Velocity factor

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Signals travel at the speed of light. However the speed of light inside a transmission line is slightly slower than the speed of light in a vacuum. It depends on the dielectric constant of the material holding the electric and magnetic fields.

Most transmission lines employ polyethylene (PE) or a PE-air mixture. The speed of light in PE is 77% of the speed of light in a vacuum. The speed of light in air is about 100% that of a vacuum. A coaxial cable with solid PE between the conductors will have a velocity factor of 0.77.

For twin-lead, the electric and magnetic fields are partly in the PE and partly in the air around the twin-lead. For this the velocity factor is somewhere between 0.77 and 1.00, usually around 0.90.

PE is somewhat lossy at high frequencies. To reduce the loss, microscopic air bubbles (foam) are often blown into the PE of coaxial cables. This also raises the velocity factor to 0.85 or 0.90, but it makes the cable less rugged.

The velocity factor is usually of no use to Average Joe installing a TV antenna. In the rare cases where the transmission delay is important, the velocity factor becomes important.