On Oct 17, 11:42*am, Alejandro Lieber alejan...@Use-Author-Supplied-
Address.invalid wrote:
Since I built my first 80meter/40meter 6aq5 + 6DQ6 transmitter with pi
output in 1972, when I want to vary the inductance of a coil in a
tunner, or loading coil in an antenna, I just short circuit some turns.
I see that this is the usual practice everywhere.

My question is why do we not just leave the turns open circuited instead
of short circuiting them.

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..

--
Alejandro Lieber *LU1FCR
Rosario Argentina

Real-Time F2-Layer Critical Frequency Map foF2:http://1fcr.com.ar

you are correct. and if the coil is on a core it can overheat the
core also. leaving them open also causes problems since it looks like
a transformer with an open circuit it can develop very high voltages
and flash over the band switch. The best method is to have separate
coils that are not coupled, but that of course gets more expensive and
larger. better amps have a combination, usually shorting turns on an
air core inductor for the high bands and then adding separate toroids
and capacitors for the lower bands.

an example of what can happen with shorted turns:

http://wiki.k1ttt.net/2008%20Mainten....ashx#hf2 500

K1TTT writes:
you are correct. and if the coil is on a core it can overheat the
core also. leaving them open also causes problems since it looks like
a transformer with an open circuit it can develop very high voltages
and flash over the band switch. The best method is to have separate
....[snip]....

Isn't that the basis for a Tesla coil?

--
-- Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
NRA Life Member & Certified Instructor for Rifle, Pistol, & Home Firearm Safety
Also Certified Instructor for the Kansas Concealed-Carry Handgun (CCH) license

On Oct 17, 10:03*pm, Myron A. Calhoun wrote:
Isn't that the basis for a Tesla coil?

The principle behind most Tesla coils is quarter-wave (90 degree) self-
resonance. There is a standing wave current maximum at the base of the
coil and a standing wave voltage maximum at the top of the coil.
--
73, Cecil, w5dxp.com

Cecil Moore wrote:
On Oct 17, 10:03 pm, Myron A. Calhoun wrote:
Isn't that the basis for a Tesla coil?

The principle behind most Tesla coils is quarter-wave (90 degree) self-
resonance. There is a standing wave current maximum at the base of the
coil and a standing wave voltage maximum at the top of the coil.
--
73, Cecil, w5dxp.com

Not really... that used to be an explanation, because for conveniently
sized coils, the length of the wire on the secondary is pretty close to
a 1/4 free space wavelength at the resonant frequency. However, you can
build tesla coils that deviate pretty strongly from that, and they still
work well, indicating that the 1/4wavelength (or slow wave transmission
line) model isn't all that hot.

The current/voltage distribution along the secondary is pretty close to
linear, especially if you have a decent sized topload.

It's resonant, but not 1/4 wavelength. You can model a tesla coil's
behavior to within about 5% using a simple lumped LC model. The
secondary is a lumped L and the self C of the inductor plus the C of the
"topload".

There's some pretty rigorous analysis out there of tesla coils these
days. Paul Nicholson's analysis is probably one of the best
http://abelian.org/tssp/

and has been confirmed by measurement.

Antonio C.M. de Queiroz has some elegant analytic models of coupled
resonators which adequately describe most tesla coil configurations
(including magnifiers) and more to the point, his analysis predicted
some new ways to operate a coil, which were proven in practice by some
experimenters. (that's sort of the proof in the pudding of theory.. it
predicts some behavior that hasn't been seen before, and when you look
for it, you find it)
http://www.coe.ufrj.br/~acmq/tesla/magnifier.html


There are some very nice finite element codes out there for Tesla coils,
as well. JavaTC is based on one of them
http://www.classictesla.com/java/javatc.html

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.
--
73, Cecil, w5dxp.com

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