Why do we short coil turns ?
 

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

Why do we short coil turns ?
 

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

Why do we short coil turns ?
 

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

Why do we short coil turns ?
 

On Oct 17, 2:36*pm, 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

that doesn't really 'solve' it, that just provides a single big
shorted turn instead of many turns.

Why do we short coil turns ?
 

On Oct 17, 9:52*am, K1TTT wrote:
that doesn't really 'solve' it, that just provides a single big
shorted turn instead of many turns.

Maybe the single big shorted turn IS the solution? :-) When there are
no actual shorted turns, i.e. all of the coil is exposed, does the
bottom section of aluminum tubing become that same single big shorted
turn? In any case, one very conductive and very wide shorted turn is a
lot less lossy than a number of small shorted turns.
--
73, Cecil, w5dxp.com

Why do we short coil turns ?
 

On Oct 17, 3:09*pm, Cecil Moore wrote:
On Oct 17, 9:52*am, K1TTT wrote:

that doesn't really 'solve' it, that just provides a single big
shorted turn instead of many turns.

Maybe the single big shorted turn IS the solution? :-) When there are
no actual shorted turns, i.e. all of the coil is exposed, does the
bottom section of aluminum tubing become that same single big shorted
turn?

no, it becomes a very small diameter and long shorted turn... but yes,
it is a shorted turn. you don't see as much effect because it is
smaller in diameter so the self inductance is smaller and it only
intercepts a fraction of the flux from the end of the coil. you get
more of an effect if you place an air core coil with its end near the
side or bottom of a metallic enclosure, the plate looks like a shorted
coil and can have large circulating currents.

In any case, one very conductive and very wide shorted turn is a
lot less lossy than a number of small shorted turns.
--
73, Cecil, w5dxp.com

that is why they get away with it, the losses in the big fat thing are
low enough that it causes less trouble than shorting turns with a
switch or relays.

Why do we short coil turns ?
 

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.)

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.

BTW, the solenoid inside an aluminium tube is a case of an inductor
surrounded by a single shorted turn... but if R in that turn is very
low, then little heat is generated in the tube.

We also sometimes use a movable shorted turn to adjust an inductor... a
brass or preferably silver plated brass slug was often used in VHF / UHF
tuned circuits.

Owen

Why do we short coil turns ?
 

On 18/10/10 07:24, Owen wrote:
....
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.)

To give a mental arithmetic example...

Assuming ideal inductors for the moment...

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))=18.75µH. Notably, the current in the s/c is
3.75/15=25% of the current in the other section, so losses are about 6%
of that in the other section... not usually a big issue.

That is, if I got the maths right on the fly!

Now, real inductors have some distributed capacitance which changes this
as you approach the inductor's self resonance frequency.

This works ok because mutual inductance is lowish. Increase k by
introducing a magnetic core material for instance, and the situation
changes.

You might see the technique applied to powdered iron core inductors. Not
necessarily a good idea, but it 'works' for some because they are low
permeability powders and flux leakage is high (ie k is not nearly 1).

Owen

Why do we short coil turns ?
 

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

Why do we short coil turns ?
 

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

Why do we short coil turns ?
 

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.

Why do we short coil turns ?
 

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.

Why do we short coil turns ?
 

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

Why do we short coil turns ?
 

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

Why do we short coil turns ?
 

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

Why do we short coil turns ?
 

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.

Why do we short coil turns ?
 

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

Why do we short coil turns ?
 

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

Why do we short coil turns ?
 

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

Why do we short coil turns ?
 

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.

Tesla Coils was Why do we short coil turns ?
 

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)

Tesla Coils was Why do we short coil turns ?
 

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

Tesla Coils was Why do we short coil turns ?
 

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.

Tesla Coils was Why do we short coil turns ?
 

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

Why do we short coil turns ?
 

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.

Why do we short coil turns ?
 

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 -

Why do we short coil turns ?
 

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.

Why do we short coil turns ?
 

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.

Why do we short coil turns ?
 

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

Why do we short coil turns ?
 

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 -

Why do we short coil turns ?
 

On Oct 21, 8:32*am, Mike Coslo wrote:
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.

So which would be better (less lossy) for a 75m Texas Bugcatcher coil
used on 40m? Short out each turn individually or use one jumper to
short out all of the turns that need to be bypassed?
--
73, Cecil, w5dxp.com

Why do we short coil turns ?
 

On 10/21/10 10:55 AM, Cecil Moore wrote:
On Oct 21, 8:32 am, Mike wrote:
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.

So which would be better (less lossy) for a 75m Texas Bugcatcher coil
used on 40m? Short out each turn individually or use one jumper to
short out all of the turns that need to be bypassed?


Kinda my original question.

Intuition tells me that ideally - in order of preference:

1. the entire unused portion of the coil should just disappear.

2. A shorting sleeve that renders the unused portion of that loading
coil as a fatter part of the mast.

3. What I have now, a #12 wire from the top of the bottom part of the
mast to the spot that I tuned the antenna. The bottom of the coil is
attached to the same point on the bottom mast.

1. is impossible without having separate replaceable tuning coils. Crazy
inconvenient.

2. This would be the world's fattest screwdriver antenna.

3. This becomes the question? Is this worth worrying about? And testing
would be interesting for each frequency to determine which ones benefit
from shorted/non shorted operation.

Which now leads me to ask, what would be a good way to set up such an
experiment? I guess if follows on that what exactly is the phenomenon
that I would be witnessing? A transformer effect in unshorted condition
certainly would be a problem even for my transmitting equipment?

- 73 de Mike N3LI -

Why do we short coil turns ?
 

Mike Coslo wrote in
:

On 10/21/10 10:55 AM, Cecil Moore wrote:
On Oct 21, 8:32 am, Mike wrote:
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.

So which would be better (less lossy) for a 75m Texas Bugcatcher coil
used on 40m? Short out each turn individually or use one jumper to
short out all of the turns that need to be bypassed?


Kinda my original question.

Intuition tells me that ideally - in order of preference:

1. the entire unused portion of the coil should just disappear.

Yes, but even better, a new coil optimised for the band. Not an original
thought though!


2. A shorting sleeve that renders the unused portion of that loading
coil as a fatter part of the mast.

Mike, some thoughts.

A word explanation of what my simple model draws out quantitatively for
an ideal coil (meaning insignificant distributed capacitance)...

Just one turn shorted / open...

If you short just one turn at the end of the coil, a current is induced
in the shorted turn. The current depends on the flux coupling factor k
and the number of turns in the rest of the coil. The loss depends on the
current squared and resistance... so whilst the current squared might be
high, resistance would be relatively low. Nevertheless, if k is high,
then the current induced in that shorted turn will increase total loss
significantly. The effect of a large current in the shorted turn is to
reduce the inductance of the combination, the higher the current (high
k), the greater the reduction in inductance.

If on the other hand, that one turn was left open a voltage is induced in
the open turn. The voltage depends on k and the number of turns in the
rest of the coil.

More turns shorted / open...

If more turns are shorted, the induced current is lower, and R is higher,
the net effect is that additional loss is reduced. Inductance is reduced,
but at a slower rate than intially.

If more turns are left open, the voltage induced in the unused turns
increases and could be many times the voltage developed across the used
turns. A first approximation is k times the turns ratio times voltage
developed across the used turns.

Shorting each of several adjacent turns...

This approach assures the highest circulating current in the shorted
turns, assuring the worst loss at all tappings.

Screwdrivers...

The approach taken in some screwdrivers uses a very low resistance
shorted turn (the tube) to make contact at the tapping point, and it
protects the unused coil turns (now inside the tube) from flux. Done
properly, that might well be a fairly good solution... I haven't tried to
measure it.

k...

Avoiding high k provides finer granularity of inductance adjustment and
lower loss when few turns are shorted... but of course lower k means a
physically larger coil for the same inductance, and potentially higher R
which increases loss.


Owen

Why do we short coil turns ?
 

Owen Duffy wrote:
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 .

I saw that yesterday and it looks good (a figure of the equivalent T
model would help)

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.


But in that situation, the voltage across the unshorted turns is likely
to be low, because you've got low reactance, right? So the net effect is
small.

Why do we short coil turns ?
 

Jim Lux wrote in
:

Owen Duffy wrote:
Mike Coslo wrote in news:i9n3jq$ds62$1
....
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.


But in that situation, the voltage across the unshorted turns is
likely to be low, because you've got low reactance, right? So the net
effect is small.

Hi Jim,

It depends what you take as a reference. A rough estimate of the voltage
across the unused turns is the turns ratio (unused /used) time the flux
coupling factor, which in practical applications is likely to be in the
range 0.2 to 0.5.

If the application was a valve amp pi coupler, the voltage impressed
across the used turns could be a couple of thousand volts, and on a 10m
tap, the voltage across the unused turns could easily be 5 to 10 times
that.

In the case of a loading coil for a multiband whip, the current in the
used turns is probably similar from band to band, but the used turns are
lower for higher bands and so the flux cutting the unused turns is lower,
but there are more unused turns. In this application, insulation problems
are probably less severe than the PA with a bandswitch.

To my mind, the interesting thing is why shorting some turns is 'ok', and
then to understand that doesn't make it 'ok' in all scenarios. For
instance, someone taking a valve amp pi coupler and implementing it on a
single powdered iron core changes the situation significantly.

Re the Tee circuit, yes, but it takes a while to draw it up... not nearly
as quick as grinding a few numbers and creating a graph! I have a backlog
of hand drawing scanned into articles to redraw already.

Owen

Why do we short coil turns ?
 

On 10/20/2010 03:04 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.

Owen

Thank you very much for the explanation.

Another interesting and very practical graph would be: for a 20 turn,
50mm diameter and various winding pitches coil, how much additional
energy loss, 1, 5, 10, 20, 30, 40 additional short circuited turns add.


--
Alejandro Lieber LU1FCR
Rosario Argentina

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

Why do we short coil turns ?
 

Alejandro Lieber wrote
in :

Thank you very much for the explanation.

Another interesting and very practical graph would be: for a 20 turn,
50mm diameter and various winding pitches coil, how much additional
energy loss, 1, 5, 10, 20, 30, 40 additional short circuited turns
add.

Hello Alejandro,

I think the graphs in the article demonstrate that there is a simple
analytical solution, giving the appropriate parameters.

There are issues about the applicability of the model to the real world,
but the model does show trends about what is desirable and undesirable
practice. Wheeler's formula for inductance has its shortcomings
(Wheeler's formula doesn't take into account wire diameter for instance),
but it is (IMHO) sufficiently accurate to expose some of the
'interesting' effects that prompted your initial question.

The model is based on basic electricity and magnetism, stuff that isn't
so appeallng to new age hams.

Owen

Why do we short coil turns ?
 

Owen Duffy wrote:
Alejandro Lieber wrote
in :

Thank you very much for the explanation.

Another interesting and very practical graph would be: for a 20 turn,
50mm diameter and various winding pitches coil, how much additional
energy loss, 1, 5, 10, 20, 30, 40 additional short circuited turns
add.

Hello Alejandro,

I think the graphs in the article demonstrate that there is a simple
analytical solution, giving the appropriate parameters.

There are issues about the applicability of the model to the real world,
but the model does show trends about what is desirable and undesirable
practice. Wheeler's formula for inductance has its shortcomings
(Wheeler's formula doesn't take into account wire diameter for instance),
but it is (IMHO) sufficiently accurate to expose some of the
'interesting' effects that prompted your initial question.


I think that for the purposes of looking at the effect of shorting/not
shorting turns, Wheeler is more than adequate. the key is the fact that
it encapsulates the difference between the Nturns^2 (zero length
solenoid with all turns coincident, fully coupled) and Nturns (zero
coupling) for dimensions that are "practical" for ham use.

And the other interesting thing is that the graph of inductance vs
length for coils of interest here is that it looks pretty linear (and
the fact that the coil stock vendors refer to "uh/inch" kind of confirms
that)

Why do we short coil turns ?
 

Jim Lux wrote in
:

I think that for the purposes of looking at the effect of shorting/not
shorting turns, Wheeler is more than adequate. the key is the fact
that it encapsulates the difference between the Nturns^2 (zero length
solenoid with all turns coincident, fully coupled) and Nturns (zero
coupling) for dimensions that are "practical" for ham use.

And the other interesting thing is that the graph of inductance vs
length for coils of interest here is that it looks pretty linear (and
the fact that the coil stock vendors refer to "uh/inch" kind of
confirms that)

An interesting bit of trivia for the models I created is that the flux
coupling coefficient doesn't vary much with turns for a given diameter
and coil pitch.

When you make relatively large diameter coils of fine pitch, k is higher,
and that creates the conditions for higher loss in shorted turns.

The implication for long loose coils is that k is low, mutual inductance
is low, inductance approaches a constant L per unit length etc.

Sensibly, most air cored solenoids operate in the midrange.

Owen