What determines the velocity factor in a transmission line?

The velocity factor of a transmission line is entirely determined by the dielectric, which is the insulating material that separates the conductors. The denser the dielectric material's permittivity compared to a vacuum, the slower the RF energy travels through it. A cable with a solid plastic dielectric will slow the wave down considerably more than a cable that uses mostly air or foam insulation. Distractors like line length, terminal impedance, and center conductor resistivity are incorrect because they affect factors like signal attenuation (loss) and standing wave ratio (SWR). They do not alter the physical speed at which the electromagnetic wave propagates through the medium.