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Old July 9th 17, 02:08 PM posted to rec.radio.amateur.antenna
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Default Magnetic receiving loop theory


I'm looking at an ad in QST regarding the MFJ Low-Noise Receiving
Loop. Since I have a lot of noise here, I am very interested in this
topic. However, I have having trouble understanding the theory. My
understanding regarding electromagnetic waves is you can't have one
without the other. RF propogates through space my having the moving
electric field create a moving magnetic field which then creates a new
electric field, etc, etc. How can one exist without the other?

Here's a quote from the ad, "The MFJ-1886 drastically reduces noise
and interference by receiving the magnetic field and rejecting the
electric field". How can a varying electric field from a noise source
not also create a corresponding magnetic field? Is this a near-field
/ far-field thing?

Pat



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Old July 9th 17, 03:29 PM posted to rec.radio.amateur.antenna
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Posts: 539
Default Magnetic receiving loop theory

On 7/9/2017 8:08 AM, Pat wrote:

I'm looking at an ad in QST regarding the MFJ Low-Noise Receiving
Loop. Since I have a lot of noise here, I am very interested in this
topic. However, I have having trouble understanding the theory. My
understanding regarding electromagnetic waves is you can't have one
without the other. RF propogates through space my having the moving
electric field create a moving magnetic field which then creates a new
electric field, etc, etc. How can one exist without the other?


You are correct, they can't.

Here's a quote from the ad, "The MFJ-1886 drastically reduces noise
and interference by receiving the magnetic field and rejecting the
electric field". How can a varying electric field from a noise source
not also create a corresponding magnetic field? Is this a near-field
/ far-field thing?


Pat


I seem to recall that within a couple of wavelengths, there is a
difference in the magnetic and electric fields and the magnetic field
diminishes rapidly beyond that but remains associated with the electric
field as you say.

I have read trusted authors who say that the real value in a small loop
is the ability to null the incoming interference.

MFJ is lacking in technical knowledge. They said that my MFJ analyzer
would measure impedance (Z). It does not measure the imaginary part. It
measures absolute value of impedance (|Z|).

Cheers,
John




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Old July 9th 17, 06:53 PM posted to rec.radio.amateur.antenna
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Posts: 1,296
Default Magnetic receiving loop theory

On Sun, 09 Jul 2017 09:08:11 -0400, Pat wrote:

I'm looking at an ad in QST regarding the MFJ Low-Noise Receiving
Loop. Since I have a lot of noise here, I am very interested in this
topic. However, I have having trouble understanding the theory. My
understanding regarding electromagnetic waves is you can't have one
without the other. RF propogates through space my having the moving
electric field create a moving magnetic field which then creates a new
electric field, etc, etc. How can one exist without the other?


No.

Here's a quote from the ad, "The MFJ-1886 drastically reduces noise
and interference by receiving the magnetic field and rejecting the
electric field".


How can a varying electric field from a noise source
not also create a corresponding magnetic field?


The transmitter generates both. You can reduce the sensitivity of a
receiving loop to the electric E field by shielding, leaving only the
magnetic H component. Examples of shielded loop antennas:
https://www.google.com/search?q=shielded+loop+antenna&tbm=isch
and unshielded loop antennas:
https://www.google.com/search?q=unshielded+loop+antenna&tbm=isch

Is this a near-field / far-field thing?


No.

I've been collecting articles on magnetic loops, tuners, theory, and
such in an apparently futile attempt to find the time to design
something. Maybe you'll find these articles useful. Most have
references and links at the end to other magnetic loop articles:
https://sidstation.loudet.org/antenna-theory-en.xhtml
https://www.nonstopsystems.com/radio/pdf-ant/article-antenna-mag-loop-2.pdf
http://www.w0btu.com/magnetic_loops.html
http://owenduffy.net/blog/?cat=31
http://www.aa5tb.com/aa5tb_loop_v1.22a.xls
https://frrl.wordpress.com/2009/03/21/limited-space-antennas-the-small-transmitting-loop-antenna/

There's some discussion of E and H fields he
http://owenduffy.net/antenna/PA0RDT-MiniWhip/
which you might find applicable.


--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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Old July 9th 17, 07:13 PM posted to rec.radio.amateur.antenna
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First recorded activity by RadioBanter: May 2011
Posts: 539
Default Magnetic receiving loop theory

On 7/9/2017 12:53 PM, Jeff Liebermann wrote:
On Sun, 09 Jul 2017 09:08:11 -0400, Pat wrote:

I'm looking at an ad in QST regarding the MFJ Low-Noise Receiving
Loop. Since I have a lot of noise here, I am very interested in this
topic. However, I have having trouble understanding the theory. My
understanding regarding electromagnetic waves is you can't have one
without the other. RF propogates through space my having the moving
electric field create a moving magnetic field which then creates a new
electric field, etc, etc. How can one exist without the other?


No.


No what? Please expand.

Here's a quote from the ad, "The MFJ-1886 drastically reduces noise
and interference by receiving the magnetic field and rejecting the
electric field".


How can a varying electric field from a noise source
not also create a corresponding magnetic field?


The transmitter generates both. You can reduce the sensitivity of a
receiving loop to the electric E field by shielding, leaving only the
magnetic H component. Examples of shielded loop antennas:
https://www.google.com/search?q=shielded+loop+antenna&tbm=isch
and unshielded loop antennas:
https://www.google.com/search?q=unshielded+loop+antenna&tbm=isch

Is this a near-field / far-field thing?



No.


So near-field is not different from far field? The textbooks disagree
with you.


I've been collecting articles on magnetic loops, tuners, theory, and
such in an apparently futile attempt to find the time to design
something. Maybe you'll find these articles useful. Most have
references and links at the end to other magnetic loop articles:
https://sidstation.loudet.org/antenna-theory-en.xhtml
https://www.nonstopsystems.com/radio/pdf-ant/article-antenna-mag-loop-2.pdf
http://www.w0btu.com/magnetic_loops.html
http://owenduffy.net/blog/?cat=31
http://www.aa5tb.com/aa5tb_loop_v1.22a.xls
https://frrl.wordpress.com/2009/03/21/limited-space-antennas-the-small-transmitting-loop-antenna/

There's some discussion of E and H fields he
http://owenduffy.net/antenna/PA0RDT-MiniWhip/
which you might find applicable.


Did you read any of the links you posted? You must believe that anything
posted on the Internet is true. Owen Duffy is an exception but you
posted his radiation pattern comparing a vertical and a loop.

You must have a http look-up fetish.


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Old July 9th 17, 07:16 PM posted to rec.radio.amateur.antenna
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Posts: 22
Default Magnetic receiving loop theory

On Sun, 09 Jul 2017 10:53:33 -0700, Jeff Liebermann
wrote:


How can a varying electric field from a noise source
not also create a corresponding magnetic field?


The transmitter generates both. You can reduce the sensitivity of a
receiving loop to the electric E field by shielding, leaving only the
magnetic H component.


I understand making antennas that are sensitive to only the H field.
My question is why would I want to? If the noise has both components,
how does an H field only antenna reduce unwanted noise?

Pat


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Old July 9th 17, 09:53 PM posted to rec.radio.amateur.antenna
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First recorded activity by RadioBanter: Jun 2007
Posts: 1,296
Default Magnetic receiving loop theory

On Sun, 9 Jul 2017 13:13:00 -0500, John S wrote:

On 7/9/2017 12:53 PM, Jeff Liebermann wrote:
On Sun, 09 Jul 2017 09:08:11 -0400, Pat wrote:

I'm looking at an ad in QST regarding the MFJ Low-Noise Receiving
Loop. Since I have a lot of noise here, I am very interested in this
topic. However, I have having trouble understanding the theory. My
understanding regarding electromagnetic waves is you can't have one
without the other. RF propogates through space my having the moving
electric field create a moving magnetic field which then creates a new
electric field, etc, etc. How can one exist without the other?


No.


No what? Please expand.


Sorry.
That should be "No, one component cannot exist without the other".
https://en.wikipedia.org/wiki/Electromagnetic_radiation#Wave_model
"The electric and magnetic parts of the field stand in a fixed
ratio of strengths in order to satisfy the two Maxwell equations
that specify how one is produced from the other. These E and B
fields are also in phase, with both reaching maxima and minima
at the same points in space (see illustrations)".

I must confess that I really don't understand Maxell's equations and
therefore cannot offer more detail.

How can a varying electric field from a noise source
not also create a corresponding magnetic field?


The transmitter generates both. You can reduce the sensitivity of a
receiving loop to the electric E field by shielding, leaving only the
magnetic H component. Examples of shielded loop antennas:
https://www.google.com/search?q=shielded+loop+antenna&tbm=isch
and unshielded loop antennas:
https://www.google.com/search?q=unshielded+loop+antenna&tbm=isch

Is this a near-field / far-field thing?


No.


So near-field is not different from far field? The textbooks disagree
with you.


In order for RF work, there has to be both E and B fields. However,
the ratio between the two components is not constant and can vary
depending on the source and distances. This described the
characteristics and differences far better than I could:
https://en.wikipedia.org/wiki/Near_and_far_field
The main characteristic is that in the far field, the signal decrease
at the usual inverse square law. In the near field, it's much more
rapid at what I guess could be called the inverse cubed law.

I've been collecting articles on magnetic loops, tuners, theory, and
such in an apparently futile attempt to find the time to design
something. Maybe you'll find these articles useful. Most have
references and links at the end to other magnetic loop articles:
https://sidstation.loudet.org/antenna-theory-en.xhtml
https://www.nonstopsystems.com/radio/pdf-ant/article-antenna-mag-loop-2.pdf
http://www.w0btu.com/magnetic_loops.html
http://owenduffy.net/blog/?cat=31
http://www.aa5tb.com/aa5tb_loop_v1.22a.xls
https://frrl.wordpress.com/2009/03/21/limited-space-antennas-the-small-transmitting-loop-antenna/

There's some discussion of E and H fields he
http://owenduffy.net/antenna/PA0RDT-MiniWhip/
which you might find applicable.


Did you read any of the links you posted?


Yes. At some point, I read most everything, but can't claim that I
remember, understood, or agree with everything that I've read.

You must believe that anything posted on the Internet is true.


Only if I write it.

Owen Duffy is an exception but you
posted his radiation pattern comparing a vertical and a loop.


Read his titles. STL is "Small Transmitting Loops".
Note the word "loop" as in "magnetic loop".

You must have a http look-up fetish.


I try never to directly attack the person posting a question or
answer, but you deserve an exception. Did you do anything useful here
to answer the original question? If I was wrong, did you do anything
to correct my errors? If you make an unsubstantiated claim, at a
minimum provide a URL for where you stole your assertion. Did you
write anything worth reading? Methinks not.


--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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Old July 9th 17, 10:29 PM posted to rec.radio.amateur.antenna
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Posts: 1,296
Default Magnetic receiving loop theory

On Sun, 09 Jul 2017 14:16:20 -0400, Pat wrote:

On Sun, 09 Jul 2017 10:53:33 -0700, Jeff Liebermann
wrote:


How can a varying electric field from a noise source
not also create a corresponding magnetic field?


The transmitter generates both. You can reduce the sensitivity of a
receiving loop to the electric E field by shielding, leaving only the
magnetic H component.


I understand making antennas that are sensitive to only the H field.
My question is why would I want to? If the noise has both components,
how does an H field only antenna reduce unwanted noise?
Pat


That should be the E and B field, not H field. My mistake.

I wish that I had a supportable answer to this question. There are
quite a few opinions on the topic. Here's one that says that all the
shield does is make it easier to build a balanced antenna:
https://electronics.stackexchange.com/questions/70262/what-if-anything-makes-shielded-loop-antennas-so-great-at-rejecting-local-nois
It also states that it is impossible to block either the E or B
fields, which contradicts what I wrote. To be uncharacteristically
honest, I don't know exactly what the shield does and how it works. I
do know that in building LF (30-300KHz) loop type direction finders,
the noise levels with a shielded loop were far lower than with an
unshielded loop. How much? I don't recall as it was a long time ago,
but it was quite noticeable. Whether this also applies at HF
frequencies is also unknown.

I have my own simplistic understanding of how a magnetic loop
operates. It works because the Q of the loop is very high. In some
cases, so high that the operating bandwidth of the loop is narrower
than modulation bandwidth. For example, if I use the default numbers
in the AA5TB loop calculator spreadsheet:
http://www.aa5tb.com/loop.html
http://www.aa5tb.com/aa5tb_loop_v1.22a.xls
it shows a Q=1746 at 7MHz.
BW = freq/Q = 7MHz/1746 = 4.1KHz
That's the width of about 2 SSB signals at 2KHz modulation bandwidth
each, which is barely acceptable. I think you can see that if I play
with the dimensions, which will increase the Q, it won't take much to
end up with an antenna that's narrower than the signal it's trying to
receive. Another problem with a high Q antenna is that it has to be
constantly tuned to compensate for changes in tuning caused by
mechanical vibrations, changes in nearby metal objects, rotation, etc,
as well a slight changes in operating frequency.

That begs the question, what does such high Q do for you? Well, it
dramatically reduces interference from other stations on nearby
frequencies. It produces a very efficient antenna. I improves
receiver sensitivity by removing quite a bit of noise, EMI, and RFI
that might sneak in through the receiver bandpass, through various
possible mixes (usually with stations on adjacent frequencies),
through receiver images, and through static buildup on the antenna.
The price you pay is having to use VERY rigid construction, expensive
(vacuum or butterfly) tuning capacitors, silver solder, a potentially
complicated automatic antenna tuner, and having to retune every time
you change ANYTHING while operating. Is it worth it? I think so.

I may soon see how well a magnetic loop really works. A friend
recently installed a 55ft tower and a collection of HF yagi antennas.
I bet him that I could build a magnetic loop antenna that would hear
the same stations as his monster yagi, but near ground level and much
smaller size and cost. The bet is for lunch at the local coffee shop.
This is going out on a limb, but I believe that it can be done
receive. Unfortunately, because of the narrow antenna bandwidth, I
can't use WSPR and PSK Reporter to compare gain and coverage.

Suggestion: Use the AA5TB spreadsheet, 4NEC2, etc to design
something. Or, just follow someone's construction instructions. Go
cheap initially so that you can see how it should be done. Improve
the design as you go along.

--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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Old July 10th 17, 03:50 AM posted to rec.radio.amateur.antenna
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First recorded activity by RadioBanter: May 2011
Posts: 539
Default Magnetic receiving loop theory

On 7/9/2017 3:53 PM, Jeff Liebermann wrote:
On Sun, 9 Jul 2017 13:13:00 -0500, John S wrote:

On 7/9/2017 12:53 PM, Jeff Liebermann wrote:
On Sun, 09 Jul 2017 09:08:11 -0400, Pat wrote:

I'm looking at an ad in QST regarding the MFJ Low-Noise Receiving
Loop. Since I have a lot of noise here, I am very interested in this
topic. However, I have having trouble understanding the theory. My
understanding regarding electromagnetic waves is you can't have one
without the other. RF propogates through space my having the moving
electric field create a moving magnetic field which then creates a new
electric field, etc, etc. How can one exist without the other?

No.


No what? Please expand.


Sorry.
That should be "No, one component cannot exist without the other".
https://en.wikipedia.org/wiki/Electromagnetic_radiation#Wave_model
"The electric and magnetic parts of the field stand in a fixed
ratio of strengths in order to satisfy the two Maxwell equations
that specify how one is produced from the other. These E and B
fields are also in phase, with both reaching maxima and minima
at the same points in space (see illustrations)".

I must confess that I really don't understand Maxell's equations and
therefore cannot offer more detail.

How can a varying electric field from a noise source
not also create a corresponding magnetic field?

The transmitter generates both. You can reduce the sensitivity of a
receiving loop to the electric E field by shielding, leaving only the
magnetic H component. Examples of shielded loop antennas:
https://www.google.com/search?q=shielded+loop+antenna&tbm=isch
and unshielded loop antennas:
https://www.google.com/search?q=unshielded+loop+antenna&tbm=isch

Is this a near-field / far-field thing?


No.


So near-field is not different from far field? The textbooks disagree
with you.


In order for RF work, there has to be both E and B fields. However,
the ratio between the two components is not constant and can vary
depending on the source and distances. This described the
characteristics and differences far better than I could:
https://en.wikipedia.org/wiki/Near_and_far_field
The main characteristic is that in the far field, the signal decrease
at the usual inverse square law. In the near field, it's much more
rapid at what I guess could be called the inverse cubed law.

I've been collecting articles on magnetic loops, tuners, theory, and
such in an apparently futile attempt to find the time to design
something. Maybe you'll find these articles useful. Most have
references and links at the end to other magnetic loop articles:
https://sidstation.loudet.org/antenna-theory-en.xhtml
https://www.nonstopsystems.com/radio/pdf-ant/article-antenna-mag-loop-2.pdf
http://www.w0btu.com/magnetic_loops.html
http://owenduffy.net/blog/?cat=31
http://www.aa5tb.com/aa5tb_loop_v1.22a.xls
https://frrl.wordpress.com/2009/03/21/limited-space-antennas-the-small-transmitting-loop-antenna/

There's some discussion of E and H fields he
http://owenduffy.net/antenna/PA0RDT-MiniWhip/
which you might find applicable.


Did you read any of the links you posted?


Yes. At some point, I read most everything, but can't claim that I
remember, understood, or agree with everything that I've read.

You must believe that anything posted on the Internet is true.


Only if I write it.

Owen Duffy is an exception but you
posted his radiation pattern comparing a vertical and a loop.


Read his titles. STL is "Small Transmitting Loops".
Note the word "loop" as in "magnetic loop".

You must have a http look-up fetish.


I try never to directly attack the person posting a question or
answer, but you deserve an exception. Did you do anything useful here
to answer the original question? If I was wrong, did you do anything
to correct my errors? If you make an unsubstantiated claim, at a
minimum provide a URL for where you stole your assertion. Did you
write anything worth reading? Methinks not.



Pot/kettle.
  #9   Report Post  
Old July 10th 17, 09:21 AM posted to rec.radio.amateur.antenna
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First recorded activity by RadioBanter: Jun 2012
Posts: 39
Default Magnetic receiving loop theory


"Pat" wrote in message
...

Is this a near-field
/ far-field thing?

Hi
Not really
But most statements about magnétic loops are false because
the ground is much too close to the antenna in terms of wavelenght.
And average ground has little magnetic effect and big electric one.
Making a VHF/FM magnetic loop is a good idea to start
understanding how it really works.
It is very cheap and small too :-)



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Old July 10th 17, 11:28 AM posted to rec.radio.amateur.antenna
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First recorded activity by RadioBanter: Oct 2013
Posts: 22
Default Magnetic receiving loop theory

On Sun, 09 Jul 2017 14:29:18 -0700, Jeff Liebermann
wrote:

On Sun, 09 Jul 2017 14:16:20 -0400, Pat wrote:

On Sun, 09 Jul 2017 10:53:33 -0700, Jeff Liebermann
wrote:


How can a varying electric field from a noise source
not also create a corresponding magnetic field?

The transmitter generates both. You can reduce the sensitivity of a
receiving loop to the electric E field by shielding, leaving only the
magnetic H component.


I understand making antennas that are sensitive to only the H field.
My question is why would I want to? If the noise has both components,
how does an H field only antenna reduce unwanted noise?
Pat


That should be the E and B field, not H field. My mistake.


E and H are fine. I think it depends on which books you are reading
or maybe how old you are? I remember E and H from school (a long time
ago).

snip

I may soon see how well a magnetic loop really works. A friend
recently installed a 55ft tower and a collection of HF yagi antennas.
I bet him that I could build a magnetic loop antenna that would hear
the same stations as his monster yagi, but near ground level and much
smaller size and cost. The bet is for lunch at the local coffee shop.
This is going out on a limb, but I believe that it can be done
receive. Unfortunately, because of the narrow antenna bandwidth, I
can't use WSPR and PSK Reporter to compare gain and coverage.


I look forward to hearing the results. Sounds like a great
experiment.

Thanks for your responses.
Pat



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