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Date: Mon, 03 Dec 2018 14:03:00 +0100
From: DK7FC <selberdenken@posteo.de>
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To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: More on unstable antenna R
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Hi Paul,

How do you measure your antenna current? I know, the scope. But do you 
measure across a small shunt resistor or do you use a current xfmr or 
something else?

73, Stefan

Am 03.12.2018 12:49, schrieb N1BUG:
> 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
>
>