On Sun, Nov 11, 2012 at 7:50 AM, a prior poster wrote:
> If voltage or heat at a low impedance feedpoint is so high you need a
> special sleeve, that is a bit of a warning flag about system design. You
> probably could use almost anything, including small plastic fuel line hose,
> insulating spaghetti, or even tape.
>
Short version:
There is always an urge to see how little we can get away with. It's an
experimenter's itch. I understand it well. It's an Olympic sport around
here in the crowd I run with. But we do not export ideas that we managed
to burn up. We only do the best things for our friends.
Long version:
Resonance is over-hyped. There are many superior performing non-resonant
antennas and antenna components, often requiring matching devices to get to
50 ohm transmitter/amplifier output impedances. With non-resonant antenna
devices comes the need to deal with higher voltages. The higher voltages
are expected and routine.
An FCP is a non-resonant solution to a counterpoise problem. It exhibits
significant capacitive reactance at the operating frequency. It was never
intended to be resonant. It was intended to be a low resistance
counter-field device with the least amount of ground field induction
possible in a very small linear footprint. All other characteristics in
the design are secondary to that goal. Practically, it requires an
isolation transformer wound on a very low mu core with proper construction
for high voltages.
Going back over a decade or more, we gleefully tried what rather resembles
the prior poster's minimalist collection of Rube Goldberg insulation
strategies. We launched a string of burn-em-up, crack-em-up, blow-em-up,
arc-over episodes under extended 1.5 kW stress and in a lightning prone
area. For quite a while it was entertaining, because we were only winding
for ourselves, and it was like being a kid again, blowing things up.
But when we started winding these things for friends, we had to get
serious.
Complaining all the way, we grudgingly worked our way up to the current
published requirements. We developed them on various apps, like an
auto-transformer for end-feeding an 80m 3/8 wave L, just one
example. Used #14 solid copper because currents could get up to 10 amps RF,
even more depending on the specifics of some vertical radiators. We went to
double polyimide for HV insulation and high temperature characteristics.
Conveniently #14 double polyimide insulated wire was easily available
in retail quantities. #12 standard wall teflon sleeving was easily
available, used for additional HV insulation, high temperature
characteristics and to protect the polyimide against nicks. The high
temperature ratings also guarded against destruction by overheating when
soldering connections and during very short interval extreme current
episodes, aka lightning induction. All before the FCP. When we knew we
needed an isolation transformer, we knew how to wind it AND keep it.
When it came time to publish the FCP and the isolation transformer, we had
no intention of publishing methods we ourselves had already seen arc,
crack, shatter, blow up, melt, burn, etc. We used our decade-long
battle-tested polyimide/teflon/low mu powdered-iron method for windings,
hardly a theoretical choice.
I have had some dozens of urgent requests to bless lesser cores and
materials, which were politely denied. Do less than the published spec and
you're on your own. Sorry. We already tried that and blew it up. Been
there, done that. BUT, good luck! :>)
Not at all did that stop everyone from trying lesser methods. In particular
one gentleman duplicated a lot of our winding demolition derby, only
compressing it into a few months. He confirmed with us his own
observations of destruction by less-than-spec means, and he finally used
the published components because he was tired of the failures and wanted to
move on to operating 160 without melting, burning or cracking something out
in the project box.
As I said previously, when we got to #14 double polyimide, standard wall
#12 teflon (PTFE) sleeve, and #2 powdered iron toroids, we just simply quit
having problems. No arcs, no smoke, no cracks, no blow-ups, no melt-downs.
And it ran stone cold used as directed.
Over the last two years I have had four trees die, that tree guy says were
split by lightning, within 500 feet of my 160 antenna, FCP, feedline and
matching box. Will soon be paying tree guy to take them down, so they don't
rot and fall on things we want to keep, like the fence around my neighbor's
swimming pool. Yet, with that kind of induced lightning stress, even with
twelve hundred feet of elevated conductors to channel lightning induction
to my isolation transformer, it is still functioning.
That's not an assumed outcome and I wouldn't dare guarantee it, but I
certainly will take it. Personally, with lightning I expect shards and
melted things in the antenna match project box, vaporized wire and all the
other familiar outcomes. One local opinion explains it saying if lightning
can't get started with an easy arc it will take an easier path close by.
Sounds interesting, but no way to prove it one way or the other.
You will not catch this writer recommending electrical tape to insulate
windings on toroids. We recommend the best we know. We like to keep our
friends. We really don't care how cheaply someone else could do it and
MAYBE get away with it. If you have OUR luck with lesser methods, it's
only a matter of time before it fails :>)
73, Guy.
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Topband reflector - topband@contesting.com
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