A Multiple-Bands Metamaterial Absorber Based in X, Ku and K-Band

Developing a highly efficient and multiple-bands metamaterial absorber is a hot issue in recent era. In this paper, A multiple-bands metamaterial absorber has been presented which is based in X, Ku and K-band. Firstly, we have designed two single layer basic unit cell of X-shape and cross-shape, then they are arranged in the multi-layers structure form for the purpose of obtaining multiple- bands and wide band absorption. The proposed absorber is able to work in multiple bands because it has six peaks in the frequency range of 8-24 GHz with having near perfect absorption. Moreover, the sixth peak has a wideband absorption which is 2.93 GHz. Furthermore, the proposed absorber is also tested for polarization insensitivity and also for oblique incidence. Absorption was found polarization insensitive with almost perfect absorption.


Introduction
Metamaterials are new forms of compounds that show incomparable electromagnetic properties. It is one of the main features which has been observed in recent times that it can perform as a perfect absorber for the incident EM wave. Metamaterials have numerous applications one of them is that it can be utilized in the stealth technology in the direction of remote sensing for a wide range of spectrums. Moreover, it has enormous characteristics like simple design, easy fabrication method, tunability, and broadband absorption. Probably, the most acknowledged description of the metamaterials is that, a material which is composed of both metal and dielectric material and shows at the same time negative Dielectric constant (Ԑ ) and Magnetic Permeability ( ) [1]. When the negative Dielectric Constant and Magnetic Permeability have been attained then the other properties in the material will also change such as negative refractive index, phase velocity, and backward wave propagation. Due to their exceptional properties, they can be utilized in numerous fields such as in cloaking [2], filters [3], lens with perfect focusing [4], and electromagnetic wave absorption [5]. In the field of electromagnetic absorber, they can also be utilized as Radar Cross Section (RCS) lessening [6], reducing sidelobe radiation in the antennas and minimization of electromagnetic interference [7]. Victor G. Veselago firstly started to study the metamaterial on the property of a negative index of refraction in 1968 [8]. Sir John B. Pendry et al. had done the first experimental verification on the property of a negative index of refraction in 1999 [9]. Metamaterials have been utilized for the first time in 2008 as an Absorber which has perfect absorption by Landy et al [10]. From that point forward, various kinds of Single-Band [11][12][13][14][15] and Multiple-Band [16][17][18][19][20] MA contemplates were designed. Moreover, polarization-independent [21][22][23][24][25] and Tunable Perfect Metamaterial Absorbers (PMA) [26][27][28][29][30] were structured in the Gigahertz, Megahertz, and also in Terahertz. Mehmat et al. [31] have proposed an Absorber which consists of a split ring resonator (SRR) and it has the perfect absorption between 4 and 16 GHz. As it has high absorption so it tends to be employed in wireless power transfer applications. In [32], they have proposed an ultra-thin triple-band metamaterial resonant structure. The unit cell has an absorption spectrum of 3-12 GHz having three absorption peaks at 3.44 GHz, 6.36 GHz, and 11.51 GHz with more than 90% absorption. The proposed unit cell has additionally demonstrated for polarization insensitivity and for oblique incidence for both Transverse Electric and Transverse Magnetic modes of propagation. The proposed unit cell is appropriate for potential microwave frequency applications. In [33], they have proposed a broadband absorber in which eight square patches were loaded on the double ring structure. They have used two layers of resistance film for the purpose of achieving better absorption. They have achieved the absorption of more than 90% at 0.95 GHz and at 18.4 GHz. It has also the characteristic of polarization insensitivity. In [34], they have used the finite integration technique (FIT) to design the metamaterial absorber based on a multi-layer resonance structure. The proposed unit cell works in the frequency regime of 5 to 8.9 GHz. Moreover, the four-layer unit cell allows absorption peaks with an intensity of over 90%. It also works for oblique incidence and polarization insensitivity. The proposed unit cell has applications in the design of radar shield or military devices.
In this article, we have proposed a multi-layers and multiple-bands metamaterial absorber which works efficiently in X, Ku, and K-band. Firstly, we have proposed the two single-unit cells of X-shaped and Cross-shaped structures. Xshaped structure has five absorption peaks in the frequency regime of 12-24 GHz and a Cross-shaped structure has three absorption peaks in the frequency regime of 10-24 GHz. Then we have put these designs in multi-layers form for the purpose of obtaining multiple frequency bands and wide bandwidth. We have utilized two cross-shaped structures and one X-shaped structure in the multi-layers form. The best Multi-layers structure has six absorption peaks with above 80% absorption at the frequency of 9 GHz, 9.7 GHz, 11.2 GHz, 13.2 GHz, 15.3 GHz, and at 20.7 GHz. Moreover, the sixth peak has wideband absorption of 2.93 GHz. Furthermore, the proposed design is also tested for the Polarization insensitivity and for oblique incidence. The proposed design was found polarization-insensitive with perfect absorption. This paper is divided into four sections, after the introduction the second section includes design details of the proposed absorber. In the third section, results are discussed. In the end conclusion of the proposed design has been discussed.

Design and Theory
We have designed two different unit cells structure for the purpose of obtaining multi-bands absorption. The first design is X-shaped structure and the second design is cross-shaped structure. The copper has been utilized for top and bottom layer whose thickness is chosen of 0.035 mm. Furthermore, fr-4 substrate has been used as a dielectric material whose relative permittivity is chosen to be 4.4, thickness of 1.6mm and loss tangent is 0.02. As can be seen from the Fig.1 that the first basic unit cell is modeled using X-shaped structure, and the second basic unit cell is made using Cross-shaped structure. The dimension of the both proposed unit cell are as follows P=24 mm, W=1.5 mm, L=18 mm, L1=5 mm, L2=6 mm. We have used these basic unit cells in the multi-layers form. We have designed three multi-layers structures. It has to be remembered that the di-electric layer of same thickness which is applied on the basic unit cells is also applied in multi-layers design between each periodic structure design layer and bottom layer is grounded with copper metal which can be seen in the Fig.2.
In the first design, we have kept an X-shaped structure on top and the Cross-shaped structure in the second and third layer. In the second design, we have kept a X-shaped structure in between the two Cross-shaped structures. In the third design, we have kept a X-shaped structure in the third layer and both Cross-shaped structures in the first and second layers which can be seen from the

Simulated Results sand Discussions
For the purpose of designing a well-organized and consistent absorber, it is very essential that the reflection and transmission must be minimum. The absorption can be calculated from equation (1)  As the unit cell is grounded with copper metal so the transmission will be zero and the modified form of Equation (1) will be as ( ) = 1 − | 11 ( )| 2 (2) Fig.4 represents the simulated absorption of the X-shaped structure and Cross-shaped structure. According to the Fig.4. there are five absorption peaks for the X-shaped structure in the frequency from 12 GHz to 23 GHz. There are three absorption peaks for the Cross-shaped structure from the frequency 10 GHz to 24 GHz. This is obvious from the Fig.4 that the cross-shaped structure has absorption peaks at 10.8GHz, 15.7GHz, and 23.7GHz with the absorption of approximately 90%. This design works in three frequency bands of X, Ku and K-band. The X-shaped structure has five absorption peaks at 13GHz, 16.7GHz, 18.2GHz, 20.4GHz and at 22.5GHz with an absorption of 90% except one absorption peak which is at 20.4GHz has an absorption of 80%. This design mainly works in two different frequency bands of Ku and K-band.
(a) We have combined these both designs in the three-layers form in three patterns to obtain the wideband absorption and multiple bands. As it is obvious from the Fig.3 that the Cross-shaped structure works in three different frequency bands of X, Ku and K-band. X-shaped structure works in two frequency bands of Ku and K-band so for the purpose of obtaining multiple frequency bands we have used Cross-shaped structure in two layers. The absorption results for all three designs are shown in Fig.5. The Fig.6 shows the best absorbance result of the multi-layers structure and according to the Fig.5 the 1st pattern has the best absorption so that is why we have chosen it as our final design because it has six absorption peaks at 9 GHz, 9.7 GHz, 11.2 GHz, 13.2 GHz, 15.3 GHz and at 20.7 GHz in the X, Ku and K-band. Moreover, the sixth peak has 2.93 GHz bandwidth from the frequency 20.74 GHz to 23.67 GHz and with the absorption of above 80%. These are the best results so far research has been done in Giga Hertz frequency regime. Cheng et al. [36] have achieved seven peaks but they are not wideband similarly Xu et al. [37] and Ranjan et al. [38] have achieved six peaks but they are also not wideband as compared to our results.   The effect of polarization insensitiveness has been examined in the Fig.7 for the different values of polarization angles (Phi=0-60). As the proposed absorber has a symmetrical shape that is why it has been observed that it shows stable absorption towards the changing polarization. The effect of Oblique Incidence has been examined in the Fig.8 for the different angles in the range between Theta=0-60. The resonant frequency and the absorption have been reduced by changing the oblique incidence, although the proposed structure is not extremely influenced by the changing oblique incidence.

Conclusion
In this article, we have presented a multi-layer absorber structure which works in the X, Ku, and K-band. We have shown that the proposed absorber is able to work in multiple bands and it has six peaks in the frequency range of 8-24 GHz with the absorption above 80%. Moreover, the sixth peak has a wideband absorption which is 2.93 GHz. These are the so far best result achieved in the Giga Hertz frequency regime. Furthermore, the proposed absorber is also tested for polarization insensitivity and also for oblique incidence, though it does not strictly work for oblique incidence. As the proposed absorber works in X, Ku and K-band so it has numerous applications like in satellite communication, terrestrial microwave communication and radar communication.