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Date: Wed, 10 Mar 2010 01:35:00 +0000
From: Paul Nicholson <vlf0308@abelian.org>
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References: <9afca2641003090337t4062bb3avc9b952612014c0ba@mail.gmail.com> <4B9678C3.7070907@iup.uni-heidelberg.de> <4B9689B9.8030805@abelian.org> <38A51B74B884D74083D7950AD0DD85E82A1B47@File-Server-HST.hst.e-technik.tu-darmstadt.de>
In-Reply-To: <38A51B74B884D74083D7950AD0DD85E82A1B47@File-Server-HST.hst.e-technik.tu-darmstadt.de>
Subject: Re: LF: VLF TX coil matching
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Stefan Sch=E4fer schrieb:

 > Who tells that they [charge] are just to find at the end?

The charge distributes itself for minimum potential energy,
so it concentrates somewhat towards the ends.  Actually
the concentration isn't all that much, only about 10% greater
at the bottom end than in the center.

  http://abelian.org/vlf/tmp/ss100309a.png

shows the linear charge density, Coulombs/metre, along the
wire having 1 volt on it.  At least that's what the model
says, I will have to check it against Maxwell's 1878 formula!

The distribution is slightly biased towards the bottom due
to the presence of the ground, so the effective height will
be a bit less than h/2.

The maximum surface field strength looks to be about 2.2
volts/cm per volt on the wire, ie 10kV on the wire will
give you 22kV/cm at the bottom end of the wire.  Take these
results with a 'pinch of salt' until I've checked the model
against some calculation.   But the figures look reasonable
and if they stand, there is no problem until 20-30kV, and
then you can suppress corona easily with rings and spheres.

 > I plan to use an inductive coupled primary coil,

OK.  There is some advantage to driving into the base, as
Markus mentioned in a post a few days ago - current is self
limiting as the system detunes.  You would need up to a
kV from the driver, I think.   If you can't manage that,
it would have to be a tuned primary.

 > The PA has a big output transformer with different numbers
 > of turns, like a universal transformer.

Perfect.

 > do you think i MUST have to built a simple band pass?

No, not yet.  Still running models.  For example, first overtone
is at 117kHz, close to 13th drive harmonic.  Coil base impedance
is quite high and harmonic voltage 1/13th of fundamental
so current is low.  But low enough to ignore?  Not sure yet.
That's with a square wave from a switching driver straight into
the base.  Maybe a broadly tuned primary (Q=3D10) will be needed?
Or just a simple trap to catch and dump the high harmonics into
a resistor?  Basic question is: can loading coil Q at 9kHz be
relied upon for a clean signal when directly driven from a simple
efficient switch-mode PA?   Yes, Q is high but the coil is a
transmission line and has a spectrum of resonances...

 > Each friday and monday i have free time, so my weekend
 > is 4 days!

I will move to Heidelberg too :)
--
Paul Nicholson
--