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论文范文
1. Introduction Metamaterials are based on periodic or nonperiodic artificial engineered structures that exhibit unusual characteristic behaviors, like negative permeability and negative permittivity , simultaneously over a common resonance frequency band [1, 2]. From the first realization of the metamaterials by periodic wire and split-ring resonator (SRR) structures [3, 4], there are various modified shapes and geometries such as square, triangle, -, Ω-, L-, S-, and P-shaped resonators [5–11] utilized in the designs in order to meet certain specifications in the applications. These geometries can be set in one-, two- or three-dimensional arrangements to control the electromagnetic wave propagation behavior through the medium. The wideband metamaterials in X-band have been designed in [10, 11]. S-shaped SRRs were printed on opposite side of the host material and transmitted power was provided to show the DNG characteristics of the designed configuration in [10]. The double P-shaped resonators in [11] were printed on single side unlike that in [10]. The -parameters were presented; however there were no possible metamaterial applications found. In this paper, a novel wideband metamaterial comprising a modified S-shaped SRR is proposed for X-band frequencies. The entire periodic structure is placed on one side of the substrate, which simplifies the manufacturing process and can be considered as an advantage of the designed structure. The retrieved constitutive parameters and the -parameters obtained via simulations and experiments demonstrate that the proposed structure exhibits a wideband left-handed characteristic in X-band frequencies. An absorption application is selected and also studied for demonstration of the performance of the metamaterial. The structure can be used as an absorber with absorption rate of over 80% for the polarization angles between 0° and 40°. 2. Design and Simulation Conditions A unit cell of the proposed structure is composed of a modified S-shaped SRR printed on the front side of a square substrate of a side length of 16 mm and thickness of 1.6 mm as shown in Figure 1(a). The two ends of the S-shaped SRR are elongated in such a way that they take the form of C-shaped SRRs. In the design, FR-4 Epoxy of permittivity and loss tangent is chosen as the substrate, and the copper of electrical conductivity S/m with coating thickness of 0.017 mm is chosen for printing the SRRs. The simulations are carried out by using the commercial program CST Microwave Studio 2016. Figure 1(b) shows the parameters that represent the electric topology of a modified S-SRR unit cell with the inductors and capacitors. Explicit expressions for and can be found in [12–14]. A unit cell of the proposed structure together with the ports applied in the simulations is shown in Figure 2. In the simulations, the unit cell is placed between two waveguide ports, which are perpendicular to the direction of the wave propagation, which is along negative -direction. To imitate the infinite structure, perfect electric boundary (PEB) conditions are set at the boundary surfaces perpendicular to the E-field while perfect magnetic boundary (PMB) conditions are at the boundary surfaces perpendicular to the H-field (see Figure 2). The frequency band is chosen as the X-band region (8–12 GHz). 
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