Modeling Strategies for Superconducting Microstrip Transmission Line Structures

Strategies are explored to reduce the electromagnetic simulation time of electrically large superconducting transmission line structures while retaining model accuracy. The complex surface reactance of an infinite thin-film superconducting sheet is evaluated with the BCS (Bardeen-Cooper-Schrieffer) theory and is used as an input to model the phase velocity and characteristic impedance of finite width transmission line structures. Commercially available electromagnetic simulation software packages are employed for the calculations, and the results are compared with limiting analytic forms from the literature. The influences of line width, metallization thickness, and substrate height on microstrip transmission line propagation are considered in detail, and a scaling approach is presented to compensate for the leading-order effect in numerical simulations. These findings are particularly important near the energy gap of the superconductor due to the influence of the kinetic inductance on the transmission line dispersion.