El 31-08-12 20:07, Dave Platt escribió:
In ikabel.net,
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 )

Assuming power of about 1 kW and a HW dipole (say 60 Ohms), the
voltage will be about 350Vp. I can hardly imagine that a plastic gap
of 0.05m will degrade to failure because of tracking.

On the other hand, if the original poster is attempting to drive this
antenna off-resonance, with open-wire feedline and a wide-
impedance-range transmatch / "antenna tuner", then it's entirely
possible that the feedpoint will sometimes be "looking into" a much
higher impedance, and that a matched drive will result in very high
voltages at the feedpoint. Even a coax-feed antenna might have this
problem, I suppose, if the split element is being driven by something
like a delta match which has a relatively high impedance-
transformation ratio.

From what I see in a short Google-search, fiberglass *can* be
hygroscopic, depending on what resin was used to bind the fiberglass.
A fiberglass rod which was made with a somewhat-hygroscopic resin
(e.g. polyamide) might tend to behave badly in the face of high RF
voltages, whereas a rod made with a non-hygroscopic resin might be
fine unless dirty or wet.


Hello Dave,

Thanks for de addition. During the simple voltage calculation my mind
said: "what if VSWR is really bad".

With 1 kVp over 0.05m I would not expect problems. You are right,
Polyamide (Nylon) is bad for RF and as far as I know, isn't fully UV
resistant without additives.

Once I had a nylon bolt in the field of a tuning capacitor running
close to breakdown over about 3mmm. The bolt produced lots of smoke
within some seconds and broke down. Changing from PA to PE solved the
problem.

Assuming 50% glass fill factor and effective loss factor of 0.2 (so Q
of the glass/PA composite material is just 5), I would expect a
dissipation in the range of 4 Watts at 1 kVp and 10 MHz in a 1.5 Inch
thick massive rod. Maybe Kba can provide us some additional info on
the working voltage across the gap (and frequency).

When using epoxy or polyester fiberglass, loss will be significantly
less.

--
Wim
PA3DJS
www.tetech.nl
Please remove abc first in case of PM