Re: Primary > secondary distance?

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

Original poster: Pyrotrons-at-aol-dot-com 

Hi Simon....

I had the same thoughts about 6 months ago with my 4.5" coil. Everyone seemed 
to have a inch or so of spacing between the primary and secondary coils. My 
coil had almost 3" worth of space, and I was curious as to how it was 
affecting performance. SO:

I soldered another 3 or so turns into the middle of the primary coil (1/4" 
copper refridgeration tubing) after advice to do so from the list.

I later found that doing this adds coupling between the two coils. More of 
the primary coil's magnetic field will be "absorbed" by the secondary coil. 
That's basically what coupling is. I imagine that adding turns to the middle 
has the same effect as increasing the angle (from horizontal plane) of the 
primary windings. This tilts the field inward to where the secondary is 
sitting - - resulting in a greater coupling coefficient (more coupling).

Depending on your system, you may find that you get better performance by 
adding turns to the inside. If you're critically coupled, that is if you have 
"perfect coupling", then you may experience problems after adding the turns 
due to overcoupling (too much interaction between the two coils).

I will say, that I was running 1kVA into a 4.5" secondary coil, using a flat 
pancake primary coil. When I added those 3 extra turns, performance really 
livened up. Output sparks weren't a whole lot longer, but they were sure 
hotter. I was very impressed with the result. Sometimes I think that coupling 
is slightly overlooked sometimes when dealing with efficiency/performance, 
especially with some medium sized coils out there (such as mine in this 
example). I'm wondering if I could've gotten better performance by going with 
a 15 degree primary, or even 30 degrees. Has anyone done work, and found a 
good coupling primary coil in a 4.5" system? I'm interested to know.

Simon by the way, I don't think rewinding your primary is necessary. I found 
that just soldering the turns in place with a propane torch and some good 
flux worked out great. You can barely tell that there's no supports holding 
the new inner turns up.....

Good luck to ya,

Justin Hays
KC5PNP
Email: pyrotrons-at-aol-dot-com
Webpage: www.hvguy-dot-com