On Thu, 31 Jan 2019 14:39:28 EST, "J.B. Wood"
wrote:

Hello, all, and in addition to my recent posting on how the subject term
came into usage (primarily by hams) there is another possible
explanation: If we make a loop antenna electrically smaller and smaller
(relative to a wavelength), we approach what the EM/antenna textbooks
refer to as a "magnetic dipole". This is a mathematical construct
(exists in the limit) but is useful for practical calculations as we
approach such a limit. The E-field equivalent is an "electric dipole"
I think this was what Jeff Liebermann alluded to in his recent post.
Sincerely, and 73s from N4GGO,

Nope. You give me too much credit. I'm not much of a mathemagician
and much prefer easy to visualize mechanical constructs to
complexicated mathematical equations. I'm also recovering from a
"minor" medical procedure earlier today and therefore need to keep
this brief (if possible).

"Magnetic Loop Antennas Receiving"
https://www.w8ji.com/magnetic_receiving_loops.htm
"In the electrically small antennas, such as the loop
antenna and the sort dipole or vertical described above,
the dominant field descriptions only apply within 1/10
distance."

"It is the energy storage or reactive induction field
response within 1/10 wavelength distance from the antenna
that gives small "magnetic loop" and "electric dipole"
antennas their names."

"Radiation in the Near Zone of a Small Loop Antenna"
http://www.physics.princeton.edu/~mcdonald/examples/smallloop.pdf
"Within one wavelength of the current loop, the magnetic
field looks essentially the same as the dipole field pattern
due to a steady current loop, multiplied by cos wt."
After that, there's far too much math for me to understand.

From what I understand, when the antenna is quite small relative to
the operating wavelength, and the measurement distance is really close
compared to the operating wavelength, the antenna acts more like a
inductively coupled transformer. I think that might be one source of
the "magnetic" term.

Another possible source is the assumption that in a shielded loop
antenna, the shield blocks the E (electric) field, while the H
(magnetic) field is unaffected. The assumption is that if the E field
is blocked, what must be radiated is only the H field. There's a
section in the above article that discusses the purpose and effects of
shielding, which contradict the assumption.

Anyway, those are my best guesses as to the origin of the term
magnetic loop.

I could use Google to search for the earliest use of the term magnetic
loop. My guess(tm) is came from a ham radio application or
publication. However, that will need to wait as I'm recovering today
from a minor medical procedure and need to be horizontally polarized
in bed for most of the next day.



--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558