To: rsgb_lf_group@blacksheep.org
References: <1677a7ef953.marcocadeddu@tin.it>
From: N1BUG <paul@n1bug.com>
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Message-ID: <6180aab4-bc94-3c2a-74be-cb16d7586fca@n1bug.com>
Date: Wed, 5 Dec 2018 16:02:31 -0500
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In-Reply-To: <1677a7ef953.marcocadeddu@tin.it>
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Subject: Re: R: Re: LF: More on unstable antenna R
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Reply-To: rsgb_lf_group@blacksheep.org

Hi Marco,

I just got a new IR heat sensing device. It could not see any
warming of the transformer or loading coil (variometer). If the
loading coil changed from heat, I would expect to see it mostly in
change of resonant frequency, not a change in resistance as I am seeing?

I think I must have some connection in the ground which changes
resistance with RF current flowing, but it's very strange.

73,
Paul


On 12/4/18 1:33 PM, marcocadeddu@tin.it wrote:
> Hi Paul,
> 
> aside of my sympathy for how you are managing "the mistery" ;-)
> just reading this message I had a though.. maybe I'm wrong but
> after checking a lot of things one more is nothing... I had to
> inspect my ATU box (it hosts the LF variometer, the xfmr for LF
> and MF and a couple of vacuum relays for band switching). In one
> of such inspections I seen that my variometer, although uses
> solid enameled 2mm copper, generates some heat.. (the top of the
> box holds signs of the warm up!) so I assume that when warming
> the winding can change in shape and spacing.. maybe is not a
> matter of cm but 1 mm change in diameter or turns spacing can
> make some difference in a sharp tuning device.
> 
> I remember you checked the possible temperature rise of almost
> all the components between the TX output and antenna but cannot
> remember if you checked the variometer.
> 
> Keep on Paul!
> 
> 73, Marco IK1HSS
> 
> 
> 
> ----Messaggio originale---- Da: paul@n1bug.com Data: 3-dic-2018
> 18.22 A: <rsgb_lf_group@blacksheep.org> Ogg: Re: LF: More on
> unstable antenna R
> 
> 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
> 
> 
> 
> 
> 

-- 
Paul
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FN55mf      ME      Piscataquis County
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