Message-ID: <16ba1ed6c34.marcocadeddu@tin.it>
Date: Sat, 29 Jun 2019 08:30:19 +0200 (CEST)
From: "marcocadeddu@tin.it" <marcocadeddu@tin.it>
To:  <rsgb_lf_group@blacksheep.org>
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Subject: R: Re: LF: LF daylight test close to the north hemisphere solstice
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Hola Luis,
may be that due to the high temp (> 30°C at 8AM local time) my CPU 
doesn't work fine and I misunderstood some info, but will share my 
thoughts anyway ;-)
I mean you can remote operate the tape switch with standard car relays 
(I mean the open frame high current relays for the light or the horn ;-
) ) rated at 10 or 20 DC amperes: they are cheap robust and keeping 
short wires should work fine. It should be easier to switch taps to 
ground in the secondary winding (thus antenna side) providing for 
instance 1:1 1:2 and 1:0,3 ratio.
This was the easy part ;-)
What I don't understand is what you are measuring..
When you have higher antenna impedance, also the output impedance of 
the ampli should rise and have about the same DC input power with lower 
RF current and higher RF voltage.

I had to do some antenna work, but really have no will to work on my 
roof with temp reaching +40 during the day.. prefer to be lazy and do 
it later at the end of the summer :-D

73, Marco IK1HSS

----Messaggio originale----
Da: luis@vigilant.es
Data: 28-giu-2019 22.04
A: "rsgb_lf_group@blacksheep.org"<rsgb_lf_group@blacksheep.org>
Ogg: Re: LF: LF daylight test close to the north hemisphere solstice

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