Loop antenna electrostatic shields
 

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

The 'slit' in the shield allows a voltage difference to exist across the
slit.
If there were no voltage difference, then you could short the slit with
no change in performance.

Thus the shield acts like a short dipole, with the ends bend up to almost
touch one another. There is a voltage gradient along this short shield
(dipole),
and current flow along the length of the shield (dipole).

The loop inside the shield thus is coupled to this short dipole. Multiple
turns of the loop 'add' (increase) the voltage since the dipole (shield) is
short compared to the wavelength.

Thus, the shield IS the antenna, and the loop is the means to couple the
energy out of the short dipole. Thus, it really isn't a magnetic antenna at
all - it's a dipole!

Tom, W8JI has a better, more detailed explanation at his website (a great
resource) at http://www.w8ji.com/magnetic_receiving_loops.htm




"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

Thus, the shield IS the antenna, and the loop is the means to couple the
energy out of the short dipole. Thus, it really isn't a magnetic antenna at
all - it's a dipole!

Tom, W8JI has a better, more detailed explanation at his website (a great
resource) at http://www.w8ji.com/magnetic_receiving_loops.htm



Looks like another "debunked" fact by "Tom noname" W8JI.
(Welcome back Tom ?:-)

The shield IS an electrostatic SHIELD and wires inside are the ANTENNA.
If you want good efficient, electrostatically shielded small loop, take copper
or aluminum tubing, bend it in a circle about 1m diameter (for around 160m
band). I made it of two pieces, joined at the bottom in metal electrical box.
Top of the loop is open and insulated with plastic tubing. Then thread wire
through the loop, three turns, bring the ends into the junction box and connect
trimmer capacitor (about 1.5k ?) across the ends (not connected to anything
else). Then thread another single turn loop through the tubing. In the junction
box, connect one end to the coax shield and the second end to trimmer capacitor
(about 500 pF ?) in series with the center conductor of the coax. Shield and
mast holding it can be grounded at the installation place. I actually used low
power transmiter and SWR bridge to tune the loop, now there are nice antenna
analyuzers to do it.
The three turns are resonated to operating frequency with capacitor and that is
the ANTENNA. The single turn loop is the coupling and trimmer provides match to
the coax feedline. The tubing is a SHIELD which helps with suppressing the
noise and interference from nearby sources.
I have also used this loop as a coupling to Beverage antenna (positioned at the
end of Beverage), where it provided less noise than beverage alone.

Don't believe everything you read at W8JI web pages.

I would like to see a short "dipole" (NOT), really short piece of tubing, which
is attached to a mast and grounded to act as an antenna on low frequencies.
Having small loop, insulated and properly fed or coupled to, is another thing.
You can have single loop or turn antenna too, but above described antenna is
"magnetic" or electrostatically shielded small loop antenna which provides more
signal and better discrimination from nearby interference.

Yuri, K3BU.us

Yuri Blanarovich wrote:
Thus, the shield IS the antenna, and the loop is the means to couple the
energy out of the short dipole. Thus, it really isn't a magnetic antenna at
all - it's a dipole!

Tom, W8JI has a better, more detailed explanation at his website (a great
resource) at http://www.w8ji.com/magnetic_receiving_loops.htm




Looks like another "debunked" fact by "Tom noname" W8JI.
(Welcome back Tom ?:-)

The shield IS an electrostatic SHIELD and wires inside are the ANTENNA.
If you want good efficient, electrostatically shielded small loop, take copper
or aluminum tubing, bend it in a circle about 1m diameter (for around 160m
band). I made it of two pieces, joined at the bottom in metal electrical box.
Top of the loop is open and insulated with plastic tubing. Then thread wire
through the loop, three turns, bring the ends into the junction box and connect
trimmer capacitor (about 1.5k ?) across the ends (not connected to anything
else). Then thread another single turn loop through the tubing. In the junction
box, connect one end to the coax shield and the second end to trimmer capacitor
(about 500 pF ?) in series with the center conductor of the coax. Shield and
mast holding it can be grounded at the installation place. I actually used low
power transmiter and SWR bridge to tune the loop, now there are nice antenna
analyuzers to do it.
The three turns are resonated to operating frequency with capacitor and that is
the ANTENNA. The single turn loop is the coupling and trimmer provides match to
the coax feedline. The tubing is a SHIELD which helps with suppressing the
noise and interference from nearby sources.
I have also used this loop as a coupling to Beverage antenna (positioned at the
end of Beverage), where it provided less noise than beverage alone.

Don't believe everything you read at W8JI web pages.

I would like to see a short "dipole" (NOT), really short piece of tubing, which
is attached to a mast and grounded to act as an antenna on low frequencies.
Having small loop, insulated and properly fed or coupled to, is another thing.
You can have single loop or turn antenna too, but above described antenna is
"magnetic" or electrostatically shielded small loop antenna which provides more
signal and better discrimination from nearby interference.

Yuri, K3BU.us

Don't believe everything you read here, either. For a good treatise on
how the shielded loop works, and what it's good for, read Glenn S.
Smith's (Georgia Institute of Technology) article "Shielded Loop
Antenna" in Richard C. Johnson's _Antenna Engineering Handbook_.
73,
Tom Donaly, KA6RUH

The value of the shield on a small loop is an overated old-wives tale.

Having erected it, performance would be better to use the shield itself as
the loop, by virtue of its much greater conductor diameter.

"Reg Edwards" ha scritto nel messaggio
...
The value of the shield on a small loop is an overated old-wives tale.

Having erected it, performance would be better to use the shield itself as
the loop, by virtue of its much greater conductor diameter.



Find some more info on loops here

http://www.dxzone.com/catalog/Antennas/Loop/index.shtml

Simone

"Tom Donaly" wrote in message
m...
Yuri Blanarovich wrote:
Thus, the shield IS the antenna, and the loop is the means to couple the
energy out of the short dipole. Thus, it really isn't a magnetic antenna
at
all - it's a dipole!

Tom, W8JI has a better, more detailed explanation at his website (a great
resource) at http://www.w8ji.com/magnetic_receiving_loops.htm




Looks like another "debunked" fact by "Tom noname" W8JI. (Welcome back
Tom ?:-)

The shield IS an electrostatic SHIELD and wires inside are the ANTENNA.
If you want good efficient, electrostatically shielded small loop, take
copper
or aluminum tubing, bend it in a circle about 1m diameter (for around
160m
band). I made it of two pieces, joined at the bottom in metal electrical
box.
Top of the loop is open and insulated with plastic tubing. Then thread
wire
through the loop, three turns, bring the ends into the junction box and
connect
trimmer capacitor (about 1.5k ?) across the ends (not connected to
anything
else). Then thread another single turn loop through the tubing. In the
junction
box, connect one end to the coax shield and the second end to trimmer
capacitor
(about 500 pF ?) in series with the center conductor of the coax. Shield
and
mast holding it can be grounded at the installation place. I actually
used low
power transmiter and SWR bridge to tune the loop, now there are nice
antenna
analyuzers to do it. The three turns are resonated to operating frequency
with capacitor and that is
the ANTENNA. The single turn loop is the coupling and trimmer provides
match to
the coax feedline. The tubing is a SHIELD which helps with suppressing
the
noise and interference from nearby sources. I have also used this loop as
a coupling to Beverage antenna (positioned at the
end of Beverage), where it provided less noise than beverage alone.

Don't believe everything you read at W8JI web pages. I would like to see
a short "dipole" (NOT), really short piece of tubing, which
is attached to a mast and grounded to act as an antenna on low
frequencies.
Having small loop, insulated and properly fed or coupled to, is another
thing.
You can have single loop or turn antenna too, but above described antenna
is
"magnetic" or electrostatically shielded small loop antenna which
provides more
signal and better discrimination from nearby interference.

Yuri, K3BU.us

Don't believe everything you read here, either. For a good treatise on
how the shielded loop works, and what it's good for, read Glenn S. Smith's
(Georgia Institute of Technology) article "Shielded Loop
Antenna" in Richard C. Johnson's _Antenna Engineering Handbook_.
73,
Tom Donaly, KA6RUH

There is also a reasonably good discussion in the ARRL Antenna Book. The
section on direction finding antennas discusses the directivity of shielded
loops vs loop antennas.

Tam/WB2TT

Find some more info on loops here

http://www.dxzone.com/catalog/Antennas/Loop/index.shtml

Simone




One has to be careful in distinguishing between small loop vs.
Electrostatically shielded ("magnetic") small loop antennas.
Small loop is typically less than 1/4 wave single loop made of tubing (low
resistance - losses) and with efficient matching network.
The shielded small loop is variation of small loop antennas using electrostatic
shield (tubing with gap - opening)

Above referenced sites seem to use the name Magnetic Loop for wrong antennas
(just simple loop).

Got that Reg, Tom, Dick and Harry?

Yuri, K3BU.us

Well, don't have access to that reference. I'm sure that it would clarify
some
questions. Access to the once great Collins technical library is gone
(sigh).

Built a couple models in MMANA. One for a shorted loop, and one for an
open loop. As expected, the shorted loop is vertically polarized, and the
pattern
is in-line with the loop. [a magnetic flux pickup antenna].

Upon opening the loop, it forms a small dipole, with the two ends near each
other.
It is horizontally polarized, and the directivity is perpendicular to the
shorted loop.
[an electric field pickup antenna].

Of course it's hard to tell exactly the voltages that would produced by
these two
different antennas, and how they would combine, magnitude wise. Rough order
of magnitude in MMANA is they are within a few dB for a diamond shape, 1m on
a side at 3.5 MHz. But this would depend on how well the magnetic coil
coupled
to the dipole, and that's hard to tell in MMANA.

It would seem from this that the electrostatic shield does not necessarily
help
things because it appears to degrade the directivity [assuming the
magnitudes of the
two components are roughly similar ]. It would be interesting to see in
real life if
the shield did degrade the nulls of the unshielded loop (accounting for the
polarization
change).

-- Tom





"Tom Donaly" wrote in message
m...

Don't believe everything you read here, either. For a good treatise on
how the shielded loop works, and what it's good for, read Glenn S. Smith's
(Georgia Institute of Technology) article "Shielded Loop
Antenna" in Richard C. Johnson's _Antenna Engineering Handbook_.
73,
Tom Donaly, KA6RUH

Managed to model the shielded loop in MMANA. Single turn loop, with
one nearby wire as shield, and two nearby wires as shield. Set the spacing
of the wires 10mm and 5mm away from the loop, didn't make any difference
in results. [using 0.8mm wires].

The single shield wire improved the E-field rejection a little bit (maybe 3
db)
over the unshielded loop. Two shield wires degraded the E-field rejection as
compared to the single shield, but still a tiny bit better than the
unshielded loop
(maybe 1 dB better).

Pretty much the shield provides very little E-field coupling compared to the
H-field response of the loop.

So I'll retract my initial statement.

-- Tom

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