[Elecraft] Backward balun

[Tom W8JI]

> 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.

You will have trouble. Lead lengths will kill SWR (as will lengths through 
connectors and connectors) because it is almost impossible to build 12.5 ohm 
line and connections without special cables (very thin dielectrics and thick 
center conductors).

Look at baluns for solid state amplifiers.

You can series connect two 25 ohm lines (each small 50 ohm cables) on the 50 
ohm end, and parallel them on the 12.5 ohm end. Use an RG Teflon cable about 
1/8th inch or so diameter.

Don't try the silly idea of a single core, build two isolated lines.

I've done this with solid state amps with bandwidths of 1-100 MHz.

> 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.

You should look at other sources anyway. A few errors in books aren't 
uncommon, and that's why we should never trust a single source for learning 
how things work unless it has been extensively critically reviewed.

You might want to look at broadband matching applications for solid state 
amplifiers. Many of them go from several ohms balanced to 50 ohms 
unbalanced. Dig through Motorola application notes on transformers by Helge 
Granberg and look at Walt Maxwell's books for theory. Walt is much more in 
tune with how antenna systems work, and Granberg spent a lot of time working 
with transformers. DeMaw might have some stuff on baluns also in the form of 
matching solid state amps to 50 ohm filters if you can borrow a copy of 
Solid State Design for the Radio Amateur.

You'll have few core worries because balanced impedance is so low, and since 
you can use coax balun windings will be easy. I think your project will be 
easy if you watch lead or connection lengths on the 12.5 ohm end.

73 Tom