23 August 2012

How to measure coax cable velocity factor

A recent (2010 AD!) discussion on an Italian forum tried to answer the question: "how to measure coax cable velocity factor?" Depending on the cable dielectric and size, vf is between 0.6 and 0.9.

If you have an antenna analyzer, such as the MFJ-259, it's easy, since the device will do it for you. Just follow the instructions on the user's manual.

If your instrument doesn't allow that, then you will have to proceed with trial and error. For example connect a known non-inductive impedance (resistance) on one end and look for the frequency where the same impedance is shown on the other side of the unknown cable. This condition occurs at multiples of lambda/2, so choose your frequency accordingly. The resistance must be different from the line impedance.
Edit 2013-10-22. OM Don adds that due to the intrinsic feed line attenuation, the resistance seen at the generator's side will be less than the face value. Obviously "how much?" depends on line length and test frequency, but could be in the order of ohms or fractions.

With a two-channel oscilloscope another comparison can be done, on two sections of the same cable, same length. Feed cables with a power splitter and observe the signal at both far ends: if waveforms are out of phase, then your two cable sections have different vf.. From this point you can use your imagination to work out other measures the o'scope can provide...