Ultra-Wide-Band and Polarization-Insensitive Metamaterial Absorber With Resistive Ink Sheet for RCS Reduction Applications

This proposed work focuses on designing and evaluating planar and optimal metamaterial absorbers for radar stealth applications. The structure of the unit cell is designed using a dielectric FR4 substrate with resistive ink patterns printed on it. With the help of computational optimization using the Ansys HFSS tool, the proposed unit cell's various geometric parameters are fine-tuned to achieve outstanding results. The unit cell, which is designed with resistive ink-based lossy material printed on an FR-4 dielectric substrate, achieves a remarkable <inline-formula><tex-math notation="LaTeX">$-10$</tex-math></inline-formula> dB reduction in reflection with a 90% fractional bandwidth between 15 and 28.9 GHz under normal incidence. Additionally, the proposed structure's dual negative feature reduces reflection by <inline-formula><tex-math notation="LaTeX">$-10$</tex-math></inline-formula> dB for Both TE and TM polarized waves can be angled up to <inline-formula><tex-math notation="LaTeX">$60^{\circ }$</tex-math></inline-formula> across all frequencies. The design has a fourfold rotation symmetry, which enables polarization-insensitive capabilities, enhancing its versatility. Furthermore, the unit cell's various properties, including permittivity, permeability, refractive index, and impedance plots, are analyzed to assess the design. The distribution of surface current and electric field is being investigated, which further bolsters the design's credibility. The prototype is developed with a novel unit cell structure, an assortment of unit cell structures is also fabricated to enhance its functionality further, and the results are measured and presented. The experimental results match the simulated results, indicating that the proposed metamaterial absorber is an optimal solution for radar stealth applications.


I. INTRODUCTION
B ROADBAND absorbers designed for microwave frequen- cies are attracting their extensive range of applications has led to these being subjects of significant attention [1], [2], [3], [4], [5], [6], [7].The first absorber ever made was the Salisbury screen [8].It consisted of a metal surface and a single resistive sheet positioned a quarter wavelength apart.However, the broadband Jaumann absorber [9], [10], [11] has gained more popularity due to its broad absorption range compared to the narrow-band characteristic of the Salisbury screen.This absorber employs multiple resistive sheets separated by quarterwavelength intervals, resulting in a wider absorption range.Even though the absorber size had to be increased to achieve this broad range, several studies have explored alternative approaches to balance the absorber's broad absorption capability with a thinner structure.Various methods can be used for FSS-based resonating structures, including circuit analog techniques and capacitive circuit analog absorbers.References [12], [13], [14], [15], [16] provide further information on these approaches [17], [18], [19], [20], [21].Lossy elements are incorporated through lumped resistors or resistive ink to create a wide absorption bandwidth, and sheets can be used to achieve either fixed or distributed resistance.
Multiple broadband absorber solutions have been presented in academic literature that utilize dielectric substrates or highpermeability magnetic materials resulting in significant magnetic loss [22], [23], [24], [25], [26].Although some of these solutions have limitations, one magnetic metamaterial absorber with a stepped structure, for instance, covers the full microwave range with a fractional bandwidth of 175.67%.Despite its fragility and restrictions on enhancing permeability values, this absorber design is still a practical and effective solution.Similarly, another design, a three-dimensional (3D) configuration [27] that incorporates the microwave frequency range from 1 to 18 GHz is covered by the ITO film and metal resonator.Mehmet Bagmanc et al. introduced a non-planar broadband metamaterial absorber in three dimensions.This absorber consists of a periodic arrangement of stepped cones made of multiple layers of metal and dielectric materials.The research shows that at normal incidence, the absorption is higher than 90% within the frequency range of 9.68 to 17.45 GHz and exceeds 95% within the frequency range of 9.91 to 14.86 GHz [28].Khalid Majeed et al. developed an ultra-wideband metamaterial absorber using a split-ring resonator-shaped structure.The absorber offers a relative bandwidth of 85.05%, covering a bandwidth of 8.02 GHz from 5.42 to 13.44 GHz [29].Zafer Özer et al. developed THz absorbers based on graphene.The first design covers the mid-infrared spectrum (6 to 14), while the second is for the far-infrared range (1 to 14 THz).Both models are for spectroscopy, imaging, communication, and sensing applications [30].Yongzhi Chenget and colleagues introduced single square-patch structure nonlinear-circuit metamaterial absorbers with absorptance bandwidths around 0.149 GHz and 0.138 GHz [31].similarly some other structures using gallium arsenide to achieve ultra-broadband and wide-angle capabilities [32], [33].Although some may consider these solutions bulky and intricate, it's important to note that achieving broad absorption comes at the cost of practical, scenario-based conformal applications.
Recent advances in ultra-wideband absorbers have revolutionized the field and made it possible to create resistive absorbers that work effectively across multiple frequency bands.Studies [34] and [35] have documented the use of lumped resistors and resistive ink to achieve this result.Kundu et al. [36] have also developed an absorber that uses a capacitive surface, operates in the ultra-wideband frequency range, and has a low profile that is lightweight and substantially reduces radar cross-section (RCS).With a bandwidth ratio (BWR) of 1:4, this absorber is a significant achievement.Similarly, Yu et al. [37] have demonstrated the use of a multi-layer absorber with single polarization that utilizes lumped resistors and covers a frequency range of 171.18%.Yao et al. [38] have introduced an absorber that has dual-polarization capabilities and uses lumped resistors to cover 172.6% of the frequency range.However, despite these remarkable advancements, there is still a need for ultra-wideband absorbers that are lightweight, easy to integrate into flat and curved surfaces using absorption techniques and effectively reduce RCS.To address this need, a new absorber type [39] has been designed that operates in the S frequency band on both flat and curved surfaces.
This article introduces innovative polarization-independent structures using specially crafted resistive ink patterns for flat surfaces.The flat design achieves a remarkable reflection bandwidth of −10 dB, covering 18 GHz to 28.6 GHz for typical electromagnetic waves.Additionally, the thin dielectric absorber covers the entire K band at a thickness of just 0.12λ 0 .Each design stage is meticulously analyzed with simulation results to provide a comprehensive understanding.The prototypes are thoroughly tested and validated, showing promising results for both flat and curved surfaces.Overall, this solution presents a significant advancement in polarization-independent structures.

II. BASIC UNIT CELL DESIGN
This section offers a detailed and comprehensive examination of the absorption mechanism, accompanied by a step-by-step design process for the unit cell structure.In-depth technical details are covered to ensure a clear understanding of the unit cell geometry, which involves a lossy layer of resistive ink patterns printed on a dielectric substrate with a relative permittivity of 4.4 and a loss tangent of 0.02.The substrate comprises a circular ring resonator and a modified version on top, as depicted in Fig. 1(a).The metal plate at the bottom of the substrate completes the design.Each element has been meticulously selected to create a high-performance absorber that offers a broad bandwidth.
The development of the resonance structure unfolds across three stages.Initially, we embark on the creation of a circular ring design (stage 1).As we progress to the second stage, a vertical box is introduced to the existing ring structure, resulting in the formation of a ring resonator.The third stage entails a horizontal cut of a circle with a distinct radius, along with the creation of a vertically positioned partial C-shaped cut, as elucidated in Fig. 1.Fig. 2 provides a comprehensive illustration  of the final unit cell design.Initially, the unit cell parameters are selected through a random process.However, the optimization journey is steered by a thorough analysis of the surface current and electric field distribution, instilling confidence in our ability to optimize these parameters effectively.The culmination of this process involves the application of parametric optimization to achieve the most optimal result with a high level of precision.We maintain a strong conviction that our meticulously crafted design will exhibit exceptional performance and perfectly align with the desired specifications.
The Ansys HFSS software has expertly optimized the geometric parameters, with detailed adjustments in Table I.The FR4 substrate is chosen for cost-effectiveness and performance.It has a resistive ink layer on top and a metallic layer on the bottom, both of which are 0.035 mm thick.In the context of the unit cell depicted in Fig. 3, periodic boundary conditions have been implemented along the X and Y-axes, utilizing primary Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.and secondary boundary conditions.Moreover, the proposed unit cell geometry originates from Floquet ports situated in the Z-axis, as highlighted in Fig. 3.By leveraging the prescribed boundary conditions, the resulting S-parameters, including reflection coefficient plots, have been computed and plotted.

III. ABSORPTION MECHANISM
The formula for absorption as it relates to frequency is expressed as [40].To achieve almost complete absorption, it is necessary to reduce both the transmission coefficient S 21 and the reflection coefficient S 11 at the same time.This can be achieved by utilizing resonating structures in the absorber.Using a metallic ground plane in a proposed unit cell structure can bring transmission values down to zero.As a result, absorption can be calculated only based on the reflection coefficient.The absorption process can be made simpler by expressing it as To achieve complete absorption, S 11 must be reduced, which can be accomplished using the proposed unit cell geometry.To achieve complete absorption of electromagnetic waves, it is necessary to have a specific structure that possesses both electrical and magnetic resonances simultaneously.Achieving maximum absorptivity in a proposed structure can be made possible by matching its impedance with the impedance of free space.This can be achieved by creating independent electric and magnetic resonances in a well-designed structure, which optimizes the unit cell's geometrical parameters.With tailored values of relative permittivity and relative permeability, the structure can meet the condition of free space impedance, resulting in minimal reflectivity and improved absorption.To ensure effective design, selecting a lossy dielectric material that can effectively dissipate EM waves within the material is crucial.

IV. SIMULATION AND DISCUSSION
The results of the simulation provide valuable insights into the performance of the conventional structure during the Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.The unit cell structure to assess its performance under transverse electric (TE) and transverse magnetic (TM) polarizations, in normal incidences.The graph in Fig. 5 displays the results of S-parameter testing.The graph shows a reflection coefficient of less than −10 dB over a frequency range of 15-28.9GHz.Additionally, the structure boasts an impressive absorptivity rate of over 90%.The obtained results strongly confirm that the proposed unit cell design is highly effective in achieving acceptable absorption rates across the specified frequency range.

A. Unit Cell Parameter Retrieval
The metamaterial parameters for unit cell geometry are determined using a highly effective modified Nicholson-Ross-Weir method [28].We have conducted a thorough analysis spanning from 15 to 28.9 GHz to accurately extract the unit cell's electromagnetic characteristics.Our findings reveal that both the real and imaginary parts of permittivity and permeability are negative within this frequency range, as shown in Fig. 6(b).Similarly, Fig. 6(a) illustrates negative values for the real and imaginary parts of permeability.The negative refractive index resulting from this unique combination of permittivity and permeability, as demonstrated in Fig. 6(c), thereby confirming the left-handed behaviour of the proposed metamaterials.Moreover, Fig. 6(a), (b) provides evidence of electric and magnetic resonances through the permeability and permittivity results.Finally, we have also discovered that the real part of the impedance matches the free space impedance, as depicted in Fig. 6(d).

B. Parametric Analysis of the Structure
The study shows how altering design parameters affects structure absorptivity.An analysis of absorptivity is presented in this report, and it significantly impacts practical applications.The analysis is conducted for various substrate heights (h) ranging from 0.8 mm to 2.4 mm with increments of 0.4 mm, as illustrated in Fig. 7.It is observed that a substrate height of 0.8 mm does not yield the desired frequency range.Similarly, at heights of 1.2 mm, 2 mm, and 2.4 mm, the absorptivity decreases for the desired frequency range due to wave penetration in the substrate material.This analysis provides a clear understanding of the impact of substrate height on absorptivity, enabling more informed decisions in the design process.
To accurately assess the relationship between the resistance structure width (w) and the behaviour of the unit cell, we varied the values from 0.2 to 1.5 mm with a step size of 0.3 mm, as shown in Fig. 8. Upon analysis of the figure, slight variations in frequency responses, bandwidth, and absorptivity efficiency are evident when each response is evaluated individually.However, these variations can be attributed to electromagnetic resonances Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.within the metamaterial structure, which may alter absorption peaks, causing shifts to different frequencies.By adjusting the width of a metamaterial absorber unit cell, we can precisely tailor its electromagnetic response to meet specific application requirements.This optimization allows for fine-tuning absorption characteristics across different frequencies, bandwidths, and incident angles with confidence.

C. Analysis of Oblique Incidence Angles
In the following section, a detailed analysis of the absorber's response to transverse electric (TE) and transverse magnetic (TM) waves at various incident angles.It is essential for the absorber to maintain consistent absorptivity across different oblique incident angles (θ), and our proposed design excels in this aspect.The simulations were carried out at incident angles ranging from 0 • to 60 • in 15 • increments.The results depicted in Fig. 9 illustrate the TE and TM polarization simulations, showcasing a remarkable 92% absorptivity as the incident angle approaches 60 • .Fig. 9(a) presents the absorptivity responses for TE mode at oblique incidence angles.The responses at oblique angles up to

45
• are comparable to the normal incidence (θ = 0 • ), with minor deviations in absorptivity.However, at an angle of incidence of 60 • , there is a slight decrease in absorptivity due to the resonance structure property for higher oblique incident angles.
In a similar fashion, Fig. 9(b) demonstrates the absorptivity responses for TM mode, showing consistent absorptions for all oblique incidences.It appears that the absorptive responses for transverse magnetic absorption are slightly better than for transverse electric mode.The stability of the metamaterial's absorptivity regardless of the angle or polarization of the incoming wave is remarkable and can be attributed to the symmetry in its design.This symmetry significantly contributes to the reliability and consistent performance of the metamaterial.The degree of absorptivity is on the properties of the materials and the angle of incidence.
The metamaterial absorber was analyzed for various polarization angle incidences, as shown in Fig. 10.The results of the simulation show that within the frequency range of 15 GHz to 26.89 GHz, the absorptivity is over 90%, as demonstrated by the reflection coefficient and absorptivity plots.Furthermore, up to 28.9 GHz, the efficiency is more than 90%.The symmetrical attributes of the design ensure consistency of reflectivity across The proposed structure undergoes thorough examination across various polarizations at normal incidence, yielding highly satisfactory outcomes.In Fig. 4(a) and (b), the incident TE and TM electromagnetic wave directions remain perpendicular to the structure, while adjustments in the electric and magnetic field orientations occur along the x and y axes through an angle phi.Across a range of phi values from 0 • to 60 • for both TE and TM polarizations, the peak absorptivity and bandwidth demonstrate notable stability, as shown in Figs. 9 and 10.This underscores the structure's remarkable consistency and reliability.Furthermore, its four-fold symmetry renders it insensitive to alterations in electric and magnetic field orientations over a wider frequency range at normal incidence, enhancing its reliability and efficacy in practical applications.
Moreover, to examine the insensitive behaviour of the proposed metamaterial absorber structure, the polarization angles (φ) were varied from 0 • to 60 • with a step size of 10 • , as depicted in Fig. 10.The simulations reveal that the proposed design is capable of maintaining consistent absorptivity for both TE and TM polarizations across the range of incidence angles, from 10 • degrees to 60 • .The figures demonstrate that the symmetry of the design plays a significant role in achieving this consistency.These findings emphasize the robustness of the metamaterial's absorptivity, underscoring the effectiveness of the proposed design.

D. Surface Current Distribution
The surface current density plot corresponds to the significant absorption peaks in Fig. 11.Fig. 11(a), (b) displays the distribution of surface current density at the top and bottom interfaces of the proposed structure in relation to the absorption peak at 20.9 GHz.Additionally, the surface current densities at the upper and lower boundaries for the absorption peaks at 26.7 GHz and 28 GHz are delineated in Fig. 11(c), (d) and Fig. 11(e), (f), respectively.As depicted in Fig. 11(a)-(f), the simultaneous occurrence of parallel and antiparallel currents originating from the upper and lower surfaces for each absorption peak creates robust magnetic resonances within the unit cell configuration.Thus, a significant presence of magnetic resonances is observed at the peak absorption frequencies, producing broadband absorption characteristics, as shown by the analysis in Fig. 11.

V. EXPERIMENTAL SETUP AND RESULTS
To ensure the effectiveness of the metamaterial absorber, thorough testing is conducted on both a prototype absorber geometry and an array of absorber geometry, as displayed in Fig. 12(a), (b).
Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.To determine the absorption properties of the prototype unit cell, a rigorous testing method is employed in an anechoic chamber, utilizing a top-of-the-line instrument, a vector network analyzer (Anritsu MS46122B) capable of operating within a frequency range of 40 GHz, and a conventional antenna (horn) to accurately assess the sample's absorption at normal incident angles, as outlined in Fig. 12(a).Rest assured that this testing procedure is designed to provide reliable and comprehensive results on the performance of the metamaterial absorber.

TABLE II TABLE OF COMPARISONS WITH PUBLISHED LITERATURE
The simulation results presented in Fig. 13 closely match the measured outcomes, indicating a maximum absorption rate of 93% across frequencies from 15 to 28.9 GHz.This design approach achieves full and broad-angle absorption within transverse electric and transverse magnetic polarizations, as confirmed by simulation and measurement correspondence.The results presented in Fig. 14 demonstrate an outstanding level of consistency between the absorptivity at oblique angles (0 • , 30 • , and 60 • ) from both simulated and measured data.The proposed metamaterial absorber surpasses others in Table II regarding bandwidth, unit cell dimensions, substrate layers, and polarization insensitivity when compared to existing literature.The data clearly shows this.These results confidently establish the proposed absorber as a highly efficient and innovative solution in the field of metamaterial absorbers.A comparison of the proposed absorber with previously reported relevant metamaterial absorbers is presented in Table II.This table shows the performance of these absorbers and how they compare to each other.The proposed absorber design exhibits large fractional bandwidth, polarization insensitivity, compact topology, and wide angular stability.It achieves an unparalleled absorption bandwidth using a single-layer thin substrate (λ/8), which is not observed in existing literature.This speaks volumes about its potential.

VI. CONCLUSION
This article describes a new design for a metamaterial absorber structure that reduces radar cross section (RCS) for planar surfaces, and is insensitive to polarization.The proposed design demonstrates double-negative behaviour, which is confirmed by measuring the metamaterial absorber properties.The study includes an analysis of the unit cell individual stages, which show broadband functionality.The simulation results show the structure's behaviour when hit at angles ranging from 0 to 60 • and with different polarizations.The frequencies examined are between 15 and 28.9 GHz.The electric field and surface current distribution are used to determine how much of the energy is absorbed.The design has several notable features, such as a broad bandwidth for reducing radar cross-section, a thickness close to the theoretical minimum, a small periodicity, and properties not affected by polarization.This makes it ideal for numerous practical applications.

Fig. 2 .
Fig. 2. The geometry of designed unit cel detail dimension.

Fig. 3 .
Fig. 3.The geometry of unit cell Isometric representation with all boundaries and excitation.

Fig. 4 .
Fig. 4. Reflection coefficient responses for all the stage examined.

Fig. 5 .
Fig. 5. Simulation results for reflection coefficient and absorption of unit cell geometry are presented for (a) TE and (b) TM polarizations.

Fig. 6 .
Fig. 6.The unit cell geometry plots for permeability, permittivity, reflective index, and impedance plots with their real and imaginary values.

Fig. 7 .
Fig. 7.The outcomes of the parametric analysis across various substrate thicknesses (h) are presented.

Fig. 8 .
Fig. 8.The outcomes of the parametric analysis across various width of unit cell (w) are presented.

Fig. 9 .
Fig. 9.A simulation of absorptivity results for oblique incidence angles (a) TE polarisation and (b) TM polarisation.