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Loop Antenna Polarization
Just to note that halo antennas were/are dipoles, not loop antennas.
https://en.wikipedia.org/wiki/Halo_antenna |
Loop Antenna Polarization
In article , says...
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, There is a difference in the mag loops and regular loop antennas. --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus |
Loop Antenna Polarization
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. -- Rick C |
Loop Antenna Polarization
On Tue, 23 Aug 2016 13:14:32 -0400, rickman wrote:
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. To add a little confusion, the convention is for the polarization to be that of the E-field (electric field) and not that of the H-field (magnetic field), even if the communications ocurrs using the H-field as in a shielded loop antenna. https://en.wikipedia.org/wiki/Loop_antenna#Radiation_pattern_and_polarization Small loops (0.1 wavelength circumference) are also a subset of loop antennas. http://www.antenna-theory.com/antennas/smallLoop.php Loop antenna users are also polarized. Some users hate them, while other users swear by them. There is some middle ground, but not in public forums. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Loop Antenna Polarization
On 8/23/2016 2:09 PM, Jeff Liebermann wrote:
On Tue, 23 Aug 2016 13:14:32 -0400, rickman wrote: 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. To add a little confusion, the convention is for the polarization to be that of the E-field (electric field) and not that of the H-field (magnetic field), even if the communications ocurrs using the H-field as in a shielded loop antenna. https://en.wikipedia.org/wiki/Loop_antenna#Radiation_pattern_and_polarization Small loops (0.1 wavelength circumference) are also a subset of loop antennas. http://www.antenna-theory.com/antennas/smallLoop.php Loop antenna users are also polarized. Some users hate them, while other users swear by them. There is some middle ground, but not in public forums. I've been told that the term "magnetic" loop is the same as "small" loop. 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. -- Rick C |
Loop Antenna Polarization
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. 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. 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/ 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. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Loop Antenna Polarization
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 |
Loop Antenna Polarization
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, -- J. B. Wood e-mail: |
Loop Antenna Polarization
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. Richard Fry, CPBE |
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