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Today I investigated a cable which is around 900m long. I used a TDR that injects a needle pulse into the system with a width of around 40ns. The needle pulse had an amplitude of around 2V. A reflection never came back. I wonder why there wasn't a reflection visible. I used a pulse with a width of around 30µs and also 2V amplitude and could see the two pulses overlapping like that:

enter image description here

But the reflection of the needle pulse couldn't be seen. I have two assumptions:

  1. the amplitude of the signal was too small, but then why could I see the wide pulse?
  2. The energy of the needle pulse got somehow fully absorbed by the cable.

Do you guys know why I couldn't see the short needle pulse?

Yoomo
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  • In your diagram, what does the plot (apparently showing a signal with two impulses) on the right axis represent? – The Photon Dec 17 '20 at 20:02
  • Not enough energy. The second pulse contained 750x the energy and your measurement tools could see it. Dispersion could have reduced the amplitude of the first below your capacity ot measure it (it may have been there, at low amplitude) A good next step would be to see what a 1 us pulse looks like. –  Dec 17 '20 at 20:02
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    What size conductor(s) are in the cable? It could be simply that the pulse is being attenuated by the 1800 m (out and back), or more than a mile length of the cable. – SteveSh Dec 17 '20 at 20:37
  • What kind of cable is it ? Coaxial cable ? – andre314 Dec 17 '20 at 20:41
  • @andre314 two parallel wires – Yoomo Dec 17 '20 at 20:53
  • @SteveSh the capacitance of the 900m long cable is around 70nF – Yoomo Dec 17 '20 at 20:53
  • @BrianDrummond ok thats also what I thought. So I will need to increase the energy. Increasing pulse width and or amplitude will help. Thanks – Yoomo Dec 17 '20 at 20:54
  • @ThePhoton the right plot should represent a pulse that gets reflected. The first plot represents a long pulse getting overlaped by its reflection. Keep in mind that these are ideal plots. – Yoomo Dec 17 '20 at 20:56

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