Topband: binocular matching transformers

Mike Waters mikewate at
Sun Mar 24 22:47:09 EDT 2013


You're correct on the turns ratio: the impedance ratio is the square of the
turns ratio. I'm wondering whether you are considering that one turn on a
binocular core is actually one pass through both holes. There's a photo
clearly showing this at
the right side of that page.

For a 4:1 transformer, I use two turns (four passes) for the 75 ohm winding
and four turns (8 passes) for the 300 ohm winding.

You can download a PDF (
of the turns ratios I use from . I think if you try and
download the PDF directly from here, you'll get an error.

I assume you are using a BN-73-202 core?

73, Mike

On Sun, Mar 24, 2013 at 2:48 PM, Pete Smith N4ZR <n4zr at>wrote:

> I'm trying to wind a binoocular core matching transformer for 75-ohm coax
> to a BOG with (for a start) 270 ohms of terminating resistance.  I figure
> that is a 4:1 stepdown, which should require a turns ratio of 2:1.
>  Initially I wound a #73 core with identical windings, 3 passes each.
> I put this on the output of my MFJ-259 with a 270-ohm resistance on the
> secondary.  To my surprise the MFJ read 135 ohms, not 270 as I would have
> expected.  Is this not appropriate as a way of measuring the transformer?
>  Should I just go ahead and wind 2:1, and if so, how many turns/passes are
> appropriate for 160M?

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