"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