Velocity Factor

Velocity Factor (VF) is ratio of the speed of propagation of a wave or signal (\(v_p\)) in an electromagnetic field to the speed of light.

In computer networking and cable industries, the terms "velocity of propagation" and "wave propagation speed" are used to mean the ratio of speeds, or VF, rather than the true velocity.

Calculations

• For optical signals, the velocity factor is the reciprocal of the refractive index.
• For radio signals in vacuum, the velocity factor is 1.0
• For electric waves
  • In the most general case, VF = \(\large{\dfrac{1}{\sqrt{\mu_r \epsilon_r}}}\) (= \(\large{\dfrac{1}{\sqrt{\mu \epsilon}} / \dfrac{1}{\sqrt{\mu_0 \epsilon_0}}}\) = \(\large v_p/v_0\) )
  • For most cases, VF = \(\large{\dfrac{1}{\sqrt{\kappa}}}\) (where there is no magnetic conducting materials)
  • For a lossless transmission line, VF = \(\large \dfrac{1}{c_0 \sqrt{L'C'}}\), (where \(L'\) is the distributed inductance and \(C'\) is the capacitance, and \(c_0\) is the speed of light in vacuum)

Values

The VF varies with the transmission medium or cable type.