Topband: impedance transformer windings
charlesh3 at msn.com
Tue Oct 28 00:43:38 EST 2003
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 ixi.net>
>To: <topband at contesting.com>
>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.
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>Topband at contesting.com
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