On 1.9.2012 19:20, Jeff Liebermann wrote:
On Thu, 30 Aug 2012 21:49:15 +0300, KBa wrote:

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

In a past life, I helped design marine radios. As others have
mentioned, there are different flavors of fiberglass, some of which
are not really suitable for outdoor or high humidity applications.

Instead of trying to fix fiberglass, you might consider machinable
glass ceramics.
http://www.matweb.com/Search/MaterialGroupSearch.aspx?GroupID=301
Something like material:
http://www.matweb.com/search/datasheet.aspx?matguid=848bdecf89b74ef986925162e6a 6255e&ckck=1
http://www.corning.com/assets/0/965/989/1081/1397D5E7-018E-4CF4-A34C-6814B815BCAC.pdf

It's sufficiently strong to handle antenna elements, but was also
somewhat brittle. RF characteristics are superb. 9.4Kv/mm is not
going to arc. However, you won't like the prices. A 1" dia 6" long
rod will cost you $150.
http://www.astromet.com/MacorPriceSheet2010.pdf
For ham radio, nothing but the best.

Otherwise, there's always the traditional dry pine dowel soaked in
bees wax.

Well, hmmm...
There seems to be several alternatives to fiberglass... and several
grades of it...

Just thinking of how glass ceramics may behave under bending force...
Element center gap is under very small vibration (wind load) and element
static load stresses the insulator center... it bends slighly depending
how elastic is the material...
Fiberglass have some elasticity... it's not brittle... on the other hand
there could be support limiting the bar bending or the element halves
might have more than two clamping points... so the center gap would
be steady...

Actually I had in mind using extra varnish layers on fiberglass after
processed to diameter and then boiling it in bee wax...
This is still the plan b or c.

tnx oh6io