An Approximate Analytical Model for the Quasi-Static Parameters of Elevated CPW Lines

Approximate analytical formulae are presented for calculating the characteristic impedance of elevated coplanar waveguides (CPWs). In such a structure, both the signal and ground traces are suspended above a substrate using airbridge technology. The analysis of this structure is formulated in terms of partial capacitances obtained through conformal mapping techniques. The paper highlights the limitations of previously published formulae. Furthermore, the paper presents a method to overcome these limitations over a limited range of practical geometries. Comparisons between measured, simulated, and calculated results are included, and excellent agreement is obtained for a wide range of geometries. In addition, the effects of CPW-to-elevated coplanar waveguide transitions and supporting posts on the characteristics of elevated lines are analyzed. Good agreement is obtained for a relatively complex fabricated structure which is nonuniform at nine points in the direction of signal propagation. The fabrication of elevated CPW structures is also discussed.