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.
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