Posted by
Cookie on
Aug 08, 2010; 1:07pm
URL: http://elecraft.85.s1.nabble.com/Re-Backward-balun-tp5383661p5386051.html
Al, your comments about Jerry's book makes me wonder even more about what you
are trying to do with a backward 4:1 balun. With a BS in Physics and 30 years
as an EE I found the book anything but a cook book. To begin to understand what
I thought previously to be a simple device I had to drag up every bit of
transformer theory I ever knew and found the book to be at the PHD level with
basics assumed to be common knowledge. I had to read the chapters several times
to feel that I understood what Jerry was telling me.
In trying to guess what you are trying to match with a 50 to 12.5 ohm balun the
only good guess is some sort of shortened dipole. This will not respond well to
a broadband approach because it will have a low impedence at resonance and high
impedance everywhere else. The approaches that I can see working for this type
of antenna are LC networks rather than broad band transmission line
transformers. You can probably make it work with a non-inductive resistance by
experimenting with different mixes of cores until you find the range of
interest. If you are trying to match a shortened whip you will need a unun
which is similar, but wired differently than a balun.
Willis 'Cookie' Cooke
K5EWJ
________________________________
From: Al Lorona <
[hidden email]>
To: Elecraft_List <
[hidden email]>
Sent: Sun, August 8, 2010 1:02:22 AM
Subject: Re: [Elecraft] Backward balun
Thanks for all of the excellent advice.
To sum up, today I measured a 4:1 current balun backwards and Tom was pretty
right on... the high frequency cutoff is so severely lowered that it is unusable
for my application. I did not test a 4:1 voltage balun.
I think one approach to take is to use a unun to transform down to 12 ohms and
then a conventional 1:1 balun following that to transform to a balanced feed.
I'll try this and report back if it doesn't work.
I know that there were several who suggested Sevick's book-- and I hope I'm not
commiting sacrilege by saying that I've gotten very little understanding of
balun theory from that book. It's a great cookbook for constructing baluns (and
ununs), but not so great to learn why one is doing what he's doing. But that's
just me.
It is also very surprising that I found almost no reference to a 50-to-12.5 ohm
balun on the web. You can certainly find one for purchase, but you can't find
the instructions on how to wind one. You have to buy the book, I guess!
This is surprising to me because I can think of many applications where one
needs to transform 50 ohms to a lower (balanced) impedance, including the one I
am currently battling.
Thanks again to everybody.
Good weekend,
Al W6LX
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