[Amps] how to wind an HF broadband 10:1 transformer

[Manfred Mornhinweg]

Hi Dan,

> I would like to wind a broad band HF, 160 to 10, transformer.
> 
> What core material is best for this range?

It depends on many factors, but typically it will be a ferrite material 
with a permeability somewhere between 100 and 1000, and most likely 
between 200 and 500. Depending on the manufacturer of the ferrite, one 
or more materials with widely varying properties can be available in 
this range. Which one to use depends on power level, core size, and not 
least, which material you can actually obtain in small quantities! The 
latter factor might limit you to Amidon, which might not carry the very 
best ferrites, but at least does sell in amateur-friendly quantities!

> Is the way to wind this a trifler winding (9:1) in an autotransformer 
> configuration?

If you mean a 10:1 IMPEDANCED transformation, and you are sufficiently 
happy with a 9:1 transformation, then yes, a trifiliar winding connected 
as a 3:1 turns ratio step down might be a good choice.

Carl,

> Iron powder #2 mix is most commonly used in broadband baluns between an 
> antenna tuner and open wire line.

I fear to open a can of worms with this, but it is beyond me how anyone 
could suggest iron powder material for baluns or other broadband 
applications! Ferrite is a very much better choice than iron powder in 
these situations. Iron powder has its place in application where there 
is high magnetizing DC, and where relatively high stability of the 
permeability is required. But a broadband RF transformer usually 
requires neither of them, and so we can take advantage of the very much 
higher permeability of ferrite, which allows a much lower turns number, 
and thus lower interwinding capacitance, and better coupling, resulting 
in much better broadband performance.

I know that some ham literature mentions iron powder cores for broadband 
transformers, and I assume that this is just for historical reasons, 
from times when iron powder cores were widely available and ferrite was 
rare or nonexistent. But today, we should really all be using ferrite 
for broadband transformers!

> I am experimenting reworking a Heathkit 104 (not A) front end. That 
> front end had a reputation for being deaf. I believe it was due in large 
> part to a gross impedance mismatch between the filters and the dual gate 
> mosfet.

So your application is really for very low power.

Broadband transformers do not lend themselves very well to high 
impedances. I suppose you want to transform 50 Ohm up to 450 or 500 Ohm. 
That should still be manageable. But much higher would be difficult. 
That would be more suited for tuned circuits on each band, with link 
coupling.

> How does this relate to 'red' or 'yellow' and 47 or 61? 

The red ones are usually mix-2 iron powder, with a permeability of 10. 
The yellow ones are usually mix-6 iron powder, with a pemeability of 8, 
if my memory is right. 47 is a ferrite material with a nominal 
permeability of 850, and 61 is another ferrite that has a permeability 
of 125. All these are Amidon/Micrometals designations. And be careful 
with toroids marked yellow but with one white face. Those are mix 26 or 
similar, which is 75-permeability iron powder, suitable mostly for 
chokes carrying DC with a rather small AC component not higher than a 
few hundred kHz.

 > I have red and yellow toroids.

They are excellent for tuned circuits, but not for broadband 
transformers. The red for 10MHz and down, the yellow from there up.

> Experimenting with red seems to work on 40 meters, 
> however it did not work well on higher frequencies. I do not have data 
> handy for these materials.

It should be available on Amidon's website. If not, I could transcribe 
the basic data from the catalogue, which I have on paper.

> A follow up, I wound a quintfiler toroid in an auto transformer 
> configuration. Does that configuration have a 125:1 ratio?

If you connected the five windings so that all five are in series for 
the input, and only one is used for the output, then this transformer 
would have a 5:1 voltage ratio, or a 25:1 impedance ratio. But not 125:1!

Manfred.

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