Design of a wideband and tunable radar absorber

To effectively address the challenges posed by intricate and dynamic electromagnetic environments, we propose a wideband and tunable radar absorber in this paper. The proposed absorber, composed of graphene capacitor, plasma enclosed within a sealed glass cavity, radar absorbing material (RAM), FR-4 and copper plate, allows for tunable radar absorbing performance through the manipulation of the electromagnetic properties of the graphene capacitor and plasma. Based on the equivalent circuit model, the reflectivity of the radar absorber is analyzed using transmission line theory (TLT). Good agreement is observed between the full-wave simulations and the TLT. The study thoroughly investigates the influence of graphene, plasma, and RAM components, as well as their sequential arrangement within the radar absorber, on its reflectivity, expounding the fundamental mechanism of these materials’ synergistic integration. Additionally, the effects of key factors, including the surface resistance R g of graphene, plasma frequency wp, collision frequency vp and plasma thickness tplasma, on the radar absorbing performance are examined. Our findings reveal that adjusting surface resistance R g controls the absorbing amplitude, and manipulation of the plasma frequency and collision frequency tunes the absorbing frequency and effective absorbing band. By appropriately adjusting the surface resistance R g of graphene, plasma frequency wp and collision frequency vp, the proposed radar absorber exhibits superior performance in the frequency range of 1 GHz to 10 GHz. The radar absorber we propose serves as a significant reference for the application of tunable radar absorbers and adaptive radar stealth techniques.


Introduction
The development of radar absorption materials has advanced rapidly along with the advancements in radar detecting capacity.Researchers have worked hard to increase the absorbing property and absorbing bandwidth, changing the morphology and size of the absorbent [1][2][3], combining different materials [4][5][6][7], optimizing the distribution of the absorbent in the matrix [8], using core-shell structure [9], sandwich structure [10], metamaterials [11,12], FSS(frequency selective surface) [13,14], and dielectric resonance unit [15][16][17], among other things.The aforementioned techniques, however, are passive radar stealth, once established, their absorption performance will not change in response to changes in the surrounding electromagnetic field.Targets are easily exposed by passive radar stealth due to the use of frequency-hopping radar and the development of radar networking technologies, dramatically increasing the likelihood of target detection.This relates to the survivability of modern military targets.
In recent years, there has been a growing interest in tunable radar absorbers due to the need to effectively handle complex and constantly changing electromagnetic environments.By adjusting the bias voltage on varactor diodes, Saptarshi Ghosh [18] created dual-band tunable absorber based on split square loops connected with cross-dipoles through varactor diodes.The amplitude and frequency of absorption can be controlled using the bias applied to PIN diodes, the radar absorber proposed by A Sharma [19] achieved dynamic control of the absorption characteristics from 8.2 GHz to 12.4 GHz.Cheng Huang [20] reported a tunable radar absorber by combining the graphene capacitor and AFFS loading the varactor.The absorber's absorption frequency and amplitude could be changed by providing the bias voltage to the varactors and graphene capacitance, respectively.Besides, Heijun Jeong modified the resonance frequency of the radar absorber by shifting the dielectric plate [21].However, the tunable frequency ranges and effect bandwidth are too small for practical applications.
Graphene, a 2-D nanomaterial composed of a single layer of carbon atoms, has high mechanical strength and excellent electrical conductivity.The conductivity of graphene can be adjusted when its chemical potential is varied by applying a bias or doping, which endows it great potential for application in tunable radar absorber and plasmonic structure [22][23][24][25][26].When voltage was supplied to the multi-layer graphene in a radar absorber created by A G D'Aloia [27], the absorber's reflectance might be as low as −10 dB from 7 to 22 GHz.A flexible radar absorber composed of continuous graphene sandwich structure and patterned graphene sandwich structure was created by Mingyang Geng, whose amplitude and frequency all could be dynamically tuned via applying bias voltages on the graphene from 9 GHz~11.5 GHz [28].
Plasma, recognized as the fourth state of matter [29], possesses distinctive electromagnetic properties that make it highly applicable in radar stealth technology.Consequently, extensive research efforts have been devoted to studying the interaction between electromagnetic and plasma [22,29].The electromagnetic properties of plasma are influenced by plasma frequency and collision frequency [30,31], which can be effectively adjusted through external excitation.This characteristic makes plasma an ideal active material for manipulating electromagnetic wave.By combining plasma with double FSS, Komlan Payne [32] created a radar absorption structure whose absorbing peak ranged from 9 GHz to 10 GHz and whose reflectivity could be less than −10 dB from 7.5 GHz to 11.5 GHz when the variable voltage was applied to the plasma, and then, the prototype plasmatuned absorber was fabricated and measured, validating the feasibility of the concept [33].
The aforementioned studies provide a useful reference for the exploration of tunable radar absorbers.However, it should be noted that the bandwidth of the presented radar absorbers is narrow, which limits their practical applicability.In order to address this problem, the concept of a novel wideband and tunable radar absorber is put forward.This radar absorber design incorporates a graphene capacitor, plasma, radar absorbing material (RAM), dielectric substrate and copper plate.To the best of our knowledge, this is the first report on the design of a radar absorber using graphene, plasma, and RAM simultaneously and then using a double control to manipulate the absorbing performance.By applying suitable external excitation conditions to graphene and plasma, their electromagnetic properties can be manipulated to achieve improved absorption capabilities.This controllability enables us to optimize the radar absorbing performance and reduce radar reflectivity.
The remainder of this paper is organized as follows, In section II, we present the proposed radar absorber and analyze its reflectivity through a combination of equivalent circuit model and transmission line theory.Section III focuses on investigating the impact of graphene, plasma, and RAM components, as well as their sequential arrangement within the radar absorber, on its radar absorbing performance.Additionally, the effect of the surface resistance of graphene, plasma frequency, collision frequency and thickness of plasma on radar reflectivity is also studied.Finally, we provide a summary of our findings and draw conclusions.

The physical model of the proposed absorber
The schematic of radar absorber is illustrated in figure 1 which consists of dielectric substrate, graphene capacitor, plasmas enclosed within a glass cavity, RAM and copper plate serving as a PEC (perfect electrical conductor).FR-4 is characterized by its low cost and excellent mechanical properties, rendering it widely utilized in the microwave system [34,35].Therefore, FR-4 is chosen as dielectric substrate with the thickness of t FR 4 -in this paper, whose relative permittivity r e = 4.3 and loss tangent tan e = 0.025 [19,36].FR-4 is implemented in the design to provide protection for the graphene capacitor and act as a separator between the graphene capacitor, plasma, and RAM.The graphene capacitor layer is composed of graphene film with a surface resistance R g as a upper electrode, high-impedance sheet with surface resistance R 5000 sq h 1 = W -as a lower electrode, and 0.025 mm thick polyethylene(PE) membrane soaked with ionic liquid electrolyte (DEME TFSI) sandwiched between them.[20,37].By applying electrostatic field bias on the graphene capacitor, the electrolyte becomes polarized and the ionic layer with opposite polarizations are formed on the interface of the two electrodes.So, the tunable high-mobility free carriers are generated on the graphene electrodes, leading to adjustment of Fermi energies of graphene and, consequently, the variation of sheet resistance of graphene.Plasmas are generated by supplying energy to neutral gas, causing the formation of charge carriers.When electrons or photos with sufficient energy collide with the neutral atoms and molecules in the feed gas, electrons and ions are produced in the gas phase [38].Various techniques, such as glow discharge, dielectric barrier discharge and radio frequency discharge, have been employed to generate plasma in experimental studies [39][40][41][42].Due to the difficulty associated with generating and maintaining a certain density of plasma in a lowaltitude open environment [43,44] the plasma in the closed cavity with low gas pressure is used in this paper.High-purity argon is used to fill the closed cavity, which is made of glass with a thickness of t glass and a permittivity of 3.8 [31,45] and the thickness of plasma is denoted as t .plasma One of the most common methods for generating plasma is by applying an electric field to the neutral gas [38,46].The plasma frequency w p and collision frequency v p are crucial parameters of plasma, and these parameters can be controlled by adjusting the conditions of plasma generator, such as bias voltage and RF power [44,46], thereby enabling effective manipulation of the electromagnetic properties of plasma.Carbonyl iron (Qingdao Jiuwei Huadun Technology Research Institute Co., Ltd) is employed as RAM to enhance the loss for incident waves, with a thickness denoted as t .

Theoretical derivation and analysis of the reflectivity
The equivalent circuit model of proposed radar absorber under normal incident is depicted in figure 2. Because a copper plate is selected as the bottom layer of the radar absorber, electromagnetic waves cannot penetrate it.Consequently, the studied radar absorber can be considered as a single-port system without the transmission channel [47].According to the transmission line theory (TLT), when an incident wave is perpendicular to the radar absorber, its reflection coefficient can be mathematically expressed as formula (1) [22].It is evident that as  the input impedance of the radar absorber, denoted as Z , in approaches the characteristic impedance of free space, represented as Z , 0 the reflectivity of the absorber decreases.Furthermore, when the input impedance of the radar absorber, Z , in equals the characteristic impedance of free space, Z , 0 the radar absorber can achieve perfect absorption.
Where Z 0 is the free-space characteristic impedance and Z in M M = is the total input impedance of the proposed radar absorber, in which M 12 and M 22 are the coefficients of the overall transmission matrix M of the radar absorber.
, plasma and M RAM represent the transmission matrix of FR-4, graphene, high resistance sheet, closed glass cavity, plasma and RAM, respectively.
The transmission matrixes of FR-4, closed glass cavity, plasma and RAM are presented as follows: The indexes of n = 1, 2, 3 and 4 denote the FR-4, glass, plasma and RAM parameters.
is the complex wave number, in which f is the frequency of incident wave, c is the light velocity in a vacuum, rn e is the complex relative permittivity and rn m is the relative permeability.t n represents the thickness of material, is the characteristic impedance of each medium, where 0 e and 0 m represent the permittivity and permeability of vacuum.The graphene capacitor in the equivalent circuit model can be treated as a parallel connection of R g and R , h which correspond to the graphene and high resistance sheet respectively [48].
Because the monolayer graphene is electrically thin in the considered frequency range, the transmission matrix of graphene can be defined as [27] Where g s is the surface conductivity of graphene.
In the previous expression, e 1.6 10 C

=
´is the elementary electron charge and, k 1.38 10 J K

=
´-is Boltzmann's constant, T(K) represents the temperature, and t(s) is the relaxation time.
h 2  = p is the reduced Plank's constant, h 6.63 10 23  = ´-Js and w = 2 p f represents the angular frequency, c m represents the chemical potential.t is around 0.2 ps when graphene is synthesized by chemical vapor deposition (CVD) [49].
When a bias voltage V g is applied to the graphene, it can alter its Fermi energy or chemical potential, as represented by formula (6) [24].
Here, E f represents the Fermi energy, V F is the Fermi velocity, approximately / m s 10 , t s is the substrate thickness, r e is the substrate permittivity and 0 e is the permittivity of vacuum.
The surface resistance R g of graphene can be represented by / 1 , g s which keeps almost constant in the microwave region, and when the voltage is applied to the graphene capacitor, g s will be tuned duo to the variation of Fermi energies [20].
Similar to graphene, the transmission matrix of high resistance sheet can be expressed as Where R h represents the surface resistance of high resistance sheet.
The complex relative permittivity of cold plasma can be represented by Drude dispersion, which is expressed as [ ´is the elementary electron charge and / 8.85 10 F m ´is the relative permittivity of free space.n e can be changed by applying a external excitation on the plasma [32].
The collision frequency of plasma is related to the gas pressure P of plasma and electron temperature T , e which can be defined as [53]: 10 10 (10), it is evident that the electromagnetic properties of plasma reply on the electron density n , e gas pressure P, and electron temperature T .e These parameters can be effectively controlled by external discharge conditions.
The transmission matrix M of the proposed radar absorber can be calculated using the above formulae.Subsequently, the reflectivity will be obtained by employing formula (1).respectively.The thickness of the graphene capacitor is neglected due to its thinness.Increasing the voltage applied to the graphene capacitor can lead to a reduction in the surface resistance of graphene and an increase in its conductivity [20,28].Consequently, we employ the variation in the surface resistance to investigate the impact of the applied voltage on the graphene capacitor on radar absorption performance.In this paper, the surface resistance can be adjusted from 400 sq 1 W -to 1600 sq 1 W -by applying a bias voltage to the graphene capacitor [37 .] Assuming that the plasma frequency w p can be tuned within the range of 1.5 10 rad s 10 1

Numerical computation results and their analysis
´to 7.5 10 rad s , 10 1 ´and the collision frequency can be adjusted between v 2.5 10 = ´Hz and 4.5 10 10 ´Hz [22,31,54].The RAM sample, as shown in figure 3(a), was fabricated into toroidal shapes, and then tested using the coaxial-line method [7] with a vector network analyzer (CETC, AV3672C).The test result is presented in figure 3(b).
The reflectivity of radar absorber is calculated using TLT under the conditions of R 700 sq , = ´and the results are presented in figure 4(a).To validate the reflectivity calculated through TLT, the electromagnetic software CST Microwave Studio (CST) was employed to investigate reflection characteristics of proposed radar absorber and the model of proposed radar absorber is depicted in figure 4(b).In the CST simulation, the graphene was modeled as a configurable resistive sheet, denoted as R g , connected in parallel with a fixed resistive sheet of R 5000 sq g = W −1 .Plasma in this study was assumed to be uniform and defined as a homogeneous material.The unit cell boundary condition was applied to all four sides of radar absorber, and Floquet ports were selected along the z-axis direction.The reflectivity of radar absorber, calculated by CST under normal incidence, is also presented in figure 4(a).Remarkably, there is a good agreement between the results obtained from TLT and CST simulations, thus confirming the accuracy and validity of the presented TLT formulation.´-

The effect of graphene, plasma and RAM on the reflectivity of radar absorber
To investigate the effect of graphene capacitor, plasma and RAM on the radar absorbing performance, different radar absorbers were constructed and compared, as illustrated in figure 5.These radar absorbers were defined as Model 1, Model 2 and Model 3, respectively.In Model 1, the graphene capacitor was replaced with FR-4.In Model 2, the plasma and glass cavity were substituted with FR-4, and in Model 3, RAM was replaced with FR-4.Importantly, it should be noted that the thickness of the FR-4 is same as that of the component being replaced.
To demonstrate the individual contributions of each component in the radar absorber, numerical calculations were conducted to determine the reflectivity of each radar absorber under different conditions.Specifically, the calculations were performed for four scenarios with the following parameters: = ´v 15 p = GHz and R 1000 sq .
g 1 = W -The results of these calculations are presented in figures 6(a)-(d), while the reflectivity of plasma with the thickness of t 30 mm plasma = and RAM with the thickness of t 0.5 mm RAM = is shown in figure 6(e).The analysis reveals several important findings.Firstly, when comparing the reflectivity of the proposed radar absorber with that of model 1 in figures 6(a)-(d), it becomes evident that the introduction of graphene to the radar absorber significantly enhances radar absorption performance, particularly in the low frequency band (form 1 GHz to 10 GHz).This can be attributed to the improved impedance matching, allowing more  electromagnetic wave to entry the radar absorber.Consequently, a greater amount of electromagnetic energy is consumed within the radar absorber, resulting in improved radar absorbing performance.Secondly, the use of plasma in the radar absorber effectively reduces the number of anti-resonance peaks and improves the radar absorbing performance, especially in the low-frequency band of 1 GHz to 10 GHz.This finding aligns with the radar absorbing performance depicted in figure 6(e), which demonstrates that plasma exhibits better radar absorbing capabilities in low-frequency band.Finally, the impact of RAM on the radar absorbing performance is observed to be minimal within the 1 GHz to 10 GHz, but it effectively improves the radar absorbing performance at the anti-resonance frequency range within the range of 10 GHz and 18 GHz.Additionally, RAM helps reduce the fluctuation in absorbing amplitude within this frequency range.This observation aligns with the absorbing characteristics of RAM illustrated in figure 6(e), which indicates that the RAM in this study has better radar absorbing performance in high-frequency band.Crucially, it is worth to noting that selecting RAM with different electromagnetic properties allows for adjustments to the input impedance of the radar absorber.This can lead to improved radar absorbing performance overall.Furthermore, the combination of graphene capacitor, plasma and RAM has been proven to achieve enhanced radar absorbing performance.

3.3.
The effect of the sequential arrangement of graphene, plasma and RAM on the reflectivity of radar absorber Furthermore, the impact of the sequential arrangement of graphene, plasma and RAM within the radar absorber on its radar absorbing performance is also investigated.Figure 7 illustrates the schematic of radar absorber, showcasing different arrangements of graphene, plasma and RAM.For simplicity, the abbreviations 'G', 'P' and 'R' are used to represent the graphene, plasma and RAM, respectively.Consequently, and the radar absorber in figure 7(a) can be denoted as 'G-P-R', the one in figure 7(b) as 'P-G-R', and so forth in a similar manner for figures 7(c)-(f).In figure 7, the thickness of plasma and RAM within radar absorber is t plasma = 30 mm and t RAM = 0.5 mm, respectively.The radar absorber's reflectivity is then calculated under the following conditions: (1) R 800 sq ,
p 10 = ´The calculated results are presented in figure 8. Observations reveal that the sequential arrangement of graphene, plasma and RAM significantly impacts the radar absorbing performance.Firstly, superior radar absorbing performance is achieved when the graphene capacitor is positioned first.This substantiates that graphene enhances radar absorbing performance, particularly in low-frequency band.Moreover, when the RAM is situated in the second position, the lowest resonance frequency shifts towards a lower frequency.Secondly, radar absorption performance deteriorates and RAM with the thickness of t 0.5 mm.RAM = when the plasma is positioned first.This suggests that more electromagnetic waves are reflected due to impedance mismatch and fewer electromagnetic waves pass through the plasma, being consumed by graphene capacitor and RAM.This demonstrates that the plasma is not ideally positioned first.Finally, when the RAM is placed first, the sequential arrangement of graphene capacitor and plasma significantly affects the radar absorbing performance.When the graphene capacitor is in the second position, the reflectivity of radar absorber is reduced.It is hypothesized that the electromagnetic wave after passing through the RAM, interacts with graphene capacitor, resulting in greater dissipation of electromagnetic energy in low-frequency band.
Therefore, it is concluded that altering the sequential arrangement of graphene, plasma and RAM within radar absorber can effectively regulate the radar absorbing performance.It is recommended that graphene capacitor be placed first to achieve the radar absorption performance.Additionally, the plasma should not be positioned first due to increased electromagnetic wave reflection.Moreover, altering the RAM serves as an effective strategy for adjusting the input impedance of radar absorber, thereby improving its radar absorbing performance.The calculated results indicate that by incorporating graphene, plasma and RAM in this paper, the proposed radar absorber exhibits superior radar absorbing performance.

The effect of surface resistance R g of graphene on the reflectivity of radar absorber
The reflectivity of radar absorber with varying surface resistance R g of graphene is numerically calculated to investigate the impact of graphene on the radar absorber.The results are presented in figure 9, where plasma frequency w 5 10 rad s , = ´collision frequency v 2.5 10 Hz.CST exhibit good agreement.As depicted in figure 9, it is evident that the lowest reflectivity is observed at 5.9GHz.Furthermore, adjusting the surface resistance of graphene can effectively fine-tune the reflectivity at the absorption peak frequency.As the surface resistance R g of graphene increases from 450 sq 1 Wto 900 sq , 1

W -
the absorbing amplitude of the radar absorber continues to rise.Conversely, when the surface resistance R g further increases from 900 sq 1 Wto 1200 sq , 1 W -the absorbing amplitude of radar absorber decreases.This finding demonstrates the potential of utilizing graphene to tune the performance of radar absorber.By controlling the surface resistance of graphene, the absorbing performance can be optimized for specific frequencies and applications.
In order to gain insight into the underlying physical mechanism of proposed tunable radar absorber, the power loss density at the incident wave frequency of 5.9GHz was examined for different values of surface resistance R 450 sq , W -and 1200 sq .W -The results are depicted in figure 10. Figure 10 illustrates that the surface resistance R g increases from 450 sq 1 W -to 1200 sq , 1 Wthe surface power loss The schematic of radar absorber with different sequential arrangement of graphene, plasma, and RAM.For simplicity, the abbreviations 'G', 'P' and 'R' are used to represent the graphene, plasma and RAM, respectively.For example, the radar absorber in (a) can be denoted as 'G-P-R'.
Figure 8.The reflectivity of radar absorber with different sequential arrangement of graphene, plasma, and RAM, the thickness of plasma is t plasma =30 mm, and the thickness is t RAM = 0.5 mm.= ´and v 2.5 10 Hz.
Mater.Res.Express 10 (2023) 106301 X Gao et al density of graphene steadily decreases, while the power loss density of plasma component continuously increases.It is speculated that as the surface resistance of graphene increases, more electromagnetic waves will penetrate the graphene, enter the plasma, and subsequently be absorbed within it.This observation provides valuable information regarding the behavior of the radar absorber.The decreasing surface power loss density of graphene indicates that it becomes more resistive, resulting in reduced energy dissipation.On the other hand, the increasing power loss density of plasma signifies a stronger interaction and absorption of the incident electromagnetic wave by the plasma component.
3.5.The effect of plasma parameters w p and v p on the reflectivity of radar absorber In order to comprehensively investigate the influence of plasma parameters on the radar absorbing performance, the reflectivity of proposed radar absorber was simulated and calculated using TLT.The calculation involved examining the reflectivity of radar absorber using various plasma frequency: w 3 10 rad s , = ´-4 10 rad s , 10 1 ´-5 10 rad s , 10 1 ´-6 10 rad s 10 1 ´and 7 10 rad s .´-Additionally, different collision frequencies were considered, namely v 10 GHz, p = 25 GHz and 40 GHz.Throughout the calculations, the surface resistance R g of graphene was fixed as at 800 sq 1 W -for providing a consistent parameter for comparison.
Figure 11 presents a visual representation of calculated results, allowing for a comparison of radar absorbing performance under different combinations of plasma frequency w p and collision frequency v .
p The figure clearly illustrates that as the plasma frequency w p increases, the multiple absorbing peaks of radar absorber gradually shift toward higher frequencies.Specifically, when collision frequency v p is set at 10 GHz, the amplitude of absorbing peaks decreases as the plasma frequency w p increases.Conversely, when the plasma frequency w p increases from 3 10 rad s 10 1 ´to 7 10 rad s 10 1 ´under the condition of v 25 p = GHz and 40 GHz, the amplitude of the absorbing peaks increases.What's more, by adjusting the plasma frequency w p when the collision frequency v 10 p = GHz, the reflectivity of proposed radar absorber can be reduced to less than −10 dB in the frequency range of 3.0 GHz to 9.1 GHz, 10.6 GHz to 11.9 GHz and 14.7 GHz to 15.2 GHz.Similarly, under the condition of v 25 p = GHz, an effective absorbing band can be achieved from 2.2 GHz to 9.7 GHz, 10.5 GHz to 11.6 GHz as well as 14.7 GHz to 15.0 GHz.Additionally, by tuning the variation range of plasma frequency w p when collision frequency v p is set at 40 GHz, the reflectivity can be reduced to less than −10 dB in the frequency range of 1.6 GHz to 9.9 GHz and 10.3 GHz to 11.2 GHz.It is hypothesized that the higherfrequency electromagnetic waves will enter the plasma and be absorbed within it as the plasma frequency increases.It is worth noting that as the collision frequency v p increases, the effective absorbing band in the range of 1 GHz~10 GHz becomes wider by tuning the plasma frequency w .
p Moreover, the upper cutoff frequency and lower cutoff frequency of the effective absorbing band shift towards higher and lower frequencies, respectively.This phenomenon may occur because a broader spectrum of electromagnetic waves can enter the plasma and then be absorbed within it due to the increased collision frequency.Therefore, it can be concluded that the plasma frequency w p and collision frequency v p can be controlled to tune the resonance frequency and the effective absorbing band.

The effect of thickness t plasma of plasma on the reflectivity of radar absorber
To investigate the correlation between plasma thickness t plasma and radar absorbing performance, calculations were conducted to determine the reflectivity of the radar absorber.The conditions for these calculations were set as follows: w 4 10 rad s , = ´collision frequency v 20 p = GHz and surface resistance R 800 sq .
g 1 = W - The calculated results are illustrated in figure 12. Upon analysis, it is evident that as the thickness of plasma increases, the amplitude of the absorbing peak in the low-frequency band also increases.This signifies that a thicker plasma layer leads to enhanced radar absorbing performance in low-frequencies band.Furthermore, the increase in plasma thickness effectively reduces reflectivity fluctuations and enhances the absorbing amplitude at anti-resonance frequencies in high-frequency band.It is speculated that as the thickness t plasma of plasma increases, more attenuation occurs for the propagating wave through the plasma medium, resulting in less incident power being reflected.This suggests that a thicker plasma layer exhibits enhanced radar absorbing performance at these specific frequencies.

Conclusion
In conclusion, this study introduces a novel design for a wideband and tunable radar absorber, composed of a graphene capacitor, plasma enclosed within a sealed glass cavity, RAM, FR-4, and a copper plate.The equivalent
circuit model and transmission line theory are employed to thoroughly investigate the radar reflectivity, and the results are verified through full-wave simulations.
The calculated results indicate that the combination of graphene, plasma and RAM yields improved radar absorbing performance, and placing the graphene capacitor first proves to be an effective strategy for augmenting the dissipation of electromagnetic energy within the radar absorber.Through the application of external excitation conditions to both graphene and plasma incorporated within the proposed radar absorber, we can manipulate their electromagnetic properties.This manipulation, in turn, enables precise adjustment of the absorber's input impedance.This dual control mechanism facilitates impedance matching between the radar absorber and free space across a broader frequency spectrum.Consequently, it promotes the ingress of more electromagnetic waves into the absorber, leading to increased absorption of electromagnetic energy within the radar absorber.
Moreover, by continuously varying the surface resistance R g of graphene from 450 sq 1 Wto 900 sq , 1 W -the absorption peak of radar absorber increases.However, further increasing R g from 900 sq 1 Wto 1200 sq 1 W - decreases the absorbing peak of radar absorber.The graphene integrated into the radar absorber can be employed to tune the absorber's input impedance by adjusting the voltage applied to the graphene capacitor, thereby optimizing the radar absorption performance.Besides, increasing the plasma frequency w p from 3 10 10 ´rad s 1 -to 7 10 10 ´rad s 1 -shifts the resonance frequency towards higher frequencies.
Simultaneously, increasing the collision frequency v p proves to effectively expand the effective absorbing band.Additionally, augmenting the thickness t plasma of plasma not only enhances the absorbing performance in the low-frequency band but also reduces the reflectivity fluctuations in the high-frequency band.By applying suitable external excitation conditions to graphene and plasma, the proposed radar absorber can achieve superior radar absorbing performance within the frequency range of 1 GHz to 10 GHz.In the future, our work will primarily focus on investigating the impact of the practical distribution of plasma on radar absorption performance and exploring its potential applications in real-world scenarios.The radar absorber proposed in this work will serve as a valuable reference for the development and practical implementation of tunable radar absorbers and adaptive radar stealth technology.= ´collision frequency v 20 p = GHz and surface resistance R 800 sq .

Figure 1 .
Figure 1.The schematic of the proposed radar absorber, composed of FR-4, graphene capacitor, plasma enclosed within a sealed glass cavity, and RAM.

Figure 2 .
Figure 2. The equivalent circuit model of the proposed radar absorber under normal incidence.
Where w p and v p represent the plasma frequency and collision frequency, respectively, and w = 2 p f is the angular frequency.The plasma frequency is defined as[50][51][52] In which, n e is the electron density of plasma, m 9.1 10 Kg e 31=´represents the electron mass, e 1.6 10 C 19 =

5 10 p 9 ´  4
Its electromagnetic parameters are shown in figure 3(b)

Figure 3 .
Figure 3. (a) Sample of RAM, and (b) its test results of electromagnetic parameters.

Figure 4 . 10 =
Figure 4. (a) The reflectivity of proposed radar absorber obtained from TLT and CST under R 700 sq g = W −1 , w 5 10 5 10 rad s , p 10 10 1 = ´´and v 2.5 10 Hz, p 10 = ´and (b) the model of the proposed radar absorber in CST.

Figure 5 .
Figure 5.The schematic of different radar absorber, in Model 1, the graphene capacitor was replaced with FR-4, in Model 2, the plasma and glass cavity were substituted with FR-4, and in Model 3, RAM was replaced with FR-4.

Figure 6 .
Figure 6.The reflectivity of different radar absorbers under normal incidence, figure 6(a) ~(d) represent the reflectivity of radar absorbers under different conditions, figure 6(e) represents the reflectivity of plasma with the thickness of t 30 mm plasma = and RAM with the thickness of t 0.5 mm.RAM =

p 10 = 1 W-
´The surface resistance of R g of graphene is varied in the range of 450 sq1  Wto 1 200 sq .The calculated results obtained from TLT and

Figure 7 .
Figure 7.The schematic of radar absorber with different sequential arrangement of graphene, plasma, and RAM.For simplicity, the abbreviations 'G', 'P' and 'R' are used to represent the graphene, plasma and RAM, respectively.For example, the radar absorber in (a) can be denoted as 'G-P-R'.

Figure 10 . 1 W 1 W
Figure 10.The schematic of calculated power loss density at 5.9 GHz under normal incidence with different values of R 450 sq , g 1 = W -900 sq 1 Wand 1200 sq , 1 W -(a) ~(c) represent the surface power loss density of graphene at surface resistance value of 450 sq , 1 W -900 sq 1 Wand 1200 sq , 1 W -respectively.(d) ~(f) represent the power loss density of radar absorber at surface resistance value of graphene of 450 sq , 1 W -900 sq 1 Wand 1200 sq , 1 W -respectively.

Figure 11 .
Figure 11.The reflectivity of proposed radar absorber versus frequency for different plasma frequency w p (w 3 10 rad s , p 10 1

Figure 12 .
Figure 12.The reflectivity of radar absorber with different thickness (t 10 mm, plasma = 20 mm, 30 mm, 40 mm, and 50 mm) of plasma versus frequency under the conditions of plasma frequency w 4 10 rad s , p 10 1 3.1.Numerical computation of reflectivityThe material parameters of the proposed radar absorber are presented in table1.In this study, the thickness of FR-4, closed glass cavity, plasma and RAM are set as t RAM =

Table 1 .
The material parameters of proposed radar absorber.