Split driven element insulation... ?
 

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

Split driven element insulation... ?
 

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 ?

The traditional approach is to use glazed ceramic insulators, I
believe.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Split driven element insulation... ?
 

KBa wrote:
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

Such a problem is going to come from one of two things:

Too much power for too little separtion and arcing; use wider spacing and
less power.

Accumulated filth and moisture leading to leakage leading to carbonization;
heat shrink tubing or tape like Coax-Seal to keep the path clean and dry.

Split driven element insulation... ?
 

"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'll leave the specifics of your question to the experts.

But, worth mentioning is that once you have selected a material, plop some
in a microwave along with something like a a container of water. Then see
if your insulating material stays cool.

Wayne
W5GIE

Split driven element insulation... ?
 

On 8/30/2012 4:15 PM, Wayne wrote:


"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'll leave the specifics of your question to the experts.

But, worth mentioning is that once you have selected a material, plop
some in a microwave along with something like a a container of water.
Then see if your insulating material stays cool.

Wayne
W5GIE

Which is valid at the microwave oven frequency. Might be relevant at
HF, or more likely, not.

tom
K0TAR

Split driven element insulation... ?
 

El 30-08-12 20:49, KBa escribió:
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

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.

Make sure you have UV resistant rods, and/or add some layers of UV
resistant (marine quality) coating.


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

Actually, a material can be perfectly fine at microwave frequencies and bad at HF. And vice versa. The microwave oven test should only be considered a *very rough* indication of material suitability.

73,
John
K4KQZ

Split driven element insulation... ?
 

"tom" wrote in message
. net...

On 8/30/2012 4:15 PM, Wayne wrote:


"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'll leave the specifics of your question to the experts.

But, worth mentioning is that once you have selected a material, plop
some in a microwave along with something like a a container of water.
Then see if your insulating material stays cool.

Wayne
W5GIE

# Which is valid at the microwave oven frequency. Might be relevant at
# HF, or more likely, not.

# tom
# K0TAR

I'm sticking with the test being relevant at HF.
A material that does not heat up at microwave, would be unlikely to heat up
at HF.

At any rate, the test can be done for free.

Split driven element insulation... ?
 

In article . net,
Wimpie 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.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Split driven element insulation... ?
 

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