The Research On Broadband Microwave Absorption Properties Of Frequency Selective Surfaces-Magnetic Dielectrics Multilayer

Broadband absorption is one of the crucial properties for the microwave absorber. Compared with the dielectric loss and ohmic loss materials, magnetic dielectrics absorbers dominate in the application of the microwave absorption, due to their magnetic permeability which is more than 1 and prior impedance matching. Thus, increasing the value of magnetic permeability is an effective way to broaden the absorption bandwidth. However, limited by the Sneok's theory, it's difficult to increase the permeability of the magnetic dielectrics materials greatly within the gigahertz range. Therefore, the research on incorporating frequency selective surfaces(FSS) into magnetic dielectrics materials was investigated in order to improve the absorption bandwidth in this paper.Firstly, three-layer magnetic dielectrics composite absorbers including a multi-resonances splits spiral FSS layer have been designed and fabricated. These absorbers with different thicknesses(total thickness 1.3 mm, 1.6 mm and 1.9 mm individually) were compared. The corresponding absorption bandwidth was increased with 159.5%?69.3% and 129.4%. Comparing to the magnetic dielectrics composite without FSS, the enhancing magnetic field and the decreasing electric field of the designed composite absorber demonstrate that the embedded frequency selective surfaces improved the equivalent permeability and decreased the permittivity, contributing to the impedance matching and bringing the more excellent reflection of the composite absorber. In the composite absorber, the absorption peak in low frequency was generated because of the FSS resonance, and the one in high frequency was due to the upper magnetic layer consuming the electromagnetic wave which dispersed by the FSS. The absorption band should be obviously expanded with the reasonable distance between the two peaks.In addition, the magnetic dielectrics materials were patterned to form magnetic sheet(magnetic FSS), which not only decreased the weight and but broadened the absorption band. Meanwhile, using optimization algorithm could obtain a more feasible size of FSS pattern improving the absorption bandwidth. Two groups of composite absorber with the magnetic FSS in the middle layer were proposed in the paper and its total thickness is merely 1.8 mm. The absorption bandwidth of them were win the range of 5.92-18 GHz and 5.36-18 GHz respectively. According to the scattering feature of magnetic FSS in high frequency, we can get the equivalent electromagnetic parameters. And with the parameters applied to the transmission line model(TL model) of multi-layer absorber structures, the reflectivity results calculated from TL model matched well with the simulation and experiment results. It is found that the ?/4 resonance is the primary reason in low frequency, while, the electromagnetic wave(EMW) scattering has significant effect on the energy loss of electromagnetics in the high frequency bands.