Topband: Insulator problems

[Don Kirk]

Greg,

One property you should look at when deciding the material to use for your
insulator (especially if exposed to water) is a property called water
absorption and normally it's listed as a weight percent.  Some nylons are
better than others, but nylon in general absorbs a lot of water and
therefore probably not your best choice of material.

73,
Don (wd8dsb)


On Fri, Dec 13, 2013 at 9:19 PM, Greg - ZL3IX <zl3ix at inet.net.nz> wrote:

> 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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