Insulator was round fiberglass tube of 65mm diameter and around one
meter long, used as center of 7MHz antenna feed element ( 60mm diam.),
the outer surface of the tube was weather ( and UV ) protected and
water proof, but the 5 cm gap inside the tube was not entirely though
element ends were not in open air... some moisture do condensate
always inside the element... I guess...

This changes the story when moisture can enter the inner side of the
tube and may stay there. I assumed you used a massive rod for
mechanical strength. At the inner side you don't have the cleaning
effect due to rain, wind, precipitation, etc. Could there be salt
built-up (due to seeping water that evaporates slowly)?

You mentioned it is 1 m long, that is really a large creepage
distance! Assuming about 5mm wall thickness and fully conducting inner
side of the tube, radial field strength (350V) will be in the 50V/mm
range, well below permissible levels to avoid insulation failure at
MHz frequencies in FR4 material.

It would be nice to inspect such a failed tube to figure out how the
tracking took place, because I have now idea.




Measured impedance was quite low, 25 ohms resistive near resonance, so
probably voltages have been also low at one kw power level.

I cannot tell the exact fiberglass grade that was used, probably it
was green FR4 or other strong quality as mechanical stress was high in
the insulator and long and heavy 7MHz element halves loaded the
insulator center.
This fiberglass insulator tube finally got to a ohmic short and didn't
recover.

Later this insulator was replaced with another type and problems
disappeared.

. . .

Now one other future split driven OWA antenna has been under plan ...
but there are the same mechanical stress problems as element center gap
should ( ? ) have center support rod but it exposes also to all
weather effects... in this case element diameter will be 25 mm...
So insulation in the gap should be good but material should be also
something capable handling bending moment of the element halves...

Now the latest plan is try to move the mechanic load away from the
center support rod to element support plate which could be much longer
in length... then the element center stress is expected to be lower
across the gap...

Then also the element clamps have more separation as this support plate
is also again FR4...
Secondly the element halves will be insulated from the clamps so there
will be less risk of this FR4 plate becoming conductive...
Insulation between element and clamps could be thin straps of teflon
sheet...

Best might be if the fiberglass center support rod can taken totally
out or be of round teflon bar... good insulator, low friction,
probably air particles etc. don't stick on it easily...

Teflon feels tough but elastic, so in the stress point it's not much
use, but probably functions as a vibration damper in the gap...
assuming
the element support plate does not transfer element end bending effects
due to wind load directly to element center gap...

Anyway it's going to be interesting to see how these precautions
will actually work... in both at mechanical and electrical
reliability... hopefully in both.

Besides mechanical cracks that may provoke solid insulation breakdown,
carbon trace forming (tracking) over wheather exposed plastic surfaces
is the most likely problem, so I think you should try to maximize the
creepage path. As E-field strength in the insulation materials will be
low, you don't need low loss materials.

Thanks for sharing the info on the insulator failure!


Hi

Yes, thanks for good material analysis. It was really very odd to find
this kind of failure *can* occur as gap was large and the material was
good quality. Visually by eye there wasn't anything unusual in the gap
inner surface, maybe they had been visible by more accurate means. But
aging exists in all materials and other environments than dry and clean
air shorten time for problems.
Insulator structure must taken account too...
Now to avoid similar troubles takes some more planning, the next
insulator is smaller in diameter and element mechanical stress is also
much lower...

tnx oh6io

"KBa" wrote in message
...
Hi all

Problem exists how to get reliable insulation to split driven element
and support rod which is exposed to RF voltage ?
Fiberglass is told to be sensitive creating conductive coal brigdes which
creates true resistive short at driving point ? (at least one fiberglass
tubing is done so... gap was 2 inches )

How to improve insulation ? Ptfe-shield over fiberglass rod, heat shrink
tubing, varnish layer ?

tnx oh6io

I used a chunk of circuit board material about 5 mm thick. I don't recall
where I got it or why it was so thick but it worked very well for a short
period. Can you go to a surplus store and root around in their junk boxes
for old circuit boards? If you spray them with electrical varnish after
making the connections, you should get some time before failure.

"Sal"