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

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

"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.

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