What is the major factor influencing the velocity factor of a coaxial cable?

The velocity factor of a transmission line is the speed at which radio waves travel through the line compared to the speed of light in a vacuum. It is primarily determined by the physical properties of the insulating material, known as the dielectric, that separates the center conductor from the shield. Different dielectric materials, such as solid polyethylene, foam, or Teflon, slow down the RF signal by different amounts. For example, a foam dielectric contains more air and thus yields a higher velocity factor than solid polyethylene. The size, spacing, or materials of the metal conductors (center or shield) dictate the characteristic impedance and power handling capability, but they do not significantly affect the speed at which the wave propagates through the cable.