Many flexible coax wires use PTFE insulating material, solid or covered in continuous overlapping layers using porous film. In semi-rigid, the outer conductor is extruded steel, usually copper or copper-plated aluminum, which requires precision-length slashes and forming. PTFE is known for its thermal development, which is often a concern when the connector end of the contract involves soldering. Insulator movement at mating interfaces and coax termination areas to change dielectric properties and performance.
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