NanoVNA – measuring cable velocity factor – demonstration – coax

The article nanoVNA – measuring cable velocity factor discussed ways of measuring the velocity factor of common coax cable. This article is a demonstration of one of the methods, 2: measure velocity factor with your nanoVNA then cut the cable.

Two lengths of the same cable were selected to measure with the nanoVNA and calculate using Velocity factor solver. The cables are actually patch cables of nominally 1m and 2.5m length. Importantly they are identical in EVERY respect except the length, same cable off the same roll, same connectors, same temperature etc.

Above is the test setup. The nanoVNA is OSL calibrated at the external side of the SMA saver (the gold coloured thing on the SMA port), then an SMA(M)-N(F) adapter and the test cable. The other end of the test cable is left open (which is fine for N type male connectors).

Using the method described at Velocity factor solver the quarter wave resonance was measured for each of the test cables with the same adapters.

Above is measurement of the short cable. The short cable measured 1.150m between the insides of the crimp sleeves (which were hard up against the connector body), the long cable measure 2.450m and resonance was at 19.614MHz.

Above is the calculation which tells us that the velocity factor of the coax itself (having deducted the effects of adapters and connectors) is 0.66.

The calculator also reports that it calculated the offset to be 363.9ps, equivalent to 72mm of RG213 length in the adapter and connectors. This reconciles well with physical measurement (allowing that part of the internal path of N connectors is air dielectric, about 13mm in a mated pair so that appears to be 8.6mm equivalent at VF=0.66).

Be aware that velocity factor is frequency dependent, though the error is small for practical low loss cables above 10MHz.

You can now cut the cable based on the measured velocity factor, allowing for the electrical length of connectors and adapters as appropriate.

Warning: do not measure cable with a loose UHF series plug at an open end. The loose collar will cause unpredictable / unreliable results, install a UHF(F)-UHF(F) adapter to fix the problem. Likewise for plugs of similar construction (eg SMA).