[SECC] TDR question

[Kevan Nason]

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.contesting.com/pipermail/secc/attachments/20191004/0556a563/attachment.html>