To: rsgb_lf_group@blacksheep.org
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From: N1BUG <paul@n1bug.com>
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Message-ID: <b00ba0af-a6f2-8e63-2dc5-68b562e9f28a@n1bug.com>
Date: Mon, 3 Dec 2018 12:22:23 -0500
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In-Reply-To: <04207F3E-523E-40B8-A93C-E1AEBE0D3A4A@md.metrocast.net>
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Subject: Re: LF: More on unstable antenna R
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Reply-To: rsgb_lf_group@blacksheep.org

Hi Rob,

I think my reply to Stefan explains what and where I am measuring...
until I get a new RF ammeter at least.

"Normal" X is zero, as I adjust the variometer (very slightly) at
the start of each operating session and don't need to change it
during the night unless weather conditions change (precipitation,
fog, etc.).

"Normal" R does not exist! It varies so widely with season and
ambient temperature I wouldn't be able to pick a figure to call normal.

At the time of the tests in question, R at the transmitter end of
the line varied from 58 ohms at the start of a transmission to 46
ohms 2 minutes later. I calculated that from measured voltage and/or
current on a 50 ohm line with no reactance. Translated to the other
end of the coax by using 'TLW' software (N6BV) and then multiplied
by the transformer impedance ratio, I calculate about 80 ohms
dropping to 65 ohms at the antenna / variometer side of the transformer.

Believe it or not I have quite a few radials and ground rods in.
Good old Maine sand... about as conductive as Teflon! :)

73,
Paul


On 12/3/18 11:01 AM, Rob Renoud wrote:
> Hi Paul,
> 
> Can you provide the values of “normal” X and R and also where and how you are measuring those values? 
> 
> 73,
> Rob
> 
>> On Dec 3, 2018, at 08:03, DK7FC <selberdenken@posteo.de> wrote:
>>
>> 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