Cecil Moore wrote:
On Oct 18, 11:30 am, Jim Lux wrote:
You can model a tesla coil's
behavior to within about 5% using a simple lumped LC model.

How can a model that presumes faster than light speeds yield a valid
outcome? Drs. Corum seem to disagree with you. Here's what I have been
quoting:

http://hamwaves.com/antennas/inductance/corum.pdf

Drs. Corum seem to debunk the lumped LC model. They also once had some
class notes titled: "Tesla Coils and the Failure of Lumped-Element
Circuit Theory", but I can't locate it on the web.

Changed the topic.. and really, this isn't r.r.a.a territory any more..
Feel free to send me an email directly.

This has been thrashed through pretty thoroughly on the Tesla Coil
Mailing List (TCML, http://www.pupman.com/) and I'd refer interested
parties to the list archives, or, to the works by Paul Nicholson.


I've had some nice discussions with Corum the younger, but, their model
makes life harder than it needs to be. Occam and all that.

At TC resonant frequencies (100kHz), the "light time delay" from top to
bottom of a 3 foot high coil is pretty small. I don't know about faster
than light, but at 100kHz, for an object that's a meter or two in size,
assuming simultaneity isn't a big stretch.

(for the propagation of the spark, though, and the current flow in the
top load, yes, the speed of light matters, and in fact, the speed at
which the charge can get off the top load and into the spark channel is
probably one of the bigger factors affecting maximum spark length.)

People have also put current and voltage probes at the top and bottom of
the secondary coil (with fiber optic connections etc.).

Getting to the "transmission line" or "slow wave structure" aspects..

You've got an inductor with a lot of stray distributed capacitance.
Indeed, that's exactly what a transmission line can be modeled as
(distributed series L and shunt C). And with an arbitrarily complex
nonlinear distribution of L and C, you can make a transmission line that
acts like a lumped L and C of the appropriate values.

So, the question really is, do you want the simple model or the complex
one. If the goal is to design better tesla coils, and the simple model
gets you to fractions of a percent in terms of agreement between
experiment and theory, why use the more complex model. It doesn't lead
to any better understanding of how it works, either.


Now, if you want to talk about modeling the spark channel as a time
varying lossy transmission line, that's something more interesting, and
it DOES have value in terms of understanding/predicting the behavior.
For that, I'd point readers to "Spark Discharge" by Bazelyan and Raizer,
which is one of the best works in the field, and a fascinating read
(and, as well, they have that cool 150 meter+ spark picture from a
really, really big Marx generator, 1.2 MJ or more, in Siberia)

On Oct 18, 6:43*pm, Jim Lux wrote:
... assuming simultaneity isn't a big stretch.
... yes, the speed of light matters, ...

These two concepts seem to be contradictory. Some simple 1/4WL Tesla
coils are obviously close to 90 degrees long and limited by the speed
of light. The traveling waves cannot travel faster than the axial
propagation factor, whatever that value might be. For those simple
Tesla coils, there is an electrical ~1/4WL between the feedpoint and
the spark. Simultaneity is impossible in the real world.

(Thanks for the email invite, but right now most of my time is
dedicated to learning how to be a square dance caller.)
--
73, Cecil, w5dxp.com

Cecil Moore wrote:
On Oct 18, 6:43 pm, Jim Lux wrote:
... assuming simultaneity isn't a big stretch.
... yes, the speed of light matters, ...

These two concepts seem to be contradictory. Some simple 1/4WL Tesla
coils are obviously close to 90 degrees long and limited by the speed
of light. The traveling waves cannot travel faster than the axial
propagation factor, whatever that value might be. For those simple
Tesla coils, there is an electrical ~1/4WL between the feedpoint and
the spark. Simultaneity is impossible in the real world.

(Thanks for the email invite, but right now most of my time is
dedicated to learning how to be a square dance caller.)
--
73, Cecil, w5dxp.com

The actual physics isn't of interest. What's of interest is "does the
model replicate the observed behavior of the real system" and "do
changes to the model produce changes comparable to that of the real
system in a fashion that is good enough to allow systems to be designed
and work on the first try".

The lumped model is sufficient for both of these.

The "traveling wave" model is also sufficient, but is substantially more
complex, and doesn't bring advantages to the design process.

Tesla coils can be modeled very well by a lumped model with a half dozen
components and nodes.

On 10/19/2010 4:51 AM, Cecil Moore wrote:

...
These two concepts seem to be contradictory. Some simple 1/4WL Tesla
coils are obviously close to 90 degrees long and limited by the speed
of light. The traveling waves cannot travel faster than the axial
propagation factor, whatever that value might be. For those simple
Tesla coils, there is an electrical ~1/4WL between the feedpoint and
the spark. Simultaneity is impossible in the real world.

(Thanks for the email invite, but right now most of my time is
dedicated to learning how to be a square dance caller.)
--
73, Cecil, w5dxp.com

Long, long ago, I possessed a few copies of some of Teslas' original
notes, thoughts, sayings, etc. condensed in a thin little book. I can
remember reading it and contemplating what he was thinking/saying. I
got a distinct feeling Tesla seen his "magnifying transmitters" and the
"receivers" as, on resonance, being connected together by an "invisible
wire" ... I still think I may have missed a lot of what he
knew/discerned/guessed/thought/etc. He was nothing short of utterly
amazing ... possibly "insane like a fox."

BTW. I Square Danced from the time I was ~17 to 26, some of the best
times I ever had. Got into exhibition dancing in front of large groups,
just for fun ... times were a lot simpler/better then ...

Regards,
JS

On 10/18/10 12:04 PM, Jim Lux wrote:
Cecil Moore wrote:
On Oct 17, 6:42 am, Alejandro Lieber alejan...@Use-Author-Supplied-
Address.invalid wrote:
It appears to me that in the short circuited turns, a very big current
must be circulating, adding heat losses and lowering the Q of the
circuit.

For a screwdriver antenna, the problem is solved by a conductive
sleeve over the outside of the shorted turns that keeps most of the RF
on the conductive sleeve instead of in the shorted turns of the coil.
--
73, Cecil, w5dxp.com

But, isn't that conductive sleeve itself a shorted turn? It's
conductive, coaxial with the rest of the inductor above the sleeve, so
the magnetic field certainly passes through it.

Whic of course means that everything everywhere is a shorted turn. 8^)


I think the real answer is that everything is a tradeoff.

That's a pretty good answer.

On 10/18/10 6:23 PM, Cecil Moore wrote:
On Oct 18, 11:20 am, Jim wrote:
You can see that for this kind of coil, the coupling from turn to turn
must be pretty low.

For an average air-core coil, the delay through the coil seems to be
in the ballpark of half of the coil wire stretched into a straight
line, i.e. the VF of the coil is about double what is the VF of the
straight wire used to wind the coil. The turn to turn coupling exists
but turn to far away turn coupling is very low.

This seems to be the most accurate inductance calculator that I have
seen and includes the characteristic impedance and axial propagation
factor.


So to return to my real world example, an air core solenoid used as a
tuning coil for a bugcatcher antenna, would I be wanting to short the
unused portions of the coil, or leave them unshorted?

Seems that unshorted would be bad.

-73 de Mike N3LI -

Mike Coslo wrote:
On 10/18/10 12:04 PM, Jim Lux wrote:
Cecil Moore wrote:
On Oct 17, 6:42 am, Alejandro Lieber alejan...@Use-Author-Supplied-
Address.invalid wrote:
It appears to me that in the short circuited turns, a very big current
must be circulating, adding heat losses and lowering the Q of the
circuit.

For a screwdriver antenna, the problem is solved by a conductive
sleeve over the outside of the shorted turns that keeps most of the RF
on the conductive sleeve instead of in the shorted turns of the coil.
--
73, Cecil, w5dxp.com

But, isn't that conductive sleeve itself a shorted turn? It's
conductive, coaxial with the rest of the inductor above the sleeve, so
the magnetic field certainly passes through it.

Whic of course means that everything everywhere is a shorted turn. 8^)

Indeed, but some shorted turns intercept more flux than others.
Hopefully the field in the passenger seat is small enough that the power
dissipated in one's spouse's ring is fairly low.

Mike Coslo wrote:
On 10/18/10 6:23 PM, Cecil Moore wrote:
On Oct 18, 11:20 am, Jim wrote:
You can see that for this kind of coil, the coupling from turn to turn
must be pretty low.

For an average air-core coil, the delay through the coil seems to be
in the ballpark of half of the coil wire stretched into a straight
line, i.e. the VF of the coil is about double what is the VF of the
straight wire used to wind the coil. The turn to turn coupling exists
but turn to far away turn coupling is very low.

This seems to be the most accurate inductance calculator that I have
seen and includes the characteristic impedance and axial propagation
factor.


So to return to my real world example, an air core solenoid used as a
tuning coil for a bugcatcher antenna, would I be wanting to short the
unused portions of the coil, or leave them unshorted?

Seems that unshorted would be bad.


Given the relatively few turns on a typical bugcatcher, and the low
coupling of flux, the voltage rise would be negligible, and the fact
that you're not having a high voltage across the coil in the first place
(compared to turn spacing), I don't think it's an issue to leave the end
free.

Practical experience: On most tesla coils, the primary is a 10-20 turn
coil with turn to turn spacing of 1/4" to 1/2" or so and runs at a peak
voltage around 20kV. One adjusts the tapping point to adjust the
primary L to bring the system into resonance, and it usually winds up
being tapped about 70% of the way into the coil. Almost never do you
get arcing/corona from the free end of the coil, which is potentially at
as much as twice the voltage, and if it were in free space, you'd start
to see corona from the relatively small radius of curvature.

Mike Coslo wrote in news:i9n3jq$ds62$1
@tr22n12.aset.psu.edu:

....
So to return to my real world example, an air core solenoid used as a
tuning coil for a bugcatcher antenna, would I be wanting to short the
unused portions of the coil, or leave them unshorted?

Seems that unshorted would be bad.

I wrote some notes based on a simple model of an air cored single layer
solenoid, they are at
http://www.vk1od.net/tx/concept/TappedCoil/index.htm .

The model suggests that shorting the unused turns is a poorer solution
when the flux coupling factor is relatively high, and a very small number
of turns are shorted. Poorer both because of loss and the granularity of
L adjustment.

In that situation, the voltage induced in open unused turns is not very
high, whereas it can be extreme in cases where most of the turns are
unused.

So, a combination of methods may be optimimum, depending on the flux
coupling factor, voltage withstand, granularity of variation of L, etc.

Owen

On 10/20/10 2:04 PM, Owen Duffy wrote:
Mike wrote in news:i9n3jq$ds62$1
@tr22n12.aset.psu.edu:

...
So to return to my real world example, an air core solenoid used as a
tuning coil for a bugcatcher antenna, would I be wanting to short the
unused portions of the coil, or leave them unshorted?

Seems that unshorted would be bad.

I wrote some notes based on a simple model of an air cored single layer
solenoid, they are at
http://www.vk1od.net/tx/concept/TappedCoil/index.htm .

The model suggests that shorting the unused turns is a poorer solution
when the flux coupling factor is relatively high, and a very small number
of turns are shorted. Poorer both because of loss and the granularity of
L adjustment.

In that situation, the voltage induced in open unused turns is not very
high, whereas it can be extreme in cases where most of the turns are
unused.

So, a combination of methods may be optimimum, depending on the flux
coupling factor, voltage withstand, granularity of variation of L, etc.

After taking a good look at the loading coil, its apparent that there
isn't much choice. The bottom of the coil is attached to the lower mast,
and a four pronged plate that the tap wire is attached to at the same
junction. So unless no tap is used, some portion will be shorted/
bypassed or the like.

And given that GLA systems is no longer in business, my loading coil
just got a lot more valuable, so I'll have to experiment on a new coil.
I have the Acrylic top and bottom pieces, next will be getting the rods
and make the wire cheannels.

- 73 de Mike N3LI -