Hi, Julio,
Sorry for delay. I didn't see this post when it hit the reflector.
Around 2009 before publishing the FCP design, the change to polyimide
coated wire and Teflon sleeve brought a string of toroid device failures to
a sudden and permanent halt and turned previously "fragile" devices into
build-it-once-keep-it-for-life items.
At root this specific choice (sleeving and coating) relates to:
1) Availability: The polyimide wire (used in automotive products) and
Teflon tubing are fairly common.
2) Hardiness: Teflon sleeving protects polyimide from nicks and abrasion in
construction and installation
3) Lightning: Since these transformers are used outdoors and near or on
lightning fodder like trees and towers, enough insulation is needed to
withstand surges from nearby lightning strikes. 55 kVDC combined insulation
between primary and secondary in isolation transformer appears to be more
than enough except in direct strikes. Nothing you can do will **guarantee**
protection from damage in a direct strike. May improve odds, but no
guarantee.
4) Static voltage: 55 kVDC is enough insulation to withstand wind static in
winter.
While not being destroyed running 1500 watts certainly is a helpful
outcome, 1) 2) 3) 4) above have a larger insulation requirement with their
higher voltages, and **apply equally to QRP, LP and QRO.**
Not breaking down at QRO gets a free ride if you have solved 1) 2) 3) 4).
Historically our principal problem with QRO power per se had to with
attempting to use ferrite and its high mu in highly reactive situations,
and in generally unsuitable antenna/feed designs with very high common mode
voltages.
If the polyimide wire is not sleeved, then easily made nicks can and
probably will break down and start carbon tracks at 100 watts. The
polyimide withstands heat and voltage well, but is fragile and easy to nick
if mishandled.
Double polyimide #14 and teflon tubing is the specific choice used by
W2FMI(SK) for cause as explained in his balun book for items with this kind
of stress. {"Understanding, Building, and Using Baluns and Ununs -- Theory
and Practical Designs for the Experimenter." Jerry Sevick W2FMI (SK),
Copyright 2003 CQ Communications, Hicksville, NY}
You can't lay coated wire directly on cores. Cores can have tiny little
sharp places which will penetrate/nick the coating as it is being wound. Or
a drop will bang the wound wire hard on the core. Been there, done that.
This is NOT speculation. It has to be protected with something. Polyimide
coated wire, if it is never ever nicked, before or after winding, will
function until an arc to the core.
Teflon sleeving, immediately pulled over the polyimide wire before winding
and exposure to environment, is protection against nicks and ordinary
handling. This creates an isolation transformer that withstands induced
voltages from nearby lightning and so will not send those surge voltages
down the center conductor to the shack.
In the beginning W0UCE and I tried all the cheepy short cuts (which turned
out to be long-cuts), but thankfully eventually learned from our dumb
mistakes.
Verbal out-takes from the FCP development saga, we were really bad...
"Wow, I didn't know we could destroy anything that fast."
"Why is the SWR curve so flat?"
"He's asking if we are running low power (we weren't)."
"That really blew up into a lot of pieces. Make a good rattle."
"When did you buy this coax?"
"This isn't soldered and it's full of water."
"Jack, your utility building is smoking. Really, it's smoking."
"Wasn't this core supposed to be painted red? Maybe the fire changed it's
color? Uh, I think that's ferrite and the wrong size core. When did we make
this? Is this one of ours? (It was)."
"We're going to have to order it and wait. In the meantime let's try this
stuff."
"Didn't we do this same thing last year?"
73 All,
Guy K2AV
On Tue, Sep 4, 2018 at 2:59 PM VE3FH via Topband <topband@contesting.com>
wrote:
> Jim,
> Did you build the isolation transformer exactly as described in K2AV's
> article?? By exactly I mean using "teflon sleeved double polyimide
> insulated" wire. I'm not planning on using it with more than 100W...
> 73,Julio VE3FH
>
> Sent from Yahoo Mail on Android over Bell Mobility Network.
>
> On Tue, 4 Sep 2018 at 12:44, James Denneny<57jndenneny@comcast.net>
> wrote:
> Gary, I use an inv L with K2AV FCP and matching balun. I purchased a
> RigExpert AA-30 to tune out the reactance. It worked great. No tuning cap
> or inductor were needed. It took just a couple cuts to dial it in. My L
> is now 126 ft long with a 65 ft vertical leg standing a few feet off a tree
> trunk. The SWR is now much sharper as it should be.
>
> We live on a small lakefront lot and there was not sufficient room for a
> decent TB ground radial system. Our location is semi-rural. So, man-made
> noise is not a frequent issue. The FCP and trimming of the antenna has
> produced a major improvement in the L’s performance. It enabled me to
> reach TB DXCC this past season.
>
> Best Luck
>
> Jim K7EG
>
> Sent from Mail for Windows 10
>
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband
>
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband
>
_________________
Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
|