HI,

Often I can see coils where some turns are shorted in order to modify there
value.
I wonder whether these shorted turns would not be seen as a short cicuit and
could decrease the quality of that coil.
How can I have an idea of the lost of quality without complex equipment ?
I plan to build this kind of coil for a shortened antenna...

regards

"Tomylavitesse" wrote in message
...
HI,

Often I can see coils where some turns are shorted in order to modify
there value.
I wonder whether these shorted turns would not be seen as a short cicuit
and could decrease the quality of that coil.
How can I have an idea of the lost of quality without complex equipment ?
I plan to build this kind of coil for a shortened antenna...

regards

Good question! I've always wondered about that too. In a former life as a
marine electronics technician, the old marine "AM" transmitter antenna
tuners uses shorted turns a lot. It always sort of went against the grain, I
thought, but in practice, it seemed to work ok. What was I missing here?
Old Chief Lynn, W7LTQ and some commercials with endorsements

In article ,
coffelt2 wrote:

Good question! I've always wondered about that too. In a former life as a
marine electronics technician, the old marine "AM" transmitter antenna
tuners uses shorted turns a lot. It always sort of went against the grain, I
thought, but in practice, it seemed to work ok. What was I missing here?
Old Chief Lynn, W7LTQ and some commercials with endorsements

My understanding is that the presence of the shorted turns in the coil
does lower the Q of the coil somewhat. If the loaded Q of the
system/application is already low enough, the reduction in coil Q
doesn't result in an unacceptable increase in losses.

The alternative, in some cases, is to leave the unused turns open at
one end. This isn't always possible, and when it is possible it can
have an unfortunate side effect. Autotransformer action can cause the
open end of the turns to develop a high voltage relative to the rest
of the system, and there can be arcing to nearby points of lower
potential. Could make a mess of the inside of your transmatch :-(

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

coffelt2 wrote:
"Tomylavitesse" wrote in message
...
HI,

Often I can see coils where some turns are shorted in order to modify
there value.
I wonder whether these shorted turns would not be seen as a short
cicuit and could decrease the quality of that coil.
How can I have an idea of the lost of quality without complex
equipment ? I plan to build this kind of coil for a shortened
antenna... regards

Good question! I've always wondered about that too. In a former life
as a marine electronics technician, the old marine "AM" transmitter
antenna tuners uses shorted turns a lot. It always sort of went
against the grain, I thought, but in practice, it seemed to work ok.
What was I missing here? Old Chief Lynn, W7LTQ and some commercials
with endorsements

In many military antenna tuners, the variable inductor inside the tuner was
open on one end. The coil was rotated such that the length of the coil
winding was changed, or the position of the tap was changed to effect the
tuning. There were also some tuners that spooled one end of the coil onto a
grounded metal drum. The mechanism changed the effective length of the
coil, but did not introduce any shorted turns.
--
David
masondg44 at comcast dot net

Tomylavitesse wrote:
HI,

Often I can see coils where some turns are shorted in order to modify there
value.
I wonder whether these shorted turns would not be seen as a short cicuit and
could decrease the quality of that coil.
How can I have an idea of the lost of quality without complex equipment ?
I plan to build this kind of coil for a shortened antenna...

regards


If the shorted turns are connected to one end of the coil then the total
shorted turns would appear as the final turn of the coil. In amateur
transmitters the band switch often shorts turns from the cold end of the
tank coil as one switches to higher frequency bands. The shorted turns
simply are not in the circuit I guess.

On 2009-09-24, Tomylavitesse wrote:
HI,

Often I can see coils where some turns are shorted in order to modify there
value.
I wonder whether these shorted turns would not be seen as a short cicuit and
could decrease the quality of that coil.
How can I have an idea of the lost of quality without complex equipment ?
I plan to build this kind of coil for a shortened antenna...

The circuit to shorten turns in such environments has to be very low
loss. Otherwise you will have lots of power lost in this short
circiuted parts of the coil. And in PA / antenna environments you
always have power. This is why switches are always "heavy duty" in PA
and antenna switching units.

If you do so, the current in this shortened turns is a complex
current. And as you know, complex currents do not consume power. Only
the real parts of current do.

This is totally different from the situation e.g. in a power
transformator. If you would shorten turns there, you would have power
loss, because the resistant of the turns in power transformators has a
much higher real part compared with rf coils in PA / antenna units.

OK?

rtfm wrote:
On 2009-09-24, Tomylavitesse wrote:
HI,

Often I can see coils where some turns are shorted in order to modify there
value.
I wonder whether these shorted turns would not be seen as a short cicuit and
could decrease the quality of that coil.
How can I have an idea of the lost of quality without complex equipment ?
I plan to build this kind of coil for a shortened antenna...

The circuit to shorten turns in such environments has to be very low
loss. Otherwise you will have lots of power lost in this short
circiuted parts of the coil. And in PA / antenna environments you
always have power. This is why switches are always "heavy duty" in PA
and antenna switching units.

If you do so, the current in this shortened turns is a complex
current. And as you know, complex currents do not consume power. Only
the real parts of current do.

This is totally different from the situation e.g. in a power
transformator. If you would shorten turns there, you would have power
loss, because the resistant of the turns in power transformators has a
much higher real part compared with rf coils in PA / antenna units.

OK?

This seemed like an insightful response - but not one with universal appeal.

Let me try embroidering on this theme a little more....

The leakage path for current in an insulated wire is strikingly
different from the leakage path in a magnetic conductor - usually called
a core or stamping.
The leakage resistance can easily be 1000 megohms compared to the
conductor's resistance of (say) 1 ohm. A ratio of a billion to one.
The leakage path for cores and stampings is lucky to be a thousand times
more "resistive" than the path through the core - if it's an iron
stamping, an iron dust core, or a ferrite core. A ratio of a thousand to
one.
For air cored coils, the leakage path is lower still, so that the
magnetic path does not couple all turns together at the best of times.

If an end turn or two is shorted, the reactive current in the shorted
turn pinches off most of the magnetic coupling from the remaining coil
altogether, so the effect is not as dramatic as we might expect.
Waddaya think of that?

Brian W

"brian whatcott" wrote in message
...
rtfm wrote:
On 2009-09-24, Tomylavitesse wrote:
HI,

Often I can see coils where some turns are shorted in order to modify
there value.
I wonder whether these shorted turns would not be seen as a short cicuit
and could decrease the quality of that coil.
How can I have an idea of the lost of quality without complex equipment
?
I plan to build this kind of coil for a shortened antenna...

The circuit to shorten turns in such environments has to be very low
loss. Otherwise you will have lots of power lost in this short
circiuted parts of the coil. And in PA / antenna environments you
always have power. This is why switches are always "heavy duty" in PA
and antenna switching units.

If you do so, the current in this shortened turns is a complex
current. And as you know, complex currents do not consume power. Only
the real parts of current do.

This is totally different from the situation e.g. in a power
transformator. If you would shorten turns there, you would have power
loss, because the resistant of the turns in power transformators has a
much higher real part compared with rf coils in PA / antenna units.

OK?

This seemed like an insightful response - but not one with universal
appeal.

Let me try embroidering on this theme a little more....

The leakage path for current in an insulated wire is strikingly different
from the leakage path in a magnetic conductor - usually called a core or
stamping.
The leakage resistance can easily be 1000 megohms compared to the
conductor's resistance of (say) 1 ohm. A ratio of a billion to one.
The leakage path for cores and stampings is lucky to be a thousand times
more "resistive" than the path through the core - if it's an iron
stamping, an iron dust core, or a ferrite core. A ratio of a thousand to
one.
For air cored coils, the leakage path is lower still, so that the magnetic
path does not couple all turns together at the best of times.

If an end turn or two is shorted, the reactive current in the shorted turn
pinches off most of the magnetic coupling from the remaining coil
altogether, so the effect is not as dramatic as we might expect.
Waddaya think of that?

Brian W

Wooo hoooooo, now we're cookin'

Old Chief Lynn

On 2009-09-27, brian whatcott wrote:
...
The leakage path for current in an insulated wire is strikingly
different from the leakage path in a magnetic conductor - usually called
a core or stamping.
The leakage resistance can easily be 1000 megohms compared to the
conductor's resistance of (say) 1 ohm. A ratio of a billion to one.
The leakage path for cores and stampings is lucky to be a thousand times
more "resistive" than the path through the core - if it's an iron
stamping, an iron dust core, or a ferrite core. A ratio of a thousand to
one.
For air cored coils, the leakage path is lower still, so that the
magnetic path does not couple all turns together at the best of times.

If an end turn or two is shorted, the reactive current in the shorted
turn pinches off most of the magnetic coupling from the remaining coil
altogether, so the effect is not as dramatic as we might expect.
Waddaya think of that?

Thanks for this posting. You are totally right. I tried to explain
thinks a little more easier. And if you try to reduce complexity in an
explanation, you have to simplyfy. Looks like I simplyfied too much.
My fault.

Hi,
Thanks everybody.
I just found your answers, so I will work with them.
I've also found some other info on my old handbook ARRL.
Thanks again

Raph