Topband: impedance transformer windings

CHARLES HUTTON charlesh3 at
Tue Oct 28 00:43:38 EST 2003

Hi Steve:

One little problem is that you can't reduce things to "inductance per turn" 
since that is not linear. The inductance varies as the square of the number 
of turns. When I do the calculations, the numbers are 336 uH for the antenna 
winding using the 4:1 rule.

That requires 6.3 turns on the antenna winding and 1.4 turns on the receiver 

I'd make that with 9 and 2 turns respectively. The extra turns will help 
just a tiny bit as the insertion loss with greater than 4:1 ratio will fall 
maybe .05 dB. On the other hand, the high end of the passband may come in 
just a little but you're way outside of a flag's frequency range that it 
does not matter. The transformer is likely to be flat past 30 MHz even with 
the extra turns.

Your other choice is to use a normal 73 or 77 core. You'll be better off 
with the BN core even with the slight amount of "extra" turns.


>From: "Steve Flood" <flood at>
>To: <topband at>
>Subject: Topband: impedance transformer windings
>Date: Mon, 27 Oct 2003 14:04:27 -0700
>I've been trying to put together an impedance transformer on the Amidon 
>BM-73-202 binocular core.  It is to be used on a flag RX antenna for 160m 
>so I need about a 950:50 (19:1) transformation.  The published AL is 8500 
>mH/1000 turns.  This translates to 8.5 mH per turn.  From a Handbook 
>formula, I calculated I need 1.4365 mH on the HI Z side and 0.0765 mH on 
>the Lo - Z  side.  As you can see, even one turn thru the core exceeds 
>these by a large margin.  Do I need a core with a lower AL?  Seems like 
>many folks recommend the BN-73-202 though with a 3:12  (W7IUV) or 1:4 
>(W8JI) turns.   But I can't arrive at these numbers by equation.  I'd like 
>to learn how this works rather than just build it without learning.
>   Thanks all.  Steve KK7UV.
>Topband mailing list
>Topband at

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