Re: Lumped vs. Helical Resonator

To: tesla-at-pupman-dot-com
Subject: Re: Lumped vs. Helical Resonator
From: Jim Fosse <jim.fosse-at-bdt-dot-com>
Date: Tue, 21 May 1996 05:02:34 GMT
On Mon, 20 May 1996 12:33:26 -0600, Tesla List
<tesla-at-poodle.pupman-dot-com>, you wrote:

Mark,
	Thanks for the reply.
[snip]
>        This is true due to the rapid falloff in mutual inductance, as shown
>in the simulations.  However, the standing waves produced on the helical
>resonator demonstrate a voltage rise along the secondary which is nearly
>sinusoidal in appearance.

Yes, which lead us back to a combined model that is a 1/4 wave
resonator driven by an decreasing volt-per-turn induced voltage. I
think that we have to agree to agree here:)


>This is demonstrated experimentally in Duane Byland's text, where he wired
>up a tesla coil secondary with multiple LED's every inch or so along the
>secondary.  
NEAT!  Question thought, were the LEDs wired turn to turn or turn to
ground? If the latter, I pity the top LEDs;)

>        I also suggest that once the air ionizes and a spark breaks out, all
>the above rules are out the window.  At that point,  The energy stored in
>the toroid is "probably" being dumped out, so you want a big "C" on top to
>make 1/2xCxVxV large.
Yes, I've seen this with the drop in E field strength on my "relative"
field strength meter. 3 before breakout, 1 or 1.1 after. (1 is the 0
point of my SWR bridge)

Idea: the top-hat dumps it's energy and starts the discharge, the coil
then acts as a current source until it has dumped all of it's energy.
I'm not sure how the 1/4 wave aspect of the coil fits in here except
to act as a delay/transmission line element
>
>        I agree.  There is a simple solution, however.  Build a magnifier.
Coming soon, to a location near me;)

>Then you can use helical resonator theory (at least until the spark breaks out).

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.

	Regards,

	jim