RE: Primary coil form

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

Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Gary.Lau-at-compaq-dot-com>

The differences between the three forms of primaries - flat, conical, and
helical, all boil down to primary/secondary coupling (k), and strike
distance from outer/top primary turn to top load.  I don't believe that the
"shape" of the magnetic field is any more or less focused among them, as
some have suggested.  Any of the three can be adjusted to achieve the same
coupling, though a helical may have to be so far down from the secondary
that it becomes awkward.  And with the smaller diameter of a helical, more
turns and more wire will be required to achieve the same inductance as a
flat or conical primary.

I don't believe that a helical form offers any advantages over the others,
although they do seem popular for tube coils, perhaps because a higher
coupling is needed(?).  A conical may allow a higher coupling to be achieved
if the construction limits how low the secondary can be lowered.  But if a
good pri/sec arrangement is worked out in advance to allow easy adjustment,
a flat primary will allow adequate coupling while maximizing primary to top
load strike clearance.  With only a 15" long secondary, you'll need as much
clearance as you can get.

Regards, Gary Lau
MA, USA


>Original poster: "Vanderputten, Gary by way of Terry Fritz
<twftesla-at-qwest-dot-net>" ><gvp-at-pvaintl-dot-com>
>
>For smaller coils, what is the performance difference between a
>helical/tubular versus the 'inverted cone" primary?  
>My past coils (40 years ago) were all helical and performance was mediocre
>compared to what I see today. However, given my modest requirements and the
>difficulty in creating and inverted cone, what do I give up with a helical
>primary?
>Is flat better than Helical?
>
>Specs = 
>3.5 x 15" secondary, 900+ turns of #28 dbl 
>12/30 nst
>Fritz/Lau protection circuit
>.0066 mf /30 kv cap
>
>Thanks
>
>Gary