In recent years, electromagnetic (EM) wave absorbing devices based on metamaterials have attracted widespread attention from researchers around the world, due to their advantages such as broadband, easy preparation and flexibility to tailor EM waves. Nevertheless, a review of the existing research reveals that the inherent sub-wavelength characteristics of metamaterials and metasurfaces impose certain constraints on their use in low-frequency ranges. In order to achieve low detectability that takes into account both low-frequency and broadband absorbing performance, this paper presents a metamaterial absorber based on 5-layer gradient resistance film and dielectric composite structure, as shown in FIG.1 (a). To begin, we introduce the structural design of the initial element, and based on this, the transmission line theory and impedance matching principle are applied to analyze the strong wave absorption conditions of the absorber element. In terms of the element structure optimization, the genetic algorithm is adopted to globally search for the optimal solution in the multi-variable domain, resulting in the rapid determination of metamaterial elements’ configurations and resistance parameters that meet the design goals. In the simulation, the wave absorption performance and mechanism of the designed absorbing element are also conducted in an in-depth manner. Simulation results show that the designed metamaterial absorber can achieve more than 90% EM wave absorption in the frequency range of 1.62- 19.16 GHz (with a relative bandwidth of 168.8%) under normal incidence of linearly polarized plane waves, which effectively expanding the absorption bandwidth to the L and K bands. In addition, the simulations for oblique incidence at different polarization provide strong evidence for the device's insensitivity to both polarization and angle. The radar cross section (RCS) curves obtained by the time domain (TD) simulation illustrate that, the novel structure can achieve more than 10 dB RCS reduction over the frequency range of 1.7- 20 GHz. During the device's performance verification process, a metamaterial absorber with 20 × 20 elements and dimensions of 1.566<i>λ<sub>l</sub></i>×1.566<i>λ<sub>l</sub></i> ×0.113<i>λ<sub>l</sub></i> is fabricated and tested using the bow method reflectivity test system. The absorptivity curves under 5° oblique incidence of different polarizations, as depicted in FIG.1 (b), show that, the proposed metamaterial absorber can realize more than 80% EM absorption over the entire frequency range from 2 to 18 GHz, the test results of different polarizations are basically consistent. The test results at oblique incidence (θ ≥ 30°) show that, despite the measured and simulated curves exhibit discrepancies in certain frequency bands due to human error or material dispersion characteristics, the overall outcomes of the experiment are consistent with our expectations, which fully proves that the designed metamaterial absorber has potential application value in the field of low-frequency and broadband EM absorption.
