Analysis of Normal and Superconducting Coplanar Waveguides in Radio Astronomy

In terahertz radio astronomy, superconducting coplanar waveguide resonators have been commonly applied in Kinetic Inductance Detectors (KIDs) to measure the absorption of photon energy in the millimeter and submillimeter bands. Here, we present an analysis on the performance of superconducting niobium coplanar waveguides (CPWs). To compute the loss in a superconducting CPW, we have incorporated the complex conductivity developed by Matis and Bardeen based on the superconducting BCS theory, into the CPW loss equation. We have made a comparison between the loss in a CPW at room temperature with that below the critical temperature Tc of the superconductor. It can be observed that at frequencies below the gap frequency fg, the low in the superconducting CPW is significantly lower than that in a normal CPW. Above fg, however, the material loses its superconductivity and the loss in both temperatures becomes comparable. In our analysi, we have also shown that the loss decreases as the gap bewteen the strip and groundplane becomes wider. Hence, with careful design, the loss in a CPW can actuall y be minimized.