On Mon, 22 May 2006 10:42:40 -0400, "Yuri Blanarovich"
wrote:


"Michael Tope" wrote in message
...

"Yuri Blanarovich" wrote in message
...

There are other examples where shield "doesn't shield" - like link
coupling made of coax with end shield open and center conductor soldered
to the shield. As I mentioned I have magnetothermia machine that produces
about 200W from single shielded loop, according to Tom, it should be
frying the coax in the gap, with all that RF power trying to make the
corner :-)


Yuri, think about how the "link coupling" magnetic loop you describe
above works. When the loop is energized where does the RF current
leaving the center conductor go? It has to flow onto the outside of the
shield. Where else could it go?

RF current "makes the corner" around to the outside surface of the
shield in coax all the time. If it didn't we wouldn't need choke
balun's.

We need RF chokes and baluns to supress curents induced on the shield from
the unbalance at the antenna feedpoint.
Sooo, according to W8JI "teachings", RF current gets induced onto the
outside surface of tubing, then crolls around the edges and goes inside the
tubing?
Sooo, we should cork the elements, or the current will get confused inside
of dark tubing elements, Eh?
Any formulas to calculate the resonance of such "antenna"??

73, Mike
W4EF.............................................. ...................

Yuri,

It is true that current will not flow on the inside of a tube from
current on the outside. The "waveguide beyond cutoff" effect keeps it
from doing so. The currents quickly cancel a short distance inside the
tube.
However, if you put a conductor inside that tube (wire) now it acts
like a coax cable and the energy on the center conductor couples to
the inside wall of the tube. At the end of the tube the current is
free to wrap around to the outside.

73
Gary K4FMX