From: "Malcolm Watts"
Date: Mon, 27 May 2002 09:47:36 +1200
Subject: Re: [TSSP] Topload breakout potentials
Hello Paul, all,
On 24 May 2002, at 8:14, Paul wrote:
> Malcolm Watts wrote:
>
> > We are talking about direct connection without disturbing the
> > external fields aren't we?
>
> Oh yes.
>
> > Somehow, we have to construct a probe which can go up the centre of
> > the coil
>
> Yes, then the coil will shield the external field from the divider,
> and we just have to shield the divider from the coil!
Agreed, and I had that in mind. In effect, a thin coaxial probe with
good insulation between the centre wire and shield and between the
shield and the windings. I realize that this is not a simple problem
to deal with. There is of course the capacitance between the central
wire and shield, not to mention distributed inductance. The
disturbances would be reduced as the coil diamater got larger
relative to the probe's diameter. Just thinking about the probe
structure: the insulating material must have a low dielectric
constant and be as low a loss as possible. It may be that the
dielectric shielding the central wire from its shield and the
dielectric surrounding the conductive probe wire shield may
themselves have to be made coaxial so that gaps (preferably vacuum)
exist between layers, the idea being that permittivity and hence
distributed capacitance between the central wire and shield is
further reduced. The probe can of course be both tested and modelled
at operating frequency independently of the coil being measured to
quantify its characteristics so these may be factored in to the
actual coil measurements. Further modelling with the probe
"virtually" in place may quantify its disturbance on various types of
windings.
Sorry for thinking out loud - I'm trying to inject ideas so that
someone who is currently in a better position than I may have a first
shot at constructing something. I have too many irons in the fire to
be able to dedicate every waking moment to this project (which I
would just love to be able to be first at succeeding with). I realize
we want to make progress and this is no time for one-upmanship. I've
realized for some time that this measurement is probably the ultimate
frontier in TC research.
Regards,
malcolm
> > (we can quantify any
> > disturbance it might cause through low power measured comparisons)
>
> Up to a point. For example a central divider column will register
> a mixture of the topvolts and the coil's voltage distribution. The
> 'mix' will be fixed by the geometry below breakout and thus the
> topvolts can be calibrated. But above breakout, all is lost unless
> the divider can be shielded. The performance metric is the ratio
> between the divider output due to conduction current entering from
> the topload, verses the divider output due to displacement currents
> induced by the E-field. For any given divider design we can
> measure this metric at small signal, and thus assign an error
> estimate to subsequent topvolts readings.
>
> But we can certainly allow for the fixed C of the divider loading
> the top, so that's not a problem unless the loading is so much that
> it upsets the breakout behaviour.
>
> Anyway, despite the apparent difficulties I don't think we should
> be too intimidated by this. After all, even a fairly rough and
> ready divider will give us some sort of result. As we study the
> data from it we'll begin to notice limitations, but it represents
> a starting point and it gets things moving.
> --
> Paul Nicholson,
> --
>
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.