Analytical Estimation of Radar Cross Section of Infinitely Long Conducting Cylin

Girish K.and Hema Singh

1 Introduction

The estimation of RCS of a complex structured target involves steps such as approximating the given structure in terms of canonical shapes, calculating RCS contribution from each of the individual canonical shapes and then appropriately summing individual contributions to arrive at overall RCS of the structure (Crispin and Siegel, 1968). Thus, the RCS estimation of canonical structures is an important study towards scattering analysis of complex structures. RCS is a function of target geometry, constituent material parameters such as dielectric permittivity and permeability and incident wave properties such as frequency, angle of incidence and polarization. By an appropriate choice of material, RCS of the target can be controlled to some extent.

The RCS of a target is traditionally controlled by coating one or more layers of lossy dielectric material over the structure. However, since about a decade, extensive research is being carried out, to employ the peculiar electromagnetic (EM)characteristics of metamaterials for RCS control. Metamaterials can be classified as μ-negative (MNG, where only permeability is negative), ε-negative (ENG, where only permittivity is negative) and double negative (DNG) materials (Capolino, 2009).

The materials with negative permittivity and permeability in the microwave frequency region were first practically realized in the year 2000 (Smith et al., 2000).In recent years, metamaterial has been used as radar absorbing materials (RAMs)(Chaurasiya and Ghosh, 2014) and frequency selective surfaces (FSS) (Shi et al.,2010), for achieving low reflectivity over certain frequency bands. A detailed theoretical analysis of electromagnetic behavior of metamaterials may be obtained in(Irci, E.; Erturk, V.B. 2007), where the FDTD computations of EM wave propagation in a DNG slab is discussed.

Various methods have been proposed for the estimation of RCS and can be categorized as numerical methods, high-frequency ray-theoretic methods, and hybrid methods (Jenn, 2005), (Eugene, 1985). The finite-difference time-domain (FDTD)method involves the numerical implementation of differential form of Maxwell’s equations. It is ideal for time domain visualization of the EM fields and frequency domain results can be extracted by applying fast Fourier transform (FFT) to the timedomain scattering parameters. However it has drawback of high computational complexity. On the other hand, the method of moments (MoM) is a numerical implementation of integral form of Maxwell’s equations and it offers an accurate solution, but again has high computational cost. In high frequency methods, the simplest is geometric optics (GO) method, a ray-based method which assumes that specular points on the target contribute dominantly to the overall RCS. The methods based on physical optics (PO) and physical theory of diffraction (PTD), are limited to scattering from edges and corners. Other high-frequency methods such as uniform theory of diffraction (UTD) and uniform asymptotic theory (UAT) provide solution for scattered field in transition region, in the vicinity of the shadow region. These raytheoretic methods mainly deal with conducting surfaces.

The scattering from coated conducting surfaces or non-conducting surfaces need the transmission to be taken into account apart from reflection and diffraction. This problem holds even for metamaterial-coated structures (Soares, 2009), (Kwon et al. ,2009). The scattering from an infinitely long circular PEC cylinder coated with metamaterial has been analytically analyzed and compared with a similar cylinder coated with normal dielectric material (Li and Shen, 2003), (Irci and Erturk, 2007).The cylindrical wave expanding theory has been applied for calculating far-field RCS of a PEC cylinder with two layer dielectric coating (Wu et al., 2015). The finitedifference frequency-domain (FDFD) method can also be used to estimate RCS of metamaterial-coated conducting cylinder and sphere (Zainuddeen and Botros, 2008).In FDFD method, the frequency-domain form of Maxwell equations is iteratively solved to calculate the transmitted and the scattered fields. The scattering properties of a multilayered metamaterial cylinder have been analyzed by Yao et. al (2006) for both the principal polarizations, using the eigen-function expansion method. By enforcing the boundary conditions, the eigen coefficients are calculated iteratively to arrive at RCS.

In this paper, the RCS of an infinitely long perfect electrically conducting (PEC)circular cylinder coated with DNG metamaterials is determined. A closed form solution of EM wave scattering by metamaterial-coated circular cylinder of infinite length is analytically derived. The incident and scattered field are expanded in terms of Bessel and Hankel functions. The appropriate boundary conditions are imposed to obtain the unknown coefficients for single-layer and double-layered lossless and homogeneous metamaterial coatings. The computed results are validated against MoM results available in open domain. The effect of frequency, constitutive parameters of metamaterial coating, and polarization has been analyzed. The parametric analysis of metamaterial-coated cylinder is geared towards RCS reduction over set of frequencies.

2 Analytical Formulation of RCS of Metamaterial Coated Cylinder

Single Layer Coating: A PEC circular cylinder of radius a coated with single layer of DNG type metamaterial is shown in Figure 1. The coating thickness is t1and the material parameters are (-μ1, -ε1). A uniform plane EM wave is incident at an angle φo,to the cylinder.

For TMzpolarization, incident field components are expressed as (Balanis, 2012),

For TEzpolarization, the incident field components are given by

Figure 1: PEC cylinder with single layer coating

The scattered fields are travelling waves and can be represented in terms of the infinite summation of cylindrical Hankel functions. Thus, irrespective of polarization of incident waves, the scattered field components are expressed as,

The fields inside Layer 1 are standing waves represented by infinite summation of Bessel wave functions [10]. For TMzpolarization, the field components inside Layer 1 are

For TEzpolarization, the field components are

Double Layer Coating: A PEC circular cylinder of radius a, with two layers of coating is shown in Figure 2. The thickness of Layer 1 and Layer 2 are t1and t2respectively. The fields inside Layer 1 are same as in (9) - (12). The field components inside Layer 2 are given by,

Figure 2: Structure of a PEC cylinder with two layers of coating

In order to obtain the scattering coefficients for single layer coated PEC circularcylinder, the following boundary conditions are imposed.

On solving (17) – (22), one arrives at

To obtain the scattering coefficients for two layer coating, the following boundary conditions are applied.

On solving (25)-(32), one obtains the scattering coefficients as follows:

where dm=d1/d2;de=d3/d4;d1=e3f1−e1f3

The far scattered field approximation is used to determine the normalized bistatic cylinder RCS, given by [9]

3 Results and Discussion

The RCS of coated cylinder is computed using the above analytical expressions. The computed results are validated against MoM results available in open domain. Further,parametric analyses of RCS of metamaterial coated cylinder are carried out towards achieving RCS reduction over certain frequency range.

Figure 3: Variation of RCS of one layer metamaterial coated PEC circular cylinder of radius a = 30 mm; coating thickness t1= 6 mm (b= 36 mm), coating material is

DNG, εr1= -4, μr1= -2; TMzpolarization.

A PEC circular cylinder with radius a = 30 mm, coated with DNG type metamaterial is considered. The constitutive parameters of coating are t1= 6 mm, εr1= - 4 and μr1=-2. The frequency is 5 GHz and polarization of incidence is TMz. The RCS of the coated cylinder as a function of aspect angle is computed (Figure 3) and compared with the MoM results available in open domain (Sakr et al., 2014). It is apparent that there is an excellent match between the results of proposed analytical method and MoM.

Next, the RCS of single layer coated cylinder is compared with that of bare cylinder for the TMzmode (Figure 4). It is observed that the RCS of bare cylinder varies linearly with frequency in TMzpolarization. The metamaterial coating results in reduction of RCS value over frequency range of 5.6-7.0 GHz.

Figure 4: Variation of RCS of one layer metamaterial coated PEC circular cylinder of radius a = 50 mm; coating thickness t1= 5 mm (b= 55 mm), coating material is DNG, εr1= -4.65, μr1= -1; TMz polarization.

Further, the RCS of a PEC circular cylinder with two layer coating is determined. The first layer coating is DNG metamaterial whereas second layer is a dielectric. The material parameters of coatings are εr1= -4, μr1= -2, εr2= 4 and μr2= 2. The thickness of metamaterial coating is t1= 6 mm (b = 66 mm). The second layer thickness is t2=6 mm (c = 72 mm). The frequency is taken as 5 GHz. The RCS as a function of aspect angle is computed (Figure 5) and compared with MoM results available in open domain (Sakr et al., 2014) for TMzpolarization.

Next the thickness of coatings is increased. A PEC circular cylinder (a = 60 mm) is considered with a metamaterial of thickness t1= 30 mm (b = 90 mm) as first coating and a normal dielectric of thickness t1= 30 mm (c = 120mm) as second coating(DNG-DPS). The material characteristics of first coating are same as previous case i.e.εr1= - 4 and μr1= -2, εr2= 4 and μr2= 2. The variation of cylinder RCS with aspect angle is computed (Figure 6) for TMzpolarization. It may be observed that the RCS values computed analytically are in good agreement with those computed using MoM based integral equation solutions (Sakr et al., 2014).

Figure 5: Variation of RCS of two layer coated PEC circular cylinder of radius a =60 mm; thickness of coatings is t1 = t2 = 6mm. DPS-DNG coating, εr1 = -4, μr1 = -2,εr2 = 4, μr2 = 2; f = 5 GHz; TMz polarization

Figure 6: Variation of RCS of two layer coated PEC circular cylinder of radius a =60 mm; thickness of coatings is t1 = t2 = 30 mm. DPS-DNG coating, εr1 = 4, μr1 = 2,εr2 = -4, μr2 = -2; f = 5 GHz; TMz polarization.

Further, the variation of RCS of two layer coated PEC cylinder with frequency is analyzed. Here PEC cylinder radius is 50 mm (a = 50mm) with two layers of DNG coatings. The constitutive parameters of the coatings are t1= 2 mm, t2= 1 mm, εr1= -4, μr1= -2, εr2= 4, μr2= 2. The variation of cylinder RCS with frequency (1-19 GHz) for TMzpolarization is shown in Figure 7. It is apparent that the RCS reduction has been achieved only over a particular range of frequencies (7–8.6 GHz).

Figure 7: Comparison of RCS of a conducting bare and two layer metamaterialcoated PEC infinitely long circular cylinder; t1=2mm, t2 =1mm, εr1 = -4, μr1 = -2, εr2 =4, μr2 = 2; TMz polarization.

As a next case, the same PEC cylinder (a = 50mm) is considered, with two layer coating of DNG material. The first coating is 2 mm thick (b = 52mm), second coating is of 1 mm thickness (c = 53 mm). The material parameters are εr1= -5, μr1= -2, εr2=-6, μr2= -3. The variation of cylinder RCS with frequency has been computed for TEzpolarization (Figure 7). It may be observed that the cylinder RCS is reduced over the frequency range of 11.2 – 12.8 GHz.

Figure 8: Comparison of RCS of a conducting bare and two layer metamaterialcoated PEC infinitely long circular cylinder; t1=2mm, t2 =1mm, εr1 = -5, μr1 = -2, εr2 =-6, μr2 = -3; TEz polarization.

In order to get clearer understanding of the RCS behavior of single layer metamaterial coated PEC cylinder, the contour plots for RCS of circular PEC cylinder for different thickness t, of metamaterial coating are shown for TEzpolarization (Figure 8) and TMzmode (Figure 9). The radius of PEC cylinder is taken as a = 50 mm at f = 5 GHz.The permittivity and permeability of the metamaterial coating are varied from 0 to -30.The contour plots for different thicknesses of coating are compared using a common scale. The color of contour shows the level of cylinder RCS. The darker blue (indigo) colored regions of the plots represent the combination of εr1and μr1which yield lowest RCS, while red corresponds to the combination which gives highest RCS. Further it may be observed that for thin coating the cylinder RCS is low for most of the combinations of εr1and μr1. As the thickness of coating increases, the RCS value increases.

Next, the RCS of two layer metamaterial coated PEC circular cylinder (a = 50 mm) is computed as a function of both frequency and thickness (t1) of Layer 1. The contour plots for different values of thickness (t2) of Layer 2, for the TMzpolarization is shown in Figure 10. The material parameters of coatings are same as those in Figure 7.The contour plots are compared with that of a bare cylinder. It is apparent that, the cylinder RCS is reduced in certain frequency ranges (represented by the dark blue color). Moreover increase in RCS is observed at higher frequencies (red color). As thickness of Layer 2 is increased, higher RCS reduction is achieved, but limited to low frequency range. A similar trend has been observed in TEzpolarization (Figure 11)for doubly coated cylinder with constitutive parameters of Figure 8.

4 Conclusions

Metamaterial due to its peculiar EM characteristics attracts the attention of researchers towards RCS control. In this paper, a PEC infinitely long circular cylinder with metamaterial coating is studied based on analytical approach. The incident and scattered field components are analytically derived in terms of Bessel and Hankel functions, imposing appropriate boundary conditions. It is shown that TMzpolarization contributes more to the RCS of the structure as compared to TEzpolarization. However RCS reduction is obtained only at certain frequencies in TMzpolarization. This may be due to constitutive parameters considered for coating material. This further emphasizes the need of optimization of material characteristics of coating.

For TMzpolarization, cylinder RCS is higher for large permittivity and small permeability of the coating. On the other hand, for TEzpolarization, cylinder RCS is high when permittivity of coating is low and permeability is large. For single layer metamaterial coating, the RCS increases with thickness. For a PEC circular cylinder coated with two layers either metamaterial or dielectric coating RCS reduction can be achieved but for certain frequency ranges. In case of two-layer coating, with both layers chosen as DNG, as the thickness of the metamaterial coating increases, the cylinder RCS decreases significantly over certain low frequency ranges. This feature can be further improved by more rigorous optimization of coating parameters.

Figure 9: Contour plots showing RCS variation of a single layer metamaterial coated PEC cylinder (a = 50 mm) with permittivity and permeability, for different values of thickness t1; TEz polarization.

Figure 10: Contour plots showing RCS variation of a single layer metamaterial coated PEC cylinder (a = 50 mm) with permittivity and permeability, for different values of thickness t1; TMz polarization.

Figure 11: Contour plots showing RCS variation of a two layer metamaterial coated PEC cylinder (a = 50 mm) with thickness t1 of Layer 1 and frequency f ; εr1 = −2.5,μr1 = −1, εr2 = −9.8 and μr2 = −3; TMz polarization.

Figure 12: Contour plots showing RCS variation of a two layer metamaterial coated PEC cylinder (a = 50 mm) with thickness t1 of Layer 1 and frequency f ; εr1= −5, μr1 =−2, εr2 = −6 and μr2 = −3; TEz polarization.

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