Hi Art.
Short answer:
Just do it. Go have some fun. SWR does not correlate with performance
unless it keeps your transmit stuff from developing output or blows it up.
Get your Sark fixed. Let us know how it got fried. Overall that is a very
handy portable package for an analyzer with a lot of neat features.
Long answer:
Very very hard to fry an isolation transformer made and wound to the spec.
So hard, that I do not have even one reported "burn up" failure in normal
operation. Unprotected windings dropped on concrete, etc, are excluded from
"normal operation".
Myself, particularly in the CQ 160 SSB contest, I operate on my 160
L/IsoT/FCP to up near 1.9 MHz, with my 8410 full 1500w output. This is
possible because the amp, using tetrodes in class AB1, no grid current,
will go to almost 4:1 SWR before faulting. There is a point where the
voltages get so out of line from shack to antenna that they can arc from
the center conductor to the shell of a PL259. In my experience,
miscellaneous arc points have been the problem, never the isolation
transformer.
I still have never made the isolation transformer warm. After I got my
8410, I put 1.5 kW through the transformer, 15 minutes brick on key. Lifted
the key, RAN out to the box and felt the core. Was not even warm. 8410 not
breathing hard. Same after extended contest operation at full QRO.
The basic concept of using bifilar windings on a #2 Powdered iron core to
deal with awful transformation tasks is NOT ours. It belongs to the late
Jerry Sevick, W2FMI, balun guru extraordinaire. Many will remember that
call sign as attached to a line of devices still sold by DX Engineering.
Jerry tested a whole bunch of cores, different ferrite and powdered iron
mixes, before settling on teflon sleeved #14 Thermaleze** wound on the
winner of his toroid contest: #2 powdered iron, Mu = 10. It was the lowest
Mu for core material that would bring about needed transformation.
(** polyimide high temperature and high voltage enamel-like insulation.
Thermaleze (Belden) and Allex (Essex) are brand names for this winding wire
using this insulation.)
W2FMI then used those conclusions to settle on his recommended balun for
the nasty end of an antenna tuner, seen in his photo 9-G on page 61, in the
chapter on baluns for antenna tuners.
All of that is in his book, published by CQ, available at:
https://www.dxengineering.com/search/part-type/reference-books/product-line/cq-communications-understanding-building-and-using-baluns-and-ununs?autoview=SKU&N=4294952867&keyword=w2fmi&sortby=BestKeywordMatch&sortorder=Ascending
The bifillar winding in Jerry's tough balun for antenna tuners is
configured as a 4:1 balun. If you change its winding's connections to the
coax jack and two balanced terminals so that both ends of one wire of the
bifillar winding are only connected to the coax jack, and the other wire's
ends only to the two terminals, you have the isolation transformer. Just as
simple as that.
Getting an EFFICIENT inverted L to play at 50 ohms can be quite a task. The
real nature and variability of an inverted L is only exposed when the
amazing amount of loss in some installations is removed and no longer
swamps the result.
When you've got your Sark back, I can get you my cell phone number and talk
you through taming the L. It sometimes gets a little deep.
73, Guy.
On Thu, Nov 17, 2016 at 9:26 PM, Art Heft <artandkaren@gmail.com> wrote:
> This is directed to Guy but some of the rest of you may find it
> interesting. I am unable to put the finishing touches on the new inverted
> L because my SARK 110 analyzer has gone belly up. Right now the sweet spot
> with close to 50 ohms is up at the top end of the band. There is some
> inductive reactance too. I want to use it at the low end of the band.
> Here is the question: can I use a tuner in the shack to make the rig and
> 500 watt amp happy without frying the FCP transformer?
> Thanks and 73, Art K8CIT Hillman, MI
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