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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 9th Oct 2018, 10:07 pm   #21
Skywave
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Arrow Re: Determining Zo for coax cable

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.
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Old 10th Oct 2018, 2:45 pm   #22
G8HQP Dave
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Default Re: Determining Zo for coax cable

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.
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Old 10th Oct 2018, 3:57 pm   #23
terrykc
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Default Re: Determining Zo for coax cable

Wouldn't a TDR be the obvious way of checking for that - or any other damage?
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Old 11th Oct 2018, 10:51 am   #24
Skywave
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Arrow Re: Determining Zo for coax cable

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.

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Old 11th Oct 2018, 12:44 pm   #25
terrykc
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Default Re: Determining Zo for coax cable

Quote:
Originally Posted by Skywave View Post
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.
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