Studies On Preparation And Microwave Absorbing Properties Of Koch Fractal ACF Felt Screen Composites

The fractal structure has been introduced into the design of multiband and strong absorption materials. Koch fractal patch and aperture frequency selective surface(FSS) have been designed, and the Koch fractal ACF felt screen composites combination of Koch fractal FSS as absorber, glass fiber fabric as the impedance-matching layer, epoxy resin as the matrix were successfully prepared. The effects of Koch fractal FSS types on the microwave absorption(MA) property have been studied systematically, and the relationship between refection loss(RL) and the ACF felt area ratio(AR) was explored. Meanwhile, the absorbing mechanism at different AR was discussed. The main research contents were summarized as follows1. The Koch fractal patch FSS have been designed, and the Koch fractal patch ACF felt screen composites were prepared. The effect of iteration number n, side length L and spacing d on the MA property have been investigated. Experimental results showed that(1) The iterations number n and initial side length L had great impacts on the absorbing properties. The RL increased first and then decreased with increasing n and L at low(6-9 GHz) frequency range, but increased with increasing n and L at high(16-18 GHz) frequency range. The RL gradually decreased with spacing d and the composites showed good frequency selectivity under two polarizations, and the resonance frequency is about 8.5GHz under vertical polarization and 8.2GHz under horizontal polarization.(2) The composites showed multiband absorption properties, and the frequency difference between adjacent peaks is between 0.7-0.9GHz, while the frequency ratio of the adjacent peaks is about 1.0, which are related with the self-similarity and iterative secondary structure2. The Koch fractal aperture FSS have been designed and the Koch fractal aperture ACF felt screen composites were prepared, meanwhile, the influences of n, L and d on the MA property have been studied.The results showed that(1) The RL decreased with the increasing n and L at low(6-9 GHz) frequency range, but increased at high(16-18 GHz) frequency range The resonance frequency shift to higher frequency range with increasing d, and the composites showed good frequency selectivity of 6-9 GHz.(2) Composites with Koch fractal aperture FSS embodied strong absorption characteristics under both vertical and horizontal polarization, and the absorption strength improved respect to composites containing Koch fractal patch FSS, especially under vertical polarization conditions, the RL of-62.8d B was obtained.3. Dual-band absorption was observed at low(6-9GHz) and high(16-18GHz) frequency range. For composites with Koch fractal patch ACF felt screen, The absorbing mechanism change from scattering effect to dielectric loss at low(6-9GHz) frequency range with increasing AR, however, the absorbing mechanism change from scattering effects to eddy current loss at high(16-18GHz) frequency range but for composites with Koch fractal aperture ACF felt screen, the absorbing mechanism may be dielectric loss at low(6-9GHz) frequency range, while the eddy current loss may be the main absorbing mechanism at high(16-18GHz) frequency range.Two types of Koch fractal structure FSS showed multiband and strong absorption properties, respectively. And the relationship between absorbing mechanism and ACF felt AR have been analyzed, and provided a theoretical basis for preparation of multiband and strong absorber in the future.