Determining Zo for coax cable

[Skywave]

Astral: my response to your Q. above.

Occasionally, I need a length of co-ax: could be anything between 3 to 4 feet or a much longer run to feed a TV signal from a distribution amp. to another room in a house, for example. Over the years, I have accumulated quite a collection of co-ax cables: some off-cuts; some salvaged from equipment; some from where and when I can't remember! (Yes, all kept in the dry!) But the point is this: sometimes there are no identifying markings of the sheath of a cable. Although 75Ω co-ax is usually thicker than 50Ω co-ax, can't always rely on that. And as I am sure you know, there are usually good reasons for choosing a cable with the appropriate Zo. And my suggested technique, even if does require an instrument or two, is quick, simple and reliable.

Al.

[G8HQP Dave]

Another reason for measuring it is that this could show up coax which has moisture inside it. Water has high permittivity so it would not take much water to increase the capacitance and so reduce the characteristic impedance.

[terrykc]

Wouldn't a TDR be the obvious way of checking for that - or any other damage?

[Skywave]

Yes, Terry - it certainly would. Unfortunately, TDRs are expensive items of equipment and are primarily designed for one purpose. An LCR meter has many uses and is probably much more common as an owned item by members here than a TDR. Also an LCR meter will probably be a lot cheaper to acquire for those who do not own one.

Al.

[terrykc]

Originally Posted by Skywave

Unfortunately, TDRs are expensive items of equipment ...

They are? You can build one quite cheaply.

There is (or was) a $10 kit for this one including PCB and additional voltage regulator: http://www.epanorama.net/circuits/tdr.html

These might also be of interest: https://www.youtube.com/watch?v=I1gfUNh5PJQ

https://www.qsl.net/g4usp/Time%20Dom...ometer/TDR.htm

I did find some more but the links to the schematics no longer work!

Obviously these designs don't include all the bells and whistles that you would find on a professional instrument such as integral display, entry of velocity of propagation to match cable manufacturer's specifications and accurate measurement of cable length and fault location(s) but that is not what is being asked for here.

[red16v]

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.

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.

[Skywave]

Re: post #25.
Thank you, Terry. I'll follow those up.

Al.