Re: TC Operation Question

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Paul Nicholson <paul-at-abelian.demon.co.uk> 

EMMETT SECREST wrote:
 > I would like to know what relationship the RF ground has to the
 > operation of the TC. I know it is to direct RF to ground but what
 > I was wondering was what effect it has on the operation of the
 > coil if it is not present at all.

Further to earlier answers, yes the RF ground is a fundamental part
of the TC.  If you don't take steps to provide one, the TC will
automatically establish its own.  And it may not be in a comfortable
place!

Note that when we use the term 'RF ground', it need have nothing
whatsoever to do with the earth outside in the garden.

If you think of the TC topload as one 'plate' of the TC's output
capacitance, then the 'RF ground' forms the other plate, and
RF current on this ground plate will be trying to make its
way back to the bottom end of the secondary coil.  If there is
no wire connection to the bottom of the sec, then the TC has to
make do with the base capacitance of the sec, ie the capacitance
between the base of the secondary and its surroundings.

Whether you provide an RF ground or not, in practice the
walls, ceiling, floor, table, chairs, people, etc, all take part
in the RF oscillating circuit of the secondary coil and its
capacitance.

Each object will find its own electrical potential, which will be
somewhere between that of the TC topload and the TC secondary base.
The potential each object takes up depends on the capacitance or
conductance between it and the topload, and between it and the
secondary base - acting just as a voltage divider - capacitive or
resistive.

So, you can just leave things to their own devices and let everything
find its own potential.  But most operators prefer to encourage
most of the objects around them (and themselves too!) to be nearer
to the potential of the secondary base rather than that of the
topload!.  That way you don't get corona discharge around your
boots and you don't get RF burns when you grab a door handle or
beer can, etc.

This can be done easily by choosing an object with a large self-
capacitance and connecting everything to it.   The earth itself
is one obvious choice.  Since most things around you have either a
half decent conductive connection to earth, or a large capacitance
to earth, then by also connecting the coil base to earth you achieve
the desired result. All the high voltage then appears on the topload,
rather than on boots and door handles.

If it's inconvenient to run a wire out to a stake in the garden,
you can achieve much the same effect by laying out a large sheet
of something conductive, eg aluminium foil, beneath the coil base
to form an artificial ground, sometimes called a counterpoise.
Connect this to the coil base, and the large capacitance of the
sheet will tend to pull objects in the surroundings towards the
coil base potential and away from the topload potential.  It makes
for a very efficient (low resistive loss) 'bottom plate' for the
TC secondary capacitance.  Use both a garden stake and a foil
ground plane if you like.

Yes, in answer to your question, the lack of explicit grounding
arrangements will not prevent the TC secondary from oscillating.
But just where the resulting high voltage develops is left to
the unpredictable balance of 'stray' capacitances.  Efficiency
will be lower and arcs poorer.  You might be able to draw better
arcs from the coil base than the topload!  Ideally you want the
topload to be 'hot' and everything else to be 'cold', and a
decent ground arrangement brings all this under control.
--
Paul Nicholson
--