Accurate data of complex permittivity of dielectric substrates are needed for efficient design of HTS microwave planar circuits. We have tested MgO substrates from three different manufacturing batches using a dielectric resonator with superconducting parts recently developed for precise microwave characterization of laminar dielectrics at cryogenic temperatures. The measurement fixture has been fabricated using a SrLaAlO3 post dielectric resonator with DyBa2Cu3O7 end plates and silver-plated copper sidewalls to achieve the resolution of loss tangent measurements of 2 {\times} 10-6. The tested MgO substrates exhibited the average relative permittivity of 9.63 and tan {\delta} from 3.7 {\times} 10-7 to 2 {\times} 10-5 at frequency of 10.5 GHz in the temperature range from 14 to 80 K.
@article{arxiv.1207.5906,
title = {Precise microwave characterization of MgO substrates for HTS circuits with superconducting post dielectric resonator},
author = {Janina E. Mazierska and Dimitri O. Ledenyov and Mohan V. Jacob and Jerzy Krupka},
journal= {arXiv preprint arXiv:1207.5906},
year = {2012}
}