[Elecraft] Is a K3 ATU with balanced outputs possible?

[Don Wilhelm]

Bob,

My analysis - while "floating the unbalanced tuner" may be possible in 
theory, in practice, it is not practical.  There are always be points 
that will be found to upset the balance.

There is a reason for creating balanced networks, and putting the balun 
on the input of an unbalanced network is just not one of them.

The bottom line is found at the end of the W8JI article.  "Moving the 
balun to  the input is only helpful when the balun is not needed".   So 
use the KAT100 or KAT500 as designed and put a balun on the output if 
you must use balanced feed.

73,
Don W3FPR

On 9/28/2012 9:47 PM, Bob Cunnings wrote:
> Maybe not. The analysis presented in this article:
>
> http://fermi.la.asu.edu/w9cf/articles/balun/
>
> leads to the conclusion that locating the choke balun at the input of
> the tuner, and floating the tuner,  offers an advantage only for
> balanced tuner designs (and good balance in the load). The stress on
> the balun in a high SWR situation is unchanged by moving it to the
> input. In short "An unbalanced tuner trades the large differential
> mode impedance for a large unbalanced impedance making the balun's job
> unchanged."
>
> W8JI also addresses the issue in this article:
>
> http://www.w8ji.com/tuner_baluns.htm
>
> and comes to the same conclusion, with emphasis on the need for good
> load balance in the case of a balanced tuner with choke balun at the
> input. He ends with: "The irony is, moving the balun to the input
> mostly works only when the balun is not needed!".
>
> Food for thought.
>
> I use a floating balanced-L tuner with choke balun at the input, but
> try to keep decent balance in the antenna system for this reason.
>
> Bob NW8L
>
>> There may be no difference in efficiency between putting the balun at the input
>> or the output of the tuner if you are operating into a nice 50ohm resistive
>> dummy load, but the situation changes if you are operating into an antenna fed
>> with ladder line on which there is a high SWR. In this case, the high SWR can
>> cause high currents that saturate the ferrite core in the balun, and causing
>> non-linearity and producing heat losses. This is why it may be preferable to
>> locate the balun at the input of the tuner, where the SWR on the transmission
>> line is low, and "float" the whole tuner. However, as Don mentioned, this can
>> present difficulties if the tuner is not a balanced design.
>>
>> 73, Matt VK2ACL
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