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A high temperature superconductor (HTS) filter has been developed recently. It is cooled by a small cryocooler and is operated at 70 Kelvin (K) in a vacuum chamber. Coaxial cables are used for the high frequency transmission line between the filter in lower temperature and other circuits at room temperature. Because HTS filters are typically used in low-loss applications, there is an important subject to optimize electrical loss and thermal leakage in the transmission line. The usual semirigid coaxial cables have a small amount of loss; they conduct too much outside heat into the HTS filter. We have developed our own coaxial cables to optimize both electrical and thermal performance. The outer conductor of this new coaxial cable was made of plated thin copper film and was designed to have an optimum thickness of 0.005 mm. SMA connectors are fitted carefully to both ends of the coaxial cable. The new coaxial cable that is 1.68 mm in diameter and 100 mm in length is evaluated. The evaluation result found the electrical insertion loss to be 0.175 dB at 2.0 GHz and thermal leakage to be 48 mW from 300 K to 77 K. We have also developed an HTS receiving filter subsystem for a CDMA base station using our own coaxial cables. We obtained excellent results.