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Loop Antenna Polarization
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? -- Rick C |
Loop Antenna Polarization
On 2016-08-20, 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? Fed at the bottom it is horizontal polarization. Fed on the side it becomes vertical polarization. You might also enjoy looking at vertical delta loops and the way they can be mounted and fed. ....Edwin __________________________________________________ __________ "Once you have flown, you will walk the earth with your eyes turned skyward, for there you have been, there you long to return."-da Vinci http://kd5zlb.org |
Loop Antenna Polarization
On 8/21/2016 10:18 AM, Edwin Johnson wrote:
On 2016-08-20, 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? Fed at the bottom it is horizontal polarization. Fed on the side it becomes vertical polarization. You might also enjoy looking at vertical delta loops and the way they can be mounted and fed. Yes, of course, I should have been able to figure that out for myself. The small loop looks like a very short dipole. Thanks -- Rick C |
Loop Antenna Polarization
On Sun, 21 Aug 2016, Edwin Johnson wrote:
On 2016-08-20, 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? Fed at the bottom it is horizontal polarization. Fed on the side it becomes vertical polarization. You might also enjoy looking at vertical delta loops and the way they can be mounted and fed. Now that makes sense, the same thing happens with quad antennas. But, there is the case of loops with the area placed in parallel with the earth. Those are horizontally polarized. I'm suddenly blank about what they were called, but fifty years ago it wasn't uncommon to see such horizontal loops on cars, for 2M and 6M use, since this was before FM so "base" antennas were generally horizontally polarized. Michael |
Loop Antenna Polarization
On Sun, 21 Aug 2016 23:18:39 -0400, Ralph Mowery
wrote: In article ple.org, says... But, there is the case of loops with the area placed in parallel with the earth. Those are horizontally polarized. I'm suddenly blank about what they were called, but fifty years ago it wasn't uncommon to see such horizontal loops on cars, for 2M and 6M use, since this was before FM so "base" antennas were generally horizontally polarized. Michael They were called halos. They were horizontal polarized. If mounted a fraction of a wavelength over a conducting surface then they became DDRR antennas (directional discontinuity ring radiator)and vertical polarized. Don't forget Squalos (same as halos, but the loop was squared off). I have no idea why I remember that. Must have been advertised in QST. |
Loop Antenna Polarization
On Sun, 21 Aug 2016, Ralph Mowery wrote:
In article ple.org, says... But, there is the case of loops with the area placed in parallel with the earth. Those are horizontally polarized. I'm suddenly blank about what they were called, but fifty years ago it wasn't uncommon to see such horizontal loops on cars, for 2M and 6M use, since this was before FM so "base" antennas were generally horizontally polarized. Michael They were called halos. They were horizontal polarized. If mounted a fraction of a wavelength over a conducting surface then they became DDRR antennas (directional discontinuity ring radiator)and vertical polarized. In the middle of the night the name came to me. And yes, there were those DDRR antennas that mounted on car roofs with suction cups. Michael |
Loop Antenna Polarization
On Mon, 22 Aug 2016, Pat wrote:
On Sun, 21 Aug 2016 23:18:39 -0400, Ralph Mowery wrote: In article ple.org, says... But, there is the case of loops with the area placed in parallel with the earth. Those are horizontally polarized. I'm suddenly blank about what they were called, but fifty years ago it wasn't uncommon to see such horizontal loops on cars, for 2M and 6M use, since this was before FM so "base" antennas were generally horizontally polarized. Michael They were called halos. They were horizontal polarized. If mounted a fraction of a wavelength over a conducting surface then they became DDRR antennas (directional discontinuity ring radiator)and vertical polarized. Don't forget Squalos (same as halos, but the loop was squared off). I have no idea why I remember that. Must have been advertised in QST. It all came back in the middle of the night. I think the Squalo was a commercial antenna, from Hygain or whatever. But the halo was in the antenna books too, so you could build or buy. I still have a Saturn 6 in the basement, a stacked set of 3 halos. Michael |
Loop Antenna Polarization
In article ple.org,
says... It all came back in the middle of the night. I think the Squalo was a commercial antenna, from Hygain or whatever. But the halo was in the antenna books too, so you could build or buy. I still have a Saturn 6 in the basement, a stacked set of 3 halos. Michael If that is the antenna I remember, the halos are not really stacked,but it does loop around 3 times to make the full length of the antenna instead of being a couple of feet in diameter that a true 6 meter halo would be. --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus |
Loop Antenna Polarization
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, -- J. B. Wood e-mail: |
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... Just to note that halo antennas were/are dipoles, not loop antennas. https://en.wikipedia.org/wiki/Halo_antenna Loop antennas are usually a full wavelength around, and the halos are about a half wavelength like a dipole, just bent in a circle. As always there can be several variations on this. --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus |
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 |
Loop Antenna Polarization
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 |
Loop Antenna Polarization
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 |
Loop Antenna Polarization
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 |
Loop Antenna Polarization
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 |
Loop Antenna Polarization
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 |
Loop Antenna Polarization
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 |
Loop Antenna Polarization
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. |
Loop Antenna Polarization
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 |
Loop Antenna Polarization
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: |
Loop Antenna Polarization
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. |
Loop Antenna Polarization
On 08/25/2016 08:23 AM, Richard Fry wrote:
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. It's a theoretical (textbook) construct but finds practical antenna modeling use in method-of-moments software such as the Numerical Electromagnetics Code (NEC). The idea is if we take smaller and smaller sections (say about 1/20 wavelength) of a conductor carrying alternating current we can consider the current to be uniform in that small conductor. Of course an actual antenna would consist of a series of these small conductors each carrying its respective value of uniform current. Programs like NEC also consider, in addition to conducted current the capacitive and inductive interactions between all the segments comprising an antenna model. Similarly we can build a transmission line using a number of identical tee or pi sections connected ladder-fashion. The currents and voltages associated with a section depend on its position along the length of the line. Sincerely, and 73s from N4GGO, -- J. B. Wood e-mail: |
Loop Antenna Polarization
On 8/25/2016 10:54 AM, J.B. Wood wrote:
On 08/25/2016 08:23 AM, Richard Fry wrote: 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. It's a theoretical (textbook) construct but finds practical antenna modeling use in method-of-moments software such as the Numerical Electromagnetics Code (NEC). The idea is if we take smaller and smaller sections (say about 1/20 wavelength) of a conductor carrying alternating current we can consider the current to be uniform in that small conductor. Of course an actual antenna would consist of a series of these small conductors each carrying its respective value of uniform current. Programs like NEC also consider, in addition to conducted current the capacitive and inductive interactions between all the segments comprising an antenna model. Similarly we can build a transmission line using a number of identical tee or pi sections connected ladder-fashion. The currents and voltages associated with a section depend on its position along the length of the line. Sincerely, and 73s from N4GGO, Richard is correct. The current at the feed point diminishes linearly (on a short dipole) from the feed point to the open end of the antenna as it must. Look at the current distribution using your NEC modelling program. |
Loop Antenna Polarization
On 08/25/2016 12:07 PM, John S wrote:
Richard is correct. The current at the feed point diminishes linearly (on a short dipole) from the feed point to the open end of the antenna as it must. Look at the current distribution using your NEC modelling program. No one said he wasn't. Did you read my last post? The uniform currents in each segment aren't the same value. Of course the end segments would be minimum. Sincerely, -- J. B. Wood e-mail: |
Loop Antenna Polarization
On 8/25/2016 11:55 AM, J.B. Wood wrote:
On 08/25/2016 12:07 PM, John S wrote: Richard is correct. The current at the feed point diminishes linearly (on a short dipole) from the feed point to the open end of the antenna as it must. Look at the current distribution using your NEC modelling program. No one said he wasn't. Did you read my last post? The uniform currents in each segment aren't the same value. Of course the end segments would be minimum. Sincerely, I did read your last post. But you also posted "Likewise we consider an "electric dipole" to be a straight conductor of very small length (compared to a wavelength) carrying uniform current." That is the one Richard and I take exception to. I think your last post explained your position better with breaking the antenna up into very small segments each with a uniform current. The currents in each segment can be considered to be uniform over that segment. However the segment currents diminish from the feed point to the open end of the element. I'm sure you know all this, but others may not. |
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