This paper deals with the improvement of the transmission characteristics of superconductive coaxial line (SCCX), and presents a quantitative relationship between the Fourier component of the dimensional irregularities in the raw material of SCCX, such as the thickness of the copper tape and the diameter of the insulator, and an abnormal increase of the transmission loss due to the periodic impedance irregularities. This paper also shows that even for the coaxial cable with very small attenuation constant (≈ 0.5 dB/km) and high operating frequency (up to several gigahertz), an approximate formula for loss peak due to the periodic impedance irregularities holds for the production length of cable. This is indicated by the comparison of an approximate formula with the result of digital computer simulation.

Superconductive coaxial cable has been investigated to survey its feasibility as a communication medium with enormous capacity. The measurements of the frequency and the temperature dependences of the attenuation of coaxial pairs show that dielectric loss is dominant at 4.2 K, but conductor loss increases and gets the attenuation sensitive to temperature as 5 K is exceeded at a few gigahertz. The effect of periodic impedance lrregularities, which is the greatest problem in the transmission characteristics, has been solved by the improvement of the manufacturing process based on spectral analysis of the dimensional variations of the materials, and it is suggested that superconductive coaxial line is applicable to wide-band communication. A thermal insulation pipe for the cable is manufactured and its insulation efficiency is measured. The feasibility of superconductive coaxial cable Will be increased not only by developing the refrigeration system, but also by applying superconductive materials with low surface resistance and critical temperatures higher than lead.