Re: Water HV probe: first pictures

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Bob (R.A.) Jones" <a1accounting@xxxxxxxxxxxxx>

Hi,

If I remember correctly your using pure water.

So you may have several problems caused by its high resistivity.

1 Its self C must be charged vie its high resistance. For the same reason
its sensitive to capacitance to the top load and ground

2 The high output impedance must charge the cable scope input C.  This may
be the major bandwidth limitation.
You could 50ohm terminate the cable at both ends so it only looks resistive
but then it will not be self compensated.

Obviously you could just lower the resistance by adding copper sulfate until
the distrusted resistance/distributed C and the resistance/cable C time
constant are higher  but I assume the loading on the coil would then be
significant.

The only other way I can imagine you can do it is by putting a high input
impedance unity gain amp at the output but you will still have the self C
charging problem.
Perhaps you can lower the resistance sufficiently to get the required
bandwidth and add the amp.
Perhaps you can shield the whole probe in an outer tube filled with water to
shield it from external C effects and it will reduce the effects of its self
C because the outer and inner are at the same voltage at each point but the
outer will still need to be sufficiently low impedance that its voltage is
primary determined by its resistance not its self C or C to the top load.

Bob


----- Original Message -----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Thursday, December 23, 2004 10:27 AM
Subject: RE: Water HV probe: first pictures


> Original poster: "Denicolai, Marco" <Marco.Denicolai@xxxxxxxxxxx>
>
> Hi B2,
>
> First of all I would like to stress that I feel like a plumber trying to
> fix a TV set. I am really not a pro and I'm probably making a fool of
> myself with this probe stuff. This is my attitude, really. I'm really
> not spreading science to ordinary people :)
> Said that, I not only welcome but I really NEED the help of some more
> experienced fellow. I really appreciate your questions and the critic
> that (hopefully) they will originate.
>
>  > May I ask a few more questions to clarify the impression that
>  > I am getting from your divider web page?
>  >
>  > 1.   The shiny piece of ribbon coming down from the top
>  > toroid is your ground?
>
> Yes. Its final position will be one meter up from the toroid and then
> horizontally straight to the wall (grounded metal sheet).
>
>  > 2.   How far is the Tesla coil toroid from the (metal ?
>  > Ferrous?) wall?
>
> About 3 meters. Ferrous, yes.
>
>  > 3.   How far is the conducting ceiling (metal? Ferrous?) from
>  > the ground
>  > side (top ?) of your metering toroid?
>
> I must ask the right guy. I guess some 20-25 meters.
>
>  > 3.   How is the coaxial cable carrying the measurement signal
>  > brought down
>  > from the upper toroid?
>
> For these first measurements we have the 1Gb/s scope on the yellow
> elevator, connected with 50 cm of coax to the probe. Final configuration
> will feature a optical link (battery operated) on the probe top toroid.
> We have so bad readings at the moment that it doesn't matter yet.
>
>  > 4.   Why did you use a BNC as opposed to an N connector?
>
> I was targeting a 1 MHz bandwidth (was I optimist or modest, I don't
> know yet...)
>
>  > 5.   What kind of impedance matching termination do you apply
>  > between the
>  > BNC and the measurement coaxial cable?
>
> I have 50 ohm in series at the probe side.
>
>  > 6.   Why are the voltage divider plates so wide?  (In my
>  > experience 1.4"
>  > (3.5 cm-approx.) worked down to 1 ns.)
>
> The basic idea was to have a self-compensated design (see the principle
> drawing on the top on my web page). I just thought the top, bottom and
> tap plate should be about the same size. Why a smaller tap plate would
> be better?
>
>  > 7.   Why wouldn"t the capacitance limit the bandwidth?
>
> We had the following preliminary thought with a friend of mine. The
> divider is compensated if R x C of the lower arm equals the one of the
> upper arm. Or, better, if every section of the divider satisfies R x C =
> constant. In a homogenous field that is ensured by the basic design (one
> pipe, one diameter). In the non-homogeneous field between the probe
> toroid and the TC toroid the same result is achieved by varying the
> probe section diameter together with the field strength. That's what I
> did.
> Do you mean that the capacity of the upper arm to gnd still affects the
> bandwidth and spoils my design?
>
>  > 8.   How do you account for voltage grading along the
>  > (apparently?) thin
>  > edges of the voltage divider plates?
>
> I thought the toroid surface would shield the top and bottom plates. On
> the tap I should have less than 500V.
>
>  > 9.   What size (RG = ?) of signal cable do you use?
>
> RG-58.
>
>  > 10.  How long is it?
>
> 50 cm.
>
>  > 11.  How do you terminate it at or before the scope, analyzer, etc.?
>
> The scope is set to Zin = 1Mohm
>
>  > 12.  Do you have access to Frungel, or Chace and Moore, or
>  > the Bell System technical journals?
>
> All of them are new names to me (shame on me...). I have to check. I
> have access to an electronic library (IEEE, IOP,etc.) but only last
> years' issues. Have you got any good reference or even some PDF stuff?
>
> Please, send me feedback about the above.
>
> Best Regards
>
> *************** And Season's Greeting to all the list!
>
>