Message-ID: <5D1726E3.9030400@posteo.de>
Date: Sat, 29 Jun 2019 10:52:51 +0200
From: DK7FC <selberdenken@posteo.de>
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; de; rv:1.9.1.8) Gecko/20100227 Thunderbird/3.0.3
MIME-Version: 1.0
To: rsgb_lf_group@blacksheep.org
CC: "marcocadeddu@tin.it" <marcocadeddu@tin.it>
References: <16ba263c867.marcocadeddu@tin.it>
In-Reply-To: <16ba263c867.marcocadeddu@tin.it>
Subject: Re: R: Re: LF: LF daylight test close to the north hemisphere solstice
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org

Hi Marco,

Oh, indeed. My old eyes, you know, im now 43 yrs old and still refuse to 
wear glasses!!! :-(
Variations from 30...100 is almost acceptable and it may be the PA 
design that makes it a real problem.

73, Stefan


Am 29.06.2019 10:39, schrieb marcocadeddu@tin.it:
> Hi Stefan, Luis
> Luis pointed out changes in impedance from 30 to 100 ohms not 1000 ;-)
> Leaving this misreading away, you are right Stefan: it should be nice 
> to know what happen to both reactive and resistive component of the 
> impedance.
> I assume that Luis has still working a variometer thus he can manage 
> change of reactance with it and change of resistance with an 
> insulating and matching transformer
>
> Marco, IK1HSS
>
> ----Messaggio originale----
> Da: selberdenken@posteo.de
> Data: 29-giu-2019 10.21
> A: <rsgb_lf_group@blacksheep.org>
> Ogg: Re: LF: LF daylight test close to the north hemisphere solstice
>
> 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 resonance
> > 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
> > cases.
> 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 15A@33V 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 power
> > 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:
> https://www.reichelt.de/steck-printrelais-1-co-16-a-12-v-dc-sensitiv-fin-40-61-7-12v-p259198.html?&trstct=pos_12
>
> 73, Stefan
> > 73 de Luis
> > EA5DOM
> >
> >
> > ________________________________________
> > De: owner-rsgb_lf_group@blacksheep.org 
> [owner-rsgb_lf_group@blacksheep.org] en nombre de DK7FC 
> [selberdenken@posteo.de]
> > Enviado: miércoles, 5 de junio de 2019 8:16
> > Para: rsgb_lf_group@blacksheep.org
> > 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
> >
> >
> >
>
>
>