Reply
 
LinkBack Thread Tools Search this Thread Display Modes
  #11   Report Post  
Old July 10th 17, 11:33 AM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Oct 2013
Posts: 22
Default Magnetic receiving loop theory

On Mon, 10 Jul 2017 10:21:42 +0200, "bilou" wrote:


"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 :-)


Interesting. I hadn't thought of ground effects. I may do some
experimenting. I would love to hear the HF bands without the constant
S9 noise I have now. (The noise is from multiple sources - power
lines, my electronic gadgets, neighbor's electronic gadgets, TVs,
etc.)

  #12   Report Post  
Old July 11th 17, 12:31 AM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Jul 2017
Posts: 1
Default Magnetic receiving loop theory

On 2017-07-09 o 15:08, Pat writes:

understanding regarding electromagnetic waves is you can't have one
without the other.


Yes, every transmitting antenna creates perturbance that spreads around
as electromagnetic (EM) wave with electric and magnetic component but...
In the _near field_, magnetic antenna creates mainly the Magnetic
component of the field and Electric antenna (in ex. dipole) creates
electric field. The same goes to receiving. Shielded magnetic loop is
quite deaf to M component of the EM field.

(Un)fortunately most of the local noise sources create perturbance in E
field. Therefore chances are that magnetic loop will be in some degree
immune to them. The other noise sources may or may not be in the near
field range but the furher away they are the less they add to the total
noise level.

Of course you've read this?
https://en.wikipedia.org/wiki/Near_and_far_field

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



  #13   Report Post  
Old July 11th 17, 02:36 AM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Jun 2007
Posts: 1,296
Default Magnetic receiving loop theory

On Mon, 10 Jul 2017 06:28:08 -0400, Pat wrote:

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


At this time, I'm 25,384 days old[1]. That's long enough to have
forgotten or confused most everything which I had pretended to learn
in skool. I'm perpetually mangling the various fields. So, I decided
to search for some clarification. This is least confusing explanation
I could find:
https://www.physicsforums.com/threads/in-magnetism-what-is-the-difference-between-the-b-and-h-fields.370525/#post-2537765
I think I understand most of it, maybe, or at least some of it:
https://www.physicsforums.com/threads/in-magnetism-what-is-the-difference-between-the-b-and-h-fields.370525/
There are 114 articles in the thread, most of which disagree with each
other. That suggests that not everyone understands the various fields
in quite the same manner.

E and B are the total electric and magnetic fields.
D and H are the free electric and magnetic fields.
P and M are the bound electric and magnetic fields.?
E = D + P (except that for historical reasons E is defined
differently, so we need to multiply it by the permittivity,
and for some reason P is multiplied by minus-one).
B = H + M (except that for the same historical reasons
B is defined like E, so we need to divide it by the
permeability).

At this point, I usually say "I hope this help". However, I think
that "I hope this doesn't hurt too much" might be more appropriate.

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


I'll post something. Right now, I don't see it happening until after
I design and build the one, true, ultimate, and best magnetic loop
antenna. Probably next year.


[1]
http://www.calculator.net/age-calculator.html?today=01%2F10%2F1948&ageat=07%2F10 %2F2017&x=54&y=14

--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
  #14   Report Post  
Old July 11th 17, 12:35 PM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Oct 2013
Posts: 22
Default Magnetic receiving loop theory

On Mon, 10 Jul 2017 18:36:07 -0700, Jeff Liebermann
wrote:

On Mon, 10 Jul 2017 06:28:08 -0400, Pat wrote:

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


At this time, I'm 25,384 days old[1]. That's long enough to have
forgotten or confused most everything which I had pretended to learn
in skool.

I am 1.088 kilodays younger than you. Not much in the overall scheme
of things. (Thats only 78 fortnights. I had a professor in school
who would measure velocity in furlongs per fortnight.)

I'm perpetually mangling the various fields. So, I decided
to search for some clarification. This is least confusing explanation
I could find:
https://www.physicsforums.com/threads/in-magnetism-what-is-the-difference-between-the-b-and-h-fields.370525/#post-2537765
I think I understand most of it, maybe, or at least some of it:
https://www.physicsforums.com/threads/in-magnetism-what-is-the-difference-between-the-b-and-h-fields.370525/
There are 114 articles in the thread, most of which disagree with each
other. That suggests that not everyone understands the various fields
in quite the same manner.

E and B are the total electric and magnetic fields.
D and H are the free electric and magnetic fields.
P and M are the bound electric and magnetic fields.?
E = D + P (except that for historical reasons E is defined
differently, so we need to multiply it by the permittivity,
and for some reason P is multiplied by minus-one).
B = H + M (except that for the same historical reasons
B is defined like E, so we need to divide it by the
permeability).

At this point, I usually say "I hope this help". However, I think
that "I hope this doesn't hurt too much" might be more appropriate.

True, but I appeciate your responses anyway.


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


I'll post something. Right now, I don't see it happening until after
I design and build the one, true, ultimate, and best magnetic loop
antenna. Probably next year.


Sounds good. As an aside, I just watched a youtube video of someone
trying out one of these magnetic loop antennas. With his particular
set of circumstances, it reduced the noise floor on 80 meters
significantly.



[1]
http://www.calculator.net/age-calculator.html?today=01%2F10%2F1948&ageat=07%2F10 %2F2017&x=54&y=14

  #15   Report Post  
Old July 11th 17, 04:50 PM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Nov 2012
Posts: 964
Default Magnetic receiving loop theory

Bartolomeo wrote on 7/10/2017 7:31 PM:
On 2017-07-09 o 15:08, Pat writes:

understanding regarding electromagnetic waves is you can't have one
without the other.


Yes, every transmitting antenna creates perturbance that spreads around as
electromagnetic (EM) wave with electric and magnetic component but...
In the _near field_, magnetic antenna creates mainly the Magnetic component
of the field and Electric antenna (in ex. dipole) creates electric field.
The same goes to receiving. Shielded magnetic loop is quite deaf to M
component of the EM field.


I think you mean the shielded loop is "deaf" to the electric field.


(Un)fortunately most of the local noise sources create perturbance in E
field. Therefore chances are that magnetic loop will be in some degree
immune to them. The other noise sources may or may not be in the near field
range but the furher away they are the less they add to the total noise level.

Of course you've read this?
https://en.wikipedia.org/wiki/Near_and_far_field

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?


In the near field the antenna itself will generate a field (either magnetic
or electric depending on the design). This field falls off rapidly with
distance. The antenna also generates an EM wave which radiates and is
dominant at distance (far field).


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?


The ad copy isn't saying there is no magnetic field from the noise source,
but most noise sources are close enough to be near field with a much
stronger E field than the magnetic component. This is *very* much a
near/far field thing.

--

Rick C


  #16   Report Post  
Old July 11th 17, 06:11 PM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Jun 2007
Posts: 1,296
Default Magnetic receiving loop theory

On Tue, 11 Jul 2017 07:35:35 -0400, Pat wrote:

Sounds good. As an aside, I just watched a youtube video of someone
trying out one of these magnetic loop antennas. With his particular
set of circumstances, it reduced the noise floor on 80 meters
significantly.


Compared to what other antenna? He probably reduced the received
signal strength by the same amount leaving the SNR unchanged. That's
why I included a link to the PA0RDT mini-antenna, which explains why
such a small antenna works:
http://dl1dbc.net/SAQ/miniwhip.html
A loop works much the same way. One way to benefit from a small
antenna is to do something to improve the SNR, which the loop does by
narrowing the RX bandwidth, as I explained in a previous rant.


--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
  #17   Report Post  
Old July 11th 17, 10:39 PM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Oct 2013
Posts: 22
Default Magnetic receiving loop theory

On Tue, 11 Jul 2017 10:11:11 -0700, Jeff Liebermann
wrote:

On Tue, 11 Jul 2017 07:35:35 -0400, Pat wrote:

Sounds good. As an aside, I just watched a youtube video of someone
trying out one of these magnetic loop antennas. With his particular
set of circumstances, it reduced the noise floor on 80 meters
significantly.


Compared to what other antenna? He probably reduced the received
signal strength by the same amount leaving the SNR unchanged.


Not really. Both signal and noise are reduced, but the SNR must be
better because you can hear an SSB conversation with the loop that is
not there with his sloper. Search youtube for MFJ-1886 and N9BC.

Of course, there is no mention of antenna patterns. Maybe the QSO he
was listening to is in a null or his other antenna. Not enough info
to really tell.
  #18   Report Post  
Old July 12th 17, 12:37 AM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Jun 2007
Posts: 1,296
Default Magnetic receiving loop theory

On Tue, 11 Jul 2017 17:39:37 -0400, Pat wrote:

On Tue, 11 Jul 2017 10:11:11 -0700, Jeff Liebermann
wrote:

On Tue, 11 Jul 2017 07:35:35 -0400, Pat wrote:

Sounds good. As an aside, I just watched a youtube video of someone
trying out one of these magnetic loop antennas. With his particular
set of circumstances, it reduced the noise floor on 80 meters
significantly.


Compared to what other antenna? He probably reduced the received
signal strength by the same amount leaving the SNR unchanged.


Not really. Both signal and noise are reduced, but the SNR must be
better because you can hear an SSB conversation with the loop that is
not there with his sloper. Search youtube for MFJ-1886 and N9BC.


https://www.youtube.com/watch?v=ECDklLp2FOk (2:56)

Nice of him to crop off most of the spectrum analyzer display at the
top of the screen so that I couldn't see the SNR changes. Also, he
didn't indicate which antenna he was testing in the first video. He
mostly fixed those problems in the 2nd video.

https://www.youtube.com/watch?v=_-GPS1Kqfec (4:13)

Of course, there is no mention of antenna patterns. Maybe the QSO he
was listening to is in a null or his other antenna. Not enough info
to really tell.


Nice of him to compress the signal strength in the spectrum display so
that it's difficult to compare SNR between antennas. There might be a
difference between antennas, but my guess(tm) is that he has the AGC
turned off in his SDR receiver. If AGC were on, the base line noise
level would be about the same for both receivers, which would raise
some questions as to whether there really was a difference. With the
AGC turned off, the higher gain of the sloper antenna will show more
baseline noise, which is what your seeing on the spectrum display.

However, if I freeze the 2nd video for each antenna, and just look at
the RELATIVE levels of the signals to the base line noise levels for
each antenna, I think you'll see that they're fairly close[1]. That's
the SNR which is what's important, and not the absolute levels of the
noise and signal.

Please note that the MJF-1886 is an amplified broadband untuned loop
which means it has a rather low Q. It obtains no benefits from the
narrowing the receive bandwidth as would be found in a high-Q transmit
loop. Strong signals anywhere in the 1-30MHz amplifier bandwidth
will create intermod products which might land where you're listening.
Incidentally, if you disconnect the MJF-1886 amplifier, and use it
like the PA0RDT miniwhip, my guess(tm) is that the loop and the
miniwhip will work almost identically.

Ok, Methinks I see the problem. The MFJ-1886 looks too good. With
antennas, the uglier it looks, the better it works. Nice looking
antennas just don't seem to work well.


[1] I have a customer on the phone who wants my attention so this
will need to wait.

--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
  #19   Report Post  
Old July 17th 17, 11:52 AM posted to rec.radio.amateur.antenna
external usenet poster
 
First recorded activity by RadioBanter: Sep 2013
Posts: 47
Default Magnetic receiving loop theory

On 07/09/2017 09:08 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?

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



Hello, and I've commented on this previously. Somehow hams have gotten
into the habit of thinking that loop antennas have to be further
described by the modifier "magnetic". Now, a loop antenna can certainly
be close-in coupled (non far-field) magnetically to a radio frequency
source but in that case the antenna is functioning more as
mutually-coupled inductor, which isn't the same as being subjected to an
incident electromagnetic wave in the far-field (at least several
wavelengths from the transmitting antenna).

Bottom line: We have loop antennas (further describable as shielded,
unshielded, multi-turn, tunable, etc). In all fairness, I think what
hams really mean by "magnetic" is an electrically-small loop antenna,
since as the loop diameter gets smaller and smaller we approach the
textbook theoretical "magnetic dipole". By contrast, hams usually refer
to a "dipole antenna" not "electric dipole antenna". The term
"magnetic" is unnecessary and readily misleading. Sincerely, and 73s
from N4GGO,

--
J. B. Wood e-mail:


Reply
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Re Magnetic Loop !!! Lee Antenna 53 October 11th 05 08:41 AM
Magnetic Loop !!! Lee Antenna 32 October 2nd 05 11:53 PM
MFJ 1786 magnetic loop vandmand Antenna 13 July 11th 04 03:12 AM
Magnetic loop for sale Peter Wilson Equipment 0 July 27th 03 11:51 PM
Magnetic loop for sale Peter Wilson Equipment 0 July 27th 03 11:51 PM


All times are GMT +1. The time now is 04:11 PM.

Powered by vBulletin® Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.
Copyright ©2004-2017 RadioBanter.
The comments are property of their posters.
 

About Us

"It's about Radio"

 

Copyright © 2017