Re: Short, Fat Resonators?

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

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Mddeming-at-aol-dot-com>

In a message dated 7/30/01 12:49:29 AM Eastern Daylight Time, 
tesla-at-pupman-dot-com writes: 



>
> Original poster: "Gregory Hunter by way of Terry Fritz <twftesla-at-qwest-dot-net> 
> " <ghunter31014-at-yahoo-dot-com> 
>
> Has anyone on the list experimented with very low 
> aspect ratio (1 to 2 diameters tall) resonators? On 
> another thread somebody made some real interesting 
> statements about the beneficial properties of wide, 
> squat secondaries, and I'm wondering if anyone has any 
> hands-on experience to share. I've been eyeballing 
> those 5 gal plastic buckets that are so common these 
> days. They are made of polyethylene, and "clean" 
> buckets with no printing on them are available at 
> Wal-Mart & similar stores. I bet one of these would 
> make a fine secondary form for a low aspect ratio 
> coil. I'm not serious about building anything yet. I'm 
> just thinking about it, doodling with designs, etc. 
>
> Greg 
> http://hot-streamer-dot-com/greg 
>



Hi Greg, All, 
        It can be shown mathematically that the highest inductance for a 
given length of wire occurs in a coil when h=0.9r. This also means that the 
Q-ratio XL/R is at a maximum, since inductive reactance, XL, is proportional 
to L, and resistance, R, is proportional to wire length. The only person I 
know of who actually built coils to these proportions was Tesla himself. (40" 
diameter coil 18" high). While this might be desirable because Vout ~ 
sqrt(L2/L1), extremely short coils require much higher insulation since the 
voltage drop per inch of coil length dV/dh is extremely high. For example, a 
30" high coil at 300KV has a nominal gradient of 10kV/inch or about 
250v/turn. Under the same conditions, a 10" high coil has a gradient of 
30kV/inch or 750v/turn. Insulation breakdown and arcing are much more likely 
under these conditions. Like everything else in a TC, it's a trade-off. The 
rule-of-thumb is usually to keep "small" coils to 5:1 ratio h/d or less and 
"big" coils to 3:1 or less. The division between "big" and "small" is, of 
course, very murky. I can provide the derivation of the opening statement off 
list, if requested. 

Hope this helps, 
Matt D.