R: Re: LF: More on unstable antenna R

To:	<[email protected]>
Subject:	R: Re: LF: More on unstable antenna R
From:	"[email protected]" <[email protected]>
Date:	Tue, 4 Dec 2018 19:33:20 +0100 (CET)
Reply-to:	[email protected]
Sender:	[email protected]

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: [email protected]
Data: 3-dic-2018 18.22
A: <[email protected]>
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 <[email protected]> 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

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