> http://www.qrz.com/db/DK7FC

Oh, rather a cone shaped coil.  That will alter my overtone
calculations.   They are important if you are driving the
coil with a switching waveform, as the system will happily
ring on each edge and radiate at the overtone frequencies.

> My antenna has C=580pF (measured on 137kHz). When theoretically
> calculating the C of a 100m vertical, you achieve almost the
> same.

Yes, I use a method of moments calculation for the quasi static
charge distribution.  We can integrate the charge distribution
to calculate the effective height fairly accurately.  The charge
tends to concentrate at the ends of the wire, but not quite a
50/50 split.  I'll see if I can do that tomorrow...

> i tried to vary the resonance by applying a ferrite rod into
> the field (0,1m long and 10mm diameter) but almost nothing
> happend!

To be expected, the ferrite is small compared to the coil area.

> Does anybody of the group want to buy such a kite?

The kite isn't the problem.  Finding someone to hold on to
a 100m kite with 10kV on it surely will be.  You need an
assistant called Hodges,

 http://www.lateralscience.co.uk/tesla/glitesla.html

--
Paul Nicholson