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 <[email protected]> To: Elecraft_List <[email protected]> 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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
