# Transmission Lines

## 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 http://www.mw0llo.com/freqtowavelength.aspx), 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
$Z 0 = 276 log 10 2D d$ Z0 = 276 log 10 (2D / d)

Where:

Z0 = Characteristic Impedance, Ohms

D = Spacing between wire centres, meters

d = Wire diamter, meters

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