On 8/24/2016 12:03 AM, Jeff Liebermann wrote:
On Tue, 23 Aug 2016 15:57:27 -0400, rickman wrote:

I've been told that the term "magnetic" loop is the same as "small"
loop.

Dunno. As I understand it, a magnetic loop is really a "shielded
electrostatic loop", where E-field operation is blocked by the shield.
It would seem that removing the E-field, and leaving the H-field,
would make it a "magnetic loop":
https://www.google.com/search?q=shielded+magnetic+loop+antenna&tbm=isch
At some point, some clever person decided to do away with the E-field
shield and tolerate the increased noise pickup, but still called it a
"magnetic loop". I don't know if this is really true, but it seems
possible.

I've seen the shielded receiving loop antennas referred to as magnetic
loops, but I was talking about transmitting loops.


It refers to the facts that the near field of a small loop is
mostly magnetic ( 1/10 lamda) and that they respond to the magnetic
component of the EM wave. I'm not sure how that matters in real world
use though as all antenna transmit both E and M in the far field.

A small loop is different. It's where the circumference of the loop
is sufficiently small, that the current through the loop is
essentially constant at all points around the circumference. This
results in something that operates like a dipole, but with the E and H
fields interchanged.

Interchanged because the constant current of the loop creates a
significant magnetic field but not so much of an electric field not
unlike a transformer.


Real world? Well, we had some kind of discussion a few years ago in
S.E.D. about WWVB polarization. I ran this study of how a loopstick
antenna in a commodity WWVB receiver responds to different
orientations:
http://802.11junk.com/jeffl/WWVB%20test/

If you look at the WWVB antenna construction, it looks like a really
big dipole:
http://802.11junk.com/jeffl/WWVB%20test/WWVB-antenna-lowered.jpg
Yet, the signal is vertically polarized:
https://softsolder.com/2010/01/02/wwvb-groundwave-signal-is-vertically-polarized/

It's not a dipole, it's a monopole. The part you see is the top loading
capacitor to improve the efficiency.


Ok, that seems counter-intuitive, so it might be useful to prove it
experimentally:
End of the loopstick pointed at Denver (lousy signal):
http://802.11junk.com/jeffl/WWVB%20test/end-pointed-at-WWVB.jpg
Loopstick perpendicular to Denver and oriented up/down (lousy signal):
http://802.11junk.com/jeffl/WWVB%20test/loopstick-vertical.jpg
Loopstick perpendicular to Denver and oriented left/right (good
signal):
http://802.11junk.com/jeffl/WWVB%20test/loopstick-perpendicular-to-WWVB.jpg
Yep, it's vertically polarized.

--

Rick C