From: =?iso-8859-1?Q?VIGILANT_Luis_Fern=E1ndez?= <luis@vigilant.es>
To: "rsgb_lf_group@blacksheep.org" <rsgb_lf_group@blacksheep.org>
Thread-Topic: LF: LF daylight test close to the north hemisphere solstice
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Date: Fri, 28 Jun 2019 20:04:13 +0000
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Subject: Re: LF: LF daylight test close to the north hemisphere solstice
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Hi Stefan, LF

I'm recovering this message were you proposed a 1:1 ferrite transformer to isolate the antenna

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. 

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)

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 ?
How many turns ? I have a pair of the big blue cores you recommended 

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