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Greetings. The first paragraph says, "This category also includes smaller loops 5% to 30% of a wavelength in circumference, which use a capacitor to make them resonant." However, as I understand it, if an antenna is too short for the intended frequency, the antenna itself becomes capacitive and would need an inductor to make it resonant. Am I correct?
Brent Woods 18:18, 13 March 2019 (UTC) — Preceding unsigned comment added by Bwoods ( talk • contribs)
A linear antenna, like a short dipole, can be made resonant by inserting inductance at the feed point. It could also be made resonant by adding capacity between the ends (from one end to the other). There are antennas that do this by putting large plates on the ends of the dipole or by imbedding the dipole in a dielectric. This reflects the fact that a short dipole is resonant at a frequency higher than it would be if the dipole were a half wave. To lower the resonant frequency of a circuit, one can increase either the inductance OR the capacitance OR both. Bending the ends of the dipole around in a circle does not change this principle. One could add inductance at the feed point, (assuming we were feeding by breaking the loop at the center) or by adding capacitance between the two ends, which are now close to each other making adding a capacitor the easier method. In both the short linear dipole and the small loop, the physical size of the antenna is too small compared to the wavelength for resonance, thus one must add fixed inductance or capacitance to achieve resonance. JNRSTANLEY ( talk) 19:42, 14 March 2019 (UTC)
G0CWT discovered that a 1/4 wave length circumference resonating transmitting loop has far better properties than common loop calculation programs found on Internet do suggest. The cause is the fact, that at a such NOT VERY small loop antenna, the current around its circumference has NOT everywhere the same value. Please look to the findings of G0CWT and write about it as a special and interesting case. + Higher feed point impedances, between 5.5 and 22.5 Ohms, depending on the feed point location (in the current max, or voltage max, or inbetween) + Relative low Q, thus much lower currents and lower capacitor voltages, much less losses. + less thick material has to be used (1" dia red copper pipe is sufficient for a 20m circumference 3.65MHz transmitting loop). The use of 8cm copper rain pipe is overkill. + Simple wide band, exactly matched feed, using a ferrite core transformer with ratio pri : sec = 3:1 (inserted in the current max.) to 3:2 (inseted between one side of the tuning capacitor and one open end of the loop). + Omny directional, but with the tuning capacitor at the lowest point, radiates mosly upwards (NVIS).
pa0nhc. 31.201.56.155 ( talk) 09:29, 8 July 2017 (UTC)
I have added a mention and reference to the G0CWT loop. JNRSTANLEY ( talk) 13:40, 5 August 2017 (UTC)
I came here looking for answers to the following questions:
I hope the article can be expanded to cover these topics.
-- ssd ( talk) 12:25, 24 September 2008 (UTC)
One way to improve reception would be to replace a ferrite loop winding connected to the tuning capacitor with a air core loop of the same inductance and make the loop as big as possible. other sections of the ferrite loop would be left in place for the oscillator section. — Preceding
unsigned comment added by
Arydberg (
talk •
contribs)
04:36, 2 January 2015 (UTC)
It is very technical and desperately needs an intro describing in general terms what a loop antenna is used for, and where. The general readership would learn little from the current article. On a second point, can anyone here identify the thingy behind the aerial in this image? Moriori ( talk) 21:58, 20 May 2010 (UTC)
I concur. This article seems to be written in engineer speak for those in the know. It hasn't told me a gosh darn thing I don't already know about automatic direction finding. 65.191.6.31 ( talk) 05:31, 10 May 2012 (UTC)
This article should probably be merged with magnetic_loop
Can't agree. Small loops can be resonant (with a tuning cap), and large loops don't have to be resonant. The difference between the two antennas is not if they are resonant or not, but their radiation pattern and theory of operation. -- ssd ( talk) 01:33, 12 January 2015 (UTC)
Maybe the reference you are quoting says that, but man, in that case I want to see a discussion of why it is not the orientation of the loop that matters. 172.5.154.148 ( talk) 14:59, 11 December 2013 (UTC)
There is also a loop antenna such as was used in the old tube radios. Here the loop was made to resonate with the tuning capacitor. It had no ferrite core. Arydberg ( talk) 04:31, 2 January 2015 (UTC)
Hi, folks.
I have added a bit of info and some references but see a lot more work needed. I hope to rearrange the material somewhat and add lots more references as time permits. All comments and suggestions welcome. JNRSTANLEY ( talk) 17:35, 28 July 2017 (UTC)
Not having received any comments in 4 days, I have gone ahead with a major reorganization of the text and outline. I removed quite a bit of repetitive text and replaced some of it with new text. I retained all of the graphics and added one additional. I retained all of the references. I think we could still use a few more and may add those once this newly organized version has met with the approval of those concerned. JNRSTANLEY ( talk) 18:14, 1 August 2017 (UTC)
More information on the frequency range of ferrite antennas and type of ferrite used would be welcome. 150.227.15.253 ( talk) 12:28, 6 March 2020 (UTC)
This statement cannot be true: This greater conductance channels thousands of times more magnetic power through the rod — Preceding unsigned comment added by 24.239.202.241 ( talk) 10:32, 21 August 2020 (UTC)
In my opinion more references should be added in the MIDI Loop Antenna label in the first image. Since the antenna in the picutre is of a particular brand and neither the proper name of the antenna model is written (MIDI Loop Antenna by I3VHF) nor credit is given to the designer (Ciro Mazzoni). The link to the corresponding site was inserted in order to provide reference to the model. If external links to wikipedia are frowned upon, I believe that at least the full model name and the inventor name should be a must to give some credit to whoever designed and built this antenna. — Preceding unsigned comment added by RontegWiki ( talk • contribs) 18:09, 18 October 2020 (UTC)
The phrase "along the plane" adds nothing and is confusing . The antenna is in a plane; voltages are induced in the element. I'm replacing it with the more appropriate "on opposite sides" . -- Steve -- ( talk) 18:58, 10 December 2020 (UTC)
There are lots of statements in this article about small loop antennas that seem like they could be general statements about electrically small antennas instead. E.g., "Wasted power is undesirable for a transmitting antenna, however for a receiving antenna, the inefficiency is not important at frequencies below about 15 MHz". Unless there's a good reason why these factors are especially important for loop antennas, I think details like these should be moved to Electrically small antenna instead. A "Main article" or "See also" template could be used to reference this article. Only issue is that a few of the statements in the article right now are not cited very well. Hddharvey ( talk) 04:27, 12 October 2021 (UTC)
I believe this statement is misleading. At the very least it applies only to small loops, not full wave loops like the quad loop. With the small loop it is the magnetic NEAR field that is picked up by the loop in preference to the electrical NEAR field that is picked up that gives the loop its noise rejecting properties, since near fields are often more E fields than H fields. As for the EM wave itself, it is unhelpful to try to separate the effects of the E and H components IMHO. There is some disagreement about calling even the small loop a "magnetic loop" even though it is commonly done. I would like to get some inputs from some of the EM wave experts before accepting this change, but to avoid starting a edit war I am mentioning it here first. JNRSTANLEY ( talk) 11:04, 13 April 2021 (UTC)
@ JNRSTANLEY: The fact that loop probes are used to sense the magnetic field doesn't necessarily say much about sensitivity to electric fields. Maybe magnetic field probes are insensitive to certain types of electric fields (they can't be insensitive to the entire electric field, because it is the electric field responsible for the magnetic induction in the loop), but maybe this noise rejection is affected by other design decisions for devices intended to be "magnetic field probes". I'm not sure these details need to be in the article (and it's probably hard to find good and explicit sources for them). Also, the article is drifting back and forth between near-field coupling and far-field reception/transmission. Usually " antenna" will tend to refer to far-field, although the word definitely is used (some might say abused) in cases where near-field transmission is intended. Note that electrically small loops and loops intended for near-field coupling is not necessarily the same. Hddharvey ( talk) 14:21, 9 October 2021 (UTC)
@ Hddharvey: Check out these references as see if you find them more convincing the the ones I had used to show that a shielded loop is indeed an H field probe with very little influence from any near E field that might be present: https://tel.archives-ouvertes.fr/tel-01757038/document https://www.esdemc.com/public/docs/Publications/Dr.%20Pommerenke%20Related/Active%20probes%20for%20creating%20H-field%20probes%20for%20flat%20frequency%20response.pdf https://www.naic.edu/~phil/rfi/antennas/NearFieldProbeSet7405_UserManual.pdf (page 8) I can assure you that H field probes exist and are effective. I have used them in the medium and short wave spectrum to insure compliance with RADHAZ regulations which many countries enforce and require that both E and H fields fall within certain limits. While the H field probes are mainly used for near field evaluation such as near the base of an AM broadcast tower, they do illustrate that even "antennas" in the present of strong E fields, can help reject local E field noise while receiving near H fields and distant EM waves well.
I think we all agree that the small loop is sensitive to EM waves and that stating it is only responding to the H component is meaningless and should be eliminated. This was my initial question that began this discussion.
I think we all agree that a small shielded loop tends to reject near E fields and is thus useful for reception, as is common in direction finding loops.
There seems to be disagreement as to calling the H field probe, designed to measure near H fields, an "antenna" and to include in it this article. My point in mentioning it is that it is identical to shielded receiving loops and would seem to verify that they do reject near E fields. I am fine with not including any reference to those in the article.
I think the statement that unshielded small loops discriminate against near E fields has not been established at this point, though it is commonly believed.
I think that calling these "magnetic loops" is controversial, although common. I have no strong opinion as to deleting all such references from the article.
If there is a consensus on these points, can we come up with a text that makes these points? What are the sources that we all agree are reliable? JNRSTANLEY ( talk) 11:20, 10 October 2021 (UTC)
Faraday shields are used in radio transmitters to allow one coil to couple to another without any electric fields present to add coupling between them. I have also seen them in medical equipment.
But meanwhile, I have had some different thoughts as to why small loops help with noise. The fields within a wavelength of the ground are mainly vertically polarized. This is because within a fraction of a wavelength a conductive ground plane shorts out the horizontal E fields. The source of the local fields (commutators, etc) perhaps is not as important as this factor. The calibrated loops used like the one Constant314 linked to are always oriented as shown to respond to vertically polarization and have a deep null in azimuth for direction finding, butl they can also serve for nulling out interference. If we use a short dipole or monopole oriented vertically, it will be omnidirectional in azimuth. Thus for nulling interference we need to use a loop. The polarization is still vertical, but the pattern has a null in azimuth. Adding a shield and being careful about balance apparently help preserve the very deep null, but it is there even without the shield as any AM radio with a loop or "loopstick" will demonstrate. Perhaps the proven superiority of small loops for local noise reduction has little to do with E or H field rejection, it is mainly about antenna patterns. I am going to look for reliable sources that discuss this issue.
I am still doing more literature studies as well as some NEC models to try to sort all of this out. It seems more complicated than I had (and many others have) supposed. JNRSTANLEY ( talk) 16:50, 11 October 2021 (UTC)
Also I should have mentioned one other thing. Although a loop antenna IS a magnetic field probe, since this article is about radio antennas I would not describe it as such, but that is still what it is. And I have never used the term "magnetic loop" (although the term is somewhat in use so the page should mention that linguistic fact), and in fact there USED TO BE a page with that very name [ [2]] that I shut down ten years ago and incorporated into this page. Interferometrist ( talk) 22:51, 17 October 2021 (UTC)
@ Interferometrist: Sure, the toroidal component of E is only present if there is also a changing magnetic field, but it is the electric field that actually exerts the force that causes the signal. There will be a magnetic field there along for the ride, but it doesn't do much to move charges along the antenna.
I want to emphasize that I am not saying we should say a loop antenna is sensitive to E and not insensitive to H, or that it measures E but doesn't measure H. As you say, clearly it does measure H, since E is what causes the signal and E is related to H. However, just as much as you could say the antenna measures (the flux of) changing H, you could also say it measures the (line integral of) toroidal E field. I have no problem with people just saying "it measures H", since EEs and others often seem to ignore the toroidal E - in our eyes we see a changing magnetic field and voila! There's voltage somehow! However, I don't like saying it "only measures H", especially in an encyclopedic article mostly about far-field transmission and reception since it is confusing (it confused me when I read it) and misleading. I do acknowledge that EEs and others use this (in my opinion, colloquial) language, especially for near field probing, since toroidal E is often ignored or considered a magnetic phenomenon.
You responded to @ Constant314: saying that ideally the loop acting as a capacitor plate would not produce a differential mode signal. However, this is not obvious to me. Depending on how close the loop is to the source of the near-field E, and the size/orientation of the loop, there's no reason this capacitive coupling has to be symmetric. Sure, the "small loop" is small - but that is relative to the far-field wavelength which is potentially completely unrelated to the various near-field E sources around it. This is all our personal conjecture anyway - if there is to be any discussion that relies on this in the article then it should be well cited.
Again, with your example of E and H cancelling out I already explained why this doesn't show that the loop is only sensitive to H. E cannot cancel everywhere - and it is precisely the places where it doesn't cancel (e.g., away from the center axis of the antenna) that causes the received signal.
And yes, a small loop is like a magnetic dipole, and a small dipole is like an electric dipole. In this sense, you could say that it is a "magnetic loop". However, just because it is like a magnetic dipole however does not mean that it in any way "only responds to the magnetic field". A tiny oscillating current loop produces both toroidal E and changing H, and by reciprocity it also responds to toroidal E and changing H (which always accompany one another). Thus, it is misleading to say that it is "insensitive to external E". If the loop is small enough that the toroidal component of E around it is insignificant then there will be no measurable signal whether you want to consider that signal to be due to the magnetic field or the electric field. Hddharvey ( talk) 00:28, 18 October 2021 (UTC)
If there is a good source that shows it, perhaps we could say the small loop is insensitive to "near-field E" or that it "rejects capacitively coupled interference" or something like that. However, we should be careful using sources specifically for magnetic field probes for this purpose since the behavior of near-field magnetic probes might rely on other design factors that are independent from the loop nature of its "antenna" (such as shielding). I understand that people often colloquially say that it is insensitive to E, especially in near-field contexts, however as this is an encyclopedic article we can and should try to be a little more precise - and I believe we can do that without being unnecessarily confusing. Hddharvey ( talk) 00:36, 18 October 2021 (UTC)
Also note that there is at least one crucial difference between a dipole (whether magnetic or electric) and a similarly-shaped antenna whose size approaches zero. The "dipole limit" is the limit as the size/separation approaches zero and the dipole moment is held constant. For an electric dipole, this means the charge has to also approach infinity. For a magnetic dipole, this means the current has to approach infinity. If we entertain the idea of a "receiving magnetic dipole", then this correspondingly means the gain of the receiver would have to approach infinity. As stated before, toroidal E and changing H are intricately linked (in fact, in free space they are directly proportional). If E around a loop is negligible, then so is the rate of change of the flux of H through that loop. Hddharvey ( talk) 00:57, 18 October 2021 (UTC)
@ Constant314: Hello Constant314, Hello Everyone. I think that the following reference is exactly about the subject matter of this discussion [1]. It can be accessed on IEEEXplore, and free of charge on the eurexcem.com and excem.fr web sites. It explains how the response of a loop shifts from being almost purely related to the derivative of the magnetic field at very low frequencies, to a more complex behavior at higher frequencies. It also interprets the "intended response" of an electrically small loop as being caused by a particular component of the electromagneticfield. Enjoy. FreddyOfMaule (F5OYE). FreddyOfMaule ( talk) 07:34, 31 October 2021 (UTC)
References
![]() | This article is written in American English, which has its own spelling conventions (color, defense, traveled) and some terms that are used in it may be different or absent from other varieties of English. According to the relevant style guide, this should not be changed without broad consensus. |
![]() | This article is rated C-class on Wikipedia's
content assessment scale. It is of interest to the following WikiProjects: | |||||||||||||||||||||||||||||||||||||||||||||||
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Greetings. The first paragraph says, "This category also includes smaller loops 5% to 30% of a wavelength in circumference, which use a capacitor to make them resonant." However, as I understand it, if an antenna is too short for the intended frequency, the antenna itself becomes capacitive and would need an inductor to make it resonant. Am I correct?
Brent Woods 18:18, 13 March 2019 (UTC) — Preceding unsigned comment added by Bwoods ( talk • contribs)
A linear antenna, like a short dipole, can be made resonant by inserting inductance at the feed point. It could also be made resonant by adding capacity between the ends (from one end to the other). There are antennas that do this by putting large plates on the ends of the dipole or by imbedding the dipole in a dielectric. This reflects the fact that a short dipole is resonant at a frequency higher than it would be if the dipole were a half wave. To lower the resonant frequency of a circuit, one can increase either the inductance OR the capacitance OR both. Bending the ends of the dipole around in a circle does not change this principle. One could add inductance at the feed point, (assuming we were feeding by breaking the loop at the center) or by adding capacitance between the two ends, which are now close to each other making adding a capacitor the easier method. In both the short linear dipole and the small loop, the physical size of the antenna is too small compared to the wavelength for resonance, thus one must add fixed inductance or capacitance to achieve resonance. JNRSTANLEY ( talk) 19:42, 14 March 2019 (UTC)
G0CWT discovered that a 1/4 wave length circumference resonating transmitting loop has far better properties than common loop calculation programs found on Internet do suggest. The cause is the fact, that at a such NOT VERY small loop antenna, the current around its circumference has NOT everywhere the same value. Please look to the findings of G0CWT and write about it as a special and interesting case. + Higher feed point impedances, between 5.5 and 22.5 Ohms, depending on the feed point location (in the current max, or voltage max, or inbetween) + Relative low Q, thus much lower currents and lower capacitor voltages, much less losses. + less thick material has to be used (1" dia red copper pipe is sufficient for a 20m circumference 3.65MHz transmitting loop). The use of 8cm copper rain pipe is overkill. + Simple wide band, exactly matched feed, using a ferrite core transformer with ratio pri : sec = 3:1 (inserted in the current max.) to 3:2 (inseted between one side of the tuning capacitor and one open end of the loop). + Omny directional, but with the tuning capacitor at the lowest point, radiates mosly upwards (NVIS).
pa0nhc. 31.201.56.155 ( talk) 09:29, 8 July 2017 (UTC)
I have added a mention and reference to the G0CWT loop. JNRSTANLEY ( talk) 13:40, 5 August 2017 (UTC)
I came here looking for answers to the following questions:
I hope the article can be expanded to cover these topics.
-- ssd ( talk) 12:25, 24 September 2008 (UTC)
One way to improve reception would be to replace a ferrite loop winding connected to the tuning capacitor with a air core loop of the same inductance and make the loop as big as possible. other sections of the ferrite loop would be left in place for the oscillator section. — Preceding
unsigned comment added by
Arydberg (
talk •
contribs)
04:36, 2 January 2015 (UTC)
It is very technical and desperately needs an intro describing in general terms what a loop antenna is used for, and where. The general readership would learn little from the current article. On a second point, can anyone here identify the thingy behind the aerial in this image? Moriori ( talk) 21:58, 20 May 2010 (UTC)
I concur. This article seems to be written in engineer speak for those in the know. It hasn't told me a gosh darn thing I don't already know about automatic direction finding. 65.191.6.31 ( talk) 05:31, 10 May 2012 (UTC)
This article should probably be merged with magnetic_loop
Can't agree. Small loops can be resonant (with a tuning cap), and large loops don't have to be resonant. The difference between the two antennas is not if they are resonant or not, but their radiation pattern and theory of operation. -- ssd ( talk) 01:33, 12 January 2015 (UTC)
Maybe the reference you are quoting says that, but man, in that case I want to see a discussion of why it is not the orientation of the loop that matters. 172.5.154.148 ( talk) 14:59, 11 December 2013 (UTC)
There is also a loop antenna such as was used in the old tube radios. Here the loop was made to resonate with the tuning capacitor. It had no ferrite core. Arydberg ( talk) 04:31, 2 January 2015 (UTC)
Hi, folks.
I have added a bit of info and some references but see a lot more work needed. I hope to rearrange the material somewhat and add lots more references as time permits. All comments and suggestions welcome. JNRSTANLEY ( talk) 17:35, 28 July 2017 (UTC)
Not having received any comments in 4 days, I have gone ahead with a major reorganization of the text and outline. I removed quite a bit of repetitive text and replaced some of it with new text. I retained all of the graphics and added one additional. I retained all of the references. I think we could still use a few more and may add those once this newly organized version has met with the approval of those concerned. JNRSTANLEY ( talk) 18:14, 1 August 2017 (UTC)
More information on the frequency range of ferrite antennas and type of ferrite used would be welcome. 150.227.15.253 ( talk) 12:28, 6 March 2020 (UTC)
This statement cannot be true: This greater conductance channels thousands of times more magnetic power through the rod — Preceding unsigned comment added by 24.239.202.241 ( talk) 10:32, 21 August 2020 (UTC)
In my opinion more references should be added in the MIDI Loop Antenna label in the first image. Since the antenna in the picutre is of a particular brand and neither the proper name of the antenna model is written (MIDI Loop Antenna by I3VHF) nor credit is given to the designer (Ciro Mazzoni). The link to the corresponding site was inserted in order to provide reference to the model. If external links to wikipedia are frowned upon, I believe that at least the full model name and the inventor name should be a must to give some credit to whoever designed and built this antenna. — Preceding unsigned comment added by RontegWiki ( talk • contribs) 18:09, 18 October 2020 (UTC)
The phrase "along the plane" adds nothing and is confusing . The antenna is in a plane; voltages are induced in the element. I'm replacing it with the more appropriate "on opposite sides" . -- Steve -- ( talk) 18:58, 10 December 2020 (UTC)
There are lots of statements in this article about small loop antennas that seem like they could be general statements about electrically small antennas instead. E.g., "Wasted power is undesirable for a transmitting antenna, however for a receiving antenna, the inefficiency is not important at frequencies below about 15 MHz". Unless there's a good reason why these factors are especially important for loop antennas, I think details like these should be moved to Electrically small antenna instead. A "Main article" or "See also" template could be used to reference this article. Only issue is that a few of the statements in the article right now are not cited very well. Hddharvey ( talk) 04:27, 12 October 2021 (UTC)
I believe this statement is misleading. At the very least it applies only to small loops, not full wave loops like the quad loop. With the small loop it is the magnetic NEAR field that is picked up by the loop in preference to the electrical NEAR field that is picked up that gives the loop its noise rejecting properties, since near fields are often more E fields than H fields. As for the EM wave itself, it is unhelpful to try to separate the effects of the E and H components IMHO. There is some disagreement about calling even the small loop a "magnetic loop" even though it is commonly done. I would like to get some inputs from some of the EM wave experts before accepting this change, but to avoid starting a edit war I am mentioning it here first. JNRSTANLEY ( talk) 11:04, 13 April 2021 (UTC)
@ JNRSTANLEY: The fact that loop probes are used to sense the magnetic field doesn't necessarily say much about sensitivity to electric fields. Maybe magnetic field probes are insensitive to certain types of electric fields (they can't be insensitive to the entire electric field, because it is the electric field responsible for the magnetic induction in the loop), but maybe this noise rejection is affected by other design decisions for devices intended to be "magnetic field probes". I'm not sure these details need to be in the article (and it's probably hard to find good and explicit sources for them). Also, the article is drifting back and forth between near-field coupling and far-field reception/transmission. Usually " antenna" will tend to refer to far-field, although the word definitely is used (some might say abused) in cases where near-field transmission is intended. Note that electrically small loops and loops intended for near-field coupling is not necessarily the same. Hddharvey ( talk) 14:21, 9 October 2021 (UTC)
@ Hddharvey: Check out these references as see if you find them more convincing the the ones I had used to show that a shielded loop is indeed an H field probe with very little influence from any near E field that might be present: https://tel.archives-ouvertes.fr/tel-01757038/document https://www.esdemc.com/public/docs/Publications/Dr.%20Pommerenke%20Related/Active%20probes%20for%20creating%20H-field%20probes%20for%20flat%20frequency%20response.pdf https://www.naic.edu/~phil/rfi/antennas/NearFieldProbeSet7405_UserManual.pdf (page 8) I can assure you that H field probes exist and are effective. I have used them in the medium and short wave spectrum to insure compliance with RADHAZ regulations which many countries enforce and require that both E and H fields fall within certain limits. While the H field probes are mainly used for near field evaluation such as near the base of an AM broadcast tower, they do illustrate that even "antennas" in the present of strong E fields, can help reject local E field noise while receiving near H fields and distant EM waves well.
I think we all agree that the small loop is sensitive to EM waves and that stating it is only responding to the H component is meaningless and should be eliminated. This was my initial question that began this discussion.
I think we all agree that a small shielded loop tends to reject near E fields and is thus useful for reception, as is common in direction finding loops.
There seems to be disagreement as to calling the H field probe, designed to measure near H fields, an "antenna" and to include in it this article. My point in mentioning it is that it is identical to shielded receiving loops and would seem to verify that they do reject near E fields. I am fine with not including any reference to those in the article.
I think the statement that unshielded small loops discriminate against near E fields has not been established at this point, though it is commonly believed.
I think that calling these "magnetic loops" is controversial, although common. I have no strong opinion as to deleting all such references from the article.
If there is a consensus on these points, can we come up with a text that makes these points? What are the sources that we all agree are reliable? JNRSTANLEY ( talk) 11:20, 10 October 2021 (UTC)
Faraday shields are used in radio transmitters to allow one coil to couple to another without any electric fields present to add coupling between them. I have also seen them in medical equipment.
But meanwhile, I have had some different thoughts as to why small loops help with noise. The fields within a wavelength of the ground are mainly vertically polarized. This is because within a fraction of a wavelength a conductive ground plane shorts out the horizontal E fields. The source of the local fields (commutators, etc) perhaps is not as important as this factor. The calibrated loops used like the one Constant314 linked to are always oriented as shown to respond to vertically polarization and have a deep null in azimuth for direction finding, butl they can also serve for nulling out interference. If we use a short dipole or monopole oriented vertically, it will be omnidirectional in azimuth. Thus for nulling interference we need to use a loop. The polarization is still vertical, but the pattern has a null in azimuth. Adding a shield and being careful about balance apparently help preserve the very deep null, but it is there even without the shield as any AM radio with a loop or "loopstick" will demonstrate. Perhaps the proven superiority of small loops for local noise reduction has little to do with E or H field rejection, it is mainly about antenna patterns. I am going to look for reliable sources that discuss this issue.
I am still doing more literature studies as well as some NEC models to try to sort all of this out. It seems more complicated than I had (and many others have) supposed. JNRSTANLEY ( talk) 16:50, 11 October 2021 (UTC)
Also I should have mentioned one other thing. Although a loop antenna IS a magnetic field probe, since this article is about radio antennas I would not describe it as such, but that is still what it is. And I have never used the term "magnetic loop" (although the term is somewhat in use so the page should mention that linguistic fact), and in fact there USED TO BE a page with that very name [ [2]] that I shut down ten years ago and incorporated into this page. Interferometrist ( talk) 22:51, 17 October 2021 (UTC)
@ Interferometrist: Sure, the toroidal component of E is only present if there is also a changing magnetic field, but it is the electric field that actually exerts the force that causes the signal. There will be a magnetic field there along for the ride, but it doesn't do much to move charges along the antenna.
I want to emphasize that I am not saying we should say a loop antenna is sensitive to E and not insensitive to H, or that it measures E but doesn't measure H. As you say, clearly it does measure H, since E is what causes the signal and E is related to H. However, just as much as you could say the antenna measures (the flux of) changing H, you could also say it measures the (line integral of) toroidal E field. I have no problem with people just saying "it measures H", since EEs and others often seem to ignore the toroidal E - in our eyes we see a changing magnetic field and voila! There's voltage somehow! However, I don't like saying it "only measures H", especially in an encyclopedic article mostly about far-field transmission and reception since it is confusing (it confused me when I read it) and misleading. I do acknowledge that EEs and others use this (in my opinion, colloquial) language, especially for near field probing, since toroidal E is often ignored or considered a magnetic phenomenon.
You responded to @ Constant314: saying that ideally the loop acting as a capacitor plate would not produce a differential mode signal. However, this is not obvious to me. Depending on how close the loop is to the source of the near-field E, and the size/orientation of the loop, there's no reason this capacitive coupling has to be symmetric. Sure, the "small loop" is small - but that is relative to the far-field wavelength which is potentially completely unrelated to the various near-field E sources around it. This is all our personal conjecture anyway - if there is to be any discussion that relies on this in the article then it should be well cited.
Again, with your example of E and H cancelling out I already explained why this doesn't show that the loop is only sensitive to H. E cannot cancel everywhere - and it is precisely the places where it doesn't cancel (e.g., away from the center axis of the antenna) that causes the received signal.
And yes, a small loop is like a magnetic dipole, and a small dipole is like an electric dipole. In this sense, you could say that it is a "magnetic loop". However, just because it is like a magnetic dipole however does not mean that it in any way "only responds to the magnetic field". A tiny oscillating current loop produces both toroidal E and changing H, and by reciprocity it also responds to toroidal E and changing H (which always accompany one another). Thus, it is misleading to say that it is "insensitive to external E". If the loop is small enough that the toroidal component of E around it is insignificant then there will be no measurable signal whether you want to consider that signal to be due to the magnetic field or the electric field. Hddharvey ( talk) 00:28, 18 October 2021 (UTC)
If there is a good source that shows it, perhaps we could say the small loop is insensitive to "near-field E" or that it "rejects capacitively coupled interference" or something like that. However, we should be careful using sources specifically for magnetic field probes for this purpose since the behavior of near-field magnetic probes might rely on other design factors that are independent from the loop nature of its "antenna" (such as shielding). I understand that people often colloquially say that it is insensitive to E, especially in near-field contexts, however as this is an encyclopedic article we can and should try to be a little more precise - and I believe we can do that without being unnecessarily confusing. Hddharvey ( talk) 00:36, 18 October 2021 (UTC)
Also note that there is at least one crucial difference between a dipole (whether magnetic or electric) and a similarly-shaped antenna whose size approaches zero. The "dipole limit" is the limit as the size/separation approaches zero and the dipole moment is held constant. For an electric dipole, this means the charge has to also approach infinity. For a magnetic dipole, this means the current has to approach infinity. If we entertain the idea of a "receiving magnetic dipole", then this correspondingly means the gain of the receiver would have to approach infinity. As stated before, toroidal E and changing H are intricately linked (in fact, in free space they are directly proportional). If E around a loop is negligible, then so is the rate of change of the flux of H through that loop. Hddharvey ( talk) 00:57, 18 October 2021 (UTC)
@ Constant314: Hello Constant314, Hello Everyone. I think that the following reference is exactly about the subject matter of this discussion [1]. It can be accessed on IEEEXplore, and free of charge on the eurexcem.com and excem.fr web sites. It explains how the response of a loop shifts from being almost purely related to the derivative of the magnetic field at very low frequencies, to a more complex behavior at higher frequencies. It also interprets the "intended response" of an electrically small loop as being caused by a particular component of the electromagneticfield. Enjoy. FreddyOfMaule (F5OYE). FreddyOfMaule ( talk) 07:34, 31 October 2021 (UTC)
References