[Elecraft] Backward balun

[WILLIS COOKE]

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 <alorona at sbcglobal.net>
To: Elecraft_List <Elecraft at mailman.qth.net>
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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