is the topload acting as a capacitive transmitter?
I mean is it transmitting using a dielectric effect
rather than electromagnetic.
The electric field dominates, therefore mostly capacitive
coupling to surroundings. Far field is negligible which
is why you don't need a transmitting licence.
does the topload turn the secondary into a parallel or
series LC network?
The TC is either parallel and series resonant - depends on
your viewpoint. Looking between top terminal and ground
you see a parallel resonance. Looking into the base of
the coil you find a series resonance. Same when you add
a topload but with increased C.
could the secondary topload be sufficiently sized to match
the inductance of the secondary?
Not sure what you mean by 'match' here? Coil and topload will
form a resonator with whatever L and C they happen to have.
If you want a particular frequency then you must choose the
topload to have the correct C to achieve your target frequency.
But usually F is not a design target.
The topload protects the top of the coil from high field
strengths, and matches the output of the coil to its load
(usually a spark loading). These considerations determine
the size and shape of topload. That fixes the topload C.
In combination with the coil, that determines resonant
frequency. Then you design a primary to match that F.
...Tesla used an elevated capacity. Was this for transmission
purposes
Probably the intention was to extend the E-field as far as
possible.
... or could it act as a delay line between the inductor
and capacitor?
No significant delay.
If the topload is too far above the coil it wont be able to
protect the top of the coil. Can consider two toploads, one
toroidal just above the coil and another of any shape which
can be placed remote from the coil.
Formation of long sparks requires short very rapid bursts of
charge delivered into the developing streamers. The coil only
provides charge slowly (compared with the streamer formation
timescale), so the topload needs to act as a charge reservoir.
A sphere is the worst possible shape for this - for a given
size and voltage a sphere is the shape that stores the least
amount of charge. A sphere is also poor for controlling the
field around the coil top. Toroids are good in both respects.
I'll just add that there is no formula to calculate the ideal
topload size/shape to give max spark length for some given
power level. Coilers have discovered over the years that
large is good, and larger can be even better. But there must
be an optimum size - too small and not enough charge stored,
too large and not enough E-field to push out the streamers.
--
Paul Nicholson
--
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