"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

Gene Fuller wrote:
"Terman certainly did not deny the existence of skin effect that keeps
fields out of the interior of conductors.'

true. The point is, shielding from magnetic fields is different from
electric fields. On page 35 of his 1955 edition, Terman writes:
"Magnetic flux in attempting to pass through a shield (copper or
aluminum) induces voltage in the shield which gives rise to eddy
currents. These eddy currents oppose the action of the flux, and in
large measure prevent its penetration through the shield."

On page 38, Terman writes:
"Electrostatic shielding is obtained by enclosing free space to be
shielded by a conducting surface."

On page 45, is problem 2-45 which contains an illustration of a grid of
open-circuit wires which "will provide electrostatic shielding without
magnetic shielding---." This works just like the picket fences used in
broadcast stations to inhibit harmonic transmission.

Terman did not make this stuff up. It was already in wide use at the
time.

Best regards, Richard Harrison, KB5WZI

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

Richard Harrison wrote:
Gene Fuller wrote:
"Terman certainly did not deny the existence of skin effect that keeps
fields out of the interior of conductors.'

true. The point is, shielding from magnetic fields is different from
electric fields. On page 35 of his 1955 edition, Terman writes:
"Magnetic flux in attempting to pass through a shield (copper or
aluminum) induces voltage in the shield which gives rise to eddy
currents. These eddy currents oppose the action of the flux, and in
large measure prevent its penetration through the shield."
. . .

Am I mistaken, but is this not a clear statement that a copper or
aluminum shield will block magnetic flux, along with an explanation of
why it happens?

Roy Lewallen, W7EL

Seems pretty clear to me, Roy. The effectiveness of a copper strap
around a mains-frequency power transformer at reducing the exterior
magnetic field is well known and often used. It's all very clear from
Faraday's law of magnetic induction: the net magnetic flux through an
area enclosed by a perfect conductor may not change, so time-varying
magnetic fields are perfectly blocked by perfect conductors. Copper's
a reasonable approximation of a perfect conductor in the case of RF
shields.

Cheers,
Tom

Roy Lewallen wrote:
Richard Harrison wrote:
Gene Fuller wrote:
"Terman certainly did not deny the existence of skin effect that keeps
fields out of the interior of conductors.'

true. The point is, shielding from magnetic fields is different from
electric fields. On page 35 of his 1955 edition, Terman writes:
"Magnetic flux in attempting to pass through a shield (copper or
aluminum) induces voltage in the shield which gives rise to eddy
currents. These eddy currents oppose the action of the flux, and in
large measure prevent its penetration through the shield."
. . .

Am I mistaken, but is this not a clear statement that a copper or
aluminum shield will block magnetic flux, along with an explanation of
why it happens?

Roy Lewallen, W7EL

It would only be a clear statement to those who understand what was
quoted from Terman.

If a person is confused by or somehow DOESN'T understand what Terman is
saying, he or she might take it to mean magnetic fields can travel
unimpeded through a shield.

It sure is difficult to drive a stake through the heart of myths like
the loop shield "shielding the electric field and not the magnetic
field" when clearly written text in dozens of engineering textbooks is
misunderstood.

73 Tom

On 23 May 2006 10:17:31 -0700, "K7ITM" wrote:

Seems pretty clear to me, Roy. The effectiveness of a copper strap
around a mains-frequency power transformer at reducing the exterior
magnetic field is well known and often used. It's all very clear from
Faraday's law of magnetic induction: the net magnetic flux through an
area enclosed by a perfect conductor may not change, so time-varying
magnetic fields are perfectly blocked by perfect conductors. Copper's
a reasonable approximation of a perfect conductor in the case of RF
shields.

Hi Tom,

However, Richard's explanation is the analogue of the effectiveness of
a copper strap (with a non-contacting overlap so as to not be a
shorted turn) between windings of a mains-frequency power transformer,
and grounded to provide electrostatic separation of the two circuits.

73's
Richard Clark, KB7QHC

wrote:
If a person is confused by or somehow DOESN'T understand what Terman is
saying, he or she might take it to mean magnetic fields can travel
unimpeded through a shield.

I have asked this simple question a number of times and, so
far, no one has answered. It should have a simple answer so
here it is again. Does a 60 Hz magnetic field travel virtually
unimpeded through a coax shield? This question involves 60 Hz
noise being coupled through coax.
--
73, Cecil http://www.qsl.net/w5dxp