Re: New formula for secondary resonant frequency

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

Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <paul-at-abelian.demon.co.uk>

Terry wrote:

> I inserted a Sonotube type form into my regular big coil form to see
> the > differences it would make.
> 
> http://hot-streamer-dot-com/TeslaCoils/Misc/SonoQ/SonoQ.jpg
> 
> Using many toys,
> 
> http://hot-streamer-dot-com/TeslaCoils/Misc/SonoQ/Stuff.jpg
> 
> I scanned the coil's frequency response with and without the extra
> tube inserted.  I swept the frequency linearly from 140 kHz to
> 150 kHz and measured the response with a whip antenna connected to
> the scope.  The differences are shown at:
> 
> http://hot-streamer-dot-com/TeslaCoils/Misc/SonoQ/SonoQ1.jpg
> 
> With the second tube inserted into the center of the big coil tube,
> the frequency dropped from 147.2kHz to 146.68kHz.  The Q dropped to
> 69.5% of the original.  Although Sonotube is fairly good for coil
> forms, its loss can affect such details...

That's a dramatic and unequivocal demonstration of the atrocious
performance of sonotube.  Terry's comments and pictures would make for
a good 'Dont Use Sonotube' webpage. Bear in mind that Terry's coil is
already wound on sonotube, so the coil's Q factor is already decimated
before the second tube is inserted. Modeling suggests that on a non-
sonotube former, Terry's coil would have a Q somewhere above 200. This
sonotube effect is more than enough to ruin the CW performance of a
coil.

I guess we still don't know what it is about the sonotube that short-
circuits the E-field like this?

As regards the effect on Fres calculations, there are numerous other
factors which, as Kurt Schraner has demonstrated, affect the resonant
frequency. The formula I gave applies to coils operated in a
reasonably open space, and might therefore be considered an upper
limit on the actual Fres.  Introduction of walls, roof, primary,
strike rings, etc, all act to reduce Fres below the prediction, and a
program such as E-Tesla6 is required to calculate these effects.
Perhaps it might be possible to define empirical adjustment rules in
order to make the formula a little more applicable to real-world coil
setups, eg subtract 5% if you are indoors, subtract another 3% if you
add a primary, etc, that kind of thing? 
 
Regards,
--
Paul Nicholson,
Manchester, UK.
--