Posted by
Jim Brown-10 on
Dec 09, 2011; 6:36am
URL: http://elecraft.85.s1.nabble.com/KAT500-update-tp7072932p7077204.html
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.pdf73, 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.
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