[Elecraft] Baluns - give me a kit!

[Dave Hottell]

Hi Jerry,

At 04:55 PM 5/31/02 +0000, k6iii@juno.com wrote:
>Eric/Wayne,
>
>Baluns are a bit of a mystery to me... I'm pretty simple minded, now, but
>know there is a need for 4:1 and even 6:1 baluns (and others) with these
>QRP rigs. I see articles on making these, but the parts are usually not
>readily available. If Radio Shack (Tandy) has the parts please let me
>know what to get (i.e. RS part number, etc...). Sure, the KAT1 or KAT2
>auto antenna tuners will probably match any antenna I string up, but
>wouldn't it be better to at least get close to the 50-ohm impedance that
>the K1/K2 want to see? Maybe I'm missing something here, but...

I would say you are missing something here.  The impedance seen by the
radio at the input end of the feedline can vary all over the place.  From a
few ohms to several thousand, and from considerable capactive reactance to
the same amount of inductive reactance.  It all depends on the impedance of
the antenna at the frequency of operation, the characteristic impedance of
the feedline, and the length of the feedline.  What you are really
interested in are the losses in the feedline, and that is about all.
Seldom, if ever, will a 450 ohm line have an actual impedance of 450 ohms
appearing at the transmit end.  Same for any other type of feedline.

So what you need is a good low-loss feedline, balun, and a good tuner to
transform whatever impedance happens to appear at the transmitter end of
the feedline to the approx. 50 ohms needed by the rig.

The only need for a balun is to force the two lines of the feedline to
carry the same current - equal in amplitude and opposite in phase.  A short
piece of coax with some ferrite beads on the outside works as well as
anything.  

There is no need whatsoever to be concerned about a 4:1 or 6:1 ratio since
the actual impedance at the input is almost certainly *not* the
characteristic impedance of the feedline.

Just use a simple 1:1 current balun or a W2DU coax w/beads type balun and
let 'er rip.

You are interested in the losses in the balun.  So keep those low.
Especially under high SWR conditions, the losses in a balun can get rather
high.  So don't get too fancy or you may end up doing more harm than good.

Hope this helps.

73 es gl,
Dave
AB9CA

>
>It seems to me it would simple thing to market a do-it-yourself balun
>kit. I suspect Elecraft has all the parts and could kit up a core with
>some wire, maybe a box and a couple of connectors along with some
>instruction sheet on how to wind various baluns for QRP (use with K1's
>and K2's). Why not include a discusson in these instruction sheet about
>baluns and when you need to use them, along with a few simple wire
>antenna examples, etc...
>
>I would imagine that you would sell quite a few of these balun kits.
>
>(Reflector readers are encouraged to post what you have done, how to, web
>references, pictures, RS part numbes, etc...)
>
>73 de Jerry/k6iii
>San Jose, CA
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