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

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

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

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

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

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.

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