Posted by
Cortland Richmond-2 on
Dec 15, 2011; 1:21pm
URL: http://elecraft.85.s1.nabble.com/Balun-at-input-or-output-of-tuner-tp7087668p7097073.html
I hesitate to step between Dean and anyone else but... it seems to me
that a Balun with good enough common mode choking should fulfill all the
isolation requirements we might have. The isolation any Balun provides
takes place at the point it is inserted, so a Balun built for 50 Ohms
may be used at the input to a (floating) tuner without our necessarily
being worse off than putting it on a tuner output -- if we can tolerate
what imbalance exists on the load side on the load and resulting RF on
the tuner chassis. We must make that RF low enough to live with, for
which we don't need perfect isolation, only _enough_. It may go
without saying that we can put CM chokes anywhere we want -- and as may
times as we want (losses permitting).
This may be moot; in his article/A Better Antenna-Tuner Balun/, QEX,
Sept/Oct 2005, ZS1AN noted the problem with "voltage" Baluns and
inherently unbalanced loads, and proposed a combined voltage and current
balun to gain the advantages of both. excerpt:
/...analyze the performance of the 1:1 current balun and the 4:1 voltage
balun in this application.
The analysis shows that the current balun operates effectively only for
small load impedances,
while the voltage balun is effective only if the load impedance is well
balanced with respect to ground.
I then introduce a new design: the "hybrid" balun, which overcomes these
limitations of the voltage
and current baluns. It can operate with much higher load impedances than
can current baluns and with
unbalanced load impedances that voltage baluns could not drive effectively.
/
Cortland
KA5S
On 12/14/2011 7:56 PM, Don Wilhelm wrote:
> Dean,
>
> The last two paragraphs of your writeup do not seem relevant to the
> discussion of balun (CM choke) at the input or output Those paragraphs
> deal with operating coax at a very high 60:1 SWR, and neither support
> nor agree with the other points.
>
> Consider the following: A situation where the windowsill connection is
> 20 feet away from the tuner output. The balun has the same loss no
> matter where it is placed, so lets assume it is placed at the output.
>
> Now, consider that the connection between the tuner output and the
> windowsill is with 20 feet of RG-213. The balanced line is connected
> directly to the coax (no balun). By the analysis presented, the loss
> will be exactly the same as with the balun connected at the windowsill
> end of the coax.
>
> Both conditions are electrically the same (If that point is arguable,
> then the balun at the tuner input is just as arguable).
>
> If we can extend this argument, we would be able to conclude that it
> makes no difference on a coax fed antenna whether the balun is placed at
> the antenna or at the tuner output - no matter whether the feedline is
> coax or balanced line. Oh, yes, both the coax or balanced line must be
> isolated and run with the same rules normally applied to balanced line.
> The point is that while theory says it makes no difference, it is
> impossible to achieve that perfect isolation, so the argument falls apart.
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