Results and Discussions The metamaterial absorber has a reflectivity less than -10 dB in the frequency range of 11.0-18.0 GHz, a thickness of 1.6 mm, and a much higher figure of merit (FOM) than most reported absorbers. Three different FSSs are designed for comparison, and the dipole square ring cross element structure possesses a larger bandwidth and a stronger absorption effect. Surface current and electromagnetic power loss density are analyzed (Fig. 10). Because the current cannot flow due to the gaps, positive and negative electrons gather on the two sides of the gaps respectively to form dipoles which can generate a strong electric field. In the electric field enhancement region, the energy loss of the incident electromagnetic wave increases significantly. Therefore, the dipole square ring cross element structure is used to design an absorber with an increased bandwidth and a reduced thickness. The effects of the thickness of the dielectric substrate and the surface resistance of the absorber on the reflectivity are investigated (Fig. 5). After the electromagnetic wave enters the alumina coating, its consumption becomes more difficult as the thickness decreases, so the absorption of the metamaterial absorber becomes less effective. The simulation results show that the optimal surface resistance results in the largest absorption coefficient and the widest absorption band of the metamaterial absorber.

With the development of information technology, electronic devices are widely used while leading to many electromagnetic interference problems. In addition, useless electromagnetic waves may pose a potential threat to human health. Therefore, electromagnetic absorbing materials have been developed to eliminate electromagnetic interference and provide information security. Conventional coated absorbing materials usually have a narrow and fixed absorbing band and are susceptible to external environmental influences. Compared with traditional absorbers, metamaterial absorbers have a larger absorbing bandwidth, a stronger absorbing capacity, and a lower thickness. To solve the problem that common absorbing materials have a high thickness and a narrow absorption band, a new dipole square ring crossed element structure with a large bandwidth and a low thickness is designed in this paper. This new structure shows good stability and high-frequency characteristics.

In this paper, the relationship between the electromagnetic parameters and the reflectivity of the absorber is calculated by the finite-difference time-domain method through simulation with CST software. The equivalent electromagnetic parameters of the absorber are obtained by inversion according to the equivalent medium theory. The cell size and circuit parameters of the dipole square ring crossed element structure are optimized by the equivalent circuit model of the absorber. The influence of two main parameters on reflectivity is studied. The surface current, electric energy density, and magnetic energy density of a unit cell at the operating frequency are simulated to analyze the working mechanism. A high-impedance surface comprising a lossy frequency selective surface (FSS) is employed to design a broadband microwave metamaterial absorber. The dipole square ring cross element structure is designed. Conductive paste and alumina ceramic are selected as the FSS raw material and the dielectric layer, respectively. Firstly, the alumina ceramic is used to make the dielectric substrate so that the limit thickness can be reduced. Secondly, the conductive paste is applied to the dielectric layer by the screen printing method, and the surface square resistance of the conductive paste is 60 Ω/sp. Finally, the reflection coefficient of the sample is measured by the free-space method in a microwave darkroom with a double-ridged horn antenna and a network analyzer.

In this paper, a thin wideband metamaterial absorber is designed and fabricated with a dipole square ring cross element structure. The reflectivities of three metamaterial absorbers based on cross element structures are solved using CST software through time-domain finite integration, and the effect of the metasurface structure on the reflectivity is investigated. According to the analysis of the surface current distribution and electromagnetic loss density at the resonance frequency, the absorbing loss mechanism is made clear, namely that the existence of the gaps makes the current fail to flow, and positive and negative electrons gather on the two sides of the gaps respectively to form dipoles able to generate a strong electric field. The energy of the incident electromagnetic wave is rapidly lost in the electric field enhancement region. The equivalent dielectric constant, equivalent permeability, and equivalent impedance of the absorber are obtained by inversion in light of the equivalent medium theory, and it is found that the loss mechanism of the metamaterial absorber is the excitation of magnetic resonance. The simulation and experimental results are in good agreement. The experimental results have a small deviation from the simulation results because the simulation model is an infinite one and has ideal boundary conditions. The metamaterial absorber has a reflectivity less than -10 dB in the 11.0-18.0 GHz band with a thickness of 1.6 mm, and the microwave absorption peaks at 12.7 GHz. The simplicity of the raw materials produced in this study and the feasibility of the metamaterial absorber fabrication make the large-scale application of the designed absorber possible.