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. Continue reading nanoVNA – measuring cable velocity factor – demonstration – open wire line