[G0HZU_JMR]

I think Alan, W2AEW has several youtube videos showing various homebrew TDR setups where he measures the Zo of a cable. The downside with the TDR method is that it generally requires a fairly long piece of transmission line unless you start using exotic means. It makes a great demo and it's very intuitive but I'm not sure how useful it would be for real world applications unless you have piles of unknown coax cable lying about. Usually the cable type is stamped on it anyway and if it isn't it probably isn't good quality cable.

I rarely 'need' to measure coax Zo but sometimes I need to measure/prove the Zo of twisted pair lines and usually this means measuring something maybe 20-50cm long. The wire would normally go in a wideband transformer, eg 1:1 or maybe a 4:1 and it helps to be able to optimise the Zo to suit the transformer. There are several ways to measure the twisted pair line but it would be tricky to do it with a TDR setup because the line is so short and the line is balanced.