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...
>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?
... 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
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.
Geri, DK8KW (W1KW)