Research On The Transmission-line Based Three-dimensional Frequency Selective Surface

The three-dimensional frequency selective surface is composed of periodically arranged composite structural units.Due to its superior regulation characteristics for spatial waves of different frequencies,polarization states,and incident angles,it can be applied to the design of basic units of line-to-circular polarization converters and transmission array antennas,achieving precise control of electromagnetic wave amplitude and phase.This paper mainly studies the transmission line type three-dimensional frequency selective surface and provides a comprehensive design method for its control of electromagnetic wave amplitude and phase.This significantly improves the control performance,enriches the control means,and thus reduces the design difficulty of the threedimensional frequency selective surface.The specific research contents are as follows:(1)By analyzing and comparing the design methods of traditional planar two-dimensional frequency selective surfaces and new three-dimensional frequency selective surfaces,and combining with the comprehensive design theory of filters,this paper studies the comprehensive design method of transmission line type three-dimensional frequency selective surfaces.By establishing the mapping relationship between the physical structure model,equivalent circuit,and filter function,this paper provides a comprehensive design process for the transmission line type three-dimensional frequency selective surface from design indicators to actual physical size determination.In addition,by using commercial software to calculate the derived theoretical formulas,the calculation efficiency is greatly improved.Finally,through the implementation of three-dimensional frequency selective surface cases with different design indicators,the feasibility and applicability of this comprehensive design method are demonstrated.(2)Based on the design concept of polarization separation,this paper has developed two line-tocircular polarization converters.Firstly,to address the issue that traditional line-to-circular polarization converters cannot independently control two branches,the three-dimensional structure is used to separate polarization,achieving individual control of two orthogonal polarizations.Then,the comprehensive design problem of the line-to-circular polarization converter is analyzed,enriching the theoretical basis and forming a complete design process.This type of line-to-circular polarization converter has the advantages of low loss,good angle stability,and low profile.Finally,a polarization converter with a 3 d B axial ratio and a frequency range of 3.1 GHz to 6.4 GHz(fractional bandwidth reaching 69%)was designed and fabricated,and the test and simulation results matched well.To further expand the bandwidth,a second line-to-circular polarization converter was designed: 3 d B axial ratio frequency range from 2.65 GHz to 6.4 GHz(fractional bandwidth reaching 82%).(3)Based on the comprehensive design method of the three-dimensional frequency selective surface,this paper has developed a transmission array antenna.The transmission unit can introduce a stable phase difference with a relative bandwidth of 58% in the range of 3.6～6.6 GHz,so the transmission array antenna formed by it has a peak gain of 19.4 d B near 6 GHz.This transmission array antenna mainly includes two types of array units based on the coupled line type six-order filter topology and the uniform stacked slot line.The coupled line type six-order filter topology unit is designed by the comprehensive design method prototype,achieving the unity of theory and practice.The unit based on the uniform stacked slot line targets the phase control characteristics of the frequency selective surface.Through theoretical analysis,the traditional ideal phase difference condition is transformed into focusing on the center frequency and phase slope,effectively standardizing the design process of the control unit phase,making it compatible with the coupled line type six-order filter topology unit,forming a stable phase difference.This forms a systematic comprehensive theory for the design of transmission array antennas.