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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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9th Oct 2018, 10:07 pm | #21 |
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Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
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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. |
10th Oct 2018, 2:45 pm | #22 |
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Location: Solihull, West Midlands, UK.
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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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10th Oct 2018, 3:57 pm | #23 |
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Join Date: Nov 2010
Location: North Hykeham, Lincolnshire, UK.
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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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11th Oct 2018, 10:51 am | #24 |
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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.
Al. |
11th Oct 2018, 12:44 pm | #25 |
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Join Date: Nov 2010
Location: North Hykeham, Lincolnshire, UK.
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Re: Determining Zo for coax cable
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. |
15th Oct 2018, 6:49 pm | #26 | |
Heptode
Join Date: Jul 2006
Location: Winchester, Hampshire, UK.
Posts: 639
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Re: Determining Zo for coax cable
Quote:
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. |
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16th Oct 2018, 11:14 am | #27 |
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Location: Chard, South Somerset, UK.
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Re: Determining Zo for coax cable
Re: post #25.
Thank you, Terry. I'll follow those up. Al. |