Faraday shields and radiation and misinterpretations
 

On Dec 1, 11:25*am, Lostgallifreyan wrote:
Art Unwin wrote in news:15904250-69bb-4aba-8a3f-
:

If you go back to the arbitary boundary of the Gaussian law of statics
and view it as a
Faraday shield it all becomes quite simple. If one adds a time varying
field you have the duplicate of Maxwells laws for radiation, *where
the outside of the boundary is the radiator.
The Faraday shield supplies the transition from a static to a dynamic
field for xmission and
the reverse action *for receiving.
Very basic my dear Watson, and a vindication that particles and not
waves create radiation
which puts it in line with deductions when other methods are applied.

Doesn't look basic, and I suspect it never will to me. The only thing I
can get from this is the idea that a particle model will do what the wave
one does, which isn't surprising but I've been told that particle based
models are usually best left to situations (usually atomic scale quantum
mechanical) where the wave model won't do, and I've never seen anyone suggest
that wave-based theories of electromagnetics were inadequate (or inefficient)
for scales involving obviously large numbers of particles. The other
explanations seemed to grip, but not this one. I'll leave well alone now, but
if anyone else takes up the discussion, I'll read it and only comment if I
can't stop myself..

Well I didn't tell all in the first place because so much untruths are
buried in people"s mind.
When the charge or particle hits the outside of the shield both the
electric and magnetic fields dissapate leaving just the static
particle adheared to the outside. Ofcourse non bound particles in the
air are immediatly attracted to the inside of the shield and move
along the inside of the shield to align themselves with the outside
static particles for equilibrium.
Now for the important stuff that will upset hams. The internal
particle moves to align itself with the outside particle. By moving it
generates a time varying current such that the electric and magnetic
fields that disapated on the outside are now REGENERATED on the
inside.
Most people see or think that the outside magnetic field can pierce
the shield, which is why the name magnetic loop came about. Fields do
NOT penetrate a Faraday shield. A electromagnetic shield is
regenerated by the newly formed internal current which then closes the
circuit.
To put this with the original explanation would be to much for hams to
digest so it is best to split it into two parts.

Faraday shields and radiation and misinterpretations
 

"Ian White GM3SEK" wrote in message
...
K7ITM wrote:
I'm asking this because calls of 'troll' and 'loony' aren't working for
me.

- snip -

Here is a link to a generalized proof of the skin effect:

http://www.ifwtech.co.uk/g3sek/misc/skin.htm

This is exactly equivalent to Tom's explanation above. The detailed proof
is quite mathematical but it is solidly based in classical physics

- further snip -
--

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek


Your statement on your web page 'It is temporarily reproduced here, under
provisions of the Berne Copyright Convention, to support technical
discussions on the rec.radio.amateur.antenna newsgroup' looks interesting
because, to the best of my knowledge, the issue of the legality of copying
parts of others' published work onto Web sites hasn't been resolved. I
can't find any specific provision in the Berne Convention that _allows_
re-publishing on the Internet - it looks more likely to inhibit it because
Web sites are automatically worldwide.

On the page 'http://www.copyrightservice.co.uk/copyright/p09_fair_use' it
is stated: 'Under fair use rules, it may be possible to use quotations or
excerpts, where the work has been made available to the public, (i.e.
published). Provided that:
a.. The use is deemed acceptable under the terms of fair dealing.
b.. That the quoted material is justified, and no more than is necessary
is included.
c.. That the source of the quoted material is mentioned, along with the
name of the author.'
But also: 'The actual specifics of what is acceptable will be governed by
national laws, and although broadly similar, actual provision will vary from
country to country.'

Presumably you have researched this matter, and I for one would be
interested to hear what you've found that appears to cover international www
re-publishing.

Chris

Faraday shields and radiation and misinterpretations
 

On Dec 1, 9:25*am, Lostgallifreyan wrote:
Art Unwin wrote in news:15904250-69bb-4aba-8a3f-
:

If you go back to the arbitary boundary of the Gaussian law of statics
and view it as a
Faraday shield it all becomes quite simple. If one adds a time varying
field you have the duplicate of Maxwells laws for radiation, *where
the outside of the boundary is the radiator.
The Faraday shield supplies the transition from a static to a dynamic
field for xmission and
the reverse action *for receiving.
Very basic my dear Watson, and a vindication that particles and not
waves create radiation
which puts it in line with deductions when other methods are applied.

Doesn't look basic, and I suspect it never will to me. The only thing I
can get from this is the idea that a particle model will do what the wave
one does, which isn't surprising but I've been told that particle based
models are usually best left to situations (usually atomic scale quantum
mechanical) where the wave model won't do, and I've never seen anyone suggest
that wave-based theories of electromagnetics were inadequate (or inefficient)
for scales involving obviously large numbers of particles. The other
explanations seemed to grip, but not this one. I'll leave well alone now, but
if anyone else takes up the discussion, I'll read it and only comment if I
can't stop myself..

Yep, that's about right. In fact, my advice if you do get into that
situation (where quantization of energy is important), is to NOT think
of particles or waves, but realize that quanta of electromagnetic
radiation behave exactly as they behave, which is neither exactly like
waves nor exactly like particles. One of Richard Feynman's physics
lectures covered what I think is a lovely example of this: how you
can NOT explain the results of the experiment he sets up, using EITHER
wave OR particle behaviour. I highly recommend it, to arm yourself
against people who get into the particle-vs-wave battle. I believe
it's the sixth of what has been published as Feynman's "Six Easy
Pieces."

Cheers,
Tom

Faraday shields and radiation and misinterpretations
 

Lostgallifreyan wrote:
I've been told that particle based
models are usually best left to situations (usually atomic scale quantum
mechanical) where the wave model won't do, and I've never seen anyone suggest
that wave-based theories of electromagnetics were inadequate (or inefficient)
for scales involving obviously large numbers of particles.

Consider that man's most ancient exposure to waves was
sea/ocean waves which, incidentally, consist of H2O
molecule particles.

Seems to me that everything that physically exists must
exist as a particle.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Faraday shields and radiation and misinterpretations
 

Lostgallifreyan wrote:
I've been told that particle based
models are usually best left to situations (usually atomic scale quantum
mechanical) where the wave model won't do, and I've never seen anyone suggest
that wave-based theories of electromagnetics were inadequate (or inefficient)
for scales involving obviously large numbers of particles.

Consider that man's most ancient exposure to waves was
sea/ocean waves which, incidentally, consist of H2O
molecule particles.

Seems to me that everything that physically exists must
exist as a particle.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Faraday shields and radiation and misinterpretations
 

On Dec 1, 3:42*am, Lostgallifreyan wrote:
K7ITM wrote in news:c52a1b1d-ef32-4d69-bf61-
:



It's fairly straightforward, actually, if you believe in Faraday's law
of magnetic induction. *That law says that for any closed loop
(through air, through a conductor, through anything), there is an
electromotive force (a voltage source, if you will) whose magnitude is
proportional to the rate of change of magnetic flux enclosed by the
loop. *As there is no voltage drop along a perfect conductor, if your
closed loop follows the path of a perfect conductor, there is no
voltage drop around that loop, and therefore the rate of change of the
total magnetic flux enclosed by that loop must be zero. *If the
perfect conductor is a closed box, then you can draw loops anywhere
through that conductor and you will never see a changing magnetic
field enclosed by that loop. *Thus, the inside of the box and the
outside are magnetically independent; things happening on one side
(magnetically) are not sensed on the other side.

You can understand how this works if you realize that a changing
magnetic field outside the box that would penetrate the box if it
weren't there will induce currents in the conducting box (or even just
in a closed loop of wire). *Those currents will (in a perfect
conductor) be exactly the right magnitude to cause a magnetic field
that cancels the external one everywhere inside the closed box (or the
net flux enclosed by a loop of wire). *An example: *if you short the
secondary of a mains transformer, the primary will draw lots of
current at its rated voltage: *it's very difficult for the primary to
change the magnetic flux in the core.

Does the electric field shielding from a perfect conductor need any
explanation?

Of course, an imperfect conductor will be an imperfect magnetic
shield. *But a perfect conductor won't let any change of field
through, no matter how slow (no matter how low an EMF it generates),
so a perfect conductor works as a shield all the way down to DC. *A
box made with an imperfect conductor is essentially a perfect shield
if the box's wall thickness is at least many skin-depths thick at the
frequency of interest.

That's a quick beginning. *You can find lots more about this in E&M
texts. *There's even useful stuff about it on the web. *;-)

Cheers,
Tom

Thanks, that helps, especially the paragraph about creating a magnetic field
in response that tends to cancel the original one, and the thickness of metal
with regard to frequency. The OP (Art Unwin) mentioned cancellation in more
complex terms, so I'm still not clear if this validates what he said or not.
It appears to but he mentions stuff I'm not likely to grasp in just an hour
or two of effort.. What I'm getting at is that I'm not sure if his calling
orthodoxy into question is all that drew the flak, or if there's something
obviously wrong in his post that I'm missing.

When you feed a time varying current to the mesh it is best to view it
in small parts, say a square in the mesh. The hole is a static field
alongside the applied current flows. This same current generates a
displacement current which encircles the static field as it returns
to the initial current flow. Of course this section is a microcosm of
the flow pattern of the applied varying current which is continually
flowing.
The initial current flow generates a field at right angles to its
axis. This field thus bisects the enclosed static field and
accellerates a particle thru this intersection in the same way a
particle is accelerated in a cathode ray tube. The particle that was
accellerated, by the way, came from the surface of the conducting wire
which is diamagnetic upon which particles or free electrons rest
without being absorbed into the matrix of the material upon which it
rests.The speed that the charge or particle attains is that of the
speed of light. So when Einstein gave up his search regarding the
standard model it seems rather natural that he came up with E=mc sqd
as it was obvious to him that light itself was generated by the same
particle or free electron that occupied his mind for so long and not
of waves that appeared to persist in the minds of physicists to this
very day.
Hope that helps you out
Regards
Art




Also (though I'll likely find out about this when I look deeper), why is it
often ok for a Faraday cage to have holes in it? :) Braided screens, meshes,
perforated metal sheets, etc, I've seen many shields that are not a complete
'seal'... UHF TV cables especially seem to be very loosely shielded but they
work. Conversely, I found some nice coax in a skip once that had two heavy
braids amounting to almost complete coverage around a single fine stranded
core. (Found outside a telephone exchange, but I don't know what frequency
they were intended for, though I used some for an outdoor VHF receiving
quarter wave dipole with good results, and I suspect it will do for a SW
longwire once I get a matching transformer for it).

Faraday shields and radiation and misinterpretations
 

On Tue, 1 Dec 2009 13:06:30 -0800 (PST), Art Unwin
wrote:


When you feed a time varying current to the mesh it is best to view it
in small parts, say a square in the mesh. The hole is a static field
alongside the applied current flows. This same current generates a
displacement current which encircles the static field as it returns
to the initial current flow. Of course this section is a microcosm of
the flow pattern of the applied varying current which is continually
flowing.

Is this true of a discone? I'm under the impression the current flow
is identical whether metal rods or wire mesh is used in the antenna's
construction.

Faraday shields and radiation and misinterpretations
 

On Dec 1, 3:22*pm, Registered User wrote:
On Tue, 1 Dec 2009 13:06:30 -0800 (PST), Art Unwin

wrote:

When you feed a time varying current to the mesh it is best to view it
in small parts, say a square in the mesh. The hole is a static field
alongside the applied current flows. This same current generates a
displacement current *which encircles the static field as it returns
to the initial current flow. Of course this section is a microcosm of
the flow pattern of the applied varying current which is continually
flowing.

Is this true of a discone? I'm under the impression the current flow
is identical whether metal rods or wire mesh is used in the antenna's
construction.

I am under the understanding that for a Faraday shield it doesn't
matter whether it is a mesh or solid. When the displacement current
flows in terms of an eddy current it produces a vortice which holds
the static field
Dinner has arrived
Art

Faraday shields and radiation and misinterpretations
 

Lostgallifreyan wrote:

Doesn't look basic, and I suspect it never will to me. The only thing I
can get from this is the idea that a particle model will do what the wave
one does, which isn't surprising but I've been told that particle based
models are usually best left to situations (usually atomic scale quantum
mechanical) where the wave model won't do, and I've never seen anyone suggest
that wave-based theories of electromagnetics were inadequate (or inefficient)
for scales involving obviously large numbers of particles. The other
explanations seemed to grip, but not this one. I'll leave well alone now, but
if anyone else takes up the discussion, I'll read it and only comment if I
can't stop myself..

It's not basic, and it's not real.

Art has made up a whole new wing of physics that has only the slightest
ties to reality. It involves neutrinos leaping from diamagnetic
materials to radiate. And only diamagnetic materials can radiate,
unless he revised his theories, which he does regularly. And there are
NO waves, just particles And antennas don't work properly unless they
are a multiple of a wavelength, but it's OK to roll all that wire up in
a ball so that a 160m antenna fits in a shoebox. And then you can use
that with a teeny Dish network dish for directionality. Despite the
fact that those dishes won't work reasonably at anything less than low
GHz frequencies.

He is, to put it very plainly, nuts.

tom
K0TAR

Faraday shields and radiation and misinterpretations
 

Richard Clark wrote:
On Tue, 01 Dec 2009 03:42:13 -0600, Lostgallifreyan
wrote:

why is it
often ok for a Faraday cage to have holes in it? :) Braided screens, meshes,
perforated metal sheets, etc, I've seen many shields that are not a complete
'seal'... UHF TV cables especially seem to be very loosely shielded but they
work.

This can be explained at super high frequency and at DC as easily.
However, before that it should be pointed out that the coverage (the
ratio of what is conductor to what is not - the air space) defines how
"good" the faraday shield will be. Not surprisingly, coverage is
wavelength dependant. To cut to the chase, a wide mesh will allow
increasingly higher frequencies (shorter waves) through.

Now, as to the how. With a separation in the mesh, and for very large
wavelength (in proportion to the opening size), you will have a very,
very small potential difference across any of the mesh openings. Very
little potential voltage across the mesh opening means very little
current flow around the mesh opening that is specifically due to that
potential difference.

This is not to say there isn't a very, very large current flow by
virtue of some very, very long wave. No, there's no denying that, but
to get through the mesh you have to satisfy local conditions that
demand what amounts to leakage (and this is exactly the term that
correlates to coverage when discussing coax weave). If that huge
current cannot induce a significant voltage across the mesh opening,
then the mesh opening loop current cannot induce a field through to
the other side. Now, if you examine the context of "huge current" in
a resistive conductor, then obviously a potential difference can
occur. Point is that reality (and science) allow for poor grade
shields, but as a one knock-off proof you can summon up any failure,
ignore simple contra-examples and create a new theory.

However, returning to what is well known. If you increase the
frequency applied to the mesh, then at some point wavelength will
allow a situation where the general current flowing through the whole
structure will naturally exhibit a potential difference across some
small scale. By this point, abstraction may be wearying.

Let's say you have a 10 meter-on-a-side cage with 1 meter mesh
openings. If your applied field were exciting the cage at 75MHz (4M),
then any spot on the cage could be at a very high potential difference
from any spot adjacent and 1 meter away (a simple quarterwave
relationship). This works for a solid conductor, it works for a mesh
conductor.

The 1 meter mesh openings can thus exhibit a substantial potential
difference across the opening, and a local current loop associated
with that potential difference. The mesh opening becomes a
quarterwave radiator (aka slot antenna) and can couple energy from the
external field into the interior of the cage (now possibly a resonant
chamber, aka RF cavity). In practice and literature, the mesh opening
loop exhibits a radiation resistance of 10s of Ohms. That compared to
its mesh loop Ohmic path loss, makes it a very efficient coupler of
energy.

Take this very poor example of mesh, and lower the frequency to 750
KHz. The mesh opening - if we originally likened it to an antenna, we
should be able to continue to do that - is now 1/400th Wave. A
1/400th wave radiator has extremely small radiation resistance. The
exact value would be 751 nanoOhms. As we are examining a poor mesh,
it becomes clear that it must have some resistance over that 1 meter
distance (this is a real example, after all).

Being generous and constructing that cage out of rebar will give us a
path resistance of, luckily, 1 milliOhm. This figure and that of the
radiation resistance yield the radiation efficiency (that is, how well
the exterior RF will couple into the interior) which reduces to
0.075%. The cage works pretty well, but not perfectly (it was, after
all, a poor example).

Now, repeat this with a poorer conductor, or a tighter mesh and
imagine the shielding effect. The mesh has an opening radius
squared-squared relationship driving down the radiation resistance
compared to the linear relationship of conductance.

*************

Now, expanding the topic to allow for the contribution of ALL openings
in the mesh, we must again return to the physical dimension compared
to the wavelength dimension. If the cage is truly large, larger than
the field exciting it, then you have miniscule radiators along it,
each very inefficient. However, each of those radiators is out of
phase with a distant neighbor (not so with its close mesh neighbors).
Those two wavelength distant mesh radiators will combine somewhere in
the interior space and build a field. This is very commonly found in
inter-cable cross coupling through leakage that is exhibited in very
long cable trays with tightly bound lines. This doesn't improve the
efficiency, but sensitive circuits running parallel to power drives
can prove to be a poor combination. What to do when conditions
condemn the small signal coax to live in proximity to the large signal
supply?

This introduces the foil shield. The foil shield is a very poor
conductor over any significant length, but over the span between mesh
openings (e.g. coax shield weave), the resistance is sufficiently low
to close the conductance gap.

73's
Richard Clark, KB7QHC

Nice explanation Richard. And I had never put together the
squared-squared relationship. That's a powerful thing to know.

I suppose this is why it ends up that a 1/10 lambda opening is
considered the rule of thumb cutoff frequency on a dish.

tom
K0TAR