Re: [TowerTalk] Balun question(s)

[Jim Lux <jimlux@earthlink.net>]

At 09:52 AM 7/13/2004 -0700, Al Williams wrote:
> A fellow ham has built a "Cebik" 44' doublet for his backpacking outings
> but his tuner won't tune it on 40.  Eznec says the feed point impedance
> at 20' high is 20.7 - j629.6 ohms.  He has tried inserting a balun.
> Most
> of the literature on baluns that I have seen never seem to describe
> what goes on with the reactance part of the impedance.
>
> Question 1
> What would the feed point impedance be if a 2:1 balun is inserted in
> an attempt to raise the impedance towards 50 ohms.  What really
> transformation of resistance and reactance occurs in baluns.

The reactance is transformed by the same ratio.. to -j1250 here...


> Question 2
> According to my limited understanding of Baluns from Sevick's book
> it is a bit of a misnomer to label baluns 2:1, 4:1 etc. as they are more
> correctly 100:50 ohm, 200:50 ohm etc.  as baluns have a characteristic
> impedance?  Furthermore is the 100:50 ohm (or 2:1?)  only meaningful
> when the connections to it are 100 and 50 ohms?

These are tightly coupled transformers, right?  Therefore, an impedance 
ratio is the best spec: 2:1 or 4:1 (which will be the square of the turns 
ratio: 2:1 turns ratio will be 4:1 impedance ratio).  The transformer 
itself will have a certain "magnetizing reactance" as well.

There ARE baluns (balanced to unbalanced) transformers that DO have a 
characteristic impedance.  For instance, using a pair of 100 ohm coaxes 
(one 1/2 wavelength longer) to transform a 200 ohm balanced line to a 50 
ohm unbalanced line.

And, there are transformers that are build around broadband transmission 
lines (transmission line transformers) which are sort of half way in between.


> Question 3
> Would a coil sized (+j629.6)  to conjugate the -j629.6 help?  How
> should it be installed?

Yes.. If you put it in the middle, it would make it look like a "shorty 
40". I think a lot of shorty forty designs use the loading coil as an 
autotransformer too, by connecting the feedline to a tap along the coil, to 
transform the impedance as well as add series L.  The usual design has an 
airwound coil with widely spaced turns, so there's a fair amount of leakage 
inductance.  You can sort of fiddle around with the design.. partly 
transforming the huge capacitive reactance down, partly cancelling it with 
leakage L to where it has an "acceptable" match.

If you put the coil(s) midway along the wires, rather than at the feed, you 
might get a better antenna (but harder to deploy), since, in general, just 
like for verticals loading coils in the middle work better than loading 
coils at the base.

In any event, you're much better off to put the coil on the antenna side of 
the transformer, so you aren't running all that reactive current through 
the transformer.


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