BROADBAND RADAR ABSORBING MATERIALS USING FRACTAL FSS FOR STEALTH APPLICATION

Abstract

In the past few years, development of radar absorbing materials (RAMs) has attracted attention of researchers because of their increasing demand for stealth application. The -10 dB absorption bandwidth, coating thickness and cost of RAM are three important parameters which are required to design a good radar wave absorber. A cost-effective absorber with wide bandwidth (RL ≤ -10 dB) and minimum possible coating thickness (≤ 1.5 mm) is highly desirable for stealth applications. Simultaneous achievement of these three parameters are still a very challenging task and the reported absorbers generally succeeded in achieving one goal by sacrificing the other due to strong thickness-bandwidth trade-off. Various kinds of novel composite RAMs (i.e., nanocomposite RAMs and waste composite RAMs) consisting of dielectric, magnetic and conducting elements have been introduced in this thesis for their possible application as a thin and broadband RAM in the frequency range of 8.2 to 12.4 GHz. Composite RAM alone is not sufficient for providing -10 dB absorption bandwidth and lower coating thickness (≤ 1.5 mm) due to certain frequency limitations. However, the use of multilayer and fractal FSS along with good composite RAM may be expected to provide the unprecedented opportunity to explore an innovative class of thin and broadband radar absorbers. Over the past decade or so, a substantial amount of research has been carried out over multilayered absorbers, which have been found to provide good absorption characteristics, but the requirement of thin absorber is far from reality. However, the use of optimization algorithms can solve such multi-objective design problem by knowing the number of layers, optimal choice of material for each layer, layer thickness and preferences of the layers. Further improvement of the bandwidth is possible by employing the concept of FSS due to their spatial filtering characteristics. Nowadays, fractal FSSs has attracted great attention of researchers for various electromagnetic (EM) applications due to their unique characteristics like multiband, self-similarity or self-affinity and space-filling. The novelty of this thesis is the development of fractal FSS embedded multilayered heterogeneous composites based thin (t ≤ 1.5 mm) and broadband (RL ≤ -10 dB) radar wave absorber for stealth applications. Chapter 1 presents the introduction which consists of motivation, problem statement and organization of the thesis. It also provides an insight into the radar wave interaction with RAMs, ii EM wave and field theory, advanced EM approaches like multi-layering, FSSs and others. The methods used for the RAM fabrication, characterization, optimization and simulation techniques are also presented in this chapter. Chapter 2 outlines a background, and brief literature review on composite RAMs, waste composites and multilayered RAMs. Different types of FSS’s based absorbing structures including traditional FSS’s, fractal FSS’s, FSS embedded RAM’s their limitations and research gaps are described. Chapter 3 explores the design, fabrication and EM characterization of heterogeneous nanocomposites for radar wave absorption. This chapter is divided into three sections. The first section of chapter deals with the effect of particle size proportion variation of hard-soft ferrite nanocomposite on the radar wave absorption. The hard-soft ferrite nanoparticles i.e., F-as, F-800 and F-1100 having different particle sizes are blended with different proportions (1:1:1, 2:1:1, 1:2:1, and 1:1:2).The experiments show that, sample with proportion (1:1:1) results in superior absorption properties with RL of -37.65 dB and wideband width of 4.2 GHz achieved at lower thickness of 2.0 mm. Further, FG based heterogeneous composites with different G contents (i.e., 10 wt%, 20 wt% and 30 wt %) are analyzed for their EM and radar absorption properties. The composite FG1 (having G in 10 wt%) with thickness of 2.1 mm provide RL of -27.68 dB at 10.8 GHz and bandwidth of 4.2 GHz. The absorption characteristics of heterogeneous composites consisting nano and micron sized Ti particles is presented in third section of this chapter. Three different types of composites (i.e., C1, C2 and C3) comprise of well optimized micron (80-90 μm) and nano (20-30 nm) sized Ti and Fe3O4 (80 - 100 nm) nanoparticles are developed. The strongest RL value of -32.5 dB at 9.4 GHz and wide absorption bandwidth of 3.0 GHz is noticed for C3 with 1.6 mm thickness. Finally, experimental validation and verification of theoretical results are carried out over best sample. Chapter 4 emphasizes on the cost-effective solution for broadband radar wave absorption which may be quite useful for stealth applications. A varieties of novel waste composite RAMs like mineral dust (M1, M2), mineral dust-beach sand composites (M1C, M2C), sugarcane bagasse and computer and mobile PCBs based electronic waste have been studied for their radar wave absorption characteristics. The peak RL value for M1 exists in the range of 8.7 to 10 GHz, with a peak RL value of -15.91 dB at thickness of 3.0 mm. The strongest RL value for sugarcane bagasse reaches up to -15.19 dB at 9.8 GHz with -10 dB absorption bandwidth of 2.2 GHz. The superior absorption characteristics have been noticed for electronic waste composite RAM with strong RL iii of -29.9 dB at 9.2 GHz for 2.5 mm optimal absorber layer thickness. An electronic waste based single layer absorber is fabricated with optimal coating thickness of 2.5 mm. The measured RL value has been found to -27.98 dB at 9.3 GHz with wide bandwidth (RL ≤ - 10 dB) of 3.9 GHz. Chapter 5 explores the multi-layering approach in order to further enhance the absorption capability of developed materials in terms of wide absorption bandwidth and lower coating thickness. Two and three layer radar absorbing structures are designed using transmission line theory, and GA based optimization has been applied for the given optimization restrictions of RL < -10 dB bandwidth and thickness (≤ 2.0 mm). The design process encompasses the proper optimization of the material for each layer, layer preferences and corresponding thickness values to obtain thin and broadband absorber. In this chapter, GA optimization has been carried out over three different classes of absorbers i.e., class I absorbers (consisting only nanostructured composites), class II absorbers (consisting of waste composites) and class III absorbers (consisting of both nanostructured and waste composites). The optimized results obtained from GA have been verified through Ansoft HFSS. Based on optimized results, two and three layer absorbers are fabricated and measured for the experimental validation and verification of theoretical results. A two layer nanocomposite absorber (i.e., class I absorber) possess strongest RL of -45.77 dB with corresponding bandwidth of 3.3 GHz (RL ≤ -10 dB) in the range of 9.1 to 12.4 GHz. The optimized coating thickness for two layer absorber is 1.4 mm. But still good bandwidth has also to be achieved which may cover whole X-band frequency. Chapter 6 presents a technique of blending fractal FSSs with single and multilayered absorbers in order to achieve the desired goal of wide bandwidth (RL ≤ -10 dB) and minimum possible coating thickness (≤ 1.5 mm). From the obtained dimensional data on the single layer composites, it was desirable to carry out a rigorous comparative study on the different fractal FSS geometries before committing it to experimental test. Towards achieving this goal, various fractal FSSs i.e., Sierpinski gasket, Sierpinski carpet and Minkowski loop with higher iteration level i.e., level 2 are investigated for their radar absorbing performance. It is found that after the Sierpinski gasket fractal FSS is incorporated to C3 (t = 1.6 mm), the bandwidth under -10 dB absorption increases up to 2.9 GHz and peak value of RL is -35.76 dB at 8.8 GHz. The obtained EM properties are found to be better as compared to Sierpinski carpet and Minkowski loop based fractal FSS. Taking into account the promising characteristics of the Sierpinski gasket an investigation is carried out with different levels (i.e., n=0, n=1 and n=2) of the fractal FSS. The strong RL is observed for higher iterated fractal FSS (i.e., n=2) and results in ultra-compact absorber design. Going one step iv further, variation of thickness with constant fractal size is studied, since it may provide any possibility to achieve absorption below a thickness of ≤ 1.5 mm. It is noticed that reducing the thickness to 1.1 mm is far better than the initial 1.5 mm thickness for C2. Considering the absorption performance requirements of broad and effective, two layer absorber with Sierpinski gasket fractal FSS has been studied with different fractal levels (n = 0, 1 and 2). A two layer absorber embedded with Sierpinski gasket fractal FSS is fabricated which shows RL of -35.57 dB at 9.5 GHz with broad bandwidth of 4.2 GHz at 1.4 mm coating thickness.