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.

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

Owen wrote:
On 17/10/10 22:42, Alejandro Lieber 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.



Only fairly basic AC circuit theory is needed to analyse the effect of
the shorted turns.

If you have a air cored solenoid inductor of n turns, and short m turns
at one end, you can treat that as two independent inductors of n-m and m
turns with some flux coupling factor k. The mutual inductance can be
calculated, and a T equivalent of Ln Lm-n Rn Rm-n M elements constructed
and solved. k of course depends on coil construction and n and m, a
value can be determined by measurement of the reactance of the
combination. (You might be surprised at how low k is.)

One could look at one of the standard equations for solenoid inductance
(e.g. Wheeler's) and get a feel for it. The ideal fully coupled multi
turn solenoid would have inductance proportional to Nturn^2.

Wheeler (for inches) is: L (uH) = r^2 * n^2 / (9 * r + 10 * l)
so there's the n^2 term on the top, but there's also the 10*length term
on the bottom.
For 2" diameter, 5 turns/inch, I calculated Wheeler L and for comparison
Length^2/6 (so that the number would be comparable at a length of around
12")

length turns Wheeler L uH/inch Length^2
2 10 3.45 1.7241 0.67
4 20 8.16 2.0408 2.67
6 30 13.04 2.1739 6.00
8 40 17.98 2.2472 10.67
10 50 22.94 2.2936 16.67
12 60 27.91 2.3256 24.00
14 70 32.89 2.3490 32.67
16 80 37.87 2.3669 42.67
18 90 42.86 2.3810 54.00
20 100 47.85 2.3923 66.67
22 110 52.84 2.4017 80.67
24 120 57.83 2.4096 96.00
26 130 62.83 2.4164 112.67
28 140 67.82 2.4221 130.67
30 150 72.82 2.4272 150.00


You can see that for this kind of coil, the coupling from turn to turn
must be pretty low.. The L looks closer to a linear function of length
than to the square of turns. If it were perfectly linear, it would be
as if there is NO turn to turn coupling, and is just a series
combination of single turn uncoupled inductors. If you look at the
uH/inch column you can see that once you get into the 10 inches long and
up range, it *is* almost completely linear.




Essentially, when the power lost in the shorted turns is low (due to the
combination of low k and low R), then the technique works fine.

We (hams) have some pretty inadequate word based explanations for some
of these kind of things when there are simple quantitative solutions at
hand.

An example is the traditional explanation of link coupling ratios. See
http://vk1od.net/tx/concept/lctr.htm for a quantitative explanation
using the same techniques as suggested above.

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

Owen wrote in news:vtKuo.256$tk4.180
@viwinnwfe02.internal.bigpond.com:

Well, I didn't get the maths right, there was a sign error in the formula
below. Here is what it should have read.

Suppose you had an air cored inductor, that when you measure the
inductance of the first half of the inductor (other terminal open) you
get 10µH. You now measure the whole inductor and get 30µH. We can
calculate that M=5µH.

Now forming a T equivalent of the inductor with one half shorted,
L=10-5+(5//(10-5))=7.5µH. Notably, the current in the s/c is
50% of the current in the other section, so losses are about 25%
of that in the other section... not usually a big issue.

Of course, the situation depends on the tapping point, and is much worse
when you short just one turn... but we don't usually do that. The example
has a fairly high coupling factor k (for an air cored coil), and losses
are lower for lower k.

I am working on a note that expands on this.

Owen

On Oct 18, 11:04*am, Jim Lux wrote:
But, isn't that conductive sleeve itself a shorted turn?

Yes, but that particular low-loss shorted turn solves the problem that
needs solving. Nobody said it was a perfect solution.
--
73, Cecil, w5dxp.com

On Oct 18, 9:57*pm, Cecil Moore wrote:
On Oct 18, 11:04*am, Jim Lux wrote:

But, isn't that conductive sleeve itself a shorted turn?

Yes, but that particular low-loss shorted turn solves the problem that
needs solving. Nobody said it was a perfect solution.
--
73, Cecil, w5dxp.com

well, if its not the perfect solution then the problem is not
completely solved... so there must be a better solution to really
solve the problem.

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

On Oct 18, 11:20*am, Jim Lux 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.

http://hamwaves.com/antennas/inductance.html
--
73, Cecil, w5dxp.com

On Oct 18, 5:13*pm, K1TTT wrote:
well, if its not the perfect solution then the problem is not
completely solved... so there must be a better solution to really
solve the problem.

The problem of transmission line losses can be solved with a perfect
lossless transmission line. Have you seen such or does reality force
us to settle for a reasonable non-perfect solution?
--
73, Cecil, w5dxp.com

K1TTT wrote:
On Oct 18, 9:57 pm, Cecil Moore wrote:
On Oct 18, 11:04 am, Jim Lux wrote:

But, isn't that conductive sleeve itself a shorted turn?
Yes, but that particular low-loss shorted turn solves the problem that
needs solving. Nobody said it was a perfect solution.
--
73, Cecil, w5dxp.com

well, if its not the perfect solution then the problem is not
completely solved... so there must be a better solution to really
solve the problem.

one could simply slit the tube (and finger stock at top). Consider that
the lower tube is serving two purposes: mechanical support and a movable
contact on the inductor.