Velocity Factor

The speed of an electrical signal travelling down a transmission line is somewhat less that the speed of light in a vacuum. The reduction in speed is expressed as a "velocity factor" (dimensionless) that is always less than 1.0

Velocity Factor Example

A half-wavelength of RG58 cable is required at a frequency of 14.2 MHz. What is the length of cable required? (The velocity factor is 0.66).

Using the Frequency to Wavelength Calculator page (links to, the wavelength in free space is given by λ = c / f = 3 x 108 / 14.2 x 106 = 300 / 14.2 = 21.13m. So a half wavelength is 21.13 / 2 = 10.563m. Multiplying this result by the velocity factor for RG58 cable gives the answer of 10.563 x 0.66 = 6.972m.

The Characteristic Impedance of Twin-Line Feeder

MathML formulaFormula ImageStraight text formula
Z 0 = 276 log 10 2D d twin-feeder-characteristic-impedance-formula.PNG Z0 = 276 log 10 (2D / d)


Z0 = Characteristic Impedance, Ohms

D = Spacing between wire centres, meters

d = Wire diamter, meters