Te> Greetings,

 Te> I was playing with a Tesla coil design program and also 
 Te> reading the postings. These made me wonder about calculating 
 Te> values such as the inductance of the secondary and conical 
 Te> primary coils.  

 Te> The program mentioned that there were other contributing factors
 Te> besides the number of turns, radius, and length of the coil. 

The internal or distributed capacitance of the coil affects these
calculations to some extent. I am pretty sure that is what the
author of the program was referring to.

 Te> My question is should I just calculate the inductance of the 
 Te> secondary based on turns, diameter/radius, and length, and 
 Te> forget about the exact inductance figures? In other words, can 
 Te> I used the classic solenoid inductance formula as a ball-park 
 Te> type figure and then get the coil in tune experimentally, or am 
 Te> I doomed to fail without greater precision?

I always go by ballpark figures. If I want exact inductance and 
resonate frequency I have found the only sure way is to use the 
inductance meter, scope and frequency generator/counter. 

Doomed to failure? Hardly. Ballpark is all you really need, for you
will find that the true resonate frequency of the coil-discharger
in a static condition is not necessarily the resonate frequency
under load. During firing a conductive ion cloud forms around the
discharger which further decreases the resonate frequency as this
cloud is conductive and it adds isotropic capacitance to the air 
terminal. You will find that this isotropic capacitance due to the 
presence of ions is not a linear problem that is easily taken into 
account. I guess I could put this another way by saying that even 
if you tuned everything in with a scope; that does not mean that 
you are in perfect tune when you throw the switch.

 Te> The second question is in the same vein as the first.  Do I just
 Te> calculate the inductance of a conical primary as if it was flat 
 Te> and just allow extra turns for tuning?

That sounds quite reasonable to me. I allow lots of extra turns.
First there is tuning, then there is the need/desire to experiment
on occasion. Extra turns come in real handy if you want to operate
at very low powers with a small capacitor, or you want to operate at
a very low frequency by top loading a large discharger on the secondary. 
I like about 15 turns on the primary to give flexibility over a wide 
range of frequencies and power levels.

Richard Quick

... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12