Exploiting your antenna analyser #26

Find coax cable velocity factor using a very basic analyser

A common task is to measure the velocity factor of a sample of coaxial transmission line using an instrument that lacks facility to backout cable sections or measure OSL calibration (as discussed in other articles in this series). The older models and newer budget models often fall into this category.

The manuals for such instruments often explain how to measure coaxial cable velocity factor, and the method assumes there is zero offset at the measurement terminals (whether they be the built-in terminals or some fixture / adapters). In fact even the connectors are a source of error, especially UHF series connectors.

It is the failure to read exactly Z=0+j0Ω with a S/C applied to the measurement terminals that adversely impacts efforts to measure resonant frequency of a test line section.

The method described here approximately nulls out offsets in the instrument, measurement fixture, and even in the connectors used and for that reason may sometimes be of use with more sophisticated analysers.

The method requires measurement of the lowest frequency impedance minimum of two different sections of the SAME cable with the SAME connectors (not just type, EXACTLY the same connectors), and from the measured lengths and frequencies, to calculate the velocity factor.

Note that the method relies upon an assumption that velocity factor is independent of frequency. That assumption may introduce some small error for lossy lines at frequencies below about 10MHz, and is probably not suitable below 1MHz.

Example

I have two lengths of EXACTLY the same cable with EXACTLY the same type of connectors on them. I need to fit an adapter to my analyser to plug the cables onto it, and I have made a female S/C adapter from a panel socket with a strap soldered across the back of it. If we use the same adapters for both measurements, the effects will me nulled out… as will the connector anomalies.

I measure the distance to the mm between the ends of the visible black sheath on both cables, and then measure the lowest frequency impedance minimum of each of them (… write the figures down as you measure them, don’t mix them up).

I then enter them into the calculator Velocity factor solver.

Above is the solution, the velocity factor is calculated to be 66.0%.

Watch the blog for continuing postings in the series Exploiting your antenna analyser. See also Exploiting your antenna analyser – contents.