Which of the following has the biggest effect on the velocity factor of a transmission line?
Why Velocity factor is the ratio of the speed of a signal in the line to the speed of light in free space, and it is set almost entirely by the dielectric surrounding the conductors: VF = 1/sqrt(er), where er is the relative permittivity of that insulation. Solid polyethylene coax (er about 2.3) gives roughly 0.66, foamed polyethylene about 0.8, and air-spaced open wire line approaches 0.95 or better because there is very little solid material between the conductors. Change the insulating material and the wave speed, and therefore the electrical length of a given physical length, changes with it.
Watch out Characteristic impedance is tempting because it also depends on the dielectric, but it is set by the conductor geometry (the ratio of shield to center conductor diameters) as much as by the insulation, and two lines of the same impedance can have very different velocity factors. Physical length and conductor resistivity affect loss and electrical length in wavelengths, not the propagation speed itself.
Velocity factor lives in the insulation: VF = 1/sqrt(dielectric constant). More air, faster line.