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

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........................

Mike, W4EF wrote:
"I would agree, Richard, but at HF frequencies, the current path around
the shield isn`t real;ly broken by the gap."

To best describe what broken means, a picture helps. There is a picture
on page 13.18 of the 2006 ARRL Handbook. Fig 13.26 has a legend which
says:
"To prevent shielding of the loop from magnetic fields, leave the shield
unconnected at one end."

I think the handbook has it right.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote in message
...
Mike, W4EF wrote:
"I would agree, Richard, but at HF frequencies, the current path around
the shield isn`t real;ly broken by the gap."

To best describe what broken means, a picture helps. There is a picture
on page 13.18 of the 2006 ARRL Handbook. Fig 13.26 has a legend which
says:
"To prevent shielding of the loop from magnetic fields, leave the shield
unconnected at one end."


I am a bit behind on ARRL Handbooks, Richard, but from
what you describe, this is the same figure that appears in my
1992 edition (chapter 38, figure 2). In any case, what is
shown in the figure agrees with my understanding of "broken",
although admittedly when I made my previous post, I was
thinking of the case where the shield is broken on the side of
the loop opposite the feedpoint. For the purposes of this
discussion, however, it doesn't matter whether the break is
at the top (opposite the feed) or at the bottom (adjacent
to the feed). In either case, current induced on the inside
of the shield by current flowing on the center conductor loop
has a continuous back to ground via the outside surface of the
shield. IOW, the gap doesn't suppress the eddy current, rather
it forces it to flow on the outside surface of the shield, thereby
causing the loop to radiate.


I think the handbook has it right.


Yes, I agree it does. If you connect the shield at both ends, the
loop can't radiate because the eddy current caused by current
flowing on the inner conductor loop will confined to the inside of
the shield. Likewise, eddy currents induced on the outside of the
shield by EM waves passing the antenna will be confined to the
outside of the shield if there is no gap (reciprocity holds - the
antenna won't receive with no gap).

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

Best regards, Richard Harrison, KB5WZI

Michael Tope wrote:
I am a bit behind on ARRL Handbooks, Richard, but from
what you describe, this is the same figure that appears in my
1992 edition (chapter 38, figure 2). In any case, what is
shown in the figure agrees with my understanding of "broken",
although admittedly when I made my previous post, I was
thinking of the case where the shield is broken on the side of
the loop opposite the feedpoint. For the purposes of this
discussion, however, it doesn't matter whether the break is
at the top (opposite the feed) or at the bottom (adjacent
to the feed). In either case, current induced on the inside
of the shield by current flowing on the center conductor loop
has a continuous back to ground via the outside surface of the
shield. IOW, the gap doesn't suppress the eddy current, rather
it forces it to flow on the outside surface of the shield, thereby
causing the loop to radiate.

Absolutely nothing, neither electic nor magnetic, couplesthrough the
wall of a conductor more than several skin depths thick. This isn't
anything that can be debated, it is simply how it works. It is very
easy to demonstrate, it takes only a few minutes and a minimum of test
equipment.

It is something very basic in physics and underlies how coaxial cables
and things with shields of all types work.

The gap is the feedpoint no matter where the gap is placed. The
radiation and coupling of any time-varying field, magnetic or electric,
occurs on a frequency where the shield is more than a few skin depths
thick comes by the gap.

This is such a very basic thing it is important everyone understand it.


I think the handbook has it right.


Yes, I agree it does. If you connect the shield at both ends, the
loop can't radiate because the eddy current caused by current
flowing on the inner conductor loop will confined to the inside of
the shield.

Absolutely. When the gap is closed there is no potential difference
across the gap the outside of the shield is not connected to the inside
of the shield via the potential developed across the gap. The outer
wall is not coupled to the inner wall, the feedpoint is shorted.

When the gap is opened, the outside of the shield IS the antenna. Not
the inside or anything inside the inside.

Likewise, eddy currents induced on the outside of the
shield by EM waves passing the antenna will be confined to the
outside of the shield if there is no gap (reciprocity holds - the
antenna won't receive with no gap).

Again true. This is a very basic thing we must understand if we are to
understand how shields, walls, or conductors of any kind or form work
with HF currents, voltages, or fields of any type.

There isn't any way to change this effect.

73 Tom

wrote in message
ups.com...

Michael Tope wrote:
I am a bit behind on ARRL Handbooks, Richard, but from
what you describe, this is the same figure that appears in my
1992 edition (chapter 38, figure 2). In any case, what is
shown in the figure agrees with my understanding of "broken",
although admittedly when I made my previous post, I was
thinking of the case where the shield is broken on the side of
the loop opposite the feedpoint. For the purposes of this
discussion, however, it doesn't matter whether the break is
at the top (opposite the feed) or at the bottom (adjacent
to the feed). In either case, current induced on the inside
of the shield by current flowing on the center conductor loop
has a continuous back to ground via the outside surface of the
shield. IOW, the gap doesn't suppress the eddy current, rather
it forces it to flow on the outside surface of the shield, thereby
causing the loop to radiate.

Absolutely nothing, neither electic nor magnetic, couplesthrough the
wall of a conductor more than several skin depths thick. This isn't
anything that can be debated, it is simply how it works. It is very
easy to demonstrate, it takes only a few minutes and a minimum of test
equipment.


I don't think we disagree on that point, Tom. Perhaps I should
have chosen my words more carefully. I didn't mean to imply
that gap somehow forces the current on the inside of the shield
to pass through shield. When I said that the gap forces the current
to flow on the outside surface of the shield, I meant that in the
sense that the eddy current flows on the inside of the shield until
it reaches the break in the shield at which point the current flow
wraps around the edge of the shield and onto the outside surface
(thereby reversing direction relative to the direction of the eddy
current on the inside of the shield). The skin effect in effect
separates the shield into two distinct conductors, the inner surface
being one conductor and the outer surface of the shield being the
other. The gap is the circuit node where these two independent
conductors are connected. The eddy current flows out of one
conductor (the inner surface of the shield ) and into the other
conductor (the outer surface of the shield).

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

wrote:
Absolutely nothing, neither electic nor magnetic, couplesthrough the
wall of a conductor more than several skin depths thick.

Do 60 Hz magnetic fields penetrate RF coax since
the conductor is not more than several skin depths
thick?
--
73, Cecil http://www.qsl.net/w5dxp

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