An electromagnetic resonator has a resonant element made of a high-temperature superconducting material such as YBa2Cu3O7−x. The resonant element has a substrate coated with a thermally conductive layer such as silver, over which the high-temperature superconductor material is placed. The thermally conductive layer distributes heat along the length of the resonant element to minimize the effects of localized heating at, for instance, the center of the resonator. The resonant element is held to a housing by a mounting mechanism including a post made of polycrystalline alumina. The polycrystalline alumina transfers heat away from the center of the resonant element and may be used to suppress spurious response due to second harmonic resonance.