The Tx antenna I have been using very successfully for the last 8+
years, is a top-fed vertical. This feed arrangement requires the yagi
used for top loading to be insulated from the mast. I have been using a
nylon insert between the two halves of the stub mast as my insulator.
This morning I noticed high SWR, firstly only on high power, but then at
any power level.
Today I brought the mast down for inspection, and the only sign of
trouble I can see is in the insulator, which has bubbled visibly. This
may (or may not) be the problem, but I propose to change the insulator
even if only to eliminate it as the culprit. I have a couple of
questions for this group.
1) Does anyone know if I can upload a jpg file to contesting.com, so
that guys can see what I am talking about? Tree, I guess I can't attach
a photo to a post to the group?
2) I always thought that nylon was a pretty good dielectric, and did
not expect problems, especially at 1.8 MHz. The gap in the insulator is
7.5mm, or about 0.3". I estimate that there will be around 2 kV across
this gap. Is nylon perhaps not as good as I thought it was?
3) If I replace the nylon with Teflon, will I lose anything in
mechanical strength?
Unfortunately this problem means that I will not be able to enter the
Stew. We are going away for a week for the festive season, next
weekend, so won't have time to fix the issue.
Comments welcome.
73, Greg, ZL3IX
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