View Single Post
#8
July 10th 17, 03:50 AM posted to rec.radio.amateur.antenna
 John S external usenet poster First recorded activity by RadioBanter: May 2011 Posts: 539
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.

Sorry.
That should be "No, one component cannot exist without the other".
"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:
and unshielded loop antennas:

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
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.

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.