Good morning to Carlo the group,
I think that Carlo, IK3LMA, who had a similar problem with his KLM
KT34XA, may have provided the key to answering my question. Carlo has
experience with the marine use of glass fibre in boat construction, and
his solution was to apply a coat of resin called a Gelcoat to the
degraded surface of the glass fibre tube on his antenna. Gelcoat is the
shiny exterior surface we see when looking at those beautiful creations.
I had wondered why a similar problem had not appeared on the many glass
fibre boats that are now in use, and the answer must be the initial
Gelcoat that is applied to the boat mould before the first layer of glass
fibre mat is put in position. I read that Gelcoat is formulated to
provide resistance to UV and hydrolysis.
I assume that glass fibre tube must be manufactured in the same way as
glass fibre fishing rods, which is to use a finely woven glass fibre
"cloth" that is pre-impregnated with resin. The pre-impregnated cloth is
then tightly wound on to a shaped mandrel, and when the final wall
thickness has been reached, the rolled "pre-preg" is held on the mandrel
by tightly wrapping it with mylar tape, the complete assembly is then oven
cured, and the mandrel withdrawn, the mylar tape unwound, leaving a glass
fibre fishing rod blank.
It would appear that the glass fibre products used by antenna
manufacturers do not have the additional protection of a gelcoat resin
layer, because it is not required in the tube manufacturing process and
for most applications using glass fibre tube, it probably does not matter.
But it does matter when used as spacers or insulators in the manufacture
of antennas, exposed to UV and rain etc. as they are.
The group solution for "weathered" glass fibre tube is:-
1. To clean off the degraded surface of the glass fibre tube and apply a
layer of Gelcoat to the exposed surface. (Gelcoat available from your
nearest boat/ships chandlers).
2. Others have suggested, clean off the degraded surface and tightly wrap
the tube with overlapping plumbers ptfe tape, followed by an additional
overlapping wrap of self amalgamating HV tape.
3. Most antenna manufacturers now include stainless steel fittings in
their assemblies, perhaps we should pressure them to select suitably
treated glass fibre products for inclusion in their designs as well.
I would thank the group for their patience and especially those who
responded with comment and advice. I will now continue with the work on my
X7.
Sincerely, John. G3JVC/GM3JVC.
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