Which of the following has the biggest effect on the velocity factor of a transmission line?

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Propagation speed in a line depends chiefly on the permittivity of the material between the conductors, since velocity factor is roughly the reciprocal of the square root of that permittivity. Solid polyethylene coax runs near 0.66, foam dielectric near 0.78 to 0.85, and mostly-air open-wire line 0.95 or better, while length and center conductor material barely matter.