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Date: Tue, 23 Oct 2001 12:16:45 +0100
From: "Claudio Girardi" <in3otd@qsl.net>
Subject: LF: Re: Loading coils and shorted turns
To: rsgb_lf_group@blacksheep.org
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Regarding variometer design, I have put on my site a simple form 
that computes the main parameters for the classical two coils
variometers, both the sliding and the rotating coils types.
It is at http://www.qsl.net/in3otd/variodes.html ; there
are also some photos of my variometer, designed using these
forms, at http://www.qsl.net/in3otd/pictures.html .
Hope this can be of some help,

                     73 de Claudio, IN3OTD

P.S.: shorting turns to adjust the inductance of a coil should
      not be harmful, if the wire has a low resistance and the turns
      are not too close; I seem to remember that I have used this
      in the output PI of an HF power amplifier some time ago...


> Hello Martin,
> 
>   
> >In Peter Dodd's book LF experiments page 55 a drawing is shown of a loading
> >coil arrangment. The the coil inductance is adjusted by shorting turns on
> >the coil. Is this right?
> >
> >Martin
> 
> ... not having Peter's book on hand, but there are several ways of
> adjusting the loading coil. The option of shorting turns on the coil can
> only give a rough adjustment of the inductivity needed.
> 
> You can for example build a variometer. If you have a look at may homepage
> (http://www.qru.de/station.htm) you can see that I use a combination the
> method described above with a variometer inside my loading coil. In the
> middle of the page you can see the arrangment, a PVC tube with a smaller
> coil wound on it, parallel to the main coil. This allows a very definite
> fine adjustment of the inductance needed. If you do not have an RF meter on
> hand, it also helps to use an oscilloscope and put the wires of the
> oscilloscope near the top of the coil (do not directly connect it, you
> might have several thousand Volts there!) and tune for maximum voltage that
> you can see on the scope. I use a very simple and cheap scope to do this,
> which also gives me an indication about the waveform.
> 
> Others have used two cylindrical coils that are being pushed into each
> other to fine-tune the inductivity. Walter, DJ2LF uses a lareg PVC pipe and
> has a small ferrit core lying inside, the position of this small core
> inside the pipe adjusts his inductivity.
> 
> Be aware, that adjusting the loading coil is the key to get the antenna
> resonating, it can take a fraction of a turn with the variometer to find
> the resonance.
> 
> Also, as an additional hint: think BIG! If you look to the loading coil I
> use in relation to the CD, you see, how big the coil should be. 40 cm in
> diamter and 50 or 60 cm length is not too big. Copper wire or litz wire
> might be fine, I myself used simple 1.5 square-millimeter wire used
> normally for electrical installations in houses, I used about 150 to 200 m
> for my coil.
> 
> Hope this helps a bit.
> 
> Best 73
> 
> Geri, DK8KW (W1KW)