Superconducting Notch Filter for RFI Mitigation in Ground-Based Radio Telescope

This article presents a high-rejection, thin-film high-temperature superconductor, microstrip bandstop filter to prevent a local, and high-power radio-frequency interference (RFI) source from interfering with NASA Goddard Geophysical Astronomical Observatory (GGAO)'s very long baseline interferometry (VLBI) global observing system (VGOS) cryogenic receiver. This filter has an excellent 2.7%, 50-dB-fractional-bandwidth, and a center stopband frequency of 9.41 GHz. It does not contain any narrow or interdigital features as found in some designs, which reduces the fringing electric fields and improves its power handling capability. The YBCO films were grown on 435-$\mu$m-thick R-plane sapphire substrate and the anisotropic behavior was modeled and simulated with a high degree of accuracy. The device was tested while cooled to 77 K and the measurements agree well with simulation.