Research On Three-dimensional Frequency Selection Structure Based On Square Coaxial

Frequency selective surface(FSS)is an infinite structure composed of two-dimensional(2-D)periodic array of unit cells,which can realize selective propagation.FSSs have been widely used in many aspects of electromagnetic wave technology,such as electromagnetic compatibility,microwave communications,military stealth,and electronic countermeasures,which indicates great research and development prospects.In recent years,compared to traditional two-dimensional frequency selective surfaces(2D-FSSs),three-dimensional frequency selective structures(3-D FSSs)have been developed because it has one more dimension for designing more complex filters.In this paper,the basic characteristics of the square coaxial line are first studied,and the closed formula of the characteristic impedance and the open-end capacitance is derived by synthetic asymptote method.Then a 3-D FSS with a square coaxial line as unit cell is optimized and designed.Based on this,a coaxial nested 3-D FSS is designed and implemented.The main contents and contributions of the paper include:(1)The closed formulas of the characteristic impedance and the open-end capacitance of the square coaxial line are derived by synthetic asymptote.For the characteristic impedance,the far-asymptote and near-asymptote equations are obtained by analyzing the two extremes of the square coaxial line,and the resultant asymptotic formula is obtained.For the open-end capacitance,the quasi-static field distribution of the open-end port is analyzed,and the corresponding closed formula is derived by the Green's function and the existing synthetic asymptote formula.(2)The original square coaxial type was optimized by hollowing out the center conductor and reducing the thickness of the metal layer,effectively reducing the weight of the overall structure and improving the performance of the structure.On this basis,one more transmission path is added by nesting a metal tube,thereby introducing more transmission poles and zeroes.After proper design,a coaxial nested 3-D FSS with dual polarization,third-order bandpass and high out-of-band rejection is proposed in this paper.Moreover,By analyzing the transmission zeros/poles of the coaxial nested 3-D FSS proposed in this paper,an effective equivalent circuit model is established.Then,guidelines is provided for structural design and optimization based on parameter scanning.Finally,a prototype of the coaxial nested 3-D FSS is fabricated and measured.The measured results are in good agreement with the simulation ones,indicating the feasibility of the structure.In conclusion,two closed formulas of the two parameters of square coaxial line are derived in this paper,which will avoid complicated numerical calculations while parameter solution.With these formulas,the design of the structure and the analysis of the principle can be achieved easily.In addition,the novel coaxial nested 3-D FSS proposed in this paper has contributed to the design and development of the FSS due to its superior performance.Finally,the theoretical analysis and equivalent circuit modeling of this 3-D FSS provide a solid theoretical basis for the subsequent design and analysis of a series of 3-D FSS.