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