From: "Malcolm Watts"
Date: Wed, 20 Jun 2001 08:25:26 +1200
Subject: Re: [TSSP] Some Considerations
Hi Bert, Marco, all,
I'd run with the capacitive divider idea as
being cleaner than a resistive chain (which would also have some
capacitance). Question then is: how much additional capacitive
loading can we tolerate and still claim a good measurement?
The problem becomes more pronounced as we edge closer to measuring an
unterminated resonator (the ideal).
Regards,
Malcolm
On 19 Jun 01, at 7:33, Bert Hickman wrote:
> Marco, Malcolm and all,
>
> Was thinking about this last night - a suspended capacitive divider as
> Marco describes OR one going through the center of the coil should work as
> long as the voltage stress is kept sufficiently low. The presence of the
> divider will unavoidably disrupt the local E-field around the top terminal
> as well as capacitively loading it. If we use low k materials around the
> divider this should be controllable, and if the voltage is kept below the
> point of corona or breakout around the divider (due to increased voltage
> stress at the triple point of metal contact/air/housing) we should be OK.
>
> Since we really don't need to capture any DC component, a simple capacitive
> divider should work as long as corona can be prevented and as long as its
> LF response is sufficient. Perhaps a chain of vacuum capacitors immersed in
> oil and housed in a low dielectric constant (polypropylene or
> polyethylene?) pipe? By immersing it in the more uniform E-field seen at
> the top or bottom of the toroid, the E-stresses can be made more uniform
> along the length of the divider. The problem with a pure capacitive divider
> versus a compensated divider is the changing response characteristic versus
> frequency, but allowances can be made for this.
>
> I have also seen professional dividers that use ceramic capacitors in the
> chain - have often wondered how they got around the effects of the
> capacitance changing as a function of applied voltage (a common problem for
> high-k ceramic dielectrics).
>
> -- Bert --
> --
> Bert Hickman
> Stoneridge Engineering
> Email: bert.hickman@aquila.net
> Web Site: http://www.teslamania.com
>
> Marco Denicolai wrote:
> >
> > Hello all,
> >
> > Malcolm Watts wrote:
> > > Where were you going to position the divider? Off-axis to the coil?
> >
> > We are talking about a 2-3 m high "tower": it is usually connected to
> > the target with an aluminium rod, supported by a rope hanging from the
> > roof (roof is maybe 10 m high).
> >
> > > Also, I wanted to eliminate as far as possible any loading effects. I
> > > was thinking of resistance of several GOhm.
> >
> > I believe capacitive dividers are really purely capacitive (not a
> > compensated resistive divider).
> >
> > >If we could get some
> > > accurate measurement under disruptive conditions but not necessarily
> > > at an arbitrarily high power level I think it would be reasonable to
> > > extrapolate the result under non-breakout conditions at least as it
> > > is possible to accurately quantify the secondary energy under those
> > > msmt conditions.
> >
> > If we recall that "disruptive" doesn't necessarily imply streamer
> > formation (with streamer loading, uncertainty, and all what follows), we
> > can run the measurement at a medium power level, thus avoiding extra
> > streamers leaving the aluminium rod.
> >
> > >
> > > > It would be also a matter of choice to retune the coil to compensate for
> > > > the tuning ratio shift or just to ignore it, as that is modeled too.
> > >
> > > It seems to me that this is a chicken and egg situation. We want to
> > > verify models so incorporating a model as part of the verification
> > > process seems self-defeating. Yes?
> >
> > The capacitive divider is a fairly simple device. It should be possible
> > to make a trustable model of it in five minutes. At least to a degree of
> > precision good enough for our purposes. It's a device used for precision
> > measurements, not a home-made low-budget toy like I have got at home.
> > We are basically looking for its capacitance (I guess), not its
> > resistive part.
> >
> > Regards
> >
> > --
> > _____________________________________________________________
> >
> > Marco Denicolai Senior Design Engineer
> > Tellabs Oy tel: +358 9 4131 2769
> > DSL Products mobile: +358 50 353 9468
> > Sinikalliontie 7 fax: +358 9 4131 2410
> > 02630 Espoo FINLAND email: marco.denicolai@tellabs.com
> > _____________________________________________________________
>
>
>
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.