Dear Paul,
I built the system that way because I had a lack of capacitors in the right
range so I had to increase the inductance.
This resulted is a long primary approximately the same wound length of the
secondary. I noticed the secondary was not getting really hot at the base
but throughout the entire coil. I then assumed it was because the magnetic
flux was spread evenly. Hence my belief it works better.
But it could have been something else. I got 40-50cm today out of the same
system with a large topload. At 1700W using a half wave voltage doubler and
a MOT. The primary wound length is 19.5cm and 380 turns. It is an unusual
coil: http://youtu.be/kWriWcVIe1Q
I build stuff as cheap as I can so it results in different looking coils.
But the principal is the same.
This is the best coil I built so far so I tend to favour it's
characteristics. However if I had a system that was different maybe my
thoughts would change. Not very scientific :)
As for the spot heating I did have a lot of trouble with coils getting
really hot near the primary. After using a longer primary the heat was
spread out more. I don't know if this changes the magnetic coupling. I will
have to check with JavaTC.
Thanks for explaining the 1/4 wave myth fully.
Cheers!
-Wil
On 03/11/2014 8:12 PM, "paul" <tcml88@xxxxxxxxxxx> wrote:
>
> William Howard wrote:
>
> > It is disappointing that Tesla is used a basis for so many
> > conspiracy theories and related pseudo science.
>
> Not so much these days, there's lots of up to date reliable
> info out there now to balance things out. This mailing list
> is firmly moderated which helps a great deal.
>
> The '1/4 wave myth' that others mention refers to an obsolete
> notion that the length of wire wound into the secondary
> continues to resonate at the frequency it used to do when it
> was a straight length of wire.
>
> For example, take 1000 metres of wire. In a straight line
> its 1/4 wave frequency would be 300e6/(4 * 1000) = 75kHz.
>
> Wind that wire into a coil with length/diameter ratio of say,
> 4:1, and the 1/4 wave resonance rises to around 120kHz, no
> longer 75kHz.
>
> A variation on the myth recognises that the frequency rises
> when you turn the wire into a coil, but goes on to suggest
> that you should then add enough topload C to pull the resonance
> back down to equal the original frequency of the straight wire.
>
> However nothing special happens when you do that, although
> it does lead to a large topload which is usually good for a
> classic TC. Perhaps that explains the persistence of the myth.
>
> > Secondary and primary work best when they are the same length
> > or a at least 80% of the secondary length.
>
> A helical primary 80% of the secondary length is unusual
> I think. Might be wrong here but 10% or 20% could be more
> typical. Where did the 'spot heating issues' occur that led
> you to such a long primary?
>
> --
> Paul Nicholson
> --
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