"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 Blanarovich, K3BU, VE3BMV

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

"Yuri Blanarovich" wrote in message
...

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.

Actually what oftentimes happens with a coax feed is that the RF
current leaving the inside of the feedline shield can flow in two
directions. It can flow down the antenna element half connected to the
shield (desired path), or it can flow down the outside of the shield
(undesired path). The electrons are dumb, all they are looking for is
the path of least resistance. They can't tell that the metal surface
on the outside of the coax isn't supposed to be part of the antenna.
The only way to keep current from flowing down the shield is make
the antenna element-half connected to the shield look like a lower
impedance than the outside of the shield. If you place ferrite beads
around the outside of the shield, this will raise the impedance of the
shield path, thereby diverting the bulk of the RF current into the
element-half and off of the shield's outside surface.

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?

As per K4FMX's comments, this can only happen if there is a
center conductor inside the tubing, or if the tubing diameter is greater
than ~1/2 wavelength in diameter, otherwise the inside of the tubing
looks like a circular waveguide beyond cutoff. This is why coax
of a given diameter becomes useless above a certain upper frequency
limit. Once the I.D. of the coax becomes a significant fraction of a
wavelength in diameter, the coax will start to support propagation of
waveguide modes (e.g. non-TEM modes). At HF frequencies, even
large diameter tubing is well beyond waveguide cutoff, so there is no
concern about "corking" open tubing with no center conductor (it
corks itself).

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

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"??

Michael Tope wrote:
The electrons are dumb, all they are looking for is
the path of least resistance.

Hmmmm, electrons that know ohm's law sound pretty
smart to me. :-)
--
73, Cecil http://www.qsl.net/w5dxp