Topband: Base Insulators for Verticals
W0UN -- John Brosnahan
shr at swtexas.net
Sun Sep 10 09:39:27 EDT 2006
While I can't argue with the logic expressed here concerning
ceramic insulators, I must point out that they were designed
during a time when there weren't many choices for materials
and for an application that often involved 50 KW or more into
a NON-resonant antenna. Most AM towers are NOT resonant
at their operating frequency, meaning that their base insulators
may have to withstand higher voltages or currents than would
be expected in a resonant system.
Ceramic has great compressive strength but performs poorly
in tension loading, which is why ceramic egg insulators are
designed so that the load is applied in compression.
But in this modern era there are a lot of other alternatives, many
with good tensile strength as well as compression strength. I have
been using fiberglass rod insulators in my 160M 4-square for more
than 15 years with NO problems, as have many others. G10/FR4
Fiberglass performs very well in tension as well as in compression
as shown by the fact that there are many fiberglass GUYS that are
loaded in tension.
The main DISADVANTAGE with fiberglass is that it does not
handle UV exposure very well compared to ceramic insulators.
But this can easily be addressed in any number of ways -- covering
with good quality electrical tape, heat shrink tubing, or with a
quality marine paint.
In my specific installation I have 160 ft towers of Rohn 25 with an
elevated feed point at the 25 ft level. While my Colorado location
is not the windiest part of Colorado it does get winds up to the
80 mph on occasion and gets "Wyoming" winds much of the winter
where it can see 20-30 mph winds for days on end.
Now that I have moved to Texas I will be taking down the 4-square
and will give the insulators a very serious inspection, but I think
my 15 years of experience with no failures speaks for itself.
As the price of insulated sections for Rohn 25 have soared and
as the alternative insulator options have also increased in price,
I have considered making my design available commercially. But
my design involves some work since the insulators are inserted
into the middle of a section rather than between sections -- in order
to avoid having to reduce the diameter of the insulators to something
that would fit into the swaged ends of the legs at the top of a section.
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