Richard Harrison wrote:
Tom, W8JI wrote:
"I did not claim that effect. Terman certainly did not. (Yuri claims the
shield "blocks electric fields" or stops "electrostatic fields".)"

I`ll requote Terman from page 38 of his 1955 edition which Tom ignored:
"It is possible to shield electrostatic flux without simultaneously
affecting the magnetic field by surrounding the space to be shielded wih
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."

Richard.

I know anything Roy Lewallen agrees with, you disagree with. I know
anything I say (or even what I don't say) sets Yuri off into a foaming
lather. I really wish you guys could put personal hate or dislike aside
and look at facts. This is an imporant issue because the myth about
shields is imbedded in amateur circles despite many clearly written
engineering texts and very simple experiments that prove the concept of
time-varying magnetic fields penetating the shield. It's just a fact
when the time-varying electric field is taken to zero so is the
time-varying magnetic field.

Static by definition is not moving or varying. Don't confuse jargon
describing a different coupling mode with the mechanics of a loop
operating at radio frequencies.

When we receive noise or signals, the fields are time-varying. Just as
with a piece of coaxial cable, the inner wall of a "shielded loop" is
isolated by the skin depth of the conductor from the outside wall. The
electric and magnetic coupling effects are what causes a coaxial cable
with a dense shield more than a few skin depths thick to ALWAYS have
the same current on the inside of the shield as the inner conductor
has, and all radiation or common mode current flow over the outside.

This isn't something I invented. It has been in nearly every textbook
long before I was born.

I'm pleased that Yuri credits me for the work, but unfortunately I had
little to do with it. It really was people from the 1700's and 1800's
that did all the work.

You (and Yuri) appear to be confusing how time-varying fields work.

I suggest you put Terman aside and actually read some textbooks on
fields.

It's helpful to actually make a few measurements. A few minutes spent
with some very simple test equipment would go a long way to "turning on
the light".

The loop shield is thus a true Faraday screen, not a Faraday car body or
screened room.

If you say so. And as one, it also must block any time-varying magnetic
field. As K7ITM points out it is the gap in the loop that is actually
the feedpoint, and it is the outside of the loop that is the actual
antenna.

If you do not think a loop behaves this way, you need to get busy doing
some real important work. You need to get all the Handbooks to quit
talking about common mode currents on shield outsides. You need to get
them to quit treating the inside of the shield as a isolated conductor
that is independent of the outside.

As I and others have suggested it only takes a moment to prove the
books are correct. You can prove it with a single sheet of copper and a
minimum of test equipment.

73 Tom

wrote in message
ps.com...

Richard Harrison wrote:
Tom, W8JI wrote:
"I did not claim that effect. Terman certainly did not. (Yuri claims the
shield "blocks electric fields" or stops "electrostatic fields".)"

I`ll requote Terman from page 38 of his 1955 edition which Tom ignored:
"It is possible to shield electrostatic flux without simultaneously
affecting the magnetic field by surrounding the space to be shielded wih
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."

Richard.

I know anything Roy Lewallen agrees with, you disagree with. I know
anything I say (or even what I don't say) sets Yuri off into a foaming
lather. I really wish you guys could put personal hate or dislike aside
and look at facts. This is an imporant issue because the myth about
shields is imbedded in amateur circles despite many clearly written
engineering texts and very simple experiments that prove the concept of
time-varying magnetic fields penetating the shield. It's just a fact
when the time-varying electric field is taken to zero so is the
time-varying magnetic field.


That really nails it! His "technical" response!
Perfect picture of a jerk parading as an engineer!
Yep, I hate your guts and I made up phony claims on your web site for all to
see, so I can "hate you"! Brilliant! Keep it up!
Halleluja, now we know that shields are antennas, praise the guru!


Bada BUm

wrote:
I know anything Roy Lewallen agrees with, you disagree with.

Absolutely false. I'll bet they agree on 99% of technical
topics, e.g. ohm's law, Maxwell's equations, etc.
--
73, Cecil http://www.qsl.net/w5dxp

Tom, W8JI wrote:
"This is an important issue because the myth about shields is embedded
in amateur circles despite many clearly written engineering texts and
very simple experiments that prove the concept of time-varying magnetic
fields penetrating the shield."

Some of that poison reached the 2006 ARRL Handbook on page 13.18. Fig
13.26 says:
"Electrostatically-shielded loop for RDF. To prevent shielding of the
loop from magnetic fields, leave the shield unconnected at one end."

Terman`s RDF loop should have better balance than ARRL`s because
Terman`s shield gap is squarely in the center of the loop
and not at one end. However, as long as the shield is broken preventing
induced current from flowing around the shield, Lenz`s law will be
thwarted and magnetic coupling to the coil under the shield will be
obtained. Electric field coupling to the coil beneath the shield will be
disallowed by the shield`s connection to ground wherever it occurs,
though not as elegantly as when care is taken to get the best balance
possible.

I`ve worked with such Faraday screens in my broadcasting career.

Best regards, Richard Harrison, KB5WZI

On Wed, 17 May 2006 10:19:39 -0700, Richard Clark
wrote:

It's still the same "1/2 inch copper water tubing (non ferrous
material passing the magnetic field)."

So, does that wire make the "shield" better, or worse?

Hmm, this one must've been experienced exactly as an existential
question about the infinite cosmos.

Super-extra credit question:
If we replaced the non ferrous material (same gap, no link) with (most
have probably anticipated this) a ferrous material, does this allow
near field region electrical field interference to pass un-impeded?

This one must never been experienced either. I've always wondered why
perfect academic set-ups like "non ferrous material" (as if it were
lossless) always appear in the context of a populist aw-shucks kind of
posting.

Sorry All,

But when such simple questions become imponderables of the century,
they merit Cecil's 5 forbidden words woven in. Of course, it makes
only the most strained of sense, but there's nothing to compete! ;-)

73's
Richard Clark, KB7QHC

p.s. as viewed through the bottom of a bottle of Dick's Working Man's
Brown Ale (Centralia, Washington)

Sooo, in shielded loop the shield is the antenna according to W8JI and
worshippers. But you take the shield (W8JI antenna) away, now the wires are
antenna, some say don't need no stinkin' shield and "antenna" to work as an
antenna.

I don't know what a W8JI antenna is, except for those I've heard on
160m... :/
But I do know that I've tested various versions of both shielded and
unshielded
loops, and have never been able to tell a lick of difference as far as
close local
noise pickup. I spent a whole week testing that very thing. It's not
something I just
made up, or picked up from W8JI.

Amazing how selective in reading and digestion of postings some people are.
They tend to ignore the reality and description of it, they pick on
selective "proof" of what they were taught and figered out.

Only my test results were used to come up with my conclusion. So I
guess
I taught myself. I've never built a shielded loop yet that was any
"quieter" to
local noise than any of my good unshielded loops. But my unshielded
loops
are well balanced. Were yours?

So shield works as a electrostatic shield, if you guys like it or not, or
refuse to admit.

I refuse to admit it, if I can't prove it. And I haven't been able to
prove it yet.
One thing...How in the heck is a solid shield going to filter one
source of RF,
and ignore another. In reality, it will shield *all* RF, unless I am
missing
something here. So the outer shield *must* be the antenna, unless the
sky is
now green. No RF is going to prevail past the outer skin depth of the
solid
shield. None. Nada...

Sooo, antenna works without shield (not just my assertion), but when you
insert it in the shield then shield becomes W8JI antenna.

It does? I'm sure if this is the case, it probably tunes 160m.... :/

So his shield,
untuned becomes antenna, but my tuned and tunable inside the shield antenna
is not the antenna? Makes as much sense as "there is equal current along the
loading coil doesn't matter what", riiiiight?

If you say so....

Let's stick to some reality in antennas.

Thats all I do. I've made a load of loops. I have a diamond loop 44
inches
per side right next to me. Almost is as tall as the ceiling... Heck, I
even
have tried using shielded loops as the coupling loop to unshielded
loops.
Works pretty well to maintain balance, but mine work just as well with
just
a simple unshielded coupling loop. Probably cuz my loops are very
symmetrical
and balanced naturally. The coax feedline itself is the only real issue
in my case,
and even it's not really very critical. I never saw any indication
that using a
shielded coupling loop made the loop quieter than not using one. Not
once.
Myself, I don't really like small loops for receiving on 160m. They are
good for
cutting the noise when working loud locals, but in my experience they
are
pretty ho-hum when receiving weak dx. For 160m, I would use the biggest
loop
I could manage. Probably outside to have enough room...
My loops are mainly for MW BC receiving, although the one next to me
tunes
500-2300 kc in two stages, by switching cap gangs. I can go LW if I
tack on
more fixed caps. The real value of small loops are not the "quiet", or
the s/n
or whatever. It's the nulls... But nulls have much more value in the BC
band,
than they do on 160m unless maybe you have a noise source in the area
you wanna null out. Thats how a loop reduces noise. Using the nulls...
:/
I do have to agree with Tom. I think the "shielded loop" theory many
hams
adhere to is just another batch of wive tailery.. Along with grounds to
cure
antenna/feedline problems, sticking coax ends in bottles hoping to
thwart
lightning, etc... And I've never once talked to Tom about small loops.
It's all my idea to shun this "shield=quiet" theory, not W8JI's.
MK

Cecil Moore wrote:
wrote:

I know anything Roy Lewallen agrees with, you disagree with.


Absolutely false. I'll bet they agree on 99% of technical
topics, e.g. ohm's law, Maxwell's equations, etc.

Just like arguing whether Coke or Pepsi tastes best. The closer the
product, the worse the arguing...

- 73 de Mike KB3EIA -

Mark, NM5K wrote:
"I refuse to admit it, if I can`t prove it."

A shield is extra work, weight, and cost but despite that, many are in
use.

As electrons move along a conductor a magnetic field expands from some
depth inside the conductor itself. The magnetic lines of force sweep
outward from the conductor while inducing an emf in the conductor
itself. The self induced emf opposes instantaneous change of current in
the inductance of the conductor. This is the basis of Lenz`s law:
"In all cases of electromagnetic induction, induced electromotive force
and resultant current are in such a direction as to oppose the effect
producing them."

Skin effect prevents penetration of RF very deep into a good conductor.
Skin effect makes RF coil shields impenetrable. Electric hields are
shorted to ground by the conductive shield. Magnetic fields induce
counter fields from the currents they induce on the surface of the
shield.

A Faraday screen breaks the current path on the shield preventing the
counter fields from being magneticly induced. Result is a shield that is
penetrable by the magnetic field but impenetrable by the electric field.
The electric field is still shorted to ground by its conductive path.
Faraday screens are used because they work.

Best regards, Richard Harrison, KB5WZI

On Thu, 18 May 2006 11:48:52 -0500, (Richard
Harrison) wrote:

A Faraday screen breaks the current path on the shield preventing the
counter fields from being magneticly induced. Result is a shield that is
penetrable by the magnetic field but impenetrable by the electric field.
The electric field is still shorted to ground by its conductive path.
Faraday screens are used because they work.

Hi Richard, Yuri,

In regard to my last question that so stumped you two, there is
absolutely nothing inherent in non-ferrous vs. ferrous materials that
changes this one particular aspect of shielding. Both materials are
conductive, and both are selected for their lowest conductivity (hence
their lowest Ohmic loss). The only substantive difference is that a
ferrous material offers the prospect of using a thinner covering for
the same isolation. Above LF, this is hardly useful unless you are
planning on using very thin foils. Art's selection of mylar films
with conductive coatings is one such example that works with a
conductive surface thickness in the 100s of microns.

The "shielded dipole" observes the one principle requirement of
insuring that a break in continuity is maintained. Otherwise, the
shorted turn snuffs the antenna for any design held within it. This
prohibition in continuity is paramount to all designs and is driven by
both magnetic as well as electric considerations, as in the RF field
they are inextricably coupled.

To cut to the chase: there is no way to separate these fields and
select one of them over the other. If you choose to run your arc
welder within a loop's diameter of the antenna while also DXing; then
it is the balance of the antenna design, not the shielding that will
determine your success. Screw up the geometry of that gap, and you
will hear as much hash as if the "shield" never existed.

For the standard single turn "shielded dipole," the arms of the dipole
are the shield. There must be 10 million examples of this particular
model on 1 million repeater installations world-wide. There are also
tri-axial or twin-axial designs of the "shielded dipole" that wholly
divorce themselves of the exterior shield. All such designs, to work
effectively, exhibit a balanced configuration that is identical to the
standard single turn. The balance is the only consideration at issue,
and shielding is a means, but hardly a necessary ends to that
achievement. As such, shielding is simply insurance and a brute force
means to force balance through what in engineering terms is called
"swamping."

73's
Richard Clark, KB7QHC

"Richard Harrison" wrote in message
...

As electrons move along a conductor a magnetic field expands from some
depth inside the conductor itself. The magnetic lines of force sweep
outward from the conductor while inducing an emf in the conductor
itself. The self induced emf opposes instantaneous change of current in
the inductance of the conductor. This is the basis of Lenz`s law:
"In all cases of electromagnetic induction, induced electromotive force
and resultant current are in such a direction as to oppose the effect
producing them."

Skin effect prevents penetration of RF very deep into a good conductor.
Skin effect makes RF coil shields impenetrable. Electric hields are
shorted to ground by the conductive shield. Magnetic fields induce
counter fields from the currents they induce on the surface of the
shield.

A Faraday screen breaks the current path on the shield preventing the
counter fields from being magneticly induced. Result is a shield that is
penetrable by the magnetic field but impenetrable by the electric field.
The electric field is still shorted to ground by its conductive path.
Faraday screens are used because they work.

Best regards, Richard Harrison, KB5WZI


I would agree, Richard, but at HF frequencies the current path
around the shield isn't really broken by the gap. Due to the skin
effect, the RF current flowing on the inside of the loop shield is free
to flow around the edge of the shield conductor and onto the outside
of the shield at the gap. At very low frequencies, where the skin
depth is large, this wouldn't necessarily be true, but at HF as long
as there are a few skin depths between the outside and the inside
surface of the conductor, then the inside surface of the shield and
the outside surface of the shield can be treated as independent
conductors.

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