Quote:
Originally Posted by G6Tanuki
I'd use a TDR and a terminated length of coax of known-impedance.
Join the unknown and known coaxes in series, then run a TDR. If you can see the transition then the two coaxes are of different impedance.
Though I've always got near-enough by measuring inner/outer diameter ratios. Oddities like RG62A/U [93 Ohm, used for IBM '3270' display-screens] are usually revealed by their construction before you bother to reach for any testgear.
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I spent some of my student apprenticeship days building tdr measurement sets in the early/mid 70’s. They were for use on large (375) solid coaxial cables (18 tubes in a bunch!) between Martlesham and the Birmingham PO Tower. The main purpose of the tdr kit was to determine the usable bandwidth of each tube. The PO couldn’t work how to easily do it, a chap called ‘Roseman’, an Australian, had worked out how to do it using TDR techniques and he came over and showed everyone how to do it - producing the necessary bits out of a plastic shopping bag that he had brought over for a small demo, he caused quite a sensation! In essence you launched a burst envelope containing an increasing frequency sine wave from low MHz up to 500Mhz down the cable terminated in an accurate resistance and then waited a specified time and measured the reflections - in this way you did not have readings coming back from the first few metres or last few metres of the cable which due to the usual manufacturers processes were likely to give high readings.
This was very leading edge back in the day, and it was a struggle to find appropriate kit to be glued together to form a complete practical robust working unit that would fit into the back of a land rover driving around the English countryside, leaping down manholes and performing these measurements (among others). Highly simplified methodology from me, great fun as a student apprentice. I think the cables were eventually extended to somewhere more useful than Martlesham. It was a big contract and was split between various manufacturers as was the GPO’s way in those days. Alliances were formed since some of the other cable companies could not successfully build the TDR kit - I worked for Pirelli.
The most magical machine I ever saw was the machine for applying (if that’s the right word) the outer flexible braiding to domestic type flexible coax. It was Italian (naturally) and you could stand next to it for as long as you liked and you could never figure out how the heck it was doing it. Quite an astounding bit of kit.