To: "rsgb_lf_group@blacksheep.org" <rsgb_lf_group@blacksheep.org>
From: N1BUG <paul@n1bug.com>
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Message-ID: <0d2ac752-44f5-e503-e2a0-a8b4c841bcee@n1bug.com>
Date: Mon, 3 Dec 2018 06:49:07 -0500
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Subject: LF: More on unstable antenna R
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Sender: owner-rsgb_lf_group@blacksheep.org
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Reply-To: rsgb_lf_group@blacksheep.org

Hi LF,

During past days I did some more investigating about that slow
antenna R change during Tx. This is making me a little crazy. I
don't like RF mysteries!

During a 2 minute Tx at 200W, antenna system R reduces about 20%
from beginning to end of Tx.

During a 2 minute Tx at 100W, antenna system R reduces about 20%
from beginning to end of Tx.

During a 2 minute Tx at 50W, antenna system R *increases* about 3%
from beginning to end of Tx!

There is almost no shift in X.

I went over everything I could get to (matching xfmr, loading coil,
connections between them, etc.) using a IR heat measuring gun. I did
not find anything warming up during a long Tx period. It's all cold.

I see a similar change happening on MF where I use a completely
separate xfmr and loading coil.

This may be a clue, but I don't know what it means. I have other
antennas around. All those cables come into the basement where there
is a disconnect point. All can be disconnected from the short cables
which run up into the radio rooms. If I disconnect all other cables
at that point, then the LF antenna R change during Tx is about 10%,
half what it is usually. The R at the start of a Tx period is the
same as always, but it does not decrease as much during the Tx period.

I tried connecting the coax shield to the LF antenna ground. That
did not make any difference.

Just trying to think of possible explanations...

Broken or bad connection somewhere up on the top hat of the antenna?
Seems something should fall down if so, but it hasn't yet. :)

Bad connection in the ground system? All above ground connections
are secure. Below ground connections are not accessible in winter
but they are heavy solid copper conductors, mostly exothermic welded
connections. The one exception is the heavy solid wire which runs
from the xfmr secondary down to the underground wire / center ground
rod. It is clamped, not welded. But I used three clamps, very tight
and with anti-oxidant paste. This connection is just one year old.

Ground return currents through some other path which is not stable?
I am sure I make RF currents in the power lines, etc.  but...???

Does anyone see a clue here?

73,
Paul