[TowerTalk] KT34XA PROBLEMS RESOLVED

Tue, 02 Sep 1997 15:36:03 EDT

The KT34XA won a bunch of battles, but K7SV won the war! This was really
a horror story, but I'll limit this to lessons learned.  In case you
didn't see my earlier posting, the swr at the low end of 15 became
excessive after an ice storm several years ago. Putting a 2 element 40
meter yagi (KLM) eight feet above the XA aggrevated the problem. 

As I said earlier, I suspended the boom alongside the tower which gave me
access to each element without having to take the antenna down. This
worked very well, and I noticed very little difference between the
antenna in this configuration (reflector at the bottom and about 20 feet
up) and at the top of the tower.

1. As I suspected the problem was in one or more of the capacitor banks,
but to my surprise the problem was not in the straps that go around the
capacitor tubes. It was in the joint where the main element meets the
strap coming from the capacitor tube and the ten meter linear loading
tube. 

2. Based on continuity testing, I would have never suspected this joint
to be the problem. Before taking the antenna down the last time, I
noticed that it had a nice looking swr curve on 15 meters, it just
happened to be centered about 200khz too high. I checked the resonant
frequency of each separate element on each band. Both driven elements on
15 appeared to be high, but I wasn't sure how they would react in the
presence of the rest of the elements.  

3. I started looking at this joint, and discovered that some of  the
plastic peanuts inside the ten meter linear loading tube had been
crushed, and in several instances, the tube was flattened. As I played
with the antenna, the last half inch or so of several of these tubes
actually broke off. I repaired any that appeared to be broken or weak. On
the rest, I put a flat washer on the main element, then put the ten meter
loading tube over the washer, and put the strap from the 15 meter
capacitor tube over the tube.  After doing this, I noticed that the
resonant frequency of the rearmost driven element dropped by 200khz and
that of the front driven element dropped by 100khz. 

4. Before discovering the above, I managed to get the swr at the low end
of 15 down to 1.5:1 by lengthingthe  reflector. Knowing that my 2 el 40
meter yagi pulls the frequency of the XA up on 15 meters, I decided to
leave the reflector an inch longer on each side. 

5. In view of the problem I had, I would suggest that anyone with a
KT34/KT34XA replace the poly peanut with a harder substance. I had some
hard plastic stand-off insulators that I used for now, but next time the
antennas is on the ground I am going to replace all the ten meter linear
loading tubes and am going to insert aluminum rod in the ends in place of
the plastic peanuts.  It's possible that KLM has already replaced the
peanuts with a harder substance, but I am not convinced that this is a
weak point.  

6. I would also suggest that anyone putting up a KT34/XA measure the
resonant frequency of each element on each band. I measure mined on top
of an eight foot step ladder and made sure it was well seperated from the
other elements and anything else that would effect the resonant
frequency. Next time I have a problem I know that I have to restore the
elements to the frequencies that I have recorded. 

7. The curve looks great on each band, and on 15, the swr at 21000 is
1.3:1 and I have to drop 150khz below the band to hit 2:1.  It's great to
have my XA working correctly again. F/B appears to be at least 20 db on
each band. 

I became very disallusioned  Saturday evening. After NJ4F helped put the
antenna back on top of the tower, things looked real good. However as I
played with it, I noticed something was intermittent. Then it just
stopped working period. The problem turned out to be the balun. One of
the wires that is soldered to the brass machine screw on one side broke
loose. 

I found use of the AEA HF Analyst to be a life saver both on the tower
and in the shack.  Without it I don't know how more trips up and down I
would have made.

Thanks to NJ4F for the encouragement and help on the tower and to Bruce
at KLM for answering all of my questions. Thanks also to several who
provided hints based on experience including N6BT and K3MM.

I thought about replacing the aluminum straps that go around the
capacitor tubes with a 2inch stainless steel hose clamp (the left over
band is just enough to reach the joint!). However Tom N6BT told me they
had tried using a stainless steel hairpin in an hf antenna and found it
to be so lossy that it got hot enough to color it. I did find that it's
fairly easy to reshape the aluminum straps to get good pressure on the
tubes. 

I still don't profess to understand this antenna, but I certainly know
more than I did a month ago! 

See y'all in the contests. I'll let those who asked know what happens
once I put the 2 el 40 back up again

73 de Larry K7SV

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