Topband: K2AV Folded Counterpoise- More Transformer Questions

[Guy Olinger K2AV]

First, read my reply to W1FMR.  Then interspersed.

On Sat, Dec 10, 2011 at 1:09 AM, <k3ky at radioprism.com> wrote:

> I echo W1FMR's question regarding possibly substituting a T200-2 core.
> Personally, I'd double it up, i.e. either a stack of two T-200-2 or else
> a T200A-2 which is 1.00 inch thick vs. 0.55 in. for a T200-2. My guess
> is that the power handling would be less than a T300A-2? But what about
>

The 20 bifilar turns of teflon sleeved #14 uses ALL the winding space
available on the 1.94" inside diameter of a T300 form factor. 1.25 inside
diameter comes up 8 turns too small. Double polyimide insulation, and it's
14 kV static and RF protection is only available on #14 wire for small
quantities unless you want to pay for a factory setup and run.  Wireman has
#14 double polyimide insulated as his #635 and will sell you 15 feet, just
exactly what you need.  I just saved you $50,000.

Jerry Sevick used teflon sleeved double polyimide #14 in his devices.  It
wasn't my idea.

The polyimide will not burn at solder temperatures and has to be
grotesquely scraped off in order to solder to, BUT it has a demonic
propensity to NICK. A nick directly over a core turns into a carbon path
(been there, done that, hereafter abbreviated BTDT).  If the core has a
little bit of a sharp edge, the polyimide will nick.  BTDT.  The teflon
sleeve PROTECTS the polyimide from nicks.  Exposed polyimide insulation is
asking for trouble.

The teflon sleeve is better than other common (usually PVC of some kind)
insulation because it has a much higher voltage rating, but not nearly as
good as the double polyimide.

Since I and my circle of acquaintances went to using double polyimide #14
inside teflon sleeve, we have not had one failure. Not one.  I am sure
Murphy will punish me for saying that out loud, but it's an important
point.  WHY design down to a failure point?  Isn't that what we all
complain about in a manufactured item that fails, that the next larger
size, better quality, etc would have prevented it.  THEY say a certain
percentage return is acceptable, but when WE are in that percentage we hate
their guts.


> smaller #2 cores for QRP? Provided you do choose combinations with the
> same A sub L, you might expect similar winding inductances/ reactances?
> I don't know about effects on Q here, but I suspect that is not an
> issue? The A sub L for a stack of two T200-2 is almost identical to that
> of a T300A-2.
>

You need 21 bifilar turns, not 20, on a T200A2, in the smaller inside
diameter, to keep the same resonance.

I can wind another one here, get the stand-alone inductance of the single
winding with the other floating, and using the same wire and sleeve, you
can see how many cores in in a T200A-2 stack you need with the smaller
number of turns possible. It's not a linear thing related to the size of
the core because of the square of the number of turns thing.  I'd estimate
three of them to deliver the same inductance.  20 squared is 400, 12
squared is 144.  You will need 2.78 * 228 = Al of 634 to accomplish the
inductance with only 12 turns.  634 / 3 = AI of 211  T200A-2 are 218, so
three of them with 12 bifilar windings of the teflon sleeved polyimide #14,
or teflon insulated silver copper #14.

The T200A-2 three-stack cylinder is two inches diameter by three inches
deep and cost $15 for the three toroids.

The single T300A-2 is three inches diameter by one inch deep and cost $14
for one toroid.


>
> My own question regards which exact types of enameled 14AWG wire might
> be acceptable. I know the double polyimide type is very rugged with good
> HV breakdown. OTOH there is so much insulation with those teflon sleeves
> that I'd think it would be OK to use most any type of enameled 14 gage
> wire? Or even bare wire? Would it make all that much difference in
> overall HV breakdown ratings? Since this is a classical transformer, and
>

Yes, it would make a difference, because we have managed to burn up this
stuff ourselves.  We apparently are direct descendants of the original
Smoke Em, Carbon Track Boys.  We have been bad.

Not all teflon is the same.  Have to look at the wire spec for teflon
insulation to know what it is.


> not a transmission line type transformer, I would think that the
> characteristic impedance of the windings would matter less here?
>
> What if the builder had on hand some 14 AWG silver stranded teflon wire.
>

The teflon insulated #14 is still not small enough to wind on a T200 toroid
(the stranded has a larger diameter, necessarily to have the same current
cross-section), but the teflon insulation does not nearly have the
resistance to static penetration as does the polyimide.  That wire is not
manufactured with that use in mind.  It will not survive the kind of
voltage shock at the toroid you could get with an unfortunate feedline
length and a branch breaking the wire or hanging on it.  BTDT.  Think about
wind static and lightning induction.


> That would also fit in the large teflon sleeving I have here, although a
> little snugger. What might happen to the performance with that one
> design change change to the wire type?
>
> If I can get away with it, I may initially try a stack of T200-2 cores-

but if the bigger T300 size is truly essential, I'll get one on order.
> BTW the most I'm usually outputting here is more like 800-1000 watts.
> I'm sure the T300 size is a much better bet for a full 1.5KW.
>

I use T400A-2 for my personal windings.  I figured that people would gripe,
moan and carry on about paying $14 for the T300A-2, forget the $40 T400A-2.
 Remember, y'all, this IS 160 meters, and everything is BIG up here.  Jerry
Sevick went to a T400A-2 for 160 meters.  He was RIGHT.  I just barely got
this squeezed on a T300A-2 WITHOUT any real take-aways other than you have
to make the wire longer for the simple setup.  One of my 160m apps for N4AF
has to use an $80 PAIR of T400A-2 taped together with fiberglass tape as
the core.  You guys are getting off light :>)

Consider 800 watts as QRO.  Anything with a 3-500 or 3CX800, or four 572B's
in it is QRO for voltage and heating stress. Remember that the T300 size is
to support the wire and insulation method. And at that is JUST barely large
enough.


> 73, David K3KY
>
> P.S. I recognize that Guy would prefer to eliminate variability and
> thereby accumulate some group experience among a user base.
> Standardizing on the T300 type transformers would be a very good idea in
> that regard. OTOH hams can be frugal, hams like to experiment, and hams
> will often want to try to adapt designs to what they have on hand. There
> is a natural tension between those two things.
>

:>)   I can speak for myself.   73, Guy.


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