TDR

[W1RKW]

Anyone have any experience with using a o'scope as a TDR?  Not sure how to go about this. I know it can be done and I know that one needs a pulse source but I have no idea what pulse rate or pulse width to use or bandwidth for the scope.

[KB2WIG]

cheeep  and dirty...... klc

http://www.epanorama.net/circuits/tdr.html

[WA1GFZ]

Bob that circuit will work great. The faster the scope the better the resolution. about 1  ns per foot prop delay so 2 ns per foot to get the reflected pulse.
cool tool. fc

[WQ9E]

Hi Bob,

Not sure how often you will need to use a TDR but the Tektronix 7S12 TDR plug-in often show up used these days and will fit any 7000 series mainframe.  I saw a couple of 7S12's at a hamfest earlier this year cheap.  If you go this route be sure and get the pulse and sampler heads with it.  I have had my 7S12 for several years and it has gotten a fair amount of use.  As I recall it cost $150 and included pulse and sampling heads and a second working 7S12 but without the plug in heads.

If you find a manual for the older Tektronix 1S2 Sampler/TDR plug in for the 500 series tube scopes grab it as it has a useful discussion of TDR usage.

Rodger WQ9E

[KB2WIG]

  "   If you find a manual for the older Tektronix 1S2 Sampler/TDR plug in for the 500 series tube scopes grab it as it has a useful discussion of TDR usage.  '

Too bad the 1S2 and the 1L5 (spectrum analyzer) cant seem to be found... i've been looking fer a while....... klc

[Tom WA3KLR]

We had a topic on TDR here a couple years ago that pretty well explained the technique but I have not been able to uncover it so far, using the SEARCH feature.

Here is a related topic that uses the MFJ-259B:

"How to Find a Bad Spot in a Feedline" by K1JJ:

http://amfone.net/Amforum/index.php?topic=8245.0

[KD6VXI]

ARRL Antenna Manual has everything you need.  Circuit, instructions, etc.

At least, mine does.  Circa 1996.


Shane

[WQ9E]

Hi Mack,

I have used mine for checking connector installations, antenna relays and switches, and finding an intermittent problem with one of my antenna installations.  I also helped out some well drilling friends find the location of an open in a LONG underground power feed to a water well.  The nice thing about TDR's is you can calibrate them to any type of cable if you have a sample of that cable with known length.  The Tektronix literature has a lot on using the TDR to look at "ground bounce" and other issues in circuit board layout but that is not something I have needed. 

To KC, several years ago I rescued a Tek 555 (dual beam tube type scope) that had been abandoned and it came with 1S1, 1S2, 1L10, and 1L20 plug-ins in addition to more mundane standard vertical amps.  It also had the proper "scopemobile" cart, probes, and all documentation.  Total cost was a few gallons of diesel to drive to the location and it had a few of the usual bad black beauties and needed a general calibration-otherwise it was fine.  It sits in the "main floor operating area" with a type CA and 1L10 plugged in to monitor the Desk KW and Viking 500.  Although I mostly use a 7L13/7623A combo now for spectrum analyzer work on the bench the 1L series are not bad.  They seem to have a bad rep because you will see a few internally generated spurs but it is very easy to differentiate the spurs from the actual signals based upon the speed and direction that they move across the CRT when tuning the center frequency.  If you play with vintage SSB phasing rigs the 1L10 makes alignment fast and easy.  The 1L10 also provides audio to the record jack and it is a fairly sensitive receiver (use with the 0 dispersion setting) complete with crystal filter.

Rodger WQ9E

[W9GT]

OK, old Telco guy here, TDR for looking at cable pairs and OTDR for fiber optic cables. So the question is, besides looking at a properly terminated section of feedline (I never was 100% satisfied with TDR measurements on unterminated cable pairs), what other uses for a TDR does a ham have?

Mack

Actually it is a pretty cool device for troubleshooting antenna systems, transmission line problems, etc.  Takes a little knowledge to run one effectively and most hams don't have them available to them, but that home brew job looks like a nice way to go. 

Mack...I worked on fiber optics systems when they were first in development...worked with a crude OTDR that was the forerunner for modern instruments.  It was indespensible for troubleshooting fiber systems and finding connector and splice problems.  The one we had had a test fixture that looked like it had been removed from a milling machine.  It had X, Y, and Z axis precision alignment adjustments and was similar to a machinist's vise table with a groove for the fiber.
Ah...those were the days...............

73,  Jack, W9GT

[W9GT]

OK, I get the general idea BUT from my personal experience with the TDR, a connector or relay that is inline on a feedline would still cause a fairly large reflection, that would register just like a fault condition. On Telco cables even a perfectly good splice would register on screen as a small impedance bump.

Also looking at an unterminated piece of cable on a cable pair would show reflections what appeared to be fault conditions but were just reflections on the cable pair from the unterminated condition. Termination made all these small reflections disappear.

I just can't grasp any real use for a TDR in the average ham shack, except for fault locating a piece of transmission line by itself, no relays or connectors inline.

Mack

The point is....you probably already know where the connectors and relays are.  What you are looking for are other discontinuities in the line that will be displayed on the 'scope.  Like  I said, it takes some knowledge (and experience?) to really understand what you are looking at and be able to interpret the results.  I have successfully located faults in heliax lines and bad connections/connectors in commercial antenna installations with a good ol' Biddle TDR.  Probably most hams would not bother with such a sophisticated device, but nice if you have one.

We also used an antenna system analyzer that in effect was a TDR extrordinaire for microwave installations to look at waveguide/combiners/antenna feeds, etc.  That really was a neat device and saved a lot of trouble......again it was not intuitive or always easy to know what you were looking at, but very good piece of gear to have.

73,  Jack, W9GT

[Steve - WB3HUZ]

It's a matter of degree. A TDR will quickly show the "quality" of a connector on a coax line. A small bump is probably OK. A big bump, maybe not. The bump will become more of an issue at higher frequencies.

[W1RKW]

In my case I have 2 direct burial coassial feed lines.  One of them is giving me grief.  With a 50ohm resistive termination I'm seeing a solid SWR over 2:1.  Don't know if a TDR will show anything but I've been fascinated by these things and like to mess around with a homebrew device. I'm not sure what sort of problem I have with the feedline. I have the high SWR but show an infinite open with an ohmmeter on the feedline when disconnected at the antenna end.  Probably easier to lay a new cable in a conduit like I should have done 5 years ago. Oh well, live and learn.

Steve,
When you say a bump on the line, what does that mean in terms for signal/power transfer.  Are we talking  reduction in dB's or fraction of dB's for power/signal transfer at these bumps?  I'm curious what degree of degradation these would cause.

[WA1GFZ]

Bob,
I had a pair of 200 foot runs when I had the phased dipoles up. One acted up so threw a tdr on it and quickly found it 200 ns away 100 out and 100 back. 100 feet out I had a coax connector splice and there was water in the bullet....   

[Steve - WB3HUZ]

Any bump means loss. How much depends on the size of the bump and the frequency of operation. You can always get a solid number by putting a watt meter on each end and measuring the difference.

As Frank said, the TDR is more useful at showing you where the problem is located.

[WA1GFZ]

Resolution of the TDR deppends on the pulse level applied to the transmission. Greater amplitude will be able to be callibrated in ohms easier.
Scope resolution. A scope that does 1 ns/cm will be more accurate than a scope with 50 ns/cm. Distance calibration. 
I worked with this German guy for a couple years who could look at a pulse and tell you how to match it with reactive components. He was a master at designing LC delay lines. He also had great stories of bombing raids in Germany when he was a kid.

[W1RKW]

I do have a feedline with a splice connector. I hope that's the feedline that's giving me grief. Just don't remember which one is contiguous and which one has the splice and where the spice is. Guess it doesn't matter at this point.  Mini project time. Thanks guys.