Simulated and experimental studies of a multi-band symmetric metamaterial absorber with polarization independence for radar applications

Hema O.Ali Asaad M.Al-Hindawi Yadgar I.Abdulkarim Ekasit Nugoolcharoenlap Tossapol TippoFatih ¨Ozkan Alkurt Olcay Alt?ntas? and Muharrem Karaaslan

1Department of Communication Engineering,Sulaimani Polytechnic University,Sulaimani 46001,Iraq

2Medical Physics Department,College of Medicals and Applied Science,Charmo University,Chamchamal 46023,Sulaimania,Iraq

3Department of Telecommunication Engineering,Faculty of Engineering,Rajamangala University of Technology Rattanakosin,Phutthamonthon,Nakornprathom 73170,Thailand

4Department of Electrical and Electronics Engineering,Iskenderun Technical University,Hatay 31100,Turkey

Keywords: multi-band,metamaterials,absorber,polarization-independent,negative parameters

1. Introduction

The absorption of electromagnetic waves in the microwave region is a crucial issue in applications such as stealth,cloaking, energy harvesting and so on. Materials which have the property of near-perfect signal absorption are required for these fields of application. Metamaterial(MTM)based structures,which have strong electrical and magnetic coupling features,can meet this need for perfect absorption at certain frequency ranges or points. MTMs with a periodic design and unusual electromagnetic properties have been used in many research fields such as sensors,[1–4]energy harvesters,[5–7]antennas,[8–10]cloaking,[11–13]perfect lenses,[14–16]and so on.They have been investigated by researchers for the last two decades and have many advantages such as ease of configuration,ease of production and low cost.

MTM absorbers (MTMAs) have been explored in a wide range of frequency bands in microwave and terahertz regimes.[17–26]Alkurtet al.proposed a microwave power imaging method with a MTMA at a frequency of 2.51 GHz.[17]Schottky diodes have been used to harvest energy obtained by the absorber. Bilalet al.suggested a wideband perfect MTMA in the 20 GHz–30 GHz frequency range.[18]The absorber structure was specially designed to be E-shaped and can be useful in 5G communications applications. Abdulkarimet al.investigated a novel multi-band MTMA with polarization insensitivity and wide incident angle stability at frequencies of 2.75 GHz, 4.3 GHz, and 9.5 GHz.[19]Hanet al.achieved a novel miniaturized tri-band MTMA in the terahertz frequency regime with angular and polarization stability. The structure provided perfect signal absorption at 0.33 THz, 0.62 THz, and 0.82 THz.[20]Mahmudet al.numerically realized a sandwich-type MTMA with a star-shaped resonator. The proposed structure offered 96.77%average absorption for the optical spectrum with wide-angle polarization insensitivity.[21]Zamzamet al.examined four models of an MTMA in the terahertz regime. These researchers achieved perfect signal absorption at four different frequency points having a wide angle to the incident wave.[22]Yadgaret al.proposed an ultra-thin dual-band perfect MTMA in the terahertz range using a new design of ZnSe substrate.[23]Ghoshet al.suggested an ultra-wideband and ultra-thin MTMA with circular split rings for the X band regime.[24]A perfect MTMA for a different polarization-independent based design was reported in Refs.[25–27].Researchers have also been interested in developing a novel design of a triple-band polarizationindependent MTMA for C and X band applications.[28,29]Yueyi and colleagues proposed independent phase modulation for quadruplex polarization channels enabled by chiralityassisted geometric-phase metasurfaces.[30]Other related work on a polarization-engineered no interleaved metasurface for integer and fractional orbital angular momentum multiplexing was carried out by Kuang and colleagues.[31]Ali and Al-Hindawi proposed a design for a broadband thin MTMA that covered the frequency range of Ku and K bands.The proposed structure provided a wide bandwidth with a compatible overall size. The designed absorber consisted of a combination of an octagonal disk and split octagon resonator.[32]

In this paper,a multi-band MTMA is proposed for C and X band applications. The proposed structure can cope with a wide range of incident angles and is polarization-insensitive due to its symmetrical design features. It is numerically designed based on three models. The electric field and surface current distribution of the proposed structure were investigated at the absorption peak frequencies. The MTMA was manufactured from 7×5 unit cells using a printed circuit board prototyping machine. The experimental studies were realized using an Agilent PNA-L vector network analyzer (VNA) with a proper horn antenna covering the C and X band frequency regime. Perfect absorption peaks greater than 99% were observed at four frequency points (5.6 GHz,7.6 GHz,10.98 GHz,and 11.29 GHz). The proposed structure is a good candidate for implementation in many areas such as the military, communications and medicine. Generally, such absorbers are used in medical applications including medical imaging,reducing electromagnetic hazards to humans and improving the performance of medical sensors. We believe that our absorber is suitable for some of these applications. In particular, frequency scaling means that the proposed structure can be used in any desired frequency range.

2. Theory of the designed structure and numerical results

In this current work, a new multi-band MTMA is designed and theoretically discussed with regard to use in satellite communications and radar applications.A Computer Simulation Technology (CST) based finite integration technique was used for the design process and to obtain numerical results.The proposed MTMA consists of three basic layers from top to bottom,as follows: the top layer is a copper resonator,the middle layer is a substrate and the bottom layer is a ground plane(see Fig.1).

The resonator is optimized to obtain high-absorption peaks and consists of a circular shape inside a quadratic and two L shapes; the resonator and ground plane are made of copper and have a thickness of 0.035 mm and conductivityσof 5.8×107S/m. The dielectric layer (the used substrate)is made from FR4 material with a relative permittivity of 4.4 and loss tangent(tanδ)of 0.02. The thickness of the substrate was chosen to be 1.6 mm. FR4 substrate is used in our design due its advantages of availability in the market,low price,high mechanical strength and low losses.

The required dimensions, structure and the position of the resonator are determined so as to work in the desired frequency range between 4 GHz and 12 GHz. The designed multi-band MTMA is optimized using a genetic algorithm which is built into a function in the software to control the impedance matching for all operating frequencies. The required dimensions of the designed structure are shown in Table 1.

Table 1. The dimensions required for the multi-band MTMA.

Fig.1. The designed multi-band MTMA:(a)dimensions and front view,(b)back view and(c)perspective view.

In this work,to obtain perfect absorption the reflection of the designed structure should be minimized and the absorber impedance matched with the free space impedanceZ(ω)=Z0.For all MTMAs,the effective permeability(μ)and the permittivity(ε)of the medium can be expressed by the equation

where the reflection coefficient and transmission coefficient are denoted byR(ω)=|S11|2andT(ω)=|S21|2,respectively.The back of the proposed structure is covered by a copper metal plate and its thickness is much greater than the skin depth,therefore there is no transmission through the structure(T(ω)=0)and Eq.(3)is reduced to the following form:

Figure 2(a) shows the simulated results of the reflection and absorption for the proposed structure in the frequency range 4 GHz–12 GHz. There are several boundary conditions within the CST software such as periodic, perfect electric conduction, perfect magnetic conduction, open add space and unit cell. In this work,XandYaxes were set to be in the unit cell direction while theZ-axis set to be open add space for compatibility with the experimental setup. The reflection coefficient of the designed structure was obtained and is illustrated by the red curve in Fig.2(a). The reflection coefficient has four peaks, with the maximum peak reaching-30 dB and the minimum peak being approximately-13 dB. By using Eq. (4), the absorptivity of the proposed structure can be determined, as shown by the blue curve. The absorber has four high-absorption peaks at 5.6 GHz,7.6 GHz,10.98 GHz,and 11.29 GHz,respectively corresponding to absorptivities of 100%, 100%, 99%, and 99%. Each of the center frequencies has a band according to the-10 dB reflection criterion.Therefore,the absorber has three operating frequency bands including 5.5 GHz–5.7 GHz, 7.5 GHz–7.7 GHz, and 10.8 GHz–11.44 GHz with absorption bandwidths of 0.2 GHz,0.2 GHz,and 0.64 GHz, respectively. It is crucial to mention that the bandwidths of the two last absorption peaks, 10.98 GHz and 11.29 GHz, are combined to achieve an enhanced bandwidth of 0.64 GHz. The proposed multi-band MTMA could be used in military applications, satellite communications and radar technology. Figure 2(b) describes the magnetic field distribution simulated by the CST program at four different peaks.At a resonance frequency of 5.6 GHz (peak 1) the magnetic field distribution is located only on the L-shaped part of the resonator, but when the frequency is increased to 7.6 GHz(peak 2) the magnetic field is strongly located at the squareshaped resonator. At a third peak (10.98 GHz resonance frequency), the magnetic field is located at both L- and squareshaped resonators, while at 11.29 GHz the magnetic field is weakly concentrated on the square resonator(peak 4).

Fig.2. (a)Simulated absorption and reflection of the multi-band MTMA.(b)Magnetic field distribution at four different peaks.

In this work,the proposed multi-band MTMA consists of the combination of two basic designs,namely design 1 and design 2,as shown in Figs.3(a)and 3(b),respectively. In design 1, two symmetric L-shaped resonators were used in different orientations. The L-shaped resonators produce a resonance frequency at 5.6 GHz. However, the vertical sides also generate further modes which resonate around 11.3 GHz with a low absorption value. Design 1 consists of the two L-shaped resonators deposited on a FR4 dielectric layer with a thickness of 1.6 mm. The backside of design 1 is covered by copper metal with a thickness of 0.035 mm and a conductivityσof 5.8×107S/m. The FR4 substrate has a relative permittivity of 4.4 and a loss tangent (tanδ) of 0.02. The parameters and the position of the resonators operate in the C and X frequency ranges. We used a genetic algorithm to optimize design 1. The dimensions of design 1 are shown in Table 1. As discussed before, the reflection values of the structure determined by the simulation software were used in Eq.(4)to get the absorption curve illustrated in Fig. 3(d). It can be seen from the red curve that there are two peaks at 5.63 GHz and 11.033 GHz corresponding to absorption of 99%and 84%,respectively. Moreover,design 2 consists of two resonators—a closed circular ring(CCR)resonator and a closed square ring(CRR)resonator. These two resonators are the basic types of microstrip resonator, and their resonance frequencies can be easily controlled through the resonator dimensions such as radius and line width. Therefore, each of these resonators can be operated at any desired frequency. Design 2 comprises a ring resonator inside the quadratic shape on the top of the FR4 substrate,while the bottom layer remains as in design 1. The absorption curve of design 2 is shown by the green curve in Fig. 3(d): note that the absorptivity of this design is greater than that of design 1 due to the resonator shape and coupling effect between the two resonators and the dielectric layer. The absorption is over 90%at 7.72 GHz and 11.55 GHz. Figure 3 shows a comparison of structure and absorption for these two designs with those of the proposed one. The blue curve in Fig. 3(d) has intensive multiple peaks at four different resonance frequencies which are compatible with the C and X frequency ranges. In our work,we tried to get the CRR to produce resonance at 7.6 GHz and the CRR to produce resonance at around 11.5 GHz. The design process includes different steps such as a parametric study and use of a genetic algorithm to understand the behavior of all resonators. Later, when designs 1 and 2 were combined, the two resonance frequencies around 11.3 GHz and 11.5 GHz combined,providing a wider bandwidth of 0.64 GHz due to a mutual coupling effect between the CCR and L-shaped resonators. Furthermore, there is a strong relation between the two resonance frequencies of 10.98 GHz and 11.29 GHz.It can be said that the vertical sides of the L-shaped resonator participated in producing the resonance frequency of 11.29 GHz as shown in Fig.5(d). Because of this coupling effect,absorption of the lower resonance frequency improved and the higher resonance shifted to the left compared with the individual reflection results for designs 1 and 2. Therefore, the positions and dimensions of both the L-shaped resonator and the closed ring resonator were carefully optimized to yield the improved bandwidth of 0.64 GHz(10.8 GHz–11.44 GHz).

Fig.3. Comparison of the three different designs: (a)design 1,(b)design 2,(c)proposed design. (d)Simulated absorption results.

In this work, the effect of changing the source polarization and incident angle on the designed MTMAs was monitored (see Fig. 4). It was assumed that the absorber was located in theXYplane. The incident angle is represented byθ(the angle in theZXorZYplane). Therefore, an absorber is said to be insensitive for changing incident angle when the absorption value is not changed by variation in the value ofθ.Polarization is represented byφ(the angle in theXYplane).Therefore,an absorber is said to be insensitive for polarization angle when the absorption value is not changed by variation in the value ofφ. To test the polarization sensitivity of the designed absorber, the direction of the propagating electromagnetic wave was fixed at normal incidence while the directions of electric and magnetic fields were changed at different polarization anglesφf(shuō)rom 0°to 60°in steps of 15°.

Fig. 4. Simulated absorption results at different (a) incident angles (θ) and(b)polarization angles(φ)for the multi-band metamaterials absorber.

As is clear from Fig. 4(a), at the lower frequency range the curves of the first and second peaks are almost the same but in the higher frequency range there is a shift in the two resonance frequencies of 10.98 GHz and 11.29 GHz while the absorption spectrum remains almost the same with changing polarization angle over the entire frequency range. Therefore,it is concluded that the proposed MTMAs are independent of the polarization of the electromagnetic waves.face current is found at the vertical sides of the square closed ring and corresponds to a resonance frequency of 7.6 GHz in Fig.5(b). The current distribution can be seen along the vertical sides of L-shaped resonators at reduced concentration;this is responsible for producing the third resonance frequency of 10.98 GHz. Moreover,the current distribution at the last resonance frequency of 11.29 GHz is gathered along the circular ring resonator and horizontal line of the L-shaped resonator with high strength.

Fig.5. Simulated surface current distribution of the proposed MTMA at the four absorption peak frequencies of(a)5.6 GHz,(b)7.6 GHz,(c)10.98 GHz and(d)11.29 GHz

In Fig. 4(b), we numerically investigate the absorption under an oblique incident angle because in some practical applications the electromagnetic waves are usually incident with an oblique angle. In this case,the angleθis changed from 0°to 60°in steps of 15°and the other parameters remain constant during the simulation process. As can be seen from the curve, when the polarization angle is changed the absorption of the proposed structure remains higher than 90%in the frequency range from 2 GHz–13 GHz. Only in the higher frequency range is a small shift in the resonance frequency observed. It can be concluded from these two curves that the proposed structure is independent of the polarization angle and has sensitivity properties.

Figure 5 shows the simulated surface current distribution of a single unit cell of a MTMA.It is observed that each resonator of the MTMA structure is responsible for producing each resonance frequency in the band of interest. The concentration of surface current is found on the right and left of the Lshaped resonator and corresponds to the resonance frequency of 5.6 GHz in Fig. 5(a). Similarly, a concentration of sur-

Fig. 6. Simulated electric field distribution at the four different resonance peaks.

Figure 6 shows the simulated electric field distributions on theXYplane of single-unit cells at resonance frequencies of 5.6 GHz, 7.6 GHz, 10.98 GHz, and 11.29 GHz under normal incidence(θ=0°). Similar to the current distribution,the strength of the electric field distribution can be seen on each resonator structure at the corresponding resonance frequency.The electric field distributions at 5.6 GHz and 10.98 GHz are due to the effect of the L-shaped resonator. The frequency of 7.6 GHz is strongly affected by the vertical sides of the square closed ring.Moreover,the final resonance frequency of 11.29 GHz is due to the closed circular ring resonator. Further explanations can be given related to the shape of the resonators with current and electric field distributions. Because of the asymmetry of the L-shaped resonators,when the structure rotates with different anglesφ, the positions (arrangement) of the L-shaped resonators change, causing a reduction of the current and electric field. Therefore, the L-shaped resonators are no longer responsible for producing resonance behavior at 5.6 GHz,especially in the worst case at 60°.

3. Absorption performance

For further verification, the electrical properties(permittivity, permeability, reflective index and impedance) of the proposed multi-band absorber are extracted in this section.The retrieval method is used for the extraction,which is based on the scattering parameters reflection coefficientS11and the transmission coefficientS21. To obtain these four parameters in simulation, the Nicolson–Ross–Weir method was used by taking the phase and magnitude of parametersS11andS12obtained in CST simulations. As can be seen from Figs.7(a)and 7(b), the permittivity and permeability parameters have very similar dispersion to each other at resonance frequencies and the real parts of them are negative. Note that the proposed design has negative characteristics. Figure 7(c) depicts the real and imaginary parts of the refractive index for the proposed design;it can be seen from the curve that the imaginary part of the refractive index is negative at resonance frequencies. Figure 7(d) shows the normalized impedance of the multi-band MTMA.The real part of the normalized impedance is close to one and the imaginary parts are close to zero at resonance frequencies,indicating that the impedance of the proposed multiband MTMA is matched with the free space impedance. The values of the real and imaginary parts of the designed absorber at resonance frequencies are shown in Table 2.

Table 2. Real and imaginary parts of the proposed MTMA at resonance peaks.

Fig. 7. Absorption performance: (a) permeability, (b) permittivity, (c) refractive index, and (d) impedance for proposed multi-band metamaterial absorber.

4. Fabrication of the proposed structure and comparison between simulated and experimental results

Two methods can be used for measuring the designed absorbers in the laboratory. In the first method a single cell of fabricated absorber is placed between two waveguide structures that are connected to either port of a VNA. In this method,the absorber should be of such as size that it fits into the aperture of the waveguide. Therefore, the dimensions of the absorber during the design process should be carefully optimized to obtain good results. In this work,we used the second method based on a horn antenna. The fabricated absorber can be put in front of a horn antenna that is connected to one of the VNA ports. In this method, the size of the fabricated absorber sheet should be taken into consideration. The larger manufactured structure leads to more accurate measurement results. Figure 8(a)shows the 7×5 unit cells of the designed absorber fabricated using a LPKF E-33 CNC circuit printer machine as shown in Fig. 8(b). The measurement setup for measuring the reflectivity of the fabricated absorber structure was prepared as shown in Fig.9. The manufactured absorber was placed on the sample in front of the broadband (3 GHz–18 GHz) horn antenna. The measurement was repeated for different distances between the absorber structure and the antenna to achieve more accurate results. In the optimal case,the distance was set to be 15 cm and the far-field condition was confirmed.

Fig.8. Manufactured multi-band MTMA:(a)7×5 unit cells and(b)fabricated design using a CNC based printed circuit board prototyping machine.

Fig.9. Experimental measurement setup of the proposed multi-band MTMA using a horn antenna connected to a VNA.

A comparison of the obtained measurements with the simulation results is presented in Fig.10. It can be seen from Fig.10(a)that the measured values of the reflection coefficient are well matched with the simulation results. The measured results show resonance frequencies at 5.98 GHz, 7.7 GHz,and 11.15 GHz with-16.4 dB,-42 dB,and-24 dB,respectively. The corresponding absorption response of the absorber is shown in Fig.10(b).As in the simulation results,almost perfect absorption can be achieved at all bands, whereas a small frequency shift is observed at the first band toward the higher frequency range.

Fig. 10. Comparison of simulated and measured results: (a) reflection coefficient, (b) absorption for the multi-band MTMA using the horn antenna method.

It is clear from Fig. 10 that the results measured using the horn antenna are in good agreement with the simulation results. The measured results are a bit different from the simulated ones due to the manufacturing process and calibration errors in the VNA connecting the coaxial cables. The results obtained in this work are compared with those of other similar published papers in Table 3. Comparisons are given for unit cell dimensions, frequency band, dielectric substrate, maximum absorption rate, number of bands, resonance frequency and bandwidth. It can be seen from Table 3 that the proposed structure is of a smaller size than in other previously reported works;it can be used for dual-band applications,has four resonance frequencies and has the largest absorption bandwidth.

Table 3. Comparison of the results of the current work with other reported works in literature.

5. Conclusion

In summary,a new design for a multi-band metamaterial absorber has been investigated both numerically and experimentally for C and X band applications. According to the obtained results,four high-absorption peaks(100%,100%,99%,and 99%)are observed at 5.6 GHz,7.6 GHz,10.98 GHz,and 11.29 GHz, respectively. Negative parameters such as permeability, permittivity, refractive index and impedance have been extracted for the desired frequency range of 4 GHz–12 GHz. Our design is polarization-independent when the angle changes from 0°to 60°. The proposed structure has been manufactured and tested with a horn antenna and the numerical and experimental results are in good agreement. The surface current and electric field distributions have been studied.The multi-band metamaterial absorber can be used in many applications such as radar applications.

Acknowledgments

The authors would like to thank the Department of Electrical and Electronics,Iskenderun Technical University,Hatay,Turkey,for fabricating the designed structure. Also,we would like to thank the Department of Electrical Engineering and the Department of Telecommunication Engineering, Faculty of Engineering, Rajamangala University of Technology Rattanakosin,Thailand,for fabricating and measuring the singledesigned structure.