Thanks for the information, everyone. I'll read up on a few of the
references. I have found a few web pages that compare the sensitivity of
the shielded vs. unshielded loops, and the shielded loop always ends up with
slightly less sensitivity.

Could anyone answer the following question? I'm trying to make things
simpler here than what happens what a real antenna: Let me take an ideal
circular conductor, say, a meter in circumference. Next I'll stick it
inside a perfectly conducting loop of pipe (the shield) that has an
infinitesimally small slit in it to prevent creating a shorted turn.
Finally I'll create a magnetic field at a very low frequency (say, 1Hz -- so
that the loop is clearly electrically small) and insert the loop into it
such that coupling is maximized.

Question: Will the current on the inner conductor be identical to the case
where there is no shield? Is there any current on the shield? Does
anything change if I ground the shield?

Hmm... I'm thinking I should move this over to the electromagnetics
newsgroup!

---Joel

The "shield" is actually the antenna, and the gap in the "shield" is
the feedpoint of that antenna. You will do well to make the "shield"
out of a good conductor, and to get the benefits of rejecting
vertically-polarized electric fields generated nearby, you should make
the antenna very symmetrical. See the discussion in King, Mimno and
Wing's "Transmission Lines, Antennas and Waveguides." I think I have
a .pdf file of the relevant section somewhere. I particularly like
that one for its qualitative explanation, clearly presented. I've
seen other decent explanations in places like Johnson and Jasik's
antenna book. The explanations in such texts that I've seen all
agree.

A key advantage of the "shield" is that it simplifies the task of
making the antenna symmetrical, though I've seen a lot of old ARRL
pubs that completely miss that point. If you realize that that's what
you're trying to accomplish in the "shielded" construction, you'll
find you can do quite well with a multi-turn "unshielded" loop, too.

Cheers,
Tom (one with a last name)

"Joel Kolstad" wrote in message ...
When you build a loop antenna, it's common to wrap it in, e.g., aluminum
foil that's grounded so as to prevent electric field pick-up (I'm thinking
of HF loops here, 30MHz). A slit is made in the wrapping so that a shorted
turn isn't created, thereby nulling out the magnetic field that the loop is
trying to detect in the first place.

Something I don't understand, though... normally, if you were thinking of
using aluminum for EMI shielding purposes, the skin depth of aluminum at
10MHz is all of ~1mil. Hence, a regular sheet of aluminum foil would
significantly attenuate both the magnetic and electric fields on its 'far'
side. Why doesn't this apply in the case of a shielded loop antenna? It
seems to me that the ~95+% 'coverage' of the shield (everything minus the
slit to prevent the shorted turn) would be what dictates the overall
shielding effectiveness, not the presence of the slit itself.

Looking for insight,
---Joel Kolstad

On Mon, 8 Nov 2004 09:58:10 -0800, "Joel Kolstad"
wrote:

Will the current on the inner conductor be identical to the case
where there is no shield? Is there any current on the shield? Does
anything change if I ground the shield?

Hi Joel,

A 1M loop in a 1Hz field, Hmm?

Why bother with the complication of shielding? Take a 1M loop of
wire, connect it to your MOST sensitive ammeter (mine will resolve at
least 1 nanoampere).

By simply going to my lab, and turning on the unit, this allowed me to
test my hypothesis as leads that long were already attached.

I used the earth's magnetic field and a nearby fluorescent fixture (80
W) to find absolutely NO sensitivity at all on AC or DC scales
(turning the loop in earth's magnetic loop to simulate the lower 1Hz
frequency you specify).

Now, I know that for studies of low frequency magnetics, the usual
antenna has 10 to 20 thousand turns on an 18" ferrite bar, I think
that a 1M open air loop is destined for deafness.

Of course, this will serve as no impediment to those who can count
angels on the head of a pin, so your question will undoubtedly be met
with great fuss.

73's
Richard Clark, KB7QHC

Hi Tom,

Those are some good additional references; thanks. I was surprised to find
that Kraus' antenna book doesn't discuss shielded loops (well, the most
recent/last edition -- I've heard that some material from earlier editions
has been pulled from it). Your explanation matches the 'other Tom's'
explanation, and sounds plausible to me, yet there are others posting here
who seem to disagree. The fact that you have several more theory oriented
books agreeing lends a lot of support to it.

If you do find a PDF version of the section of the King/Mimno/Wings book,
I'd love to get a copy.

---Joel

"Richard Clark" wrote in message
...
A 1M loop in a 1Hz field, Hmm?

That was meant to setup the problem such that the loops is electrically
small and such that only quasi-magnetostatic analysis would be necessary.
In actuality I'm thinking more along the lines of WWVB loop antennas --
60kHz.

Why bother with the complication of shielding?

The idea is that there's a lot of predominently electric field interference
around (60Hz power lines, for ones) and that the shield -- if grounded --
can short out that component of the field and get the loop to respond
primarily to the magnetic field.

---Joel

"Tom Bruhns" wrote in message
m...
The "shield" is actually the antenna, and the gap in the "shield" is
the feedpoint of that antenna. You will do well to make the "shield"
out of a good conductor, and to get the benefits of rejecting
vertically-polarized electric fields generated nearby, you should make
the antenna very symmetrical. See the discussion in King, Mimno and
Wing's "Transmission Lines, Antennas and Waveguides." I think I have
a .pdf file of the relevant section somewhere. I particularly like
that one for its qualitative explanation, clearly presented. I've
seen other decent explanations in places like Johnson and Jasik's
antenna book. The explanations in such texts that I've seen all
agree.

A key advantage of the "shield" is that it simplifies the task of
making the antenna symmetrical, though I've seen a lot of old ARRL
pubs that completely miss that point. If you realize that that's what
you're trying to accomplish in the "shielded" construction, you'll
find you can do quite well with a multi-turn "unshielded" loop, too.

Cheers,
Tom (one with a last name)

Hi Tom (WALN),

I thought the original question related to whether or not the magnetic
field penetrates the "shield".

I have a problem calling the shield the antenna because I have built loops
(144 & 440 MHz) which do not have this classic form of shield, but a
unidirectional screen, and good nulls off both sides. I made the shield
from a series of vertical wires connected to a conducting strip only at the
bottom.
I also wonder about the symmetrical concept as a standard "Hazeltine"
loop had the opening of the shield at one end near the bottom, near the
feed.
I suspect that a symmetrically constructed loop will have less
electrostatic response, however. This seems to make sense. Then there is
the matter of the practicality of construction. If it is easier to make the
loop unsymmetrical and shielded, then that's ok.

73, also with a last name,
--
Steve N, K,9;d, c. i My email has no u's.

On Mon, 8 Nov 2004 10:53:06 -0800, "Joel Kolstad"
wrote:

"Richard Clark" wrote in message
.. .
A 1M loop in a 1Hz field, Hmm?

That was meant to setup the problem such that the loops is electrically
small and such that only quasi-magnetostatic analysis would be necessary.
In actuality I'm thinking more along the lines of WWVB loop antennas --
60kHz.

Hi Joel,

When I was calibrating my Atomic Clock, years distant, my antenna was
a simple 20' whip above the fantail of my ship. We used WWVB and an
unique modulation envelope to perform the job. This didn't make the
antenna particularly efficient, but it was suitably sensitive. Still
and all, there are roughly 5 orders of magnitude down to the 1Hz
region.

You may as well use WWVB as your testbed, or one of the submarine
fleet frequencies around 12KHz. One station is nearby here at Jim
Creek. They use a T antenna of about 1000' tall with a top hat of 6
cables strung between two mountain tops (about 4000 to 6000 feet
between them). The numbers may be off, but you can judge that the
scale is impressive (quite a sight).

Why bother with the complication of shielding?

The idea is that there's a lot of predominently electric field interference
around (60Hz power lines, for ones) and that the shield -- if grounded --
can short out that component of the field and get the loop to respond
primarily to the magnetic field.

1Hz noise field would come from meteors striking, and ionizing the
upper atmosphere. I doubt you could judge against any local sources
of noise compared to that chatter. Given that line frequency would be
6 Octaves away, if you tuned your antenna (it would be tuned, wouldn't
it?), you would have to be next to a noisy power plant to hear it.

73's
Richard Clark, KB7QHC
73's
Richard Clark, KB7QHC

That's some interesting history Richard!

"Richard Clark" wrote in message
...
1Hz noise field would come from meteors striking, and ionizing the
upper atmosphere. I doubt you could judge against any local sources
of noise compared to that chatter. Given that line frequency would be
6 Octaves away, if you tuned your antenna (it would be tuned, wouldn't
it?), you would have to be next to a noisy power plant to hear it.

Well, I've read that down around 60kHz there's plenty of interference due to
noise on the power lines (I doubt harmonics, probably just plain old noise
from, e.g., switching power supplies). But in any case, I'm at the point
where I need to just build something and see how it performs.

---Joel

Tom Bruhns, K7ITM wrote:
"See the discussion in King, Mimno, and Wing`s "Transmission Lines,
Antennas. and Wave Guides"."

Ronold W.P. King did a fine job explaining "closed circuits as
antennas". It starts on page 224. The shielded loop is diagrammed on
page 234.

As Tom Bruhns says, the exterior of the loop is driven from the gap at
the midpoint in its shield.
The shielded wire loop is coupled by its proximity to the interior
surface of the shield. The outside of the loop is coupled by conduction
across the open circuit edges to the inside surface of the loop. The gap
is the drivepoint. The secret to noise immunity is symmetry and balance
in all aspects of the loop.

Best regards, Richard Harrison, KB5WZI

(Richard Harrison) wrote in message
The secret to noise immunity is symmetry and balance
in all aspects of the loop.

Exactly. But to me, there is no such thing as noise immunity as far as
far field noise. The only noise that really can be reduced, is any
possible common mode related noise due to lack of a balun device,
whatever, at the feedpoint. And that assumes you actually have shack
noise to reduce. Not all do. IE: using a loop out in the middle of the
woods with a battery radio, etc...You can change polarity to reduce
maybe 20 db or so, but thats another issue...As far as far field
noise, the only way to reduce, is to ensure better balance. Better
balance gives you deeper nulls, and hence, lower noise, IF!, you are
using the loop to *NULL* the noise source. If you don't turn the loop
to null the noise source, it will be just as noisy as any other loop.
But as mentioned, this good balance can be attained with unshielded
loops also. As far as I can tell, and I have tested this in the real
world, balance is what separates a good loop from a medeocre loop. Not
a shield, etc.
The split shield is just one method used to provide good balance.
Thats why they are often used for DFing. With good balance, the nulls
will be sharp, and there will be little skewing of the pattern. None
of the shielded loops I've tried were any quieter to far field noise
than my unshielded loops. Other than small differences in signal
strength due to size, turns differences, the overall s/n ratio was the
same. I could not hear anything on one loop, that couldn't be heard on
another due to any increase or decrease in s/n ratio. In other words,
all my loops act pretty much the same, shielded, or unshielded. MK