Coax cable velocity factor

- an overview of the velocity factor and propagation speed of signals within a coax cable.

The speed at which a signal travels within a coax cable is not the same as an electromagnetic wave travelling in free space. Instead it is affected by the dielectric that is used within the coax cable, and this has the effect of slowing the signal down. This can be of great importance in some applications, although for many purposes it does not need to be known.

Velocity factor

The speed at which the signal travels is normally given the designation Vp or Vg and this is the faction of the speed at which the signal travels when compared to a signal travelling in free space. Thus Vp for a signal travelling at the speed of light would be 1.0, and for one travelling at half the speed of light it would be 0.5.

The velocity factor of the cable is found to the reciprocal of the square root of the dielectric constant:

Vp     =     1 / SQRT (dielectric constant)

Coax cable electrical length

One important factor of a coax cable in some applications is the wavelength of the signals travelling in it. In the same way that the wavelength of a signal is the speed of light divided by the frequency for free space, the same is also true in any other medium. As the speed of the wave has been reduced, so too is the wavelength reduced by the same factor. Thus if the velocity factor of the coax cable is 0.66, then the wavelength is 0.66 times the wavelength in free space.

In some instances lengths of coax cable are cut to a specific length to act as an impedance transformed or a resonant circuit, then this needs to be taken into consideration when determining the required length of coax cable.

The advantage of using a coax cable with a low velocity factor is that the length of coax cable required for the resonant length is shorter than if it had a figure approaching 1. Not only does this save on cost, but it can also be significantly more convenient to use and house.

Dielectric materials

There is a variety of materials that can be successfully used as dielectrics in coax cables. Each has its own dielectric constant, and as a result, coax cables that use different dielectric materials will exhibit different velocity factors.

Material	Dielectric constant	Velocity factor
Polyethylene	2.3	0.659
Foam polyethylene	1.3 - 1.6	0.88 - 0.79
Solid PTFE	2.07	0.695

Dielectric constants and velocity factors of some common dielectric materials used in coax cables.

Summary

If resonant lengths of RF coax cable are to be used, then it is necessary to know the velocity factor of the coax cable. It is often possible to determine this to a sufficient degree of accuracy from a knowledge of the dielectric material.