A Central Spiral Split Rectangular-Shaped Metamaterial Absorber Surrounded by Polarization-Insensitive Ring Resonator for S-Band Applications

This paper reports a central spiral split-rectangular-shaped metamaterial absorber surrounded by a polarization-insensitive ring resonator for s-band applications. The rated absorption is 99.9% at 3.1 GHz when using a three-layer structure where the top and ground are made of copper and the center dielectric material is a commonly used FR-4 substrate. The central split gaps have an impact on the unit cell by increasing high absorption, and an adequate electric field is apparent in the outer split ring gap. At 3.1 GHz, the permittivity and permeability are negative and positive, respectively, so the proposed unit cell acts as an epsilon negative (ENG) metamaterial absorber. In a further analysis, Roger4450B was used as a substrate and obtained excellent absorption rates of 99.382%, 99.383%, 99.91%, and 95.17% at 1.44, 3.96, 4.205, and 5.025 GHz, respectively, in the S- and C-band regions. This unit cell acts as a single negative metamaterial (SNG) absorber at all resonance frequencies. The S11 and S21 parameters for FR-4 and Rogers4450B were simulated while keeping the polarization angle (θ and φ) at 15, 30, 45, 60, 75, and 90 degrees to measure, permittivity, permeability, reflective index, absorption, and reflection. The values of the reflective index are near zero. Near-zero reflective indexes (NZRI) are widely used in antenna gain propagation. The unit cell fabricated for the FR-4 substrate attained 99.9% absorption. S-band values in the range of (2–4) GHz can be applied for low-frequency radar detection.


Introduction
Complex research into super lenses, antenna designs, image sensing, low-cross-section materials for radar, and absorber technologies requires a material that shows negative permittivity, permeability, and reflective index properties. However, naturally available materials do not exhibit such characteristics. Unlike natural materials, metamaterials are constructed artificially with unique electromagnetic (EM) properties. Metamaterials can be used for operations at desired frequencies. A super lens constructed from a metamaterial that exhibits negative unity permeability with a 35.3 mm wavelength at 8.5 GHz has been proposed by Scarborough [1]. A lens close to 23 cm square and 4 cm thick was designed to construct a functional prototype. Metamaterial-based antennas [2] are a very attractive research area for scientists. Planer antennas are widely used for wideband [3] and ultra-wideband [4][5][6][7][8][9] (UWB) applications. UWB antenna design aims to create a tiny

Design and Methodology
Materials 2023, 16

Theory of Absorption
As the ground of the unit cell is copper, the transmission and reflection of the EM waves may become weak when they pass through the MM structure. Thus, the waves are  The proposed unit cell's two resonators have three 0.3 mm gaps that enhance the electric field, and the entire spit ring can be formed by copying at 90 • , 180 • , and 270 • shifts. The 0.3 mm gap rotates circularly and ends at the center. The complete construction of the unit cell can be easily understood from Figure 1b. Figure 1c,d illustrate the copper parameter for the proposed design. The back side of the whole substrate surface area (20 mm × 20 mm) is covered by ground. The ground is cut at the end of the area (20 mm × 0.2 mm). The commercially available computer simulation software CST-2019 is used to conduct high-frequency numerical analysis. CST has an excellent finite integration technique that can determine reflection, transmission, and absorption. A perfect EM wave can be obtained along the Z-axis by applying an ideal electric and magnetic field along the X-axis and Y-axis, respectively.

Theory of Absorption
As the ground of the unit cell is copper, the transmission and reflection of the EM waves may become weak when they pass through the MM structure. Thus, the waves are absorbed by the structure. The absorption can be calculated from: From Equation (1), by calculating the reflection co-efficient R(ω) and transmission co-efficient T(ω), the proposed metamaterial absorbance can be obtained. R(ω) and T(ω) can be represented by R(ω) = |S 11 | 2 and T(ω) = |S 21 | 2 , where S 11 is the reflectance wave from the unit cell to port 1 and S 21 is the transmittance through the cell from port 2 to port 1. To extract the primary parameters (S 11 , S 21 ), Smith et al. [37] proposed an S-parameter method. S-parameters can be represented by: where R H = (z − 1)/(z + 1), which can calculate the reflection coefficient for a half-space; z is the impedance of the MM unit cell medium; η is the reflective index; and d is the substrate thickness. Here, and where p = 1 − S 2 11 + S 2 12 . The absorber's permittivity and permeability can be calculated by ε = η z and µ = ηz. From Equations (3) and (4), the reflective index η can be calculated as follows: The proposed unit cell's impedance, reflective index, and absorption capacity can all be calculated easily from Equations (5) and (6). According to Equations (2) and (3), all the parameters rely on S 11 and S 21 .
In the proposed design, the ground is covered by copper. Therefore, the skin depth equation can be analyzed from the perspective of the electromagnetic wave blocking the unit cell from port 2 to port 1. Here, in Equation (7), resistivity ρ = 1.72e − 8 Ω-m, conductivity σ = 5.8e + 7 s/m, and permeability µ = 1. At 3.1 GHz resonance frequency, the value of Equation (7) is δ = 0.001186 mm. The skin depth result explains that the S 21 transmission of the EM wave is completely interrupted from port 2 to 1 by copper (annealed). Equation (1) in this manuscript can be rewritten as: However, there is no direct relation between the MM's absorption and the negative reflective index [38] at resonance frequencies. An MM that has both absorptivity and negative refractive index can be added as an advantage, leading to many guided wave applications in the microwave range [39]. In a perfect MM absorber, to prevent EM energy dissipation from the absorber, the conservation of energy laws' should be applied following the complex Poynting vector [40]. As a result, the MM does not act as a transient medium and absorbs the desired EM energy frequencies. The permittivity and the reflective index values must be real [41] and negative to conserve the energy that leads to the MM's characteristics, which can be set accordingly at different resonance frequencies. These technical aspects cover many vital performances parameters for some applications such as radar cross-section reduction, EM stealth mood, sensing, etc. [42], where MM properties and EM energy conservation are needed for the required performance.

Characterization of the Absorber
The unit cell must be designed in a way whereby it shows high absorption in the S-band. For this purpose, the resonators and the rectangular snake-cutting-shaped structure must include 0.3 mm gaps. These gaps create the capacitive property. The ground that is covered with copper is cut at the edge for excellent absorption [43] with a negative reflective index, as described in Equation (6). The total areas for the two split-ring resonators are 73.4 mm 2 and 58.7 mm 2 . Additionally, the center rectangular-shaped structure has an area of 30 mm 2 . The two split-ring resonators and a center rectangular-shaped structure are constructed sequentially to ensure the highest absorption. There are five different methods, as shown in Figure 3a, of achieving high absorption. The absorption is highest (99.9%) for the final design. Another resonance can be produced with the capacitive element around the unit cell's circumference. Hence, five distinct absorption peaks are obtained, as shown in Figure 3b. The shifted resonance frequencies are achieved by cutting the edge of the ground. This can be seen in Figure 3a, where the central rectangular-shaped copper split is cut in the shape of a snake with 0.3 mm gaps. The gap in the center creates more absorption than the other four designs.
The reflection (S 11 ) and transmission (S 21 ) co-efficient performance at the dB scale is shown in Figure 4. There are differences between the measured and simulated results of the S 21 parameter. When we attached the waveguide ports at the time of calibration using a Keysight N4692A MW electronic calibrator (Ecal), some air interference may have affected the propagation from the excitation port to the unit cell, which may have triggered performance variations. Another possible reason for the result deviation could be the lower concentration of copper in the layer at the edge of the ring resonator during fabrication. A 0.01 mm copper layer on the banding area of the ring resonator of the unit cell is missing. We measured the S 21 at different gap variations between the unit cell and waveguide ports to assess the mutual resonant effects of the waveguide ports. At 0.1 mm distance, the S 21 is −40 dB at 4.3 GHz, but the noise concentration is high. When the unit cell is in the optimized position concerning the waveguide ports, Figure 4 shows the final effect with less noise.

Analysis of Absorber
The proposed MA was designed to absorb microwave frequencies successfully and efficiently in the S-band range. A rectangular-shaped structure surrounded by two splitring designs was selected for the highest absorption. The central rectangular structure has a 0.3 mm snake-shaped gap, and the two resonators have three 0.3 mm gaps. Solid copper was used on the opposite surface of the resonator as a ground to block the transmittance and reflectance waves in the expected frequency ranges with a refractive index near zero or negative. After simulation in CST, the real and imaginary values of S 11 and S 21 were exported to excel and then plotted in MATLAB to obtain the perfect values of permeability, permittivity, reflective index, and absorption shown in Figures 5a-c and 3b. The proposed absorber was also simulated with a Roger4450B (dielectric constant = 3.7, thickness= 1.575) substrate. The variations in permittivity, permeability, and reflective index with respect to polarization angle are listed in Table A2 in Appendix A and displayed in Figure 6a-d. The S11 and S21 parameters were extracted from MATLAB. Table  A2 shows four resonance frequencies at 1.44, 3.96, 4.205, and 5.025 GHz with 99.382%, 99.383%, 92.91%, and 95.17% absorption, respectively. The permittivity is positive at 3.96 GHz and 5.025 GHz and negative at 1.94 GHz and 4.205 GHz and vice versa for permeability. Hence, the design is an excellent absorber with SNG properties in the S-and C-band regions. From the numerical analysis, it can be concluded that FR-4 has a wider band (3.09-3.14 GHz) than Roger4450B and a high absorption of 99.94% and −32.15 dB for S11, whereas Roger4450 exhibited values of 99.38% and −22.10 dB for S11. Moreover, the dielectric constant, loss tangent, and thermal conductivity of FR-4 were 4.3, 0.025, and 0.3 w/k/m, respectively, whereas Rogers4450B showed values of 3.6, 0.004, and 0.6 w/k/m, respectively. The loss tangent of FR-4 was greater than that of Roger4450B, but the dielectric property we found for FR-4 was comparatively stable compared to that of Roger4450B. Thus, absorption was obtained more for FR-4. We did not achieve the dielectric property that Roger4450B showed during the simulation. In this scenario, the absorption rate was decreased for Rorger4450B. The maximum absorption of 99.94% was achieved at 3.1 GHz frequency with negative permittivity and positive permeability with a 4.6 dielectric constant FR-4 substrate. As the proposed unit cell has negative permittivity and positive permeability, it can be applied as an ENG absorber. The incident waves at the different polarization magnetic and electric angles (0 • , 15 • , 30 • , 45 • ,60 • , 75 • , and 90 • ) were also analyzed to obtain permittivity, permeability, and reflective index values, as shown in Table A1 in Appendix A for the S 11 parameter. The permittivity is always negative for the polarization angle ϕ and most values are positive for the polarization angle θ, and vice versa for the permeability. The lowest value (0.06) of reflection was observed at 3.1 GHz, and the reflective indexes were near zero. MATLAB was also used to extract S 21 values for polarization angles (θ, ϕ) 0 • , 15 • , 30 • , 45 • , 60 • , 75 • , and 90 • , as listed in Table A1. The reflection was 99.68% at 1.95 GHz, which was the highest value; at the same time, the absorption is 0.3%, which is the lowest value. From Table A1, the absorption value is at its maximum at the S 11 resonance frequency and at its minimum at the S 21 resonance frequency.
The proposed absorber was also simulated with a Roger4450B (dielectric constant = 3.7, thickness = 1.575) substrate. The variations in permittivity, permeability, and reflective index with respect to polarization angle are listed in Table A2 in Appendix A and displayed in Figure 6a-d. The S 11 and S 21 parameters were extracted from MATLAB. Table A2 shows four resonance frequencies at 1.44, 3.96, 4.205, and 5.025 GHz with 99.382%, 99.383%, 92.91%, and 95.17% absorption, respectively. The permittivity is positive at 3.96 GHz and 5.025 GHz and negative at 1.94 GHz and 4.205 GHz and vice versa for permeability. Hence, the design is an excellent absorber with SNG properties in the S-and C-band regions. From the numerical analysis, it can be concluded that FR-4 has a wider band (3.09-3.14 GHz) than Roger4450B and a high absorption of 99.94% and −32.15 dB for S 11 , whereas Roger4450 exhibited values of 99.38% and −22.10 dB for S 11 . Moreover, the dielectric constant, loss tangent, and thermal conductivity of FR-4 were 4.3, 0.025, and 0.3 w/k/m, respectively, whereas Rogers4450B showed values of 3.6, 0.004, and 0.6 w/k/m, respectively. The loss tangent of FR-4 was greater than that of Roger4450B, but the dielectric property we found for FR-4 was comparatively stable compared to that of Roger4450B. Thus, absorption was obtained more for FR-4. We did not achieve the dielectric property that Roger4450B showed during the simulation. In this scenario, the absorption rate was decreased for Rorger4450B.   Absorption at the different polarization angles is depicted in Figure 9a,b. The proposed metamaterial absorber (MA) is insensitive to polarization because the absorption curves did not change with 0°, 30°, 60°, and 90° θ and φ rotations. This performance has increased MA eligibility. Typically, polarization insensitivity can be acquired by symmetrical design. The symmetrical patch arrangement of the design is the main mechanism that has a crucial impact on the absorption rate with the angular rotation of the unit cell. Figure 9a,b were plotted for FR−4, and data were collected from Table A1 in Appendix A.

Analysis of Current and Field Distribution of Unit Cell
A clear explanation of the electric field, magnetic field, and surface current distribution can be gleaned from Figures 10 and 11. All the fields are shown at the resonance frequencies. The proposed unit cell exhibits a perpendicular E-field and H-field at four resonance frequencies. The cell was constructed from two substrates, FR−4 and Rog-ers4450B. The E-field concentration, H-field concentration, and surface current density Absorption at the different polarization angles is depicted in Figure 9a,b. The proposed metamaterial absorber (MA) is insensitive to polarization because the absorption curves did not change with 0 • , 30 • , 60 • , and 90 • θ and ϕ rotations. This performance has increased MA eligibility. Typically, polarization insensitivity can be acquired by symmetrical design. The symmetrical patch arrangement of the design is the main mechanism that has a crucial impact on the absorption rate with the angular rotation of the unit cell. Figure 9a,b were plotted for FR-4, and data were collected from Table A1 in Appendix A. Absorption at the different polarization angles is depicted in Figure 9a,b. The proposed metamaterial absorber (MA) is insensitive to polarization because the absorption curves did not change with 0°, 30°, 60°, and 90° θ and φ rotations. This performance has increased MA eligibility. Typically, polarization insensitivity can be acquired by symmetrical design. The symmetrical patch arrangement of the design is the main mechanism that has a crucial impact on the absorption rate with the angular rotation of the unit cell. Figure 9a,b were plotted for FR-4, and data were collected from Table A1 in Appendix A.

Analysis of Current and Field Distribution of Unit Cell
A clear explanation of the electric field, magnetic field, and surface current distribution can be gleaned from Figures 10 and 11. All the fields are shown at the resonance frequencies. The proposed unit cell exhibits a perpendicular E-field and H-field at four resonance frequencies. The cell was constructed from two substrates, FR-4 and Rog-ers4450B. The E-field concentration, H-field concentration, and surface current density

Analysis of Current and Field Distribution of Unit Cell
A clear explanation of the electric field, magnetic field, and surface current distribution can be gleaned from Figures 10 and 11. All the fields are shown at the resonance frequencies.
The proposed unit cell exhibits a perpendicular E-field and H-field at four resonance frequencies. The cell was constructed from two substrates, FR-4 and Rogers4450B. The E-field concentration, H-field concentration, and surface current density can clearly be seen for the FR-4 substrate from Figure 10a-c. The concentrations of the E-field and H-field are high at the edge of the patch. At 3.1 GHz, the absorption and the E-field concentration were high. The permittivity, permeability, and reflective index were also measured, and 99.94% was the highest absorption rate for this resonance frequency. At 1.95 GHz, the electric field is high at the outer patch, inner patch, and the red region in the gaps of the patch, but the magnetic field concentration is visible only at the band of the internal split ring. Consequently, the permeability value is higher than the permittivity, and the surface current is less dense than 3.1 GHz. The red region indicates the high-density field concentration. The E-field concentration moves into the inner patch, and the H-field becomes higher for the outer split-ring resonator at 3.49 GHz. At this point, the permittivity is positive and the permeability and reflective index are negative. Gradually, the magnetic field increases and the electric field concentration decreases. The permittivity is positive, and permeability is negative at 4.155 GHz. Therefore, the unit cell exhibits SNG absorption and near-zero reflective indexes at all the resonance frequencies in the S-and C-bands. can clearly be seen for the FR-4 substrate from Figure 10a-c. The concentrations of the field and H-field are high at the edge of the patch. At 3.1 GHz, the absorption and the field concentration were high. The permittivity, permeability, and reflective index w also measured, and 99.94% was the highest absorption rate for this resonance frequen At 1.95 GHz, the electric field is high at the outer patch, inner patch, and the red region the gaps of the patch, but the magnetic field concentration is visible only at the band the internal split ring. Consequently, the permeability value is higher than the permitt ity, and the surface current is less dense than 3.1 GHz. The red region indicates the hig density field concentration. The E-field concentration moves into the inner patch, and H-field becomes higher for the outer split-ring resonator at 3.49 GHz. At this point, permittivity is positive and the permeability and reflective index are negative. Gradua the magnetic field increases and the electric field concentration decreases. The permitt ity is positive, and permeability is negative at 4.155 GHz. Therefore, the unit cell exhib SNG absorption and near-zero reflective indexes at all the resonance frequencies in the and C-bands.
Electric field Magnetic field Surface current For Roger4450B, the electric field, magnetic field, and surface current are dem strated in Figure 8 at 2.05 GHz, 3.735 GHz, and 4.4 GHz. In Figure 11a−c, we can see the surface current is high where the electric field is also high, and the magnetic fie less intense in this portion. Therefore, Roger4450B also behaved as an SNG absorber achieved near-zero reflective indexes similar to FR-4 substrate.

Electric field
Magnetic field Surface current For Roger4450B, the electric field, magnetic field, and surface current are demo strated in Figure 8 at 2.05 GHz, 3.735 GHz, and 4.4 GHz. In Figure 11a-c, we can see th the surface current is high where the electric field is also high, and the magnetic field less intense in this portion. Therefore, Roger4450B also behaved as an SNG absorber an achieved near-zero reflective indexes similar to FR-4 substrate.

Electric field
Magnetic field Surface current The E-field, H-field, and surface current results show that the electric field conce tration is proportional to the surface current density and that magnetic field lags behin the electric field. Maxwell's equation can explain this property: = +∈ ( From Equation (9), the variation between current density and electric field can explained as:

=
(1 J is the current density and E is an electric field. The relation between J and E show that J depends on E. The maximum concentration of the electric field in the patch occu with the highest concentration of the surface current with respect to time. E and J al follow the H-field direction. Hence, the H-field concentration is sparser than the E-fie concentration, which can be seen in Figures 10 and 11.

Array Analysis
The proposed unit cell was also analyzed for array structures.  Figure 11a-c, we can see that the surface current is high where the electric field is also high, and the magnetic field is less intense in this portion. Therefore, Roger4450B also behaved as an SNG absorber and achieved near-zero reflective indexes similar to FR-4 substrate.
The E-field, H-field, and surface current results show that the electric field concentration is proportional to the surface current density and that magnetic field lags behind the electric field. Maxwell's equation can explain this property: From Equation (9), the variation between current density and electric field can be explained as: J is the current density and E is an electric field. The relation between J and E shows that J depends on E. The maximum concentration of the electric field in the patch occurs with the highest concentration of the surface current with respect to time. E and J also follow the H-field direction. Hence, the H-field concentration is sparser than the E-field concentration, which can be seen in Figures 10 and 11.

Array Analysis
The proposed unit cell was also analyzed for array structures. Figure 12a,b show the design structure, and Figure 12c,d depict the absorption and reflection percentages. The reflection is 0.3118 and the absorbance is 96.88% at 3.09 GHz for the 1 × 2 array structure. The reflection is 0.0431 and the absorption is 95.69% for the 2 × 2 array structure. In Figure 12c,d, dispersion is present in the array structure but is at a minimum level. Therefore, absorption is decreased here. Another possible scenario is that the mutual coupling patch of the array structure creates the additional capacitance. When we designed the array, the mutual coupling of patch structures formed the additional capacitance, and as a result, a circuitry operation formed. This circuitry operation generated additional absorption. Due to this operation, the absorbance rate decreased.
The performance of unit cells with 1 × 1 and 2 × 2 arrays are shown but not shown for any single-unit cells or for any infinitely periodic cells. Figure 13a,b depict simulated boundary conditions where a perfect electric (PEC) and a perfect magnetic field (PMC) are applied along the X-axis and Y-axis, keeping the Z-axis open. reflection is 0.3118 and the absorbance is 96.88% at 3.09 GHz for the 1 × 2 array structure. The reflection is 0.0431 and the absorption is 95.69% for the 2 × 2 array structure. In Figure  12c,d, dispersion is present in the array structure but is at a minimum level. Therefore, absorption is decreased here. Another possible scenario is that the mutual coupling patch of the array structure creates the additional capacitance. When we designed the array, the mutual coupling of patch structures formed the additional capacitance, and as a result, a circuitry operation formed. This circuitry operation generated additional absorption. Due to this operation, the absorbance rate decreased. The performance of unit cells with 1 × 1 and 2 × 2 arrays are shown but not shown for any single-unit cells or for any infinitely periodic cells. Figure 13a

Comparison
Around the world, researchers have modified their metamaterial absorber structures to achieve maximum absorption for different applications. For example, In Table 1 Lshaped [44] and T-shaped [45] modified square metamaterial absorbers have been proposed for radar absorption and electromagnetic interference/electromagnetic compatibility (EMI/EMC) configurations. FR-4 substrate was used for the fabrication of these unit cells as it is commercially available and of low cost; however, in terms of absorption, the operating range of these MAs is only 90-96% in the X-band. On the other hand, Borah [46] and Prakakash Ranjan [47] have achieved high absorption above 98% in the X-band. However, they have not shown whether or not their proposed designs are polarization inde-

Comparison
Around the world, researchers have modified their metamaterial absorber structures to achieve maximum absorption for different applications. For example, In Table 1 L-shaped [44] and T-shaped [45] modified square metamaterial absorbers have been proposed for radar absorption and electromagnetic interference/electromagnetic compatibility (EMI/EMC) configurations. FR-4 substrate was used for the fabrication of these unit cells as it is commercially available and of low cost; however, in terms of absorption, the operating range of these MAs is only 90-96% in the X-band. On the other hand, Borah [46] and Prakakash Ranjan [47] have achieved high absorption above 98% in the X-band. However, they have not shown whether or not their proposed designs are polarization independent. A polarization-independent C-shaped ring MMA has been designed for Ku-band sensing applications, but its absorption level is 99.6%. Our proposed design's absorption level is 99.94%. It is also polarization-independent and suitable for S-band sensing applications.

Sensing Application
The S-band is designed for low-power microwave equipment, including Bluetooth headphones, Wi-Fi, baby monitors, garage door openers, keyless vehicle locks, and microwave diathermy devices. This band is also used for radar applications to control air traffic, ships, and weather stations. Among these, microwave diathermy (MD) is the specific S-band application area that we are interested in. The therapeutic procedure known as diathermy is most frequently recommended for disorders affecting the muscles and joints. Shortwave, microwave, and ultrasound are the three main types of diathermies. Microwave diathermy (Md) uses electromagnetic radio waves. During diathermy treatment, the heat induced in tissues depends on the conductivity of the tissues involved. Muscle tissues are more conductive than skin and fat tissue. When MD generates heat at the particular area of the human muscle that may suffer from physical problems such as joint pain, deformations, injuries, and so on, the properties of S 11 in that certain muscle area are also altered. In this case, we can measure changes in the S 11 values with our unit cell as a passive sensor, and this process is non-invasive. Non-invasive techniques do not employ instruments that penetrate the skin or inside the body and typically do not spread to or harm other tissues and organs. Figure 14a shows the complete simulation setup, where the sensor was installed scientifically with much thought. The thickness, permittivity and permeability of the skin, fat and muscle are listed in Table 2. The ground plane of the metamaterial absorber (MA) was placed a 0.1 mm air gap from the particular human body part, then 2 mm thick skin, fat, and 2/3/4 mm muscle layers were sequentially placed so that the electromagnetic wave from the proposed MA can penetrate properly into the muscle layer. Table 2 demonstrates the permittivity and thermal conductivity values of skin, fat, and muscle. From Figure 14b, when the muscle is 2, 3, and 4 mm, the S 11 values are −28.7269 dB, −30.189 dB, and −30.359 dB at 3.1GHz, respectively. Absorption levels of 99.8659%, 99.9042%, and 99.9079% were obtained for the three individual muscle layers, respectively. Due to different muscle layers, the absorption curve and S 11 are changed. The simulation result showed that the values of S 11 and absorption increased when the muscle thickness increased. This investigation concludes that the sensor can detect deformation, injuries, and other abnormalities in muscle layers by monitoring the S 11 and absorption values of the specific body parts.

Conclusions
A single-negative spiral split-rectangular-shaped resonator surrounded by two splitring copper patches having almost perfect absorption for S-band application was studied in this paper. The effect of the physical structure of the patch on absorption was thoroughly studied, including the central snake cutting gaps. A numerical analysis was conducted in a CST simulator, and the results of absorption, permittivity, permeability, and refractive index were further analyzed using MATLAB. The model achieved similar results in both programs. The absorption rate was 99.94% at 3.1 GHz in the S-band with FR-4 substrate, and 99.382%, 99.383%, 92.91%, and 95.17% at 1.94, 3.96, 4.205, and 5.025 GHz, respectively, in the S-and C-band regions with Rogers4450B. The design exhibits a singlenegative metamaterial (SNG). The wide band and absorption of the FR-4 are better than those of the Rogers4450B. The polarization angles θ and φ moved in the direction of the E-field and H-field. The E-field and H-field concentrations and current distribution were shown. The proposed unit cell was applied on 1 × 2 and 2 × 2 arrays, and the results were

Conclusions
A single-negative spiral split-rectangular-shaped resonator surrounded by two splitring copper patches having almost perfect absorption for S-band application was studied in this paper. The effect of the physical structure of the patch on absorption was thoroughly studied, including the central snake cutting gaps. A numerical analysis was conducted in a CST simulator, and the results of absorption, permittivity, permeability, and refractive index were further analyzed using MATLAB. The model achieved similar results in both programs. The absorption rate was 99.94% at 3.1 GHz in the S-band with FR-4 substrate, and 99.382%, 99.383%, 92.91%, and 95.17% at 1.94, 3.96, 4.205, and 5.025 GHz, respectively, in the S-and C-band regions with Rogers4450B. The design exhibits a single-negative metamaterial (SNG). The wide band and absorption of the FR-4 are better than those of the Rogers4450B. The polarization angles θ and ϕ moved in the direction of the Efield and H-field. The E-field and H-field concentrations and current distribution were shown. The proposed unit cell was applied on 1 × 2 and 2 × 2 arrays, and the results were 96.88% and 95.69%, respectively. The absorption of the proposed unit cell is around 99.94%, which is high and more promising than that of some of the recent papers with which it is compared in Table 1. The sensing performance was analyzed for microwave diathermy. These properties indicate that the described MA can be widely implemented for S-band applications.

Institutional Review Board Statement:
There is nothing relevant to ethical issues. As a result, it is not relevant to us.

Informed Consent Statement:
This research does not involve any human/animal trial and is executed using a numerical approach and material-based experimental process. Therefore, no consent is required.
Data Availability Statement: Data availability will be provided if any interesting part is requested. Otherwise, the data will be stored as per research tools.

Conflicts of Interest:
The author declare no conflict of interest.