Velocity Factor

Created Saturday 08 May 2021

The Velocity Factor (VF) is a property of transmission lines, specifically used with coaxial cables. Since any dielectric permittivity greater than 1 (air/vacuum) will have the effect of slowing the phase velocity of a wave, the VF is given as a percentage representing that reduced phase velocity.

The velocity factor is simply the inverse of the square root of the relative permittivity. Some common phase velocities are given below.

Coax Type Dielectric Material VF εr Z0
RG316 PTFE 69% 2 50 ohm
T10 Drop Cable Foam/HDPE 85% 1.38 75 ohm
RG8/U Foam/HDPE 78% 1.64 50 ohm

High Speed Design Wiki:Glossary
High Speed Design Wiki:Signal Integrity:Transmission Lines:Wavelength, Physical Length and Relative Permittivity
High Speed Design Wiki:Glossary:Velocity Factor

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