Hi Rick,
No magic. Just the right stuff, for the FCP, anyway.
To your particulars, when Jerry Sevick measured various ferrites, he
measured the lowest loss at mu=40. This was very low, as was the #2
powdered iron at mu=10, the lowest of the powdered iron formulas. So use
of ferrites introduces a significantly reduced number of turns, with a much
more coarse granularity of possible inductive values, that has to be worked
out to use with the FCP, AND the behavior of the mu=40 core under highly
reactive loads is not spec'd by the manufacturer. The ferrite will have far
less radiating surface if it does heat up under extreme reactive load, AND
the length of the parallel bifilar wires may not be enough to cover 160
with the needed behavior in this app.
But my admonition about no ferrites has more to do with my knowing that
people have misc T240 and T200 form factor ferrites of ALL KINDS laying
around, and want to use the one they just found down in the junk box, which
does NOT have the material # or mu marked on it anywhere. And these
requests seem to pop up just before a contest, when most hope of getting
the correct stuff before contest has gone by the wayside. You will note
that I have pounded in Amidon T300A-2 #2 powdered iron toroid or strict
equivalent, over and over again. And I get back "please, please, please
tell me that I can use my junkbox ferrite toroid". But don't hold your
breath. In over four years of working with non-resonant antenna solutions
on 160, **NONE** of the ferrites we tried to use made it. We burnt or
cracked ALL of them at QRO. ALL of them. Really. Ferrite demolition
derby.
I have a new collection of #31 ferrite stuff, but I use those in low band
RFI suppression, not transformers. So I'm NOT going to tell anyone it's OK
to use a ferrite toroid for feeding an FCP.
Beyond that, what is NOT in doubt is that the #2 powdered iron choice works
and works well. The installations where I have been able to run QRO brick
on key and quick go check toroid temp have all been stone cold. Anyone has
contrary experience please let us know ASAP. We will certainly want to
investigate and determine why in one place and why not in another.
As to why he made his #2 powdered iron choice, Jerry Sevick W2FMI, covers
this convincingly in his book, pages 58-63, which I will NOT try to
reproduce on the reflector, as even if I did, I can't pass along the
essential graphs, photos and diagrams. ("Understanding, Building, and
Using Baluns and Ununs -- Theory and Practical Designs for the
Experimenter", Jerry Sevick W2FMI, Copyright 2003, CQ Communications, Inc.,
Hicksville, New York)
I have found, over the time that I have been working on top-band issues,
that dropping back after blowing something up and consulting W2FMI or his
material has been most valuable.
Alas, Jerry is a silent key, and I can no longer phone him to further
expound upon his choices. He particularly chose the large #2 powdered iron
cores for applications with a lot of stress on the core, particularly 160
meters. Some key quotes:
"Because my simple loss measurements indicated that the higher permeability
powdered-irons had more loss than the No. 2 material, I decided to design a
4:1 Ruthroff Balun using this material--but with a larger core and more
turns than the McCoy Balun. Although McCoy's design has enjoyed
considerable success over the years, I felt that a larger inductive
reactance was desirable in order to assure better performance on the lower
frequency bands (particularly 160 meters). [Ibid p.59]
"I knew ... that loss with ferrite materials was related to the voltage
drop along the length of the [bifilar winding] and to the ... permeability.
Permeabilities of 40 (No. 67 Ferrite) exhibited the lowest loss. ...
powdered iron #2 material with a permeability of 10 also showed the very
same low loss. Because powdered-iron material has been known to be more
rugged and linear than ferrite material, this suggested that other
powdered-irons ... should be investigated. ... However all four materials
showed a definite lower input impedance than #2 material, ..." [Ibid p.59]
Sevick had access to all of the ferrites, but did not choose any of them
for his monster hang-on-the-back-of-the-tuner 4:1 Balun, especially
intended to deal with wild voltages, very reactive loads, etc, and handle
them as well on 160 as 80-10. Instead Sevick chose the T400A-2 and #2
powdered iron to lay out there as his personal method. So we've done the
same. And everywhere we've used teflon-sleeved #14 double polyimide on the
#2 powdered iron cores, bad stuff has just quit happening, they work, and
they run stone cold.
At various scattered places in the book Sevick talks about all the issues
that come to bear on designing an isolation transformer on 160 meters. I
may not be able to develop a federal level proof case satisfactory so some,
but I am sticking with Mr. Sevick. His guidance has always panned out and
explained what was ailing. He did his high-stress 160 meter windings on
large #2 powdered iron toroids after research. So are we. We have the
hard-won results that prove out Sevick's and our choices.
73, Guy.
On Thu, Dec 15, 2011 at 2:58 PM, Rick Karlquist <richard@karlquist.com>wrote:
> Guy Olinger K2AV wrote:
>
> > Martin has the right instincts, says it feels fishy for this. But it's
> > worse than he thinks. And don't even consider using ferrites.
> >
> > The T300A-2 core and 20 bifilar turns were SPECIFICALLY chosen to produce
> > a
> > residual inductance value in the right order of magnitude to cancel a
> > typical residual capacitive reactance from a 160 meter 5/16 wave single
> > wire folded counterpoise, AND provide enough coupling to make the
> > transformer work, AND maintain a low enough loss to operate QRO without
> > heating, or especially to operate QRP without further handicapping the
> > operator with needless loss. This allows the builder of the "simple
> > solution" to prune the wire to get resonance and remain somewhere around
>
> I'm trying to understand here what is magic about powdered iron.
>
> It is true you can't just use any random piece of ferrite.
> But if the T300A-2 were replaced with LOW PERMEABILITY ferrite
> having the same permeability as a T300A-2 core, it would produce
> the required residual inductance. The loss of low permeability
> ferrite is extremely low, probably lower than powdered iron.
> Coupling is a function of how the turns are wound, not the core material.
>
> Am I missing something?
>
> Rick N6RK
>
>
>
>
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