Re: Top-load altering Q-factor of secondary cct

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Gav D <gdingley@xxxxxxxxx>

Hi Malcolm,
what I meant was that if adding top-capacity does increase Q-factor, 
then there must be a limit due to the basic Q-factor equation 1/R * 
sqrt(L/C) would start to manifest. I read somewhere that displacement 
currents from the top-load make their way back onto the secondary 
increasing Q-factor, the greater the mass of metal forming the 
top-load, the greater the effect of these displacement currents. 
However there comes a point where the capacity of this mass of metal 
forces the standard Q-factor equation to kick-in; due to its 
sqrt(1/C) factor it does not happen for a while.

There are a couple of ideas that come to mind in explaining how 
voltage output may be altered by top-load size, but I don't know if 
they would manifest in a Q-factor measurement, and the theory behind 
them is very Tesla. One is that output voltage may increase with 
increased top-load because the capacity at this point is much greater 
than the self-capacity of the coil, a kind of potential divider 
effect - I think?. I suggest this as second hand, it is something 
Tesla explains in the CS Notes. The second idea is that as you 
increase top-load capacity it can start to approximate your ground 
capacity, shifting the ground node up and so a lower voltage appears 
at the top of the secondary. Again this is second hand, Tesla 
suggests this in the True Wireless article. This latter idea tends to 
presume a standing wave, or transmission line model; a taboo topic I believe.

In sstc coils I have noted a massive increase in Q-factor with 
stupidly sized top-loads; the bandwidth is so narrow I can tune the 
oscillator circuit with the proximity of my hand. Only have to look 
at the secondary circuit and it goes out off tune.

Anyway, I'm drifting out of my depth here.

Thanks again,

Gavin

On 10/26/05, Tesla list <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx> wrote:
Original poster: "Malcolm Watts" 
<<mailto:m.j.watts@xxxxxxxxxxxx>m.j.watts@xxxxxxxxxxxx >

Hi Gavin,

On 25 Oct 2005, at 23:03, Tesla list wrote:

> Original poster: Gav D <<mailto:gdingley@xxxxxxxxx>gdingley@xxxxxxxxx>
>
> Hi Malcolm,
> if I've got this right the top load can increase current flow in the
> top part of the secondary coil, resulting in a higher voltage and
> giving the appearance of a higher Q, but it's just that more current
> is flowing?

A tricky point since you are adding to the overall capacitance of
the secondary. Q is a measure of the energy loss per cycle/total
energy and it measures the same whichever way you look at it. Note
that in practice, the shape of topload is going to be a major
determinant of just how high a voltage is produced before energy is
bled off by corona. I haven't actually measured the voltages in
disruptive operation so can only conduct thought experiments in that
realm. Marco's research might better enlighten us.
     The measured Q in the case of _some_ secondaries with the
addition of a moderate amount of topload definitely measures higher
than with no topload but adding more beyond a certain point makes it
go back down.

Would this increased current effect manifest as a measure
> Q based on bandwidth and resonant frequency (Q = f / BW)?

It does exactly that. I am not presently in a position to be able to
quantify the exact causes (e.g. as Gerry suggests I'm sure the change
in frequency and skin and/or proximity effect is a factor also).

I thought
> there may be a trade-off due to the basic Q-factor equation.

Can you explain further please?

Malcolm