Posted by
Don Wilhelm-4 on
Dec 15, 2011; 12:59am
URL: http://elecraft.85.s1.nabble.com/Balun-at-input-or-output-of-tuner-tp7087668p7095775.html
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.
Because most would not consider connecting a balanced line to coax
without an intervening balun because we have been taught that we must
preserve balance in order for things to be correct.
The fallacy I see with the balun at the input vs. balun at the output
argument is that with the balun on the input, everything that follows
must be perfectly isolated from ground - and that is difficult to
achieve when all physical things are considered - if there are *any*
strays, it defeats the principle of perfect isolation. That is also
what makes my example of the 20 feet of coax at the balanced output of a
tuner not a practical consideration - one cannot easily achieve equal
coupling of the center conductor and shield if there are any surrounding
objects. That fact makes the unbalanced tuner with an input balun not
practical because that perfect isolation is just as (or more) difficult
and expensive to achieve than it would be to implement a balanced network.
73,
Don W3FPR
On 12/12/2011 3:38 PM, Dean Straw wrote:
> While we're at it, let's look at the potential loss due to line
> losses at a CM choke balun placed in the wrong place in an antenna system.
> Assume the common scenario where a balanced antenna is fed with open-wire
> transmission line to a 1:1 common-mode choke balun located at the shack
> window. From the balun at the window the ham uses, say, a 20-foot section of
> RG-213 to the antenna tuner (which in this case is an unbalanced tuning
> network). Assume again that the CM choke balun uses three feet of RG-213
> wound on the appropriate ferrite donuts to achieve the target common-mode
> impedance of 5000 ohms so that common-mode currents are choked off properly.
>
> The total length of RG-213 is now 23 feet. Again, we'll present the
> balun at the windowsill with a load of 3000 ohms. The overall
> differential-mode loss in 23 feet of RG-213 is 4.534 dB, nearly 4 dB worse
> than connecting the open-wire line directly to a tuner with a CM choke balun
> at its output! Ouch, that's a lot of wasted power.
>
> 73, Dean, N6BV
>
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