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"Cecil Moore" wrote in message
et...
Michael Tope wrote:

"Cecil Moore" wrote:
So 60 Hz magnetic fields penetrate shielded coax?

Cecil, if ever I had the feeling that I was about to answer a loaded
question, this is it, but here goes anyway - "Yes, I believe a 60 Hz
magnetic field impinging on a piece of shielded coax would penetrate
the shield of that coax significantly if the shield were made of a
non-ferrous conductor."

It's not a loaded question. I just always assumed that coax
would shield the system from 60 Hz noise and I guess I was
wrong.
--
73, Cecil http://www.qsl.net/w5dxp

Cecil,

Here's a link to an interesting post on TowerTalk by Jim K9YC
discussing the subject of shielding effectiveness of coax at very low
frequencies:

http://lists.contesting.com/_towerta.../msg00663.html

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

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Were you interested in a field coaxial to the axis of the line, or one
that is transverse? Perhaps I could get my assistant, Beaker, to run
some tests for us. He sometimes get a little, ah, involved with his
experiments, though, so it may take some time.

Regards,
Bunsen

Cecil Moore wrote:
Michael Tope wrote:

"Cecil Moore" wrote:
So 60 Hz magnetic fields penetrate shielded coax?

Cecil, if ever I had the feeling that I was about to answer a loaded
question, this is it, but here goes anyway - "Yes, I believe a 60 Hz
magnetic field impinging on a piece of shielded coax would penetrate
the shield of that coax significantly if the shield were made of a
non-ferrous conductor."

It's not a loaded question. I just always assumed that coax
would shield the system from 60 Hz noise and I guess I was
wrong.
--
73, Cecil http://www.qsl.net/w5dxp

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On Tue, 23 May 2006 15:31:13 -0700, Roy Lewallen
wrote:

Richard Harrison wrote:
Roy, W7EL wrote:
"It`s a myth that there`s no magnetic field in the space between a
capacitor`s plates."

Maxwell`s great speculation was that "displacement current", as between
a capacitor`s plates, produced magnetic flux as does conduction current.
His speculation is now proved.

Yes. So how does a capacitor between two inductors constitute "E-field
transfer with zero magnetic coupling" as you stated?

Hi All,

Really, this contretemps seems to be over a matter of scale and
application.

Ramo, Whinnery, and Van Duzer make clear distinctions between mutual
couplings and radiative couplings. Most of the discussion in this and
related threads appear to discard these distinctions.

Richard's application of screened air linked couplers and using the
illustration of power transformers is found in "Fields and Waves..."
by these authors:
"Where there is a component of the electric field in phase with
the current, the integral of the electric field cannot be
considered either as a pure "capacitive" or "inductive" voltage
drop since there will be real energy transfer (radiation) from
these terms."
Richard's applications and illustrations do not push this boundary. In
fact, Ramo et. al distinctly offer the case of "electrostatic
shielding" and clearly support the separation of magnetic and electric
flux (fields). And so as to anticipate the conundrum of the "static"
in electrostatic, the authors show no issue. However, they do provide
a rational warning:
"It often happens that electrodes, although grounded
for direct current, may be effectively insulated or floating
at radio frequencies because of impedance in the grounding
leads. In such cases the new electrodes do not accomplish
their shielding purposes but may in fact increase capacitive
coupling."

Insofar as Yuri's complaint, it is an ego trip that wholly ignores the
scales of wavelength, the application of materials, the nature of
balance, and the misapplication of mutual coupling to explain far
field effects. In short, he has been bitten by the "lumped vs.
distributed" distinction once again. The only saving grace of his
argument may be found in that there are two forms of the "shielded
dipole" where one supports Tom's claim, and the other support's
Yuri's.

Unfortunately, as correct as Richard's examples are, they too are
misapplied to the "shielded dipole." The "shielded dipole" may be
small in relation to wavelength, but its response mechanism is NOT
found by using mutual coupling math, but rather through radiation
math.

73's
Richard Clark, KB7QHC

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Richard Clark wrote:
. . .
Richard's applications and illustrations do not push this boundary. In
fact, Ramo et. al distinctly offer the case of "electrostatic
shielding" and clearly support the separation of magnetic and electric
flux (fields). . .

Can you direct me to where in the text they do so? All I've found is a
short section (5.28) on "Electrostatic Shielding" where they explain
that introducing a grounded conductor near two others will reduce the
capacitive coupling between them. Obviously this will alter the local
E/H ratio, but in no way does it allow an E or H field to exist
independently, even locally, let alone at any distance.

Roy Lewallen, W7EL

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On Wed, 24 May 2006 12:11:52 -0700, Roy Lewallen
wrote:

Richard Clark wrote:
. . .
Richard's applications and illustrations do not push this boundary. In
fact, Ramo et. al distinctly offer the case of "electrostatic
shielding" and clearly support the separation of magnetic and electric
flux (fields). . .

Can you direct me to where in the text they do so? All I've found is a
short section (5.28) on "Electrostatic Shielding" where they explain
that introducing a grounded conductor near two others will reduce the
capacitive coupling between them. Obviously this will alter the local
E/H ratio, but in no way does it allow an E or H field to exist
independently, even locally, let alone at any distance.

Hi Roy,

Article 5.12 "Circuit Concepts at High Frequencies or Large
Dimensions"

Figure 5.28(a) shows a complete shielding. Of course this is entirely
electric, and arguably magnetic. However, magnetic flux can penetrate
thin shields, electric flux cannot.

This is part and parcel to the world of isolated and shielded
circuits. The electrostatic shields are as effective as they are
complete in their coverage. Their contribution is measured in mutual
capacitance between the two points being isolated. With a drain wire
to ground, and a low enough Z in that wire, then that mutual
capacitance tends towards zero (however, near zero is a matter of
degree as I've offered in past discussion).

Figure 5.28(a) shielding is quite common in medical circuit design,
and mutual capacitance does equal zero; and yet signals and power pass
in and out through magnetic coupling. Isolated relays are a very
compelling example of magnetic transparency in the face of total
electric shielding.

Magnetic shielding operates through reflection or dissipation
(absorption loss due to eddy currents). This loss is a function of
permeability µ. Unfortunately, permeability declines with increasing
frequency, and with declining field strength. Basically, all metals
exhibit the same characteristic µ above VLF; hence any appeal to
"magnetic materials" used to build antennas is specious.

This is not to say the magnetic shield is ineffective, merely derated
seriously from what might be gleaned through poor inference by reading
µ values from tables.

However, it is quite obvious that transformer inter stage shielding
and the faraday shield found in AM transmitters is not seeking to
optimize this attenuation, far from it. Thus the degree in isolation
is found in the ratio of the mutual capacitance between the two points
before and after shielding; and the attenuation in magnetic flux
induction introduced between the two circuits after shielding.

Returning to Ramo, et. al, the introduction of a partial shield.
Figure 5.28(c) is effective insofar as its ability to reduce mutual
capacitance.

73's
Richard Clark, KB7QHC

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Richard Clark wrote:
Figure 5.28(a) shows a complete shielding. Of course this is entirely
electric, and arguably magnetic. However, magnetic flux can penetrate
thin shields, electric flux cannot.

Only when the shield is thin compared to the skin depth.
When the shield is thick relative to skin depth nothing gets through.

This is very easy to prove. I have been making measurements of a ~ .032
inch thick copper wall and with 0dB reference on a small resonant pick
up loop my analyzer is in the noise (-90dB) on the side directly
opposite that loop.

The same is true for a direct soldered connection to the wall on each
opposite side.

One inch to the side on the same side levels are -10dB. That would be a
two inch long path shorted by the copper the entire way. Go to the
direct opposite side through only .032 thick copper and levels are not
even detectable.

This is part and parcel to the world of isolated and shielded
circuits. The electrostatic shields are as effective as they are
complete in their coverage. Their contribution is measured in mutual
capacitance between the two points being isolated.

I don't have that reference and so cannot see that shield, but the only
thing the shield can do is reduce field impedance by changing the ratio
of electric to magnetic fields. In order to take either one to zero the
other must also be at zero.

I think the confusion comes from misapplying a grid forming a shunt
capacitance to reduce direct capacitance between two objects (forming a
"T" divider) to the shielded loop antenna or shielded link.

Consider a loop of any size, even a link in a tank coil. That conductor
has a field impedance and radiation characteristics largely set by the
diameter of the coil.

Once we put a wall around that conductor more than a few skin depths
thick NOTHING goes through that wall. The "shield" actually becomes the
coupling coil, the link inside simply develops a voltage across the
shield to drive that external coil. Both electric and magnetic fields
are present on the outer wall of the shield, and while they may be
modified by shield balance they really are not much different than we
had with just the inner conductor alone. We really just change the
balance.

With the grid, we have substantial air gap segmenting the "wall".
Naturally the coupling mechanism is different than we have with a solid
wall. We, in effect, have dozens of long gaps.

Each conductor in that grid is indeed excited by the magnetic and
electric fields, and each conductor has a potential difference across
area and a current flowing. Part of the field, both electric and
magnetic, leaks through. Part is reradiated by the currents and
voltages in the grid.

I think at some point of time MRT or Dave Saloff patented a right angle
grid of two layers with opposing ends in each adjacent conductor in
each layer grounded that I designed. The idea was to more evenly
distribute the fields and prevent "hot spots". This was for a medical
application.

Rest assured this applicator produced both time-varying electric and
magnetic fields, but the balance was so much improved tuning was more
stable. The improved balance and evenly distributed field meant the
feedline did not radiate any significant amount when brought near the
patient, unlike a regular multiple turn loop.

I still have some prototype applicators here, as well as the field
measurements required by the FDA. The applicator actually had to match
with lowest return loss into the buttocks of an average size 30 year
old female.

73 Tom

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On 24 May 2006 17:31:59 -0700, wrote:

This is part and parcel to the world of isolated and shielded
circuits. The electrostatic shields are as effective as they are
complete in their coverage. Their contribution is measured in mutual
capacitance between the two points being isolated.

I don't have that reference and so cannot see that shield, but the only
thing the shield can do is reduce field impedance by changing the ratio
of electric to magnetic fields. In order to take either one to zero the
other must also be at zero.

Hi Tom,

There are too many contra-examples too sustain your point. What you
are talking about is radiation, this does not account for common
induction that occurs on the very short scales I've offered.

73's
Richard Clark, KB7QHC

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There are too many contra-examples too sustain your point. What you
are talking about is radiation, this does not account for common
induction that occurs on the very short scales I've offered.

Will you give me an example where the electric field is zero and all
coupling is via magnetic flux?

73 Tom

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On 25 May 2006 03:38:14 -0700, wrote:

There are too many contra-examples too sustain your point. What you
are talking about is radiation, this does not account for common
induction that occurs on the very short scales I've offered.

Will you give me an example where the electric field is zero and all
coupling is via magnetic flux?

Tom,

As this has already been discussed not but two postings ago, the
posting your responded to, why are you asking for that content again?

73's
Richard Clark, KB7QHC

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Richard Clark wrote:
On 25 May 2006 03:38:14 -0700, wrote:

There are too many contra-examples too sustain your point. What you
are talking about is radiation, this does not account for common
induction that occurs on the very short scales I've offered.
Will you give me an example where the electric field is zero and all
coupling is via magnetic flux?

Tom,

As this has already been discussed not but two postings ago, the
posting your responded to, why are you asking for that content again?

I was going to ask the same question but Tom beat me to it. And I must
have missed the example, too. Would you be so kind as to repost it?

Roy Lewallen, W7EL