[SECC] TDR question

[dave]

It is correct that a TDR will show a lower impedance as a downward 
deflection of the trace. Upward deflection indicates a higher Z. Going 
all the way to zero would indicate a near zero Z. The trace going 
nearly to the height of the initial pulse would indicate an open. 
Water in the cable will be a variable downward deflection. Distance to 
fault is measured by the time it takes for the pulse to go out and 
back. Combine this time with the velocity factor of the cable to get 
the distance. This will be twice the distance to the fault as the 
energy must go out and back.

You cannot measure distance through any water in the cable. You can 
measure only to the beginning of the water, not through it or to the 
other end. The presence of the water severely alters the velocity 
factor of the cable and destroys the ability to calculate any distance 
through the section with water.

Depending on the sophistication of the AA-170 it may do these 
calculations for you. Or you may need to do them yourself. Study the 
manual to see what it says.

Tektronix wrote the book on TDR's and the full manual for the 1503C is 
available online. Once you know how to use one they are quite capable 
instruments. The Tek manual says it is for a 'metallic' TDR to 
differentiate it from the OTDR's (Optical Time Domain Reflectometer) 
which we used to look at fiber optic cable at the phone co. Great 
instruments!

73 de dave
ab9ca





On 10/4/19 9:15 AM, Kevan Nason wrote:
> Anyone used a Time Domain Reflectometer before?  Looking for some 
> newbie hints. Things to avoid or tricks you've picked up, how to 
> interpret readings, etc. Used one for troubleshooting for the first 
> time yesterday.
> 
> A local retirement village has a ham club with a 64 ft tower. The 
> tribander stopped working. Ohm meter on coax shows a short. Besides my 
> shoulder being a bit messed up making me reluctant to climb much for a 
> year or so, the village rules won't allow it anyway. They want 
> commercial certified climbers. So I brought along my RigExpert AA-170 
> antenna analyzer which has TDR capabilities.
> 
> The analyzer shows a flat 9.4 SWR across the entire 20 meter band. 
> Since a short exists and there isn't any reason for it (i.e.: no Bias 
> Tee or Surge Protector having an inductor to ground) I didn't bother 
> looking at the other bands. The TDR curve shows a slight positive bump 
> where the coax connects to the analyzer. The next deflection is about 
> where the top of the tower should be. There is a strong negative drop 
> well below the zero reference. Looked online and found out that likely 
> means a short -- which agrees with the ohm meter reading. There is a 
> balun at the yagi feed point. Those readings make me think there is 
> either a balun failure or damaged coax. Or (less likely) maybe a wire 
> came loose and is touching something up there. The constant SWR is 
> probably due to the coax dielectric capacitance.  They were given used 
> coax of an unknown age and used that for their setup. They weren't 
> sure when asked what type it was so I assumed a general RG8 type.
> 
> The club doesn't have an amplifier. Since the commercial Cushcraft 
> antenna SWR was okay and no prior indications of antenna problems I'm 
> thinking a balun failure due high power is probably not  the culprit. 
> There were strong thunder storms before the short was discovered. The 
> club trustee thinks the failure was from a near miss lightning strike. 
> Whatever the cause, the club members had already purchased new coax 
> and made an air choke before I got there. They had also contacted the 
> village owners who are calling in a commercial climber to replace the 
> feed line.
> 
> Replacing the feed line and choke/balun makes sense if you have to 
> convince someone to pay for a commercial climber to come in. I'm 
> pretty much done with what I can do for them, but still curious about 
> how this TDR stuff works and want to learn a bit more before the next 
> problem crops up. Any thoughts on my curve interpretation or tips for 
> future TDR use?
> 
> Kevan
> N4XL
> 
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