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Re: A coil design (fwd)

Hello All,
         Malcolm Watts said in part as a reply to Tom McGee:
<snip>
>I decided to stick my neck out and post the reply to the list despite 
>coming up with this one off the cuff. It shows the sort of approach I
>take to a specific design knowing what I do now. So here goes......
>
>Primary Cap : 13nF or so for a static gap (based on US mains 
>frequency and transformer).
>Wire Size : 3 skin depths at final resonant frequency (1 skin depth 
>in copper is about 66/SQRT(f) mm. ).
>H/D ratio 2.5 - 3.5.
>Secondary Height : about 18"
>    One person on the list has scored 4 feet from the transformer you 
>name running a synchronous gap system. I have scored 3 feet using a 
>static gap. H=18" allows about 3 times the secondary length in output.
>If that were so, I'd go for a coil around 6" diameter or more bearing 
>in mind that it has a significant effect on Cs which in turn is going 
>to affect Vo. The other side of the balance demands the lowest 
>frequency you can run at for efficiency. Cself for those figures is 
>around 9.3pF. You would want around the same amount or more in the 
>top hat since you want top storage and a decent ROC for long sparks.
>Turns : around 1000 or so (too few yields a higher than optimum 
>operating frequency and too many yields unacceptably small wire size
>compromising secondary performance). 
>   Using Wheeler's inductance formula, this gives Ls around 43mH. If 
>Ctot is about 18pF, this gives fr about 180kHz. Check to see whether
>the wiresize is suitable: Using the skindepth formula -
>dwire = 200/SQRT(180,000)mm = 0.47mm/turn. The insulation for this 
>size wire would add around .05mm/turn, so I would use 0.45mm (copper 
>diameter) wire and accept a few less turns which brings dwire within 
>cooee.
<snip>
        I wound a coil last Saturday similar to what you describe above.
Here is your reality check for comparison.  My coil is 6.25 inches in
diameter, 22.25 inches tall (3.56:1 H:D), wound with 24 AWG wire (0.56
mm/turn).  I have around 1000 turns on it.  The measured inductance is 41
millihenries, the calculated self capacitance is 12.1 picofarads, based on
the measured self resonant frequency of 228.5 kilohertz.  Qualitatively, the
coil Q is excellent, exhibiting a very narrow bandwidth at resonance (too
much nearby equipment for quantitation).
        I'd say your suggested parameters are right on target Malcolm!
Regards,
Mark S. Rzeszotarski, Ph.D.