Hi Rich,
This DXE balun is just a hefty 1:1 choke balun It's a high grade
coax (50ohm) wrapped through ferrite cores. The balanced terminals
are floating and the unbalanced end is connected to the aluminium
housing and the shield of the 6 foot length of RG213 coax which
connects the balun and my tuner. I have a balun inside my tuner too,
but I don't use it: it's a 4:1 current balun and in this application
with a multi-band loop, I prefer to use the external 1:1 balun.
I agree - with my loop , anything that changes the feedline length,
even by smallish amounts, alters the impedance seen by the tuner at
the end of the line and the manual tuner settings change on (usually)
multiple bands and the KAT3 has to reconfigure its LC settings. I
expected that, as a function of the impedance transforming effect of
the feedline, but what I didn't expect was the very significant
difference that different shack "grounding" arrangements would make
to the whole process of matching the antenna system at the shack end
of the feedline.
About the common mode current that I'm measuring on the coax between
the balun and the tuner: Yes, I assume the 1:1 balun has sufficient
common mode impedance to be preventing common mode current standing
waves on unbalanced side of it. But I think what I'm measuring is
current that's being induced in the coax shield because it's lying in
the near field of radiation of the loop. Again, what is a mystery to
me is how changing the grounding arrangement on the balun could affect
the magnitude of that.
I find this intruguing, but for the moment, I've put the clip on
ammeter away. I'm going to forget it, while i digest this, and go
and play on 20m instead.
Thanks for all the comments I've rec'd, on and off the list.
73
John
VK7JB
-----Original Message-----
From: elecraft-boun...@mailman.qth.net
[mailto:elecraft-boun...@mailman.qth.net] On Behalf Of Richard Fjeld
Sent: Friday, 21 September 2012 1:00 PM
To: elecraft posting
Subject: [Elecraft] Fw: Balun Ground Connection
John,
It is unfortunate that we can't see a schematic of how the balun is
wired.
I have thought of doing that same thing to be able to use an ATU.
Presently, I have an 80 meter loop fed with homebrew 450 ohm ladder
line to
a tuner with a 1:1 balun within. It works beautifully on all
bands. I wish
I had done it many years ago.
As for your grounding the coax at both ends, I effectively do that
here also
on my coax switch with coax to my K3 which is also grounded. So
that is why
it would be necessary to see the schematic to see what is different
about
your situation.
I can tell you that I have everything grounded to a 'ground tree', or
earth
tree as our friends call it. I buried a 6 ga. wire from the central
point
over to my tower. I grounded it at 2 or 3 places along the way, and
the
tower is grounded at each leg with a separate ground rod. When I
did make
the final connection to the tower ground, I saw a change in the swr
of my
beams which are at DC ground, and coax fed again similar to what you
are
doing. The swr got higher, but not bad. Maybe it went up point
five or
less. From my experience, any time I change ANYTHING relating to my
antenna
system, the SWR changes.
As for #2, the common mode current, I thought a balun is supposed to
stop
that from happening.
Rich, n0ce
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