I have been lusting after a TDR and finally created one by building up one
of the simple circuits you see on the internet and hooking it up to my
scope.
This made it easy to find the location of open feedline problem I have with
my European beverage. This is what it looked like. The small impedance
discon
I next wondered if this would be useful for helping me find the right
termination value for a working beverage.
I now believe it can. Here is a plot of my 70 degree beverage using the
termination value I have been using for some number of years:
http://www.kkn.net/~tree/TDR/NE%20Beverage%20Normal.jpg
This is with 500 nanoseconds per horizontal division. Just over 1
microsecond, you can see an impedance discontinuity. This is where the 9:1
impedance transformer is located. This corresponds to a distance of about
150 meters electrically or about 375 feet of physical cable (foam CATV
hardline). You can also see a small negative going blip around 2.5
microseconds out - which corresponds to the far end of the beverage.
To prove I am not making this up - I walked out and disconnected the
termination at the far end of the beverage and this is the plot I got:
http://www.kkn.net/~tree/TDR/NE%20Beverage%20Open%20Termination.jpg
This was a "WOW!" moment for me.
I now feel like I can adjust the termination value for minimum reflection
(which would be no blip either up or down) and perhaps improve the
performance of the beverage.
The circuit I am using can be found here:
http://www.epanorama.net/circuits/tdr.html. Instead of a 15K resistor, I
used a 33K and also put a switch in to make it 66K. This helps when looking
at long transmission lines as there is more time for all the echoes to come
back. I also use used a 220 pf capacitor instead of having a switch. I am
using a Tek 465 Scope - nothing fancy.
This is probably the most useful battery powered device I have ever built.
Parts cost is easily under $30 including the box and a $10 PC board to mount
the surface mount chip on. Next step is to get plots done for all of my
existing feedlines to make sure they are working the way I expect.
Tree N6TR
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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