Penta band single negative meta-atom absorber designed on square enclosed star-shaped modified split ring resonator for S-, C-, X- and Ku- bands microwave applications

This paper represents a penta band square enclosed star-shaped modified split ring resonator (SRR) based single negative meta-atom absorber (MAA) for multi-band microwave regime applications. FR-4 low-cost material has been used as a substrate to make the MAA unit cell with 0.101λ0 × 0.101λ0 of electrical size, where λ0 is the wavelength calculated at the lower resonance frequency of 3.80 GHz. There are two outer square split ring and one inner star ring shape resonator of 0.035 mm thickness of copper placed on the one side, and another side of the substrate has full copper to construct the desired unit cell. The MAA unit cell provides five absorption peaks of 97.87%, 93.65%, 92.66%, 99.95%, and 99.86% at the frequencies of 3.80, 5.65, 8.45, 10.82, and 15.92 GHz, respectively, which covers S-, C-, X-, and Ku- bands. The properties of MAA have been investigated and analyzed in the E-, H-fields and surface current. The EMR and highest Q factor of the designed MAA is 9.87 and 30.41, respectively, and it shows a single negative (SNG) property. Different types of parametric analysis have been done to show the better performance of absorption. Advanced Designed System (ADS) software has been used for equivalent circuit to verify the simulated S11 result obtained from the CST-2019 software. Experimental outcomes of the MAA unit cell have a good deal with the simulated result and measured result of the 24 × 20 array of unit cells also shown. Since the unit cell provides superior EMR, excellent Q-factor, and highest absorption so the recommended MAA can be effectively used as a penta band absorber in microwave applications, like notch filtering, sensing, reducing the unintended noise generated with the copper component of the satellite and radar antennas.

.82 GHz, and 15.92 GHz, respectively. The designed MAA unit cell provides high EMR, high quality factor and maximum absorption bands, moreover its size is small compared with many published works 11,13,14,[20][21][22]26,27,29,31 . In order to explain these innovations, this article offers a complete analysis of meta-atom properties of the formed absorber, accompanied by a study of surface current, E and H fields. Figure 1a,b depicts the SES-MAA unit cell's basic layout with the required dimensions. To construct the MAA unit cell, we have used a minimal-price material FR-4 as a substrate which has a thickness of 1.5 mm, dielectric constant 4.3, and loss factor 0.025. There is a 0.035 mm thickness of copper on both sides of the substrate. Two square SRR with various dimensions are intended at one side of the substrate, and another side is full copper. Each square ring's width is 0.50 mm, and the gap of the split is 0.50 mm. Each square ring has been divided into two parts by the split gap. One-star shape resonator has been placed inside the split square ring. The various dimensions of the suggested MAA unit cell are shown in Table 1.

Meta atom unit cell design structure in numerical environment
The recommended MAA unit cell's simulation procedure is exhibited in Fig. 2, where a normal incident EM wave is applied in the z-axis, a perfectly electric boundary condition (PEC) is applied in the x-axis, and a perfectly magnetic boundary condition (PMC) is applied in the y-axis. A finite element method (FEM) is used to design the proposed MAA's unit cell. The simulation procedure is done in widely used microwave studio suite CST-2019 software 33 where 2-18 GHz frequency range has been used. EM wave sources are applied from port 1 to all modes.

Absorption characteristics investigation of the suggested meta atom absorber unit cell
The absorption A(f ) , reflection R(f ) , and transmission T(f ) coefficients dependent on frequency are introduced to characterize the MAA unit cell. The relationships among these coefficients are given as 34,35 .
With the aim of increase absorptivity A(f ) , we could decrease the transmission T(f ) = |S 21 | 2 and the reflection R(f ) = |S 11 | 2 at the same time. The absorptivity could be estimated by A(f ) = 1 − R(f ) because the metallic plate blocked the proposed MAA on the bottom layer (So T(f ) = |S 21 | 2 = 0 ). Hence, the absorptivity of the recommended MAA could be estimated by  www.nature.com/scientificreports/ It is seen from Eq. (2) that the absorption will be near 100% ( A(f ) ≈ 100% ) while the reflectivity is close to zero (R(f ) ≈ 0) . It is noticeable that the S 11 components include the reflection of co-polarized EM waves and the reflection cross-polarized EM waves 36,37 . So, the S 11 components can be expressed as: Accordingly, based on Eq. (3), Eq. (2) could be evaluated by Here the xx is co-polarization and xy is the cross-polarization. In the suggested MAA design, |S 11 | encompasses the components of the co-polarization and cross-polarization.
The reflection coefficient (S 11 ) , transmission response (S 21 ), and absorptance A(f ) of the suggested MAA unit cell are shown in Fig. 3. By monitoring this curve, it is found that several absorption peaks occur at 3.80, 5.65, 8.45, 10.82, and 15.92 GHz, along with maximum peaks of absorption 97.87%, 93.65%, 92.66%, 99.95%, and 99.86%, respectively. Figure 4 depicts the change of absorption and resonance frequency due to the different design steps. The final design has been obtained after several steps. When the unit cell contains one large square split ring resonator comprising a width of 0.5 mm, four absorption peaks are available. The absorption peaks are 99.99%, 86.09%, 85.87%, and 92.58% at the resonance frequencies 3.83, 8.74, 11.62, and 15.50 GHz, respectively, denoted by the red dash line in Fig. 4. In design 2 of Fig. 4, one small square SRR exist in the unit cell, and the absorption peaks are 99.67%, 69.92%, and 98.05% at the resonance frequencies 5.76, 11.08, and 15.98 GHz, respectively denoted    Fig. 4. The absorption property is regulated by the distance between two metallic particles, either horizontally or vertically. The absorption efficiency is enhanced by the interaction and coupling between localized surface plasmon (LSP) modes. Different absorption achieved by varying the size and shape of the metallic patch on the substrate is shown in Fig. 4. The two square SRR enclosed one star shape resonator provides high capacitance and inductance, creating perfect impedance matching and effective permittivity and effective permeability. The absorption frequency can also be tunable by changing the geometrical parameters based on L-C resonance circuit theory. Due to the perfect impedance matching and effective permittivity, effective permeability high absorption peaks are achieved, i.e., the absorption peaks are 97.87%, 93.65%, 92.67%, 99.95%, and 99.86% at 3.80, 5.65, 8.45, 10.82, and 15.92 GHz resonance frequencies, respectively. The summarize of the resonance frequency, maximum absorption, and covering bands are listed in Table 2.
The absorber's frequency selectivity is achieved as it provides a narrow bandwidth with a high absorption level in five selected frequencies of 3.80 GHz, 5.65 GHz, 8.45 GHz, 10.82 GHz, and 15.92 GHz. The half-maximum bandwidth (HMBW) at these five selected frequencies were 124 MHz, 220 MHz, 380 MHz, 520 MHz, and 800 MHz, respectively. The following Eq. determines Q-factor where f 0 is the maximum absorption peak point frequency, the Q-factors are 30.41, 25.98, 22.53, 20.92, 19.88, and the average Q-factor 23.95 for the suggested MAA cell. The high discrimination of the recommended unit cell of MAA has the potential applicability for sensing and detecting purposes. The unit cell of MAA, whose bandwidth and Q-factor are shown in Table 3.
Resonance frequency has been changed very slightly at a higher band, though the absorption remains identical for all polarization angles. The bandwidth of the suggested MAA unit cell's absorption has been accomplished,  www.nature.com/scientificreports/ summing up each resonance peak frequency range with a maximum value of − 10 dB of the corresponding reflection and transmission coefficients. The absorptivity curve for EM wave's normal incident, the change of polarization angle is presented in Fig. 5a. In this study, E and H fields are rotated at an angle ϕ . From this Figure, it is observed that the value of ϕ = 0 • , 30 • , 60 • , 90 • the absorption coefficient is similar. For the oblique incidence angle θ = 0 • , 45 • , 90 • the coefficient of absorption is almost the same, which indicates that the proposed MAA unit cell represents polarization-insensitive behavior. Figure 5b shows the absorption for the oblique incidence angle θ = 0 • , 45 • , 90 • , respectively, along the x-axis the electric field ( ϕ = 0 • ). As the incidence angle increases, the peak absorption value would be almost unchanged 38 . So, the MAA can be achieved near-unity absorbance in a wide range of incidence angles when. Therefore, in a wide variety of incidence angles, the MAA can be accomplished with near-unit absorption when ϕ = 0 • and θ = 90 • . The amplitude and phase of the reflection response S 11 with co-and cross-polarized EM waves of the MAA is shown in Fig. 6.

Meta-atom characteristics of the recommended unit cell
Meta-atom properties, relative permittivity, and permeability were extracted from the proposed unit cell, as shown in Figs. 7 and 8. Transmission response (S 21 ) and reflection response (S 11 ) is obtained from the CST simulation 39 can be used to determine the relative permittivity, relative permeability, and normalized impedance by using Nicholson-Ross-Wier (NRW) approach. If the wave number, k 0 = 2πf c , where C is the speed of light, f is the frequency, then for a substrate having a thickness of d , the relative permittivity and relative permeability can be characterized by Eqs. (6), (7) The following Eqs. can express the refractive index and normalized impedance with the understanding of permittivity and permeability. Table 3. MAA unit cell's Q factor and bandwidth for various application bands. www.nature.com/scientificreports/   www.nature.com/scientificreports/ When permittivity is negative, then the permeability is positive; if the permittivity is positive, the permeability is negative, as shown in Figs. 7 and 8. Table 4 represents the negative region of the relative permittivity, relative permeability, transmission, and reflection coefficient. Table 4 shows that the transmission coefficient around − 200 dB, which is very low. This low value of the transmission coefficient is seen due to the effect of the full copper on the backside, which obstructs the transmission of the EM waves. Thus, the absorption varies only with the coefficient of reflection. The reflection coefficient exhibits below − 10 dB, which is a narrow band. S 11 Table 4. Figure 9 indicates the normalized input impedance. Input impedance has a significant contribution to monitoring the reflection of the incident wave. The meta-atom is classified by the frequency-dependent relative permittivity and permeability to perform the effective medium's characterization. In this incident, the device impedance can be stated as 40 .
where ε 0 and µ 0 are the free space permittivity and permeability. Thus, zero reflectance can be reached by designing ε r and µ r to get them identical from each other. From the impedance graph presented in Fig. 9, it is observed that at all different frequencies real and imaginary part of impedances deviates unity and zero values; thus, absorption is low. www.nature.com/scientificreports/

Surface current, E field and H field distribution of MAA unit cell
In order to make explicit, the idea of absorption processes, surface current and electric field analysis be able to perform on the front side and back side of the MAA unit cell device. The surface current forms five major absorption resonances are shown in Fig. 10. At the resonance frequency 3.80 GHz, current flows via the lower and upper left side of two outer rings and the lower right side of the outermost ring. The left segment of the inside ring provides significantly to flow a sufficient quantity of current. The other parts of split rings of horizontal current flows are very small, as depicted in Fig. 10a. The current components of this portion are anti. In the backside, the evenly distributed moderate current flows, which is anti-parallel to the front-end current parallel presented in Fig. 10f. This front and back layer anti-parallel current initiates a current loop, which produces magnetic dipole resonance 41 . At 5.65 GHz, current flows via the left top outermost square split ring and middle ring, as shown in Fig. 10b. The current intensity in the backplane is unevenly dispersed. In the 8.45 GHz absorption peak, a significant current flow through the edges of the left outermost ring, through the top left and bottom left part of two square rings, as revealed in Fig. 10c. It is vital that all over the outer ring, the current density is high. A high dense oppositely flowing current is noticed in the metallic backplane, as shown in Fig. 10h. The current intensity gradually decreases from high to low as it considers the top side. In the top half, and anticlockwise rotating current flows all through the copper. The current circulation shape at 10.82 GHz is shown in Fig. 10d,i for front and backside, respectively. In this frequency, current density becomes low in both front and back. A moderate current flow via the right side of two outer rings. Top coupling point of middle and inner ring, top left corner of outermost ring aids to a significant extent of current. In the backside's metal layer, lower intensity anti-parallel current is noticed that flows from left to right of the structure. This current shows a whirling pattern in the lower right corner. The last absorption peak at 15.92 GHz, where the right bottom portion of the outer two rings and the innermost ring shows the current flow as revealed in Fig. 10e. In the copper backplane significantly lower intensity of current flows is shown in Fig. 10l. The current distribution, E-and H-fields linked to each other in a time-varying EM wave can be explained by using Maxwell's curl equations, shown in Eqs. (14) and (15) 42 . Equation (12) describes the EM induction of Faraday's law, though Eq. (13) is a revised form of Ampere's law.
To determine the value of D, the material permittivity ( ε ) may be applied and for B's determination, the permeability ( µ ) can be applied. The connection can be considered by applying Eqs. (14) and (15) Take on e jωt for time enslavement and substituting time derivative of Eqs. (14) and (15) with jω , Eqs. (16) and (17) as Maxwell's curl Eqs. may be stated in phasor This effective induced electric field reverts to the electric field incident, which in turn energizes the electric field than the electric field incident 43 ensuring electrical resonance. The z-component of the E-field vector by using Maxwell's Eqs. in free space which can be written as follows: Taking the curl operation from Eq. (18), we obtain This expression can be simplified as follows: After substitution of the corresponding expressions from Eqs. (19) and (20) we derive This expression can be re-written in the following way: To derive the expression for the z-component of the electric field vector, one should project this expression on to the z-axis. The magnetic field distribution is presented in Fig. 12a-e for five different absorption peaks at the resonance frequencies. At 3.80 GHz, a sharp magnetic field is observed in the left lower and higher part, the right lower portion, and the MM absorber unit cell's middle star part. At 5.65 GHz, A sharp magnetic field is noticed left part in the upper and lower portion of the middle square and right top part of the star ring. At 8.45 GHz, the magnetic field high for the left is lower to the upper portion of the MAA unit cell. The major amount of H filed is also noticed at 10.82 GHz frequency, as exhibited in Fig. 12d. The H field is circulating on the suggested MAA unit cell. The inner edges of the innermost ring. At 15.92 GHz, the medium H field is seen surrounding the star ring as shown in Fig. 12e. Figure 13 indicates an estimated circuit for the unit cell. Since the mentioned form involves passive components, i.e., inductive-capacitive (LC) elements, the resonance frequency (f) is given below 44 .

Analysis of the MAA unit cell's equivalent circuit
where L represents the collective inductance, and C denotes the collective capacitance. In the designed structure, inductance is formed by the metal strip and assembled of capacitance by the splits. The electrical resonances are created by coupling between the gaps and electric fields. Besides, magnetic resonances are generated by coupling the magnetic fields with the loops. The capacitance where ε 0 = 8.854 × 10 −12 F/m, ε r = relative permittivity, A is the split region, and d is the split distance. Besides, the total inductance (L) can be determined by 45 where µ 0 = 4π × 10 −7 H/m. Therefore, the capacitance (C) can be achieved by  www.nature.com/scientificreports/ Microstrip lines are presented with different inductors L1, L2, L3, L4, L5, and capacitors are represented by C1, C2, C3, C4, C5, C6, and C7 due to the split gaps. Because of the two-ring coupling row, L3 is the inductor, and C4 is the coupling capacitor. By using advanced design system (ADS) software 46 , the values of these elements are modified. The reflection coefficient is calculated by tuning the parameter values such that the ADS result shows a near resemblance to the outcome obtained from the CST. Figure 14 shows the CST simulated and ADS analysis result of reflection response.

Analysis of the different array configuration of the MAA
The unit cell arrays are prepared and simulated to observe the performance of unit cell arrays 1 × 2, 2 × 2, and 4 × 4. The arrays' absorption is compared to the recommended unit cell, and the absorption plot is revealed in Fig. 15. The EM interaction in the array is complex when the built array is positioned side by side horizontally and vertically. The absorption peaks change magnitude and frequency due to the mutual coupling effect. The Harmonic influence is also evident in the series. Figure 15 expresses the change of absorption and frequency with the different array configurations. Here to show the effect of absorption peaks and frequencies, we have analyzed 1 × 2 array, 2 × 2 array, and 4 × 4 array. For the unit cell, the obtained absorption peaks are 97.87%, 93.65%, 923.67%, 99.95%, and 99.86% at the frequencies   To match the arrays' discrimination, Q-factor has been computed and shown in Table 5 alongside the highest absorptance and frequency of the resonance peak. Table 3 remarked that all the arrays deliver adequate Q-factor within S-, C-, X-and Ku-bands. The results of the arrays are summarized in Table 5.

Parametric analysis
Effect of the copper backplane. Figure 16 illustrates the effect of different sizes of copper backplane on the absorption peaks and frequencies. To show the impact on absorption with the different sizes of the copper backplane, we have used a full copper backplane, half copper backplane, and without copper backplane. When the copper has been used as a full backplane, the peaks of the absorption are 97.  www.nature.com/scientificreports/ the best performance, so full copper backplane has been used for the suggested MAA unit cell. The response of the backplane on the resonance frequency and absorption peaks are listed in Table 6.
Effect of the substrate size. We Figure 17 shows the effect of different size of the substrate on the absorption peaks and the resonance frequency. Table 7 depicts the effect of substrate size on absorption peaks, resonance frequencies, and covering bands. From Table 7, it is clearly seen that for substrate size 7.2 × 7.2 mm 2 , the absorption peaks are low and covering band C-, X-, and Ku-whereas the covering band is S-, C-, X-, and Ku-and absorption peaks are high for substrate size 8 × 8 mm 2 . Since the substrate size 8 × 8 mm 2 covers maximum bands and absorbs high peaks, hence we have considered the substrate size of 8 × 8 mm 2 .
Effect of the patch size. Figure 18 shows the effect of different size of the patch on the absorption peaks and resonance frequency. Different size of the patch has been used for the performance analysis of the proposed absorber. The different size of the outer square patches is 7.88, 7.5, and 7.13 mm. If the patch size is 7 Figure 17. Effect of size of the substrate on the absorption peaks and resonance frequency.  Table 8 depicts the effect of patch size on absorption peaks, resonance frequencies, and covering bands. From Table 8, it is clearly seen that for patch size 7.13 × 7.13 mm 2 , the absorption peaks are low and covering bands are C-, X-, and Ku-whereas the covering bands are S-, C-, X-, and Ku-and absorption peaks are high for patch size 8 × 8 mm 2 . Since the patch size 7.5 × 7.5 mm 2 covers maximum band and absorb high peaks, hence we have considered the outer patch size of 7.5 × 7.5 mm 2 . Figure 19 indicates the effect of the split gap on absorption peak and resonance frequency. When the split gap is increased, the resonance frequency is also increased because the resonance frequency depends on the capacitor. The capacitor is inversely proportional to the split gap. Also, the capacitor is inversely proportional to the resonance frequency. For split gap 0.2 mm, the absorption peaks are 95.85%, Figure 18. Effect of size of the patch on the absorption peaks and resonance frequency.  Figure 19. Change of absorption with the variation of the split gap.  10.96, and 16.03 GHz respectively for 0.7 mm split gap. Since the best performance shows for the split gap of 0.5 mm, the split gap has been selected 0.5 mm for the suggested MAA unit cell. Figure

Measurement results and discussion
First, we confirmed the proposed MAA unit cell and array design. Then we have produced the arrangement by using the LPKF Laser and Electronics AG, Promat E33 model Computerized Numerical Control (CNC), and printed circuit board (PCB) machine. Produced fabricated unit cell and array structure have been presented. A unit cell and 24 × 20 (192 × 160 mm 2 ) array fabricated prototype is revealed in Fig. 21a,b. Since 3.80 GHz is the first resonance frequency of this MAA unit cell's absorption peak, the absorbed signal wavelength is 78.95 mm this frequency. The value of the wavelength of MAA will continue to decrease as the frequency increases. As a result, the array's designated prototype makes sure its greater length contrasted to the incident signify wavelength. Owing to this pattern's more significant imprint, there is a strong probability of better response during the measurement. Several pairs of waveguides have been applied to determine the unit cell result for different frequency ranges. Figure 22 shows the unit cell measurement setup, where the MAA cell has been positioned between one pair of waveguides. Using the waveguide and vector network analyzer first measured the real and imaginary value of S 11 and S 21, and using these values in the MATLAB Code, we have got the absorption result. The measured absorption graph of the designed MAA is depicted in Fig. 23    www.nature.com/scientificreports/ To measure the MAA unit cell, we have used several pairs of waveguide model. These waveguides model and frequency range are given in Table 10.
The array prototype has been kept on one side of the horn antennas for measurement purposes, as shown in Fig. 24. The operating lower frequency is 700 MHz, and the higher frequency is 18 GHz of these horn antennas and formed in the shape of double edges guided. The whole volume of the antenna is 13.9 × 24.4 cm 2 . To measure the array prototype, the distance between the antenna and the prototype will be far field. The far-field distance ≥ 2D2/λ, for an antenna, where D is the aperture antenna dimension, and λ is the wavelength at the lowest operational frequency. The manufactured array prototype is located 40 cm from two horn antennas at a far-field distance; antennas are used simultaneously for transmitting and receiving signal purposes. The array measurement has been taken in the anechoic chamber to avoid the surrounding noise.  www.nature.com/scientificreports/ First, to measure the reflection coefficient, a copper sheet is placed with the same size as the fabricated array size. Subsequently, the blank copper plate is substituted by the fabricated array and measured the response of reflection; the authentic response of S 11 is achieved by normalization the array data with copper. The measured absorption result of the MAA array is shown in Fig. 25 10.39, 15.11, and 15.87 GHz, respectively. The resonance frequencies of the array are slightly shifted compared with the unit cell, and some harmonics are also shown in the array result due to the effect of coupling and some errors in the measurement procedure. Another cause may be frequency shifting, noise, and harmonics for the used lossy cable to connect between horn antennas and VNA.

EMR calculation
The EMR of a meta-atom expresses the structure's compactness, and it is a vital property of meta-atom. To fulfill the criteria of meta-atom, the value of EMR must be ≥ 4. The calculated formula of EMR can be revealed by the expression EMR = 0 L , here 0 and L are the wavelength and MAA unit cell large dimension, respectively 47 . Since 3.80 GHz is the first peak of absorption, so the value of EMR is 9.87 of the recommended MAA unit cell. A comparison among the proposed MAA unit cell and some other existing MMA papers are given in Table 9. This comparison's main parameters are unit cell size, absorption points, frequencies where highest absorption occurs, EMR, cover bands. From reference 11,14,21,22,26,31 we can realize that the dimension of this MMA unit cell is high, the number of resonance frequency, EMR, and cover bands are low, so our proposed MAA unit cell performance is better than the reference 11,14,21,22,26,31 . Though the MMA unit cell dimension of reference 27,29 are small compared to the proposed MAA unit cell, the number of resonances, EMR, and frequency bands are low than the designed cell, so the suggested MAA unit cell performance is good quality than reference 27,29 . Since the size of the reference 13,20 are equal to our fabricated MAA unit cell, but the other parameters, such as EMR, are low, and the covering band is low. The number of resonance frequencies is low; hence, the MAA's performance is best rather than the reference 13,20 . Hence the recommended MAA shows a better performance than some published works mentioned in Table 11.

Conclusion
In this article, an SNG meta-atom absorber has been constructed by two square SRR enclosed one-star ring resonator for the penta band microwave applications. The suggested MAA indicates five peaks of absorption of 97.87%, 93.65%, 92.66%, 99.95%, and 99.86% at 3.80, 5.65, 8.45, 10.82, and 15.92 GHz resonance frequency which covers S-, C-, X-, and Ku-band. FR-4 low-cost material has been used as a substrate to make the MAA unit cell which physical dimension is 8 × 8 mm 2 . The unit cell shows SNG performance along with five resonances of S 11 at 3.80, 5.65, 8.45, 86, 10.82, and 15.92 GHz. In addition, the Q-factors are calculated to determine the selectivity within the desired bands at each resonance frequency. The ADS software has been used to verify with the simulated S 11 result obtained from the CST-2019 software of the equivalent circuit. The measured result of the fabricated prototype has been a good match with the simulated result. The output of different array configuration of 1 × 2, 2 × 2, and 4 × 4 was taken by the simulation and 24 × 20 array arrangement has been measured and presented. The value of EMR is 9.87, and quality factor (Q factor) of the designed MAA unit cell is 30.41. Since the unit cell provides high EMR, excellent quality factor maximum absorption and the cell size is compact so the recommended MAA can be effectively used as a multiband microwave application.