[Elecraft] KAT500 update

[Jim Brown]

First we must define what sort of "balun" we are talking about. I know 
of a half dozen different devices/circuit elements that are commonly 
called baluns, and most are very different from each other.  I abhor the 
word "balun," and try to avoid using it.  I have done considerable 
research, and published what I've learned, on the subject of COMMON MODE 
CHOKES wound on ferrite cores, especially LOSSY ferrite cores. The 
efficiency of such a common mode choke is essentially independent of the 
SWR, except to the extent of the increased loss in the short length of 
transmission line wound through the core to form the choke.

Since I have no idea what sort of "balun" Elecraft plans for their 
tuner, I have no comments on it. I have, however, measured some common 
mode chokes bifilar wound using #14 and #12 THHN to form a parallel wire 
transmission line that have excellent choking performance for the HF 
bands. The data are included in the latest version of my RFI tutorial 
(since the summer of 2010).

The concept of a totally isolated tuner with common mode chokes at input 
and output is so non-nonsensical to me that I've never contemplated it.  
It is also nonsense to consider parallel wire line as a balanced system 
unless everything connected to it is also balanced -- that is, tuner, 
line, and antenna. Since most ham antennas are at least somewhat 
unbalanced by their surroundings, even when fed by these parallel wire 
lines, there will be SOME imbalance in the line, and thus some common 
mode current. That common mode current causes trouble in at least three 
ways -- it puts RF in the shack to excite Pin One Problems (very common 
in ham gear); the common mode current will radiate, potentially causing 
RFI to consumer gear in your living room (and your neighbor's); and by 
reciprocity, the transmission line will act as a receiving antenna, 
coupling noise from your neighbor's computers and battery chargers to 
your receiver.  THAT"S why we need common mode chokes at the feedpoint 
of EVERY antenna, whether fed with coax or parallel wire line.

Loss in common mode chokes is addressed in the sections of my RFI 
tutorial where transmitting chokes are discussed.

http://audiosystemsgroup.com/RFI-Ham.pdf

73, Jim Brown K9YC

On 12/8/2011 4:36 PM, Ron D'Eau Claire wrote:
> Is it true that the efficiency of a balun is not affected by the impedance
> of the line in which it is used?
>
> The justification I had seen for putting the balun at the tuner input where
> it will see a load very close to 50 ohms resistive (when the tuner is
> adjusted for a "match") while at the output the balun may see anything over
> a huge range of impedances.