On 8/24/2016 6:43 AM, J.B. Wood wrote:
On 08/23/2016 01:14 PM, rickman wrote:
On 8/23/2016 6:22 AM, J.B. Wood wrote:
On 08/20/2016 04:39 PM, rickman wrote:
I've been studying loop antennas for some time now and I don't recall a
mention of polarization. I would think that a loop antenna mounted
vertically would provide a vertically polarized signal. Is that right?


Finally, someone on the ng said "loop antennas". You can't be a ham
because you didn't say "magnetic loop" ;-). Sincerely, and 73s from
N4GGO,

Not sure what you mean. You are aware that magnetic loops and loops are
not the same thing. Magnetic loops are a subset of loop antennas.


Sorry, guys but it ain't so. It's either a loop (shielded or
unshielded) or something else. This "magnetic" stuff appears to have
originated with hams. A receiving antenna (be it a loop or something
else) in the far (radiated) field of a transmitter samples an incident
electromagnetic (EM) wave. That EM wave has a magnetic and electric
component but you can't have one without the other. J.C. Maxwell (and
others) says so. Anyone, ham or other, who claims that an antenna in
the far (several wavelengths from the transmitter) field "receives" (or
favors) an E-field or an H-field is demonstrating a lack of
understanding of basic electromagnetic theory.

Now, consider two loops, one transmitting and one receiving. If the
receiving loop is in the near field of the radiating loop then it can be
magnetically coupled. In this instance the loop behaves more like a
mutually coupled inductor than an antenna. Perhaps this is where the
"magnetic" loop idea had its genesis. (Just like the immobilizer system
in your motor vehicle that has a loop embedded around the ignition
switch and which couples to the loop in the capsule inside your
transponder key.)

Textbooks on EM and antenna theory do talk about "magnetic" and
"electric" dipoles as theoretical constructs but that's another
discussion. Sincerely, and 73s from N4GGO,

Perhaps you can explain what the shield does on a receiving loop
antenna? Your explanation clearly says an antenna can be magnetic in
the near field. That is what the term means for receiving antennas. At
lower frequencies much interference is in the near field and is electric
rather than magnetic I am told. Think 100 kHz and household appliances.

The term "magnetic" is usually used in context of a transmitting antenna

--

Rick C

On 8/24/2016 9:29 AM, Richard Fry wrote:
J.B. Wood clip: " ... Anyone, ham or other, who claims that an antenna in
the far (several wavelengths from the transmitter) field "receives" (or
favors) an E-field or an H-field is demonstrating a lack of
understanding of basic electromagnetic theory. ..."
_____________

For far-field conditions, it is a given that the E field and the H field of an e-m wave are orthogonal to each other. Neither field can exist without the other.

A simple experiment will illustrate that a single antenna can favor one field but not other, even though that other field exists.

AM broadcast stations transmit using vertical polarization (polarization is defined as the physical orientation of the E-field vectors with respect to the horizontal plane). Vertical polarization maximizes their groundwave coverage areas.

A conventional AM broadcast band receiver (other than in an automobile) uses a loopstick antenna consisting of a close-wound loop of wire wound along a ferrite core. It responds to the H field of the arriving e-m wave, and for maximum r-f output it must be oriented in the horizontal plane -- even though that arriving wave is "vertically polarized."

Such a receiver can work very well when the axis of its loopstick lies in the horizontal plane, and normal to the direction of the arriving e-m wave. But when that receiver is vertically rotated 90° around the bearing to the transmit site so that the loopstick axis is vertical, reception is much poorer than before.

So the loopstick does not respond well to the E field, even though the E field is present at the receive site.

My experiment using a Tecsun PL-880 portable receiver had about s 30 dB reduction in the value of the signal strength shown on its front-panel display, when changing its loopstick orientation from horizontal to vertical.

I do not agree that your explanation holds water at all. The loopstick
antenna will respond to a vertically polarized EM wave maximally when
horizontal. That says nothing about whether it is responding to the E
field or the H field.

To determine that you need to generate a calibrated E field without the
H field (or very low) and an H field with small E field (obviously only
possible in the near field) and compare the results.

Polarization is an entirely different matter.

--

Rick C

Rick C (rickman) clips:

I do not agree that your explanation holds water at all. The loopstick
antenna will respond to a vertically polarized EM wave maximally when
horizontal. That says nothing about whether it is responding to the E
field or the H field.

RESPONSE: Actually it does, because the maximum H field of a vertically-polarized, far-field, e-m wave always lies in the horizontal plane. So if the maximum r-f output of a loopstick receive antenna occurs when its axis lies in the horizontal plane, that output necessarily was produced by the H field.

To determine that you need to generate a calibrated E field without the
H field (or very low) and an H field with small E field (obviously only
possible in the near field) and compare the results.

RESPONSE: This was an assumption made by the developers of the E-H and Cross-field antennas --which was disproven in their field trials, as well as by theory. Neither the E field or the H field component of a far-field e-m wave can be produced or radiated independently. If one field exists, they both exist, and are related to the radiated power by the 377-ohm impedance of free space.

RF

On 8/24/2016 2:50 PM, Richard Fry wrote:
Rick C (rickman) clips:

I do not agree that your explanation holds water at all. The loopstick
antenna will respond to a vertically polarized EM wave maximally when
horizontal. That says nothing about whether it is responding to the E
field or the H field.

RESPONSE: Actually it does, because the maximum H field of a vertically-polarized, far-field, e-m wave always lies in the horizontal plane. So if the maximum r-f output of a loopstick receive antenna occurs when its axis lies in the horizontal plane, that output necessarily was produced by the H field.

The part you are missing is that you have no basis to assume the antenna
responds in any particular way to the E field or the H field. You
*assume* that a horizontal loop stick antenna is responding to the H
field because the ferrite is horizontal. How do you know which
orientation of the antenna makes it sensitive to which field?


To determine that you need to generate a calibrated E field without the
H field (or very low) and an H field with small E field (obviously only
possible in the near field) and compare the results.

RESPONSE: This was an assumption made by the developers of the E-H and Cross-field antennas --which was disproven in their field trials, as well as by theory. Neither the E field or the H field component of a far-field e-m wave can be produced or radiated independently. If one field exists, they both exist, and are related to the radiated power by the 377-ohm impedance of free space.

The E and H fields are always present in the far field. Not so in the
near field where one can dominate over the other.

You have a weird way of replying to a post.


--

Rick C

rickman: I've responded to you twice now with accurate information, but you haven't shown that you understood it. Suggest that you give the subject more thought and study using antenna engineering textbooks. Regards,

RF

On 8/24/2016 4:18 PM, Richard Fry wrote:
rickman: I've responded to you twice now with accurate information, but you haven't shown that you understood it. Suggest that you give the subject more thought and study using antenna engineering textbooks. Regards,

Dude, I get what you are saying, but you don't have a clear basis for
your statements. The results are clear... your reasoning is *not*.

--

Rick C

On 8/24/2016 3:18 PM, Richard Fry wrote:
rickman: I've responded to you twice now with accurate information,
but you haven't shown that you understood it. Suggest that you give
the subject more thought and study using antenna engineering
textbooks. Regards,

RF


Richard: I understood all you posted and found it accurate. rickman is a
troll. It does not matter what you post to him, he will argue with you.

On Wed, 24 Aug 2016 01:54:59 -0400, rickman wrote:

On 8/24/2016 12:03 AM, Jeff Liebermann wrote:

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.

Oops, your right. It's a monopole and top hat. I looked at the photo
with all the wires in the air and immediately assumed it was a dipole
without double checking. Sorry.

--
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 08/24/2016 01:12 PM, rickman wrote:


Perhaps you can explain what the shield does on a receiving loop
antenna? Your explanation clearly says an antenna can be magnetic in
the near field. That is what the term means for receiving antennas. At
lower frequencies much interference is in the near field and is electric
rather than magnetic I am told. Think 100 kHz and household appliances.

The term "magnetic" is usually used in context of a transmitting antenna


Hello, and before we get too far afield, I submit that well-respected
EM/Antenna theory textbooks (e.g. those by Jackson, Stratton,
Kraus,Jasik, Terman) don't use the term "magnetic loop antenna" just as
they don't use "electric dipole" antenna". EEs who design antennas
don't either. Hams seem to coin their own terms but not always for
valid theoretical reasons IMO. EM theory says if we make the area of a
single loop of conductor carrying uniform current very small then it can
be considered to function as a "magnetic dipole". But EM texts would
call this a small loop vice magnetic loop antenna. Likewise we consider
an "electric dipole" to be a straight conductor of very small length
(compared to a wavelength) carrying uniform current.

Finally, it's not my intent to imply one has to have an EE degree to
enjoy ham radio and build and experiment with various types of antennae.
Just like you don't have to understand all the nuances of fluid
dynamics to enjoy sailing or flying an airplane. Sincerely, and 73s
from N4GGO,

--
J. B. Wood e-mail:

J.B Wood clip: ... Likewise we consider an "electric dipole" to be a straight conductor of very small length (compared to a wavelength) carrying uniform current.
________

Just note that while the currents along the two sides of a dipole can be equal, they can never be uniform. Essentially no r-f current exists at the far ends of a dipole, no matter how short or long it is in terms of wavelengths.