R: Re: R: Re: LF: More on unstable antenna R

["marcocadeddu@tin.it" <marcocadeddu@tin.it>]

Hi Paul,

yes, I do agree: dimensional change in the variometer should drive to changes both in reactance and resistance and if you have the variometer included in the loading coil you had already checked it.
Sorry this was not the culprit..

I guess that your thought about a (faulty or semi-faulty) ground connection at this point should be the better candidate but not easy to check (specially if the bad point is not close to the antenna feed point but elsewhere).

Bad insulation in the high part of the antenna will give problems but mainly arching so in this case you can see rapid changes in DC and RF currents when arch is active.. (already experienced.. )

You need some luck when you can check the ground connections and find a lazy connection without have to rebuild the full ground system.. probably you have to wait for springtime to access properly the ground system but we stay tuned ;-)

73, Marco IK1HSS

----Messaggio originale----
Da: paul@n1bug.com
Data: 5-dic-2018 22.02
A: <rsgb_lf_group@blacksheep.org>
Ogg: Re: R: Re: LF: More on unstable antenna R

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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