New Materials for Superconductive Communication Cables

The feasibility of superconductive coaxial cables as a communication medium with an enormous capacity may be increased by applying superconductive materials with low surface resistances and high critical temperatures as well as dielectrics with low loss. This paper discusses the mechanisms affecting metallic conduction and dielectric properties of materials in relation to their application in a low temperature transmission line. It starts with a brief report on the present state of rf-superconductivity, but it particularly deals with the measurements of low temperature dielectric microwave losses of polymeric materials, because the dielectric loss mainly determines the overall-attenuation of the cable. Our investigations resulted in the smallest loss tangent of any solid material measured so far:
\tan \delta (4.2 K, 6.5 GHz) = 6.6 \dot 10^{-7}\pm 5%
for polyethylene. This corresponds to a cable attenuation of roughly 0.5 dB/km at 10 GHz and extends the bandwidth of superconductive communication lines appreciably.