Well folks I knew that pulling my KT34XA and 2el KLM down for
refurbishing was going to be a task, but it's turning out to be more than
I bargained for. I'm having a problem with the XA that's got me close to
wits end.
I rebuilt it using new capacitor tube caps (blue ones) four years ago. It
worked great and the VSWR curve was the classic curve shown in the
manual. I think it was in the 94/95 winter we had a nasty ice storm. Once
everything melted, the swr curve looked correct, except there was only a
single swr dip up in the fone band on 15. At 21000 KHZ I had an swr of
about 1.9:1. I lived with it.. I put the two element 40 8 feet above it a
year ago, and that pulled fifteen up to make it almost useless in the cw
band.
I came up with a method of pulling the driven elements and the rear
director off the antenna without taking the whole thing down. Last
summer, I went through those elements, pulling them completely apart,
removing any crap in the capacitor tubes, cleaned up all the joints etc.
Put everything back together, making sure all the joints were good and
tight, using penatrox or similar conductive grease. Checked all the
joints for continuity. Each displayed uniform low resistance.
After putting everything back in place, I found the swr curve to look
exactly the same as before the element rebuild exercise. Well several
weeks later, I was exchanging notes with W1GD on the Internet and he told
me about this KLM KT34XA "from the horses mouth" hint sheet on
maintaining the KT34 series. The very first paragraph addresses high VSWR
on the low end of 15M. It says "The tuning of the reflector controls the
VSWR on the Low end of each band."
So I now suspect that my problem is with the reflector, which I can't get
to without pulling the whole thing down. (One forgets how long a 32 foot
boom is until it's sitting in your (100 ft wide) back yard)!
Well, I pulled it down and spent most of a weekend cleaning out all the
tubes, cleaning up all the joints and put all the elements back
together using a conductive paste and making sure all joints were snug.
All of the capacitor tube caps are in good shape with no cracks etc. I
let the elementst sit to temperature cycle over night, then tightened
everything again. I checked each joint and once again found uniform low
resistance at each. For future use, I checked the resonant frequency of
each band on each element by itself, sitting on top of an 8 foot step
ladder. Just something for future reference. With the effects of
coupling between multiband elements it's kind of hard to guess at what
frequency each element should be. The one interesting note is that the
rearmost director was actually higher in resonant frequency than the
front one.
Have any of you measured the resonant frequency of each element on each
band as I did. If so, I'd appreciate your findings.
Anyway, before putting the antenna back up, I checked each joint and
found them to be good.
Once the antenna was back in place, the swr curve looked exactly like it
did after the ice storm. That is I don't see a dip around 21000. The only
dip is around 21350 or so.
I've swapped feedlines with one known to be good. As a stretch I even
swapped out the balun. No difference. The antenna is at the top of 55
feet of Rohn 25. The tower is guyed with Phillstran. The only other
antenna on the tower is a 5 element 2 meter yagi.
The one thing that doesn't make sense to me is the resonant frequency of
the rearmost director on 15. I'm going to pull it down this weekend and
see if I can figure out why it's a high as it is.
Now, questions.
Have any of you experienced loss of one of the swr dips as I have?
Anyone clean the antenna up and get low resistance readings at all
joints, but still have problems? Does it make sense that the reflector
which is 74 inches from the rear driven element could pull the rear
driven element to the point that it doesn't work on 15? I think I'm going
to pull it back down, to reassemble the antenna with the boom running
paralle with the tower so I can get at each element.It'll be pointing
stright up with the reflector about 20 feet off the ground. Before
someone asks, the dimensions now are the same as they were when I first
put it up.
Bruce Scott of KLM has been very helpful, but I obviously still have a
problem. I'm hoping that perhaps someone else has experienced this. (I
really wouldn't wish it on anyone!).
I'd appreciate hints from anyone!
73 de Larry K7SV in VA - k7sv@juno.com
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