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Re: LF: LF daylight test close to the north hemisphere solstice

To: [email protected]
Subject: Re: LF: LF daylight test close to the north hemisphere solstice
From: DK7FC <[email protected]>
Date: Sat, 29 Jun 2019 10:21:56 +0200
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Hi Luis,

Am 28.06.2019 22:04, schrieb VIGILANT Luis Fernández:
My problem is that the antenna impedance varies a lot with humidity. This days 
we have a hot and
dry weather about 30% moisture. VNA shows an impedance value of just 30Ohms at 
Previous week having sea moisture with 80% values, which are normal here over 
summer, the
impedance rised to 100Ohm at resonance. Of course, PA works in a completely 
different way in both
What happens with the reactive part of the impedance? Does it stay constant or do you have to retune all the time? Then, the relays and tapped transformer wouldn't help you.
The best antenna impedance is about 70 Ohm, were I can get 3.2A RF current with 
[email protected] PA current
Higer impedance causes PA current to rise and RF current drops. Mosfets also 
get much hotter
Low antena impedance produces high RF currents but low PA current and so, less 
and higer voltages in the LPF which causes arcs. (LPF is also pending resize. 
Is the only part still
remaining from the old Ropex PA)
Hmmmm, hard to belive that it is just the humidity that leads to a variation from 70 to 1000 Ohm. Maybe it is a bad ground contact at some point?
You could do a test: Tune to resonance when the system is at 70 Ohm.
Then run low power, maybe 5W. Run a carrier transmission for some days.
Observe the signal level at your remote site in 7 km distance. Use
SpecLabs plotter and plot the signal level over time. Watch the
variations. When the system changes slowly to 1000 Ohm, the antenna
current will drop and so the signal will be lower. If this happens
slowly, when it starts to rain, then it is ok. But maybe you will find a
sudden jump of the current, then it could be a bad contact somewhere.
You know what i mean?
So, I need a matching transformer, but must be variable and remotely 
selectable. May be selecting
taps with a set of relays or any other mechanical selector. What do you propose 
for that ?
It is all possible but you will need time again! ;-)
How many turns ? I have a pair of the big blue cores you recommended
Which ones? The very big ones? I used 18 turns primary for 50 Ohm. Calculate the voltage at 50 Ohm and your power level. Then apply this voltage to the core and the primary winding only, without a load. The core should stay cool! Or, if you don't have a voltage source, build a 1:1 (18:18) transformer and apply full power through it and connect a dummy load. The transformer should stay cool. If you want to transform from 50 to 1000 Ohm (which is not a good solution somehow) then the secondary winding needs 18*sqrt(1000/50) = 80 turns.
I would focus on finding the reason for the heavy changes. I would doubt
that it is just humidity, although i would certainly expect that
humidity has a significant influence in your configuration.
Relays: I personally would select such one:
73, Stefan
73 de Luis

De: [email protected] [[email protected]] en 
nombre de DK7FC [[email protected]]
Enviado: miércoles, 5 de junio de 2019 8:16
Para: [email protected]
Asunto: Re: LF: LF daylight test close to the north hemisphere solstice

Hi Luis,

No, i just mean a second transformer between PA and the other
transformer, into the coax line. A simple 1:1 ferrite transformer.
Your ground conductors into and arround the building are a part of the
antenna, obviously causing a high voltage between that ground and your
shack ground. All you can do is reducing the coupling impedance
(capacitive) and the electrical field strength. A well dimensioned 1:1
transformer will provide a good decoupling and low losses. The improved
decoupling may change the resonance a bit (because your shack earth is
less a part of the system taking RF currents), so a retuning is
required. Hopefully your shack earth is not the dominant part of the
antenna ground :-) The less it is a part of the ground system, the less
is the detuning of the antenna when inserting the second transformer.

73, Stefan

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