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Re: Grounding



Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <paul-at-abelian.demon.co.uk>

Gary Lau wrote:

> To the amazement of many including myself, at least one person on
> this List has reported not connecting the bottom of the secondary
> and seeing no significant loss in performance. 

With some coils this behaviour is to be expected. 

In a recent post I described the continuous deformation of a free 
bipolar coil into a grounded monopole. To begin with a horizontal
coil with both ends open-circuit, the current maximum is in the center
and the thing is symmetrical, with the center at a 'virtual' earth
potential. Tilt the coil and bring one end closer to the ground and
the virtual earth point moves towards the end with the greatest
capacitance, ie the end near the ground. As the coil is brought close
to the ground the virtual earth is maybe 10% to 25% of the way up,
and it is only a minor step then to short out the bottom-end
capacitance by actually grounding that end.

In cases where the grounding of the coil base is not too effective to
begin with, it's quite possible that little difference in performance
will be noticed by removing the ground connection. 

There are some disadvantages to operating without a well defined 
grounding of the coil base. By relying on the end capacitance to
effect a partial grounding, the position within the coil of its
'virtual' ground will wander about according to the distribution of 
capacitance and loading of the coil. For eg, toroid capacitance or arc
loading from the coil top will raise the virtual earth point further
up the coil and suddenly the coil base will be at quite a high RF
potential, with the risk then of arcing to the primary or worse.

Weazle wrote:

> Use a heavy guage wire for the ground and avoid sharp bends if
> at all possible, as R.F. doesn't like to turn corners.

There's little point in using a gauge larger than the coil wire gauge
except perhaps for mechanical strength. Sharp bends are not really an
issue at the low frequencies used by TCs. If you're having to run a 
long way to reach a ground then you may do better by laying out radial
conductors beneath the coil and connecting to those instead. You
should still connect your radials to a distant earth connection but
that will be more for a safety/static ground than for the benefit of
the RF.

Although you may in fact get a slightly higher end-end voltage without
a base ground, the disadvantages are more significant, and off-hand I
can't think of any good reason to operate without anchoring the coil
base to at a least a 'local' ground wrt the coil E field.

Terry Oxandale wrote:

> I had severe arcing from the primary winding to the secondary
> winding whenever I left the ground wire off of the coil, BUT, if the
> ground wire was connected to the coil, but not connected to the
> ground rod outside (about 30 feet of wire over the garage floor) the
> coil operated as if the ground was completed to the rod outside.

Yes, that's a good observation. The additional end capacitance applied
through the 30 foot wire is enough to clamp the coil base potential,
think of it as a single 'radial'. You might get better performance
when using a few of these radial wires than you do by going through
the ground rod, since you're bypassing the earth return resistance
of the soil beneath your garage floor. Do connect your radials to
your grounding rod as well though, because if the top of the coil
arcs to something not connected to your radials, the radials can
suddenly find themselves at a significant RF potential wrt the earth,
with possibly entertaining results.

Cheers
--
Paul Nicholson,
Manchester, UK.
--