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

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

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

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.

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

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.