Re: Lumped vs. Helical Resonator
Tesla List wrote:
> >From jim.fosse-at-bdt-dot-comTue May 21 13:29:23 1996
> Open Question. Could a small magnifier, like TCBOR's latest(?) 1
> footer, be placed inside a metal shield (can) filled with transformer
> oil and have it's Q raised to say, 6K like a true helical resonator?
> > The interesting part of all of this to me is the influence the
> >toroid has on the proximity effects near the top of the coil. This may be
> >due to a change in the voltage standing wave along the secondary, resulting
> >in a more linear voltage response, with reduced turn to turn proximity
> >effects due to the similarity of voltage between adjacent turns.
> Minds eye experiment: visualize the helical coil as a straight open
> air transmission line, at the far end place a "guard cylinder" around
> it. Connect this to the end of the transmission line. This represents
> the electrostatic shielding effects of the toroid. The mutual (turn to
> turn) capacitance has been canceled out and the capacitance to ground
> has been canceled out by virtue of the "equal potential" (to a first
> order minds eye rigor) environment. My only question is: what happens
> to a transmission line when the C portion of it is eliminated? Do we
> eliminate the series LC voltage rise on that section and turn it into
> a pure inductor? A simple delay function?
> Another thought, just to muddle matters even more, could the "guard
> cylinder" be acting in just the opposite way that I describe above and
> have a multiplying effect on the voltage rise on that section of the
> transmission line?
> > Anyone have a good method for measuring high RF voltages?
> Last year Fred Bach posted on the construction of a Field Mill,
> perhaps Fred could repost the construction and calibration info?
> I've just seen an ad for (Cadwell?) 30Mohm 30kV noninductive resistors
> for $1 US apiece. How about 30 or so in series, inside a plastic tube
> filled with transformer oil to get a 1MV 900Mohm voltage divider? The
> only problem is building the peaking cap to place in parallel;)
> otherwise: 900 ohm // 10pF => 110kHz. we would be off 3db.
I don't know the ansewer to all your questions. I would imagine the turn
to turn C would not go away on the coil, even in a guarded arrangement,
only the isotropic C would disappear. A new capacitance due to the guard
itself might develop, too. Only an experiment would tell.
Voltage at RF frequencies on the order of what comes from a big TC would
not be a matter of mere resistive drops. A capacitive divider might be a
better choice for a TC coil voltage measurement device.
Ricahrd Hull, TCBOR