Richard,

Think again about what you wrote. The "Faraday screen" is full of
openings between the wires of the picket fence. There is no evidence
that anything magnetic or electric penetrates the walls of the
conductors beyond a very shallow layer.

Terman certainly did not deny the existence of skin effect that keeps
the fields out of the interior of conductors.

73,
Gene
W4SZ

Richard Harrison wrote:
Tom, W8JI wrote:
"Absolutely nothing, neither electric nor magnetic, couples through the
wall of a conductor several skin depths thick."

That`s wrong for a "Faraday screen".

Terman is right. At the bottom of page 38 of his 1955 edition he writes:
"It is possible to shield electrostatic flux without simultaneously
affecting the magnetic field by surrounding the space to be shielded
with a conducting cage that is made in such a way as to provide no
low-resistance path for the flow of eddy currents, while at the same
time offering a metallic terminal upon which electrostatic flux lines
can terminate."

An example exists in the AM broadcast stations I`ve worked in. Every
tower was coupled to its transmission line through a 1:1 air-core
traansformer. Two identical single-layer solenoids sharing the same
axis. Between the coils was a metal picket fence. One end of the pickets
was firmly grounded to the coupling cabinet. The other end of all
pickets was an open circuit. Electric lines of force were intercepted by
the pickets and directly shorted to ground. However, the fences had no
effect on the magnetic coupling between them because the open circuit at
the ends of the pickets prevented circulating currents which would have
opposed magnetic coupling according to Lenz`s law.

Voila! Magnetic coupling but no electrostatic coupling between coils of
a transformer.

It`s time for W8JI to turn-off his misinformation machine.

Best regards, Richard Harrison, KB5WZI

Gene Fuller wrote:
Richard,

Think again about what you wrote. The "Faraday screen" is full of
openings between the wires of the picket fence. There is no evidence
that anything magnetic or electric penetrates the walls of the
conductors beyond a very shallow layer.

Terman certainly did not deny the existence of skin effect that keeps
the fields out of the interior of conductors.

Gene,

You might have to find a book that quotes the description of a screen
with parallel wires and large air gaps as compared to a wall or
cylinder several skin depths thick.
:-)

73 Tom

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

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

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

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

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