for the face sheets. Polyurethane foams containing MWNT were used as the core material. Their permittivity in the X-band was measured using the transmission line technique. The reflection loss characteristics for multi-layered sandwich structures were calculated using the theory of transmission and reflection in a multi-layered medium. Three kinds of specimens were fabricated and their reflection losses in the X-band were measured using the free space technique. Experimental results were in good agreement with simulated ones in 10-dB absorbing bandwidth.     

嵌入分形频率选择表面的低频超薄吸波层的设计

研究了频率选择表面对超薄多层微波吸波体在低频(L和S频段)吸波性能的影响. 分别采用硫化工艺和激光刻蚀方法制备出传统的微波吸收材料(MAM)--橡胶板和FSS层, 然后利用它们合成多层微波吸波体(MMA)样品, 在NRL弓形法测试系统中测量该样品的反射率. 发现随着FSS层在传统吸波材料层中的引入, 确实可以增强整个多层吸波体在低频段的吸波性能. 实验结果显示, 当两个FSS层在多层吸波体中适当排列时, 可以在1 GHz得到一个–3.49 dB的反射率峰值, 最大反射峰值可达–9.35 dB, 这时的样品厚度是1.8 mm. 本研究为吸波材料的吸波性能向低频段的拓展提供了一种有效的方法.     

吸波材料与FSS复合的隐身技术研究进展

吸波材料通过把电磁波转化为其它形式的能量来降低目标的雷达散射截面,从而实现目标隐身的目的.频率选择表面(FSS)是一种具有二维周期性结构的滤波结构,主要在滤波器上使用.在总结了吸波材料作用原理和FSS结构分析方法的基础上,介绍了FSS结构和吸波材料复合构成的隐身技术的研究进展.     

Computational method for radar absorbing composite lattice grids

Composite lattice grids reinforced by glass fibers (GFRC) and carbon fibers (CFRC) filled with spongy materials can be designed as lightweight radar absorbing structures (RAS). In the present paper, a computational approach based on periodic moment method (PMM) has been developed to calculate reflection coefficients of radar absorbing composite lattice grids. Total reflection backing (TRB) is considered directly in our PMM program by treating it as a dielectric material with large imaginary part of permittivity. Two different mechanisms of reflection reduction for radar absorbing lattice grids are revealed. At low frequency, reflection coefficients increase with the volume fraction of the grid cell wall. At high frequency, several grating lobes propagate away from the doubly periodic plane, and reflection coefficients depend on both the cell wall volume fraction and interelement distance.     

频率选择表面(FSS)在雷达吸波材料中的应用及最新进展

研究了频率选择表面(FSS)在雷达吸波材料中的应用,阐述了其原理和研究现状.重点介绍了频率特性可调的含主动式FSS的吸波结构.从研究的情况来看,经过恰当的材料选取以及结构设计,FSS可以改进雷达吸波材料的性能.最后展望了FSS在雷达吸波材料中的应用前景.     

Broadbanding of A-sandwich Radome Using Jerusalem Cross Frequency Selective Surface

Enhancement of electromagnetic performance of A-sandwich radome using aperture-type Jerusalem cross frequency selective surface (FSS) is presented. The Jerusalem cross FSS array is embedded in the mid-plane of the core of Asandwich radome to enhance the EM performance parameters over the entire Xband. For modeling the Jerusalem cross FSS embedded radome panel and evaluation of its EM performance parameters, equivalent transmission line method in conjunction with equivalent circuit model is used. A comparative study of Jerusalem cross FSS embedded A-sandwich radome and A-sandwich radome of identical material and thickness (core and skin layers) indicate that the new wall configuration has superior EM performance as compared to the A-sandwich wall alone configuration. The excellent EM performance of Jerusalem cross FSS embedded A-sandwich radome makes it a desirable choice for the design of normal incidence radomes (hemispherical/ cylindrical), near-normal incidence radomes (paraboloidal) and highly streamlined airborne nosecone radomes.     

Characterization of electromagnetic properties of polymeric composite materials with free space method

The introduction of microwave radars during the second World War altered the air defense scenario significantly, and this led to the development of the “stealth” techniques. By reducing the detectability of aircrafts or warships, of which the radar cross section (RCS) is a measure, they could evade radar detection, which affected not only the mission success rate but also survival of them in the hostile territory. In the very early stage of the research on stealth techniques, many researches were mainly concentrated on the reduction of RCS and development of radar absorbing materials (RAM), but nowadays studies on investigating the radar absorbing structures (RAS) using fiber reinforced polymeric composite materials are becoming popular research field.

In this study, electromagnetic characteristics of unidirectional E-glass fiber reinforced epoxy composites were tested with free space methods, which can overcome drawbacks of conventional cavity and waveguide methods. Complex relative permittivities of low-loss composite were measured with respect to the angle between the fiber orientation and the electric field vector of EM wave in X-band frequency range. From the experimental data, empirical relation between the dielectric properties of composites and test variable was suggested and verified.     

Polarization Independent Dual-band Metamaterial Based Radar Absorbing Structure (RAS) for MillimeterWave Applications

The EM analysis of multi-layered metamaterial based radar absorbing structure (RAS) with dual-band characteristics in millimeter wave frequency regime has been carried out in this paper using transmission line transfer matrix (TLTM) method for TE and TM polarizations. The proposed metamaterial-based RAS exhibits dual-band characteristics at centre frequencies 120 GHz and 175 GHz with very low power reflection. It absorbs more than 90% power of incidence wave over the frequency range from 111-131 GHz at first resonance and from 164.5-185 GHz at second resonance without metal backing plate, which is desirable for stealth applications. It also showed very low (< 1.6%) transmission over the frequency of interest for both TE and TM polarizations. The proposed metamaterial-RAS has potential applications in the design of multi-band sensor systems and RCS reduction in millimeter wave frequency regime.     

Aramid/epoxy composites sandwich structures for low-observable radomes

Low-observable radomes are usually made of E-glass/epoxy composite due to its low dielectric constant which is necessary not to interfere electromagnetic (EM) wave transmission characteristics. Since aramid fibers have lower dielectric constant and higher strength than those of E-glass fiber, aramid fiber radome structures may have better the EM transmission and mechanical characteristics than those of E-glass/epoxy radomes. In this work, the low-observable radome was constructed with a sandwich construction composed of aramid/epoxy composites faces, foam core and Frequency Selective Surface (FSS) which had the abilities of transmitting EM waves selectively in the X-band range. The EM wave transmission characteristics of the low-observable radome were simulated by a 3-dimensional electromagnetic analysis software and also measured by the free space measurement method with respect to the pattern size of FSS and foam cores. The mechanical properties of the low-observable radome made of aramid/epoxy composite were measured by the 3-point bending test and compared to those of the conventional low-observable radome made of E-glass/epoxy composite.     

Nanocomposite stealth radomes with frequency selective surfaces

The stealth function of the radome (Radar + Dome) is to transmit or reflect the EM (electromagnetic) wave selectively through the radome. In this work, the stealth radome for aircrafts and warships was developed with the FSS (frequency selective surfaces), PVC foam, and nanoclay-dispersed E-glass fabric/epoxy composite. The water diffusivity of nanocomposites, which changes the stealth characteristics, was measured with respect to the contents of nanoclay. The EM transmission characteristics were measured by the free space measurement system in the X-band frequency range (8.2–12.4 GHz) with respect to the content of nanoclay. Also, the flexural strength of the sandwich construction composed of the nanocomposite, PVC foam, and FCCL (flexible copper clad laminate) was measured by the 3-point bending test.     

EM Analysis of Metamaterial Based Radar Absorbing Structure (RAS) for MillimeterWave Applications

The EM performance analysis of a multilayered metamaterial based radar absorbing structure (RAS) has been presented in this paper based on transmission line transfer matrix (TLTM) method for millimeter wave applications. The proposed metamaterial-RAS consists of cascaded DPS and MNG layers of identical configurations. It exhibits extremely low reflection (< 42 dB) at 95 GHz and absorbs more than 95% power of incident wave over the frequency range of 90.4- 100 GHz without metal backing for both TE and TM polarizations. In view of aerospace applications, the reflection, transmission, and absorption characteristics of the proposed metamaterial-RAS are also studied at different incident angles (0°, 30°, and 45°) for both polarizations.     

Microwave absorption properties of FSS-impacted composites as a broadband microwave absorber

The development of a cost-effective microwave absorber with wide bandwidth corresponding to reflection loss (RL)?≤??10 dB is still a very challenging task. A sugarcane bagasse-based agricultural waste composite has been analyzed for its elemental contents. The combination of elements is suitable for its possible usage as a cost-effective microwave absorbing material. Therefore, this composite has been subjected to morphological and electromagnetic studies to analyze its microwave absorbing behavior. The frequency dependent complex dielectric permittivity and complex magnetic permeability values were obtained using a transmission/reflection waveguide approach in the X-band. Furthermore, the effect of the Minkowski loop frequency selective surface (FSS) was studied over the absorption capability of the composite. It was found that the application of FSS leads to a reduction in thickness up to 2.9 mm and an enhancement in absorption bandwidth up to 3.6 GHz. The FSS patterned composite shows a remarkable performance with peak RL of ?28.4 dB at 10.7 GHz and absorption bandwidth of 3.6 GHz.     

十字型电阻贴片频率选择表面吸收体吸波性能研究

本工作用有限元法对十字型电阻贴片频率选择表面(FSS)吸收体的吸波性能进行了研究,结果表明:改变十字型电阻贴片FSS的周期排列方式、周期尺寸、FSS单元的尺寸、FSS的方阻、介质层厚度均可对其吸收峰的位置、峰值以及带宽进行调节,并且调节范围较大,计算结果与实验结果基本吻合.研究表明这种结构具有传统Salisbury吸收体所不具有的优势.     

Broad bandwidth of thin composite radar absorbing structures embedded with frequency selective surfaces

The three-layer ultrathin radar absorbing structure (RAS) involving a frequency selective surface (FSS) exhibiting excellent broad bandwidth properties is designed and fabricated. The EW and flaky carbonyl iron powders were used to produce two kinds of silicone rubber matrix magnetic composites for the top and the bottom layer, respectively. The electromagnetic parameters of the composites were measured in the frequency range of 2–18 GHz. The middle layer is an FSS in the form of double-square loops with four micro-split gaps in the middle of the outer loop. The results show that the proposed RAS can provide a 10 dB absorbing bandwidth of 13.2 GHz from 4.8 to 18 GHz (1.7 mm thickness) and a 10 dB absorbing bandwidth of 14.1 GHz from 3.9 to 18 GHz, covering C-band, X-band and Ku-band (2.0 mm thickness). A good match between simulation and measurement results demonstrates the validity of our design.