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On 10/14/20 6:34 PM, Gary Lau wrote:
It also needs to be said that highest coupling is not the goal - more is
not necessarily better. Performance will generally increase with
increasing coupling, until an ill-defined threshold is reached. Increasing
coupling beyond that point will result in what's called "racing sparks" -
sparks across portions of the surface of the secondary coil that can
quickly destroy the secondary. You need to design the coil in such a way
that the distance between primary and secondary can be experimentally
adjusted to vary the coupling, typically by raising the secondary. For
most coils, using a flat spiral primary provides adequate coupling when the
plane of the primary is near the bottom end of the secondary.
ANd the flat primary also serves as a sort of counterpoise to make the E
field uniform between topload and primary.
Go back in the archives and you'll see all sorts of primaries,
solenoids, inverted cones (I did that one), and flat spirals.
The advent of modeling tools (like Inca and JavaTC) that could
*calculate* coupling and fields pretty much drove the "make it flat" -
except for ones requiring high coupling (DRSSTC, for instance), where
solenoid primaries are popular.
There's not much you can do with a conical primary that you can't do
with a flat or solenoid, with some control of the spacing.