I agree the usual rule of thumb (Transformer winding Z should be several
times larger than nominal line impedance) would cause you to think you
should have more turns.
The old Clifton Labs website is no more. But an archived page of
measurements of transformers shows that the frequency response extends well
below what you might think, from the rule of thumb. Archived page:
https://groups.io/g/BITX20/attachment/27529/0/clifton%20Labs%20IMD%20in%20Broadband%20Transformers.pdf
He finds that BN73-202 transformers wound with a single turn winding, have
a -3dB point at 270kHz.. A two turn winding would be good 4 times as low.
So the rule of thumb seems very conservative.
I have made step-up power converters using these cores and have been super
impressed how well they work at frequencies well below the rule of thumb. I
have run 30+ watts through these dinky cores with them just barely getting
warm.
Tim N3QE
On Sun, Jul 15, 2018 at 6:29 PM, Chuck Hutton <charlesh3@msn.com> wrote:
> The commercially available Beverage transformers I have seen are like the
> W8JI model: 2 primary turns on a BN73-202 core.
>
> My calculations say 4 turns are needed at 630 m.
>
>
> Does anyone know of commercially available transformers with isolated
> windings that operates well down to 630m?
>
>
> Chuck
>
>
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