nanoVNA – measuring cable velocity factor – demonstration – open wire line

The article nanoVNA – measuring cable velocity factor – demonstration demonstrated measurement of velocity factor of a section of coaxial transmission line. This article demonstrates the technique on a section of two wire copper line.

A significant difference in the two wire line is that we want the line to operate in balanced mode during the test, that there is insignificant common mode current. To that end, a balun will be used on the nanoVNA.

Above, the balun is a home made 1:4 balun that was at hand (the ratio is not too important as the fixture is calibrated at the balun secondary terminals). This balun is wound like a voltage balun, but the secondary is isolated from the input in that it does not have a ‘grounded' centre tap. There is of course some distributed coupling, but the common mode impedance is very high at the frequencies being used for the test.

Above, the underside of the terminal block has a 2 pin machined pin socket to allow convenient OSL calibration using the parts shown in
Antenna analyser – what if the device under test does not have a coax plug on it?. The screw terminals are not quite on the reference plane, but the small error will be adjusted out by the Velocity factor solver.

The fixture was OSL calibrated before measuring the line sections.

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 long cable. The long cable measured 8.240m end to end, the short cable measure 4.104m.

Above is the calculation which tells us that the velocity factor of the coax itself is 0.85.

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.