> From: chip (Chip Atkinson)

 CA> Thanks for the answers.  A few other questions:  How big     
 CA> should the primary coil be?  I figure that the inside radius 
 CA> should be big enough to allow the secondary to fit in it and 
 CA> allow some room to attach to the tail of the secondary coil. 

On flat pancake or saucer type primary coils I allow from 1 to 1-
1/2 inches all around the secondary coil for spacing. This is to
prevent arcing and overcoupling. Figure the diameter of the
inside, smallest, turn of primary needs to be 2 - 3 inches larger
than the diameter of the secondary coil. 

 CA> I heard something like the primary should be 3x the diameter 
 CA> of the secondary. Is that true?

If you are using a vertical, helical wound, primary with 2 - 4
turns this is barely enough spacing, and the primary may need to
be wound on a heavy plastic insulator (like a cut down bucket) to
prevent heavy corona losses and occasional arcing. I do not
recommend this type primary coil.

 CA> How much space between the secondary and inner turn of the   
 CA> primary?  

Figure about 1 to 1-1/2 inches from the secondary to the first
turn of the primary at any point. 

 CA> I guess I should just allow 1/4" to 3/8" spacing between     
 CA> turns, calculate the inductance at about 7 turns, and say,   
 CA> 20 turns and see if it spans the calculated necessary value  
 CA> for primary inductance and start tuning from there eh? 

20 turns is a bit much even for me, and your ratio of
transformation will begin to suffer. Figure your model from 8 -
15 turns.

 CA> Also, am I correct in assuming that all the wires in the     
 CA> primary circuit should be able to carry at least as much     
 CA> current as the primary winding?  I always  hear about        
 CA> "heavy" wire, but it doesn't make sense to use beefy tubing  
 CA> in the primary coil if most of the wire in the primary       
 CA> circuit is any smaller  (higher resistance).

You figure correctly. The tank circuit conductors must be heavy,
regardless if you are talking about spark gap and capacitor
connectors, or the primary coil.

Richard Quick

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