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Old July 9th 17, 10:29 PM posted to rec.radio.amateur.antenna
Jeff Liebermann[_2_] Jeff Liebermann[_2_] is offline
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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