The driven element on a TH6DXX was recently changed which was easy, figuring
out why it didn't work after being replaced was the tough part.
Turned out that one of the reworked traps blew when 1000 watts plus was
applied. The ant. worked okay at low power, but didn't work at high power -
indicated something was arcing.
The problem was found when power was applied and the sissling going on in
the 15 meter driven element trap could be heard.
The element was removed from the TH6DXX at the top of a 125 ft tower.
Findings: Approximately 30 percent of the end of a new trap cap located at
the open end of the 15 meter trap was melted. Once the trap was taken
apart, the two plastic spacers at the open end of the trap were found to be
burnt through and the residue was all over the trap.
These spacers weren't in perfect shape when the trap was rebuilt (plated
screws replaced with stainless steel and all the crud from years of exposure
cleaned out) along with the rest of the ant. However, at 1000 plus watts,
the RF found the path of least resistance!
The ant. was purchased used and it had new trap caps. I figure the old trap
caps deteriorated allowing the spacers to weather before the new trap caps
were installed. The new trap caps were installed by the previous owner and
the ant. was never driven with 1500 watts RTTY.
The weathered spacers were "dried out" and didn't have the same
resilience(?) as the ones that weren't exposed to the weather. A good
spacer will have a greenish-yellw color. The failed spacers were whiter.
Hope this bit of information helps someone reworking a THX that not only the
trap end caps need to be looked at, but the trap spacers as well. Replace
any of the trap caps and spacers that looks dried out.
Now to figure out how to get the ant. adjust for a good SWR in the CW
portion of 10 and 15 meters.
73,
Mike, K4GMH
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