HTS filters are known for their low insertion loss and high out-of-band rejection. Yet the performance of these filters may be negatively impacted by the thickness of the HTS film, dielectric-constant variations, and more. Tuning work is therefore required to guarantee high performance in an HTS filter. Such tuning may be mechanical, electrical, or magnetic in its approach. At Tsinghua University in Beijing, China, researchers successfully used a mechanical tuning method to correct the center frequency shifts of HTS filters. This work was carried out by Huili Peng, Xiaoping Zhang, Xubo Guo, Lijuan Mu, Bin Wei, Baoxin Gao, and Bisong Cao. Their mechanicaltuning method uses sapphire and copper rods for high-temperature superconducting filters. A three-pole microstrip filter and two-pole patch filter were fabricated and tuned. Depending on the local electromagnetic-field distribution, the center frequency shifted toward either the lower or higher end in dielectric tuning. See “Mechanism Study of Mechanical Tuning in HTS Filter Application,” Microwave and Optical Technology Letters, July 2007, p. 1565.