[SECC] TDR question

[Jerry Flanders]

I have no tips on the TDR, but you might want to prepare yourself and the lineman for unexpected problems by equipping him with a two way  comm channel that he can use to talk to you on the ground while leaving his hands free. Also get one of these wireless rear-view car camera kits and get it working with battery packs and show him how to point it to whatever you need. Also show him how to work your ohmmeter so you can watch the monitor and direct him to make ohmmeter checks in case simple replacement of the line doesn't fix it. Might save a return trip.


GL


Jerry W4UK



> On October 4, 2019 at 9:15 AM Kevan Nason <knason00 at gmail.com> 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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