Synthetic Asymptote Formulas of Square Coaxial Line

This paper derives simple computer-aided design (CAD) formulas of characteristic impedance and open-ended capacitance of square coaxial line by synthetic asymptote and moment method. These formulas can be used for the design of the 3-D frequency selective surface (FSS) with square coaxial line as unit cells. Both of the formulas can give good physical insights and have only one or two arbitrary constants to be matched with numerical results. A good agreement was found between the results by the formulas and numerical method.