I just read the wikipedia article on small loop antennas and it seems I
was laboring under a misapprehension. I thought receiving loops were
"magnetic" because they were shielded (this is often stated in various
web pages about constructing such loops). But the wikipedia article on
small loop antennas says the nature of a small loop is to not be very
sensitive to the E field in near field.

So if the shield has little to do with rejecting near field electrical
noise, what does the shield do? A lot of antenna designs make a big
deal of the shield. So I assume it must be a useful addition to the
small loop antenna for some purpose.

--

Rick

On Wed, 14 Oct 2015 14:34:10 -0400, rickman wrote:

I just read the wikipedia article on small loop antennas and it seems I
was laboring under a misapprehension. I thought receiving loops were
"magnetic" because they were shielded (this is often stated in various
web pages about constructing such loops). But the wikipedia article on
small loop antennas says the nature of a small loop is to not be very
sensitive to the E field in near field.

So if the shield has little to do with rejecting near field electrical
noise, what does the shield do? A lot of antenna designs make a big
deal of the shield. So I assume it must be a useful addition to the
small loop antenna for some purpose.

Indeed it is and why do you worship Wikipedia.


w.

On Wed, 14 Oct 2015 14:34:10 -0400, rickman wrote:

I just read the wikipedia article on small loop antennas and it seems I
was laboring under a misapprehension. I thought receiving loops were
"magnetic" because they were shielded (this is often stated in various
web pages about constructing such loops). But the wikipedia article on
small loop antennas says the nature of a small loop is to not be very
sensitive to the E field in near field.

So if the shield has little to do with rejecting near field electrical
noise, what does the shield do? A lot of antenna designs make a big
deal of the shield. So I assume it must be a useful addition to the
small loop antenna for some purpose.

The shielded loop reduces local noise pickup by eliminating much of
the electric component of that noise in the near field. Since the
ability of a small loop antenna to hear properly is primarily an
exercise in improving the SNR, any reduction in noise levle, without a
corresponding reduction in signal level, is a very good thing. More
detail:
http://electronics.stackexchange.com/questions/70262/what-if-anything-makes-shielded-loop-antennas-so-great-at-rejecting-local-nois

I've built small loops that were not shielded and measure the SNR of
some stable signal, such as WWV. I then wrapped the loop in aluminum
duct tape, leaving a gap to prevent a shorted turn problem, retuned,
and found that the baseline noise level had decreased and the SNR had
improved. It works.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

rickman wrote:

I just read the wikipedia article on small loop antennas and it seems I
was laboring under a misapprehension. I thought receiving loops were
"magnetic" because they were shielded (this is often stated in various
web pages about constructing such loops). But the wikipedia article on
small loop antennas says the nature of a small loop is to not be very
sensitive to the E field in near field.

So if the shield has little to do with rejecting near field electrical
noise, what does the shield do? A lot of antenna designs make a big
deal of the shield. So I assume it must be a useful addition to the
small loop antenna for some purpose.

I have read that the electric field sensitivity is non-directional, and
therefore interferes with directivity even though the sensitivity is
low. I have no idea if this makes sense when worked out quantitatively.

--
Roger Hayter

rickman wrote:
I just read the wikipedia article on small loop antennas and it seems I
was laboring under a misapprehension. I thought receiving loops were
"magnetic" because they were shielded (this is often stated in various
web pages about constructing such loops). But the wikipedia article on
small loop antennas says the nature of a small loop is to not be very
sensitive to the E field in near field.

So if the shield has little to do with rejecting near field electrical
noise, what does the shield do? A lot of antenna designs make a big
deal of the shield. So I assume it must be a useful addition to the
small loop antenna for some purpose.

The single-turn tuned magnetic loop as used for transmitting is a
different animal than the aperiodic loop of usually a couple of turns
that is used for receive-only applications.

The tuned loop cannot be shielded because of the parasitic capacitance
that would add, it would limit the high end of the tuning range.

Of course a shielded loop also will resonate at some frequency due to
parasitic capacitance.

On 10/14/2015 1:34 PM, rickman wrote:
I just read the wikipedia article on small loop antennas and it seems I
was laboring under a misapprehension. I thought receiving loops were
"magnetic" because they were shielded (this is often stated in various
web pages about constructing such loops). But the wikipedia article on
small loop antennas says the nature of a small loop is to not be very
sensitive to the E field in near field.

So if the shield has little to do with rejecting near field electrical
noise, what does the shield do? A lot of antenna designs make a big
deal of the shield. So I assume it must be a useful addition to the
small loop antenna for some purpose.


I bought a "Pixel" shielded magnetic loop from Pixel. It included a 30db
LNA. It works better than my dipoles for receive on the 40 meter band on
up. I guess I should be clear. I don't have 6 meters, so I am talking
about 40, 20, 17, 15, and 10. The SNR is better than my dipoles on all
these bands. It is significantly worse on 75 and 160.

It was well worth the money. It is probably the best 400 bucks I have
ever spent on ham radio.

I just bought a used FTDX-3000. It has a special coax connector just for
a receiving antenna. I can switch receive antennas on the front of the
radio. A nice feature.

On 10/14/2015 3:23 PM, Jeff Liebermann wrote:
On Wed, 14 Oct 2015 14:34:10 -0400, rickman wrote:

I just read the wikipedia article on small loop antennas and it seems I
was laboring under a misapprehension. I thought receiving loops were
"magnetic" because they were shielded (this is often stated in various
web pages about constructing such loops). But the wikipedia article on
small loop antennas says the nature of a small loop is to not be very
sensitive to the E field in near field.

So if the shield has little to do with rejecting near field electrical
noise, what does the shield do? A lot of antenna designs make a big
deal of the shield. So I assume it must be a useful addition to the
small loop antenna for some purpose.

The shielded loop reduces local noise pickup by eliminating much of
the electric component of that noise in the near field. Since the
ability of a small loop antenna to hear properly is primarily an
exercise in improving the SNR, any reduction in noise levle, without a
corresponding reduction in signal level, is a very good thing. More
detail:
http://electronics.stackexchange.com/questions/70262/what-if-anything-makes-shielded-loop-antennas-so-great-at-rejecting-local-nois

I've built small loops that were not shielded and measure the SNR of
some stable signal, such as WWV. I then wrapped the loop in aluminum
duct tape, leaving a gap to prevent a shorted turn problem, retuned,
and found that the baseline noise level had decreased and the SNR had
improved. It works.

I hope you realize that your experiment is not at all conclusive since
wrapping the duct tape around your loop changes many things other than
just adding a shield. Those other effects may or may not improve any
given loop antenna.

Do you understand the details of how such a shield should work? The
link you provided gives several conflicting opinions on this including
one very detailed post which claims there is little or no suppression of
the E-field, rather it is only the nulls that are useful.

It was finding posts like this that have made me doubt the suppression
of the E-field by the shield.

--

Rick

On Wed, 14 Oct 2015 20:38:23 -0400, rickman wrote:

I hope you realize that your experiment is not at all conclusive since
wrapping the duct tape around your loop changes many things other than
just adding a shield. Those other effects may or may not improve any
given loop antenna.

Yep. However, wrapping did improve the SNR a few dB, which is a sure
sign that I must have done something right.

Do you understand the details of how such a shield should work? The
link you provided gives several conflicting opinions on this including
one very detailed post which claims there is little or no suppression of
the E-field, rather it is only the nulls that are useful.

It gets worse, I just found this link, which says my explanation
doesn't work:
http://www.w8ji.com/magnetic_receiving_loops.htm
What little is mentioned about shielded loops claims that it does not
suppress the E-field and details how skin effect makes it work. I
gotta work through this again to make sure I understand it.

It was finding posts like this that have made me doubt the suppression
of the E-field by the shield.

Yep. The author of the above article definitely agrees with that. It
may take me a while before I agree, but only after I understand how a
shielded loop really works.



--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

In message , Jeff Liebermann
writes
On Wed, 14 Oct 2015 14:34:10 -0400, rickman wrote:

I just read the wikipedia article on small loop antennas and it seems I
was laboring under a misapprehension. I thought receiving loops were
"magnetic" because they were shielded (this is often stated in various
web pages about constructing such loops). But the wikipedia article on
small loop antennas says the nature of a small loop is to not be very
sensitive to the E field in near field.

So if the shield has little to do with rejecting near field electrical
noise, what does the shield do? A lot of antenna designs make a big
deal of the shield. So I assume it must be a useful addition to the
small loop antenna for some purpose.

The shielded loop reduces local noise pickup by eliminating much of
the electric component of that noise in the near field. Since the
ability of a small loop antenna to hear properly is primarily an
exercise in improving the SNR, any reduction in noise levle, without a
corresponding reduction in signal level, is a very good thing. More
detail:
http://electronics.stackexchange.com...-if-anything-m
akes-shielded-loop-antennas-so-great-at-rejecting-local-nois

I've built small loops that were not shielded and measure the SNR of
some stable signal, such as WWV. I then wrapped the loop in aluminum
duct tape, leaving a gap to prevent a shorted turn problem, retuned,
and found that the baseline noise level had decreased and the SNR had
improved. It works.


I've a 5 foot Octagonal loop for MF. The shield is copper water pipe,
with a gap , 7 turns inside plus a coupling winding. It does a good job
eliminating local noise (mostly ASDL hash from the phone lines) compared
with a vertical. However the capacitance between the shield and turns
seems to load it quite a bit meaning I can't get the tuning range I'd
like.

Brian GM4DIJ
--
Brian Howie

On 10/14/2015 02:34 PM, rickman wrote:
I just read the wikipedia article on small loop antennas and it seems I
was laboring under a misapprehension. I thought receiving loops were
"magnetic" because they were shielded (this is often stated in various
web pages about constructing such loops). But the wikipedia article on
small loop antennas says the nature of a small loop is to not be very
sensitive to the E field in near field.

So if the shield has little to do with rejecting near field electrical
noise, what does the shield do? A lot of antenna designs make a big
deal of the shield. So I assume it must be a useful addition to the
small loop antenna for some purpose.

Hello, and that seems to be ham radio jargon. Hams seem to think the
adjectives "magnetic" and "electric" are needed when referring to loop
and dipole antennas, respectively. Textbooks on electromagnetics and
antennas don't use those terms except in the case when discussing
theoretically small radiators, i.e. "magnetic dipoles" and "electric
dipoles".

My hypothesis on the ham terminology is that a loop is viewed as an
inductor. That's OK for close-in (non-radiative) mutual coupling to
some source but when you're several wavelengths away (in the far field)
then the loop (or dipole antenna for that matter) responds to the
electromagnetic field (the electric and magnetic far fields can't be
considered separately). The fact that an axis of either antenna lines
up with the electric or magnetic field vector in the far field is moot.
Does this mean that the loop doesn't have inductance? Of course not
and it plays a role in establishing the feedpoint impedance of the loop
at the operating frequency. Now if folks would just stop using that
word "literally" so damn much...

Sincerely, and 73s from N4GG0,





--
J. B. Wood e-mail: