What is the velocity factor of a transmission line?

Radio frequency waves travel at the speed of light in a vacuum, but they slow down when traveling through the dielectric materials used in practical transmission lines. The velocity factor is a specific measurement of this slowdown. Mathematically, the velocity factor is defined as the velocity of the radio wave traveling on the transmission line divided by the velocity of light in a vacuum. It is usually expressed as a decimal or a percentage, such as 0.66 for standard coaxial cable, indicating the wave travels at 66% the speed of light. Options suggesting you multiply the velocities or use the characteristic impedance are incorrect mathematical definitions. The velocity factor always compares the slower speed in the cable to the absolute speed of light to yield a fraction less than one.