Talk:Loop antenna

Antenna Resonance

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)

1/4 wave length circumference resonating transmitting loop

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)

Looking for answers

I came here looking for answers to the following questions:

• How can I construct an antenna that's superior to the AM loop antennas that typically ship with stereo receivers?
• How difficult is this, and how much additional gain can I expect to get from such an effort?

I hope the article can be expanded to cover these topics.

• Wikipedia is not a HOWTO
• Making an antenna better tuned, larger than the one you already have, or more directional would help. There are plenty of guides on making antennas on the net, just choose an antenna type and build it for the frequency you want.

--ssd (talk) 12:25, 24 September 2008 (UTC)

Agree. If you're looking for antenna how-to information, you might be better served at ARRL or instructables or DIY Wiki or wikiHow. --68.0.124.33 (talk) 06:11, 5 September 2009 (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)

This article is far too geeky

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)

Redundancy?

This article should probably be merged with magnetic_loop

Yes, I agree it should. Or otherwise have two articles: 1) One on "small loop" antennas (according to the notation in this article) which are identical to "magnetic loop antennas" referenced in the other article. 2) "Large/medium" loop antennas, but actually what I'd call resonant loop antennas with a circumference of about 1 (or N = odd) wavelengths.

And the distinction between "medium" and "large" loops needs to be removed. A resonant loop antenna is 1 wavelength long, but mention (if you must) that you can make it 3 or 5 times larger for a higher gain. BTW, I was inclined to remove the tagged sentences in the "large loop" section, but didn't because the whole section should rather be deleted. Interferometrist (talk) 17:27, 25 February 2011 (UTC)

I'm sorry, I should have also mentioned a third distinct type of antenna which might be described as a "loop" : the halo antenna, which there already is an article about so it doesn't need to be considered (except as a link to the other loop antenna articles). Interferometrist (talk) 17:33, 25 February 2011 (UTC)

Actually I'm even more sorry: a halo antenna isn't really a loop antenna because the conductor neccessarily has a break in it. So there is NO loop involved! But sure, include a link to it anyway ;-) Interferometrist (talk) 17:41, 25 February 2011 (UTC)

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)

WTF? Resonant Loops "transmits less toward the sky or ground"

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)

You're absolutely right, it absolutely depends on orientation. I'll edit that to remove the ambiguity.Interferometrist (talk) 13:19, 27 February 2014 (UTC)

third type

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)

Actually, that's just a variant of the small loop. The small loop is typically non-resonant, but can be made resonant with a tuning cap. The ferrite core does not change the antenna design, it just makes it have a larger aperture than it would for the same size air core. The overlapping windings add some complexity to the analysis and reduce the efficiency, but don't really change the antenna design per se. --ssd (talk) 01:31, 12 January 2015 (UTC)

reorganization needed

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)

Lacking information

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)

Well actually that is covered somewhat in the Ferrite core page, saying that there are such materials that work up to 300MHz. But that's not important because their usefulness is at low frequencies where a practical receiving antenna must be much MUCH less than a wavelength in size but poor electrical efficiency isn't problematic (see last paragraphs of sec 2.2). If you have more information about ferrite materials' characteristics, please add that to the ferrite core page. It would be a distraction here. Interferometrist (talk) 19:15, 6 March 2020 (UTC)

Error

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)

Adding references to first Loop Antenna image

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)

Radiation Pattern and Polarization

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)

Statements pertaining to electrically small antennas

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 don't think that accurate statements are a problem. Reliable sources do usually discuss antenna efficiency of small loop antennas and the implication to transmit usage. I can add cirtes to Balanis.Constant314 (talk) 20:36, 12 October 2021 (UTC)

Fair enough. Hddharvey (talk) 23:27, 12 October 2021 (UTC)

I just added some content in that regard, mentioning two ways that a small loop antenna contrasts with a hertzian dipole (N>1 and mu_r >1).Interferometrist (talk) 22:53, 15 October 2021 (UTC)

About "receiving predominantly the magnetic component of the electromagnetic wave"

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)

The concept of picking up the "magnetic component" makes no sense to me. In the frame of reference of the antenna, the magnetic field does not produce any force that will move charges along the conductor's axis. The magnetic field will be responsible for things like the skin effect and the Hall effect, but these are not along the axis of the wiring. There are references to Faraday's law of induction, however the EMF due to Faraday's law of induction is often purely due to the electric field and not the magnetic field. For example, consider a transformer when you are in its frame of reference: the electromotive force comes purely from the rotational electric field caused by time-varying currents and not the magnetic field. Faraday's law of induction does not say that changing magnetic fields "cause" a current to flow. What Faraday's classic "flux rule" says is that if there is a changing magnetic field through a loop, then there will be some kind of force (it could be electric or magnetic, depending on frame of reference, and so on) that acts around the loop whose line integral around the loop is proportional to the rate of change of flux. Furthermore, in the far field, the electric and magnetic fields are closely related to each other anyway. Some of these issues just come from old terminology - I personally prefer to use the term "electromagnetic induction" or just "inductive coupling" as opposed to "magnetic induction" or "magnetic coupling". I also try to say that a rotational electric field accompanies a changing magnetic field and not that a changing magnetic field "causes" a rotational electric field. But that could just be me being overly pedantic. Hddharvey (talk) 03:43, 6 October 2021 (UTC)

And regarding this idea of the near field... Wouldn't this purely depend on the separation of the antenna from the source (relative to wavelength and regardless of the size of the loop)? I suppose it would be having some near field interactions with itself regardless... Hddharvey (talk) 03:52, 6 October 2021 (UTC)

The original statement was correct although poorly/ambiguously worded. Certainly a radio wave requires both E and H and in a specific proportion (η_0) and orientation. And both act on a full sized antenna (dipole etc.). BUT a small/Hertzian dipole is ONLY sensitive to an electric field (whether a radio wave or not) and a small loop is ONLY sensitive to a magnetic field (whether a radio wave or not).

Consider two equal radio waves in phase lock, one from the west and one from the east. At a particular location the H will cancel and the E doubled. Place a small loop there (axis in N-S direction), and you will receive NO signal. Move λ/4 to the east, and now the E cancels and H doubles. A hertzian dipole receives NO signal. Clear?? Interferometrist (talk) 19:54, 6 October 2021 (UTC)

I think there should be a clear distinction between “is caused by” and “is calculated from”. If you have an electrically small loop antenna far away from the source, you can calculate the received voltage rather easily from the magnetic field alone. That doesn’t mean the antenna is only sensitive to the magnetic field. You can also calculate the signal from the line integral of the electric field, but that is more difficult. Constant314 (talk)

I agree with Constant314 regarding causality. Interferometrist you raise a good point about E and H potentially having a more complex relationship (even in the far field). However the Maxwell-Faraday equation still holds in these cases, and if there is a changing magnetic flux passing through the loop, then there will be a circulating electric field around the loop. In the frame of reference of the loop antenna, it's still the electric field that acts on charges along the conductor axis. How is this possible if the electric field has cancelled? The electric field cannot cancel to zero everywhere - if there are plane waves travelling in opposite directions, then both E and H will form standing waves with their nodes 90deg out-of-phase. If you position the loop antenna so that it is on an antinode of H, then it will be centered on a node of E - however E is only zero in the very center. It is non-zero further out. If you make the loop negligibly small compared to the wavelength so that this effect is negligible, then E.dl around the loop will decrease - but so will the magnitude of the oscillating flux through it (as per Maxwell-Faraday equation) so you end up with the same result whether you think of only the electric field or only the magnetic field (with Maxwell-Faraday equation to bridge the gap). Hddharvey (talk) 23:53, 6 October 2021 (UTC)

But regardless, it is the electric force acting along the direction of the conductor. Off the top of my head, it looks like this might be is the case for any frame of reference in the example you provided, not just one where the loop antenna is stationary. So I don't think it's really fair to say that the loop antenna exclusively picks up the magnetic component. I'd be open to arguments that the magnetic component of the impingent wave has some effect on the final signal seen by the receiver, but it seems to be the case that the electric force is dominant in either case. Hddharvey (talk) 00:17, 7 October 2021 (UTC)

@Hddharvey: I agree, but reliable sources do say things like (1) “the loop antenna is a B-field antenna,” or (2) “the loop antenna responds to the magnetic field.” Those phrases are sloppy language for (3) “the response of loop antenna can be calculated from the magnetic field.” In the frame of the electron, it is the electric field that pushes the electron around the loop, but that is too esoteric to ask the reader to understand. Phrases 1 and 2 are a form of lies-to-children, that may hold the reader back from the next level of understanding. It is always a balancing act between too simple and too complicated, but I feel that phrase 3 is not significantly more complicated. It states a simple pragmatic fact without implying any deeper (and flawed) causality. There is compromise language such as (4) “the loop antenna is regarded as being sensitive to the magnetic field in the sense that the response can be calculated from the magnetic field.” That way, it is an accurate statement without forcing the reader to comprehend new (for them) physics.Constant314 (talk) 01:01, 7 October 2021 (UTC)

No I'm sorry but you're really wrong. A small antenna can be made solely sensitive to only the E or H field e.g. Hertzian dipole and small loop. Using a small loop you can receive my radio signal but you cannot possibly tell me if it even IS a radio signal or a local magnetic field with that signal. Even by rotating it (unlike a large loop which I believe could well discriminate if rotated). Now, a current in a coil caused by the integral of the E field (regardless of gauge, reference frame) over that loop is the ONLY way a magnetic field is ever detected electronically, so calling that detection of an E field is just wrong, it isn't the E field of the radio wave itself.Interferometrist (talk) 01:16, 7 October 2021 (UTC)

Also, I should have pointed out, that the loop we're talking about is in the X-Z plane thus partially (but that's enough!) longitudinal w/r/t the transverse EM wave (if any) polarized at least partially in the X direction and possibly partially in the Y direction to which this antenna is insensitive. The Z component causing the current in the loop is clearly not part of any such EM wave. Interferometrist (talk) 01:25, 7 October 2021 (UTC)

Well I take it back. I guess you can see the loop as including the electric field in the X direction of the wave itself which is also the result of the wave's magnetic field in order to propagate. But the main point is that you don't need a EM wave in the first place for the small loop to detect something, just a changing magnetic field and never just an electric field (one of its advantages: rejection of nearby ignition noise etc.). Interferometrist (talk) 01:35, 7 October 2021 (UTC)

@Interferometrist: Sorry if I am being obtuse, but it is not obvious to me what you are responding to or whether you are responding to user:Hddharvey or me. Rather than shot-gunning a response, I’ll just ask you to clarify. I am trying to visualize two interfering plane waves intersecting at an angle, if that is the case that you are considering. Given the freedom to orient the antenna as needed, I cannot think of any case where you could extract power with an electric dipole but where you could not also extract power with a loop or vice versa, extract power with a loop but not a dipole. But maybe I suffer from insufficient imagination.Constant314 (talk) 01:44, 7 October 2021 (UTC)

And to be clear, I'm not saying we should say that a loop antenna is sensitive to the E field. In the far-field E and H are very tightly linked, and I'd prefer just to think in terms of radiation patterns. I'm just saying that the wording "picks up the magnetic component" seems a bit much. I agree with @Constant314: that thinking in terms of the magnetic field can be helpful (even when it is the electric force that is actually responsible). For near field inductive coupling, including transformers (especially with non-air cores), thinking in terms of the magnetic field helps a lot. However it is important to keep in mind that it is often still the electric force that is finally responsible for the signal you're measuring - even in different frames of reference. Hddharvey (talk) 01:53, 7 October 2021 (UTC)

@Constant314: @Interferometrist: Some additional thoughts: I can understand people associating a dipole antenna with the electric field and the loop antenna with the magnetic field - since a transmitting dipole antenna is similar to an electric dipole, whereas a transmitting loop antenna is similar to a magnetic dipole. I've also noticed that, very roughly speaking (details depend on gauge):

• When people say "electric" what they often mean is $-\nabla V$ (you might call this "electrostatic" or "capacitive coupling") and not the entire electric field, which is $-\nabla V-{\frac {\partial A}{\partial t}}$.
• When people say "magnetic" they often mean "magnetic force AND $-{\frac {\partial A}{\partial t}}$ in the near field" since it is often the rotational component of the electrical force that is finally responsible for what people refer to as "magnetic" (if you ignore magnetostatic fields then you might call this "inductive coupling").
• When people say "electromagnetic" they are often referring to far-field radiation (where the electrostatic and magnetostatic fields are negligible and the far-field electric and magnetic fields are more closely related and are both produced by accelerating charges).

Sometimes these words are used in slightly different ways - but that's a rough picture of what I've noticed. Because this terminology is pervasive, it can't really be ignored - however I think you can still avoid completely abusing it. I can't see any way in which "receiving predominantly the magnetic component of the electromagnetic wave" is meaningful, since in the far field (which this statement seems to refer to) there are no electrostatic or magnetostatic fields and the electric and magnetic fields are closely linked (even - it appears - when multiple waves are interfering) and since it appears to be the electric force from the incident waves that is mainly responsible for moving charges along the antenna. There are similar iffy statements in the article. One thing that I noticed was "Due to its direct coupling to the magnetic field, unlike most other antenna types, the small loop is relatively insensitive to electric-field noise from nearby sources. No matter how close the electrical interference is to the loop, its effect will not be much greater than if it were a quarter wavelength away.[8]" - The citation of this statement seems to directly contradict it! The citation says: "Nothing is further from the truth! At relatively small distances a small magnetic loop is more sensitive to electric fields than a small electric field probe.". Hddharvey (talk) 01:44, 8 October 2021 (UTC)

That source appears to be a personal website. As such, it is not a reliable source. Constant314 (talk) 01:59, 8 October 2021 (UTC)

I didn't intend to say that it was reliable. I think both the statement from the article and that source are questionable. Perhaps the statement from the article could be correct if you interpret "electric-field noise" as electrostatic noise or something? However in either case it needs a better citation. Hddharvey (talk) 02:04, 8 October 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.

@JNRSTANLEY:I’m going to ramble a bit. You can decompose the electric field into two components. One component is rotational (or non-conservative) and the other is irrotational (or conservative). Where there is changing magnetic field, there is a proportional, non-zero, rotational component. The loop antenna responds to the rotational component. A measuring system with a loop antenna can be designed to mostly ignore the irrotational component of the electric field. It is correct to say (1) that loop antennas are useful for measuring the magnetic field. It is incorrect to say (2) that loop antennas respond to the magnetic field. It is also incorrect to say (3) that loop antennas do not respond to the electric field. But reliable sources do say both (2) and (3).Constant314 (talk) 21:47, 9 October 2021 (UTC)

@JNRSTANLEY: My issue isn't that I don't believe H field probes exist that are insensitive to certain types of near-field E fields (which as Constant314 says, probably refers to the "charge-induced" part of the E field that EE's seem to consider responsible for voltage). My issue is that I'm not sure what this has to do with an article about "loop antennas". As you said yourself, there are other design factors that come into play for H field probes, such as shielding - so statements that are really intended to be about near-field H field probes don't seem like they belong in an article about loop antennas (although the article currently has a section at the very end discussing some near-field applications but that is nicely separated from the rest of the article and is also pretty explicit that it's technically not an antenna in that case). In my opinion these details should be removed - the entire article (except for maybe a section at the end) should be talking about antennas (and this word traditionally refers to far-field uses). I also think that sections about "magnetic vs. electrical antennas" and references to it as a "magnetic loop" should be removed unless there are reliable sources that use that terminology. Although even if there are reputable sources that discuss it, I still think the terminology is pointless/confusing/misleading - at least for far-field cases. Hddharvey (talk) 23:57, 9 October 2021 (UTC)

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)

@JNRSTANLEY:I am sure we can find compromise language. But, I ‘m going to ramble again. Here is a picture of a calibrated loop antenna: [[1]]. The grey tube is not the antenna. It is the electric field shield; it is a conductive tube. The antenna is inside the grey loop. The antenna is continuous. The shield has a gap. The white plastic part at the top covers and spaces the gap. The shield is there because the wire loop does respond to the electric field. In the near field, the wire loop is essentially the plate of a capacitor. The E-field can pump that plate. Differential amplification helps, but an E-field shield also helps.

I suppose that it is sloppy language. "Loop antenna' can mean literally the loop of wire or it can mean the entire assembly including the shield, the box, and the electronics. I was presuming that the article was about the fundamental loop element, but just be clear what is being described and use reliable sources. I reverted your previous edits because the sources were not reliable. Constant314 (talk) 05:59, 11 October 2021 (UTC)

@JNRSTANLEY: @Constant314: Is shielding like that (to reduce the effects of certain types of electric fields) specific to loop antennas? Hddharvey (talk) 08:53, 11 October 2021 (UTC)

I do not know. Constant314 (talk) 15:52, 11 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)

@JNRSTANLEY: Good luck with your research. If the benefit of this kind of shielding is that it mitigates near-field phenomena like electrostatic/capacitive coupling, then this seems unrelated to concepts like polarization and radiation patterns which only apply to EM waves (far-field). My main concern is that it seems like the topic of shielding is somewhat orthogonal to the loopiness of an antenna, so unless (a reliable source shows) that this shielding has a special relevance to loop antennas, then we shouldn't try to give that impression. IMO the section "Insensitivity to locally generated interference" should be removed for the time-being until we have better details since it is unsourced and (in my opinion) wrong as currently written. Hddharvey (talk) 00:27, 12 October 2021 (UTC)

I'm sorry but I had been too busy to follow all this, but I can see the discussion (partly) is going in the wrong direction. A small (meaning -> 0) loop is dual to a hertzian dipole and is ONLY sensitive to the magnetic field (which MIGHT come from a radio wave) and NOT to any external electric field beyond that, just as the hertzian dipole is ONLY sensitive to E and no sensitivity to H. These are exact duals, the only difference being the non-existence of magnetic monopoles, but if you include that in maxwells equations then no difference at all, now that we can detect changing electric fields by their acceleration of magnetic monopoles around a loop.

I'll say it again: You have two identical radio signals from opposite (or just different) directions (or a standing wave due to a reflector at one end), and at one point E cancels and the magnetic loop detects NOTHING just as the hertzian dipole detects NOTHING where E is zero (and H large). To say that when you detect a magnetic field electronically you are "really" detecting the torroidal electric field, is like saying that when you say you're measuring temperature, no you're REALLY measuring the height of a mercury column, or that your altimeter doesn't measure altiitude, it measures barometric pressure, or that my electronic camera doesn't measure light, it measures electron-hole pairs. Of course in any case where a device measures something and uses wires then you are measuring an electric field/current in the final analysis, but you don't say that, you tell about what you are measuring, and in this case it is nothing more or less than the magnetic field (which your apparatus turned into an electric current), wherever it came from, whatever it was due to. Since this page is about radio antennas I wouldn't add it, but you could well have a section on magnetic near-field communication in which case there is NO EM wave generated but you DO use a magnetic loop identical to these to transmit/receive the H-only signal. But there is no way of telling from the antenna terminals whether there ever was an EM wave involved or what direction it's coming from, even when you rotate the antenna (and get a sin(phi) response, always), just a great circle of where it could come from if it were from a single source in the far field (in other words, you CAN determine the polarization of H, but know nothing about E because you never measured it).

I also see that "small" loops aren't always as small as I'm talking about, as it extends to 1/3λ circumference where this is all no longer so true. Even at 1/3λ though, the formula for R_r is correct to 2% according to Balanis.Interferometrist (talk) 23:21, 15 October 2021 (UTC)

Also I should have responded to this:

You can decompose the electric field into two components. One component is rotational (or non-conservative) and the other is irrotational (or conservative).

Right. And what's important is that there is no way to create an E field with a torroidal component EXCEPT with a changing magnetic field, so when EVER you are measuring that torroidal component you ARE measuring dB/dt whether you like it or not. And someone said:

The citation of this statement seems to directly contradict it! The citation says: "Nothing is further from the truth! At relatively small distances a small magnetic loop is more sensitive to electric fields than a small electric field probe."

Well indeed nothing could be further from the truth than that statement. At any distance it has NO sensitivity to (only) an electric field, and this is an advantage when you have electric field noise (sparking etc.) in the very near field which a dipole would very well pickup.Interferometrist (talk) 23:32, 15 October 2021 (UTC)

Lots of tangents here but let me just address one. The surface of the wire in a loop antenna is effectively a capacitor plate. The antenna will respond to the plain old ordinary irrotational electric field. So much so that laboratory loop antennas incorporate an electric field shield. If there was no electric field response, there would be no need for the E-field shield.Constant314 (talk) 01:22, 16 October 2021 (UTC)

Oh wait a minute! I have a sensitive piece of equipment detecting a faint signal and you setup an arc welding operation next door, and I'm not supposed to be concerned because it's on a different frequency or something? Of course you could get such interference at SOME level if the receiver isn't perfect. In other words, if the tuner input responds to common mode signals on the two antenna terminals. But if only differential, that is, having to do with the voltage generated by the magnetic field, then nothing. So no, the differential voltage across the coil ends cannot be effected without a changing magnetic field (including thus, a changing electric field that causes a changing magnetic field and visa-versa, better known as an EM wave). Interferometrist (talk) 05:03, 16 October 2021 (UTC)

@Interferometrist: Glad you have come back into this discussion. As to the duality of the infinitesimal dipole and the infinitesimal loop, I see one difference. The dipole can detect truly static E fields, but the loop cannot detect truly static H fields. Only a changing H field can be detected, and this generates E fields. This of course is expressed in Maxwell's equations as well as being common sense. When considering quasi static fields, how does this manifest itself? Is the dipole capable of totally rejecting H fields, but the loop unable to totally reject E fields? Or are changing H field generated E fields a necessary part of the detection of the magnetic fields? How do these compare to E fields that are generated by noise sources? JNRSTANLEY (talk) 15:59, 16 October 2021 (UTC)

Well listen, I can answer that, but I sort of resent doing so because I'm getting the impression that you (perhaps not consciously) are just trying to come up with objections to what I'm putting out rather than really clarifying the issues.

I am truly sorry to have left that impression. I very much respect your understanding and contributions, I am still confused about this issue, although your latest response does make sense. Rotating the E or H probes does establish the duality of rotating probes. As to stationary ones, I am still pondering that. And I think I agree with your "main point", but would like to be more sure of it, which will require more reading.JNRSTANLEY (talk) 23:50, 17 October 2021 (UTC)

And I'm not sure it's even of fundamental importance, but your point about detecting a DC E or H field is appealing because everyone knows a loop only detects the time derivative of B. In practice you can detect a static magnetic field with a rotating coil, and you can detect a static E field with a rotating electric dipole (I've seen these used by atmospheric electricity physicists to measure the atmospheric vertical electric field, which I think you'd have trouble detecting with any other apparatus; I can only think of one: the raised metal can dripping water). In practice, even if you have a voltmeter with truly infinite impedance (like the leaf electrometer of yesteryear) connected to an electric probe, it can only be used to measure the DC field if you have been watching it since time zero when you know (or caused) there was zero E field, so after that you only detected changes in the static E field just as if you were measuring a voltage through a large capacitor, in effect integrating current coming "through" that capacitance in another capacitor with no discharge path. So similarly in principle you could integrate the output of a loop since time zero (when B==0) until now to find B(t). In order to duplicate the performance of the first case where you detected static E, you would need to integrate current with a superconducting coil, or integrate the loop voltage over time from the readings of a truly zero-offset voltmeter; in practice that's much more difficult than finding a low-leakage dielectric but in principle these problems are the same.

Again, I don't know if there is any point here relevant to the original discussion. Again I stand by my main point: that an ideal infinitesimal electric dipole (capacitively coupled to the local E field) is TOTALLY insensitive to H and an ideal infinitesimal loop (inductively coupled to the local H field) is TOTALLY insensitive to E, and neither is capable of detecting the direction of even a single incoming radio wave, only its polarization (or of detecting if there even is a radio wave involved).Interferometrist (talk) 22:27, 17 October 2021 (UTC)

Usage

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)

Further discussion on semantics

@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)

Some comments. (1) I haven’t heard of loop antennas being called “magnetic loops,” but google shows many hits for the term “magnetic loop antenna.” I don’t have an opinion on whether this term should be mentioned. (2) You cannot extract energy from the EM field unless the Poynting vector is non-zero on the surface of the antenna. That means that both E and H must both be non-zero at some points on the antenna. (3) In the case of a standing wave at a point where E is strong and there is total H cancelation, you can extract power by placing an electric dipole there. Currents set up in the dipole produce an H field such that E x H is non-zero. There is nothing in the Poynting theorem that says E and H must have the same source.

Moving forward, here is my opinion about what goes into the article. If we say

• Loop antennas are used to measure the magnetic field. That is absolutely true, and I have no objection.
• Loop antennas respond to the magnetic field. That is a partial truth, but appropriate for this article. I have no objection.
• Magnetic field measuring systems that use loop antennas can be desensitized to the incident electric field. Another partial truth that is appropriate and I have no objection.
• Loop antennas do not respond to the electric field. That is untrue and I would object. A common mode response is still a response. Arguments about the receiver being differential or balanced are out of scope. The statement is about loop antennas, not systems using loop antennas. Constant314 (talk) 22:04, 18 October 2021 (UTC)

Interesting point about Poynting's theorem. I agree with your suggestions. A few additional opinions:

• If we talk about applications of small loops for magnetic field probing, or mention any facts that use sources specific to certain applications of small loops, then it should be clear that it's not about small loops in general (ideally in a separate section or subsection). I'd personally like any discussion of near-field applications to have explicit sections (similar to the section at the end of the current article).
• "can be desensitized to the electric field" seems a lot better than saying a magnetic probe "rejects E" or "does not respond to E". I'd personally like some more specific wording than that, but this is a good compromise - especially if no good source can be found to use more specific wording.
• My objection to "loop antennas do not respond to the electric field" applies even in the far-field case. For any stationary receiving antenna, the received signal still needs a non-zero E component around the antenna for it to pick up anything. This fact cannot be compensated with by the antenna's self fields, since the charges in the antenna won't do anything in the first place if there is not an E field to move them (the magnetic force is zero if the antenna is stationary).

Thanks. Hddharvey (talk) 23:52, 18 October 2021 (UTC)

This discussion is getting repetitive. I will not answer Hddharvey's failure to see why a fixed dipole (or monopole referenced to ground) without knowing its history since a known state cannot measure the local electric field, because I'm pretty sure he can figure that out himself, likely in less time than it took him to write that. But the main point again:

Loop antennas do not respond to the electric field. That is untrue and I would object.

Again, the objection is wrong because it is based on the behaviour of non-ideal components (but already pretty damn good!) which can be enhanced by electric shielding which you obtain by calling it a "magnetic field probe" and paying 10x as much. Again I believe that Constant314 could figure this out himself. Or I could ask him to compute what the response SHOULD be according to his analysis and see if he doesn't come up with zero.

Also untrue:

(2) You cannot extract energy from the EM field unless the Poynting vector is non-zero on the surface of the antenna. That means that both E and H must both be non-zero at some points on the antenna.

Indeed you cannot measure anything without the transfer of energy, but that isn't what you said. I have twice given an example that contradicts both sentences: two counterpropagating EM waves, measured by an electric dipole at a position where H==0, or measured using a small loop where E==0. Before you inserted an antenna Re{E×H^*}==0 everywhere (no net power flow) though energy present in both E and H.

Many times repeated by Hddharvey is that there IS an E field wherever you have a changing H field, so one could say (yes you COULD) that the loop is detecting an electric field (torroidal component). According to Maxwell's equations, the curl of E and the time derivative of B are IDENTICAL, so you COULD use either terminology but the way everyone else refers to what you're measuring is to call it dB/dt. What's more, it was asked if one could measure a static (dB/dt=0) magnetic field with it, and of course the answer is no, unless you want to spin it in which case d(flux)/dt is no longer zero so you DO measure B, but in this case there is no way you could talk about an E field being present. So I would say that again, it is measuring a changing magnetic field, but the reason it's changing is because of the rotating reference frame. So again, I would say the correct formulation is as written: the infinitesimal magnetic dipole detects the magnetic field, period. And if someone asks "oh, isn't there a torroidal electric field there too?" I'd answer "yes, that's what I just said." Interferometrist (talk) 15:27, 19 October 2021 (UTC)

@Interferometrist:If you are not insisting that the article contain a statemen to the effect that the loop antenna does not respond to the E-field, then we can terminate this discussion.Constant314 (talk) 19:31, 19 October 2021 (UTC)

@Interferometrist:I agree this discussion has gotten repetitive and taken up a lot of space. If you want to continue the discussion, we can do it on my talk page. The latter parts of your reply make it sound like you agree with me. As I said, the main problem I had was saying that the small loop does not respond to E at all. My problem is not that you say a small loop measures (the rate of change of the flux of) H - because clearly it does (if I gave that impression anywhere, then I'm sorry). It clearly also measures (the loop integral of) E, but I'm happy with ignoring that - I just don't want to give the impression that it is "TOTALLY insensitive" to E, because that is wrong, and Constant314 seems to agree on that. We will need to remove the statements in the article that a loop antenna is "insensitive to an external electric field" and the faulty reasoning based on the limit to an infinitesimal current loop. In the limit as the loop radius approaches zero and the receiver gain correspondingly approaches infinity, all you get is that the loop measures $\mu _{0}\partial \mathbf {H} /\partial t$ and $\nabla \times \mathbf {E}$ - which doesn't make it "insensitive" to E. It is true that if you place the antenna on a node of E and reduce its size to zero, then E will approach zero everywhere along the antenna; however, the flux of H will also approach zero - and to actually read anything you'd need an infinite gain to compensate. In real situations, you need the received signal to have non-zero E somewhere along the (stationary) antenna for anything to happen. Hddharvey (talk) 22:19, 19 October 2021 (UTC)

OK, I didn't want to prolong the discussion, but what I did say is that it doesn't respond to an external electric field, it isn't useful at all in detecting an electric field that someone might want to measure, and it only can be said to respond to an electric field insofar as it is sensitive to a changing magnetic field and whenever you have a changing magnetic field you have a torroidal electric field associated with it. So if you want to say that a compass needle, or a hall effect sensor are capable of measuring the curl of the electric field, then I guess you could go ahead and say the same for a coil. Or if you want to say that an ohmmeter measures current (how else could it work?). Or if you want to say that a Foucault pendulum measures the earth's gravity, or that a thermometer measures the expansion of mercury. And then I guess you would also have to say, with equal justification, that a hertzian dipole measures the curl of the magnetic field.

But the direct thing a coil measures is normally just called a magnetic field (just as one would say a thermometer measures temperature), and there is no way you could generate an electric field and detect it in this way except by creating a changing magnetic field. If you ask anyone how to create an electric field I'll bet that's the last answer you'd ever get (I wouldn't have thought of it if you had asked!). So I stand by what I wrote into the article, and believe that it could only be disputed through nit-picking, the sort of objections we generally reject when a sentence otherwise is valid, informative, and essentially accurate:

The small loop antenna is also known as a magnetic loop since the signal at its terminals is dependent solely on the magnetic field present (as per Faraday's law of induction) and insensitive to an external electric field.

PS I hadn't seen the latest from Hddharvey when I wrote this, but I doubt I'd neeed to add anything.Interferometrist (talk) 22:32, 19 October 2021 (UTC)

@Interferometrist: It might be the case that a small loop used as a magnetic field probe gives much more useful magnetic field measurements than electric field measurements. However, this is a completely separate issue to whether the loop antenna "responds to" or "is sensitive to" the electric field. The article currently says the antenna is "insensitive" to the electric field and uses some questionable reasoning to arrive at that conclusion. I discussed this further above in my previous reply (and the replies before that). Hddharvey (talk) 23:27, 19 October 2021 (UTC)

Just to answer two issues raised by by Hddharvey:

It is true that if you place the antenna on a node of E and reduce its size to zero....

No I never said anything about reducing its size to zero (and when you do, we both know how to deal with differentials without having to mention that d(anything)==0). I offered the case of two counterpropagating plane waves which at a particular plane Z=z_null has (in normal parlance) zero electric field (the two cancel) but a doubled magnetic field which the loop responds to. And:

In real situations, you need the received signal to have non-zero E somewhere along the (stationary) antenna for anything to happen.

Indeed for a stationary antenna you have the torroidal E field that you always have with a changing magnetic field. But I pointed out before that with zero E field (or either sort) and a rotating coil you detect the magnetic field regardless according to the same formula (which you perhaps attribute to a "different reason" but that's beside the point). Or just considering inertial reference frames, if you have a STATIC nonuniform magnetic field and the coil is moving through it (constant velocity, any direction) then it responds again to d(flux)/dt with no E field present. Of course you can then point out that in the (inertial) reference frame of the loop there IS a (torroidal) E field (or a yet different E field in another inertial reference frame), but here the point again is that in ALL inertial reference frames and with no free charges we agree on what B(r,t) is everywhere and we observe the same response of the magnetic probe due to the same d(flux)/dt that we would all agree on, even though they disagree about what E is (if you include the torroidal component). So what all observers agree with is what I said, and what they disagree about the loop is insensitive to.Interferometrist (talk) 23:39, 19 October 2021 (UTC)

Alright, perhaps what we can agree on is that a loop antenna has zero response to the electric field of an incoming radio wave. Interferometrist (talk) 23:44, 19 October 2021 (UTC)

I disagree. Given that a stationary receiving antenna requires a non-zero electric field from the incoming radio wave somewhere along it to produce any signal, clearly you cannot say that a loop antenna is insensitive to E. Sure, you could concoct scenarios where it produces a response without E, e.g., by rotating it in a static magnetic field - but this doesn't contradict what I'm saying. I'm not saying that a small loop insensitive to the magnetic field - I'm just saying that it's not insensitive to the electric field. We should remove the part of the article that claims this (and the infinitesimal dipole reasoning for it). Hddharvey (talk) 00:06, 20 October 2021 (UTC)

And as I said before, if the antenna is stationary, then it is the electric force that produces the signal and not the magnetic force. The magnetic force will have zero force on stationary charges - and even once current starts flowing, the force will be perpendicular to the direction of the conductor. So clearly it is not the case that the small loop is insensitive to E when it is actually the electric force responsible for the signal (at least in the stationary case - but most antenna theory assumes the antenna is stationary anyway). Hddharvey (talk) 00:16, 20 October 2021 (UTC)

I apologize to Hddharvey. Indeed I had invoked the loop size approaching zero (but perhaps with the number of turns increasing according to 1/area) in order to make a valid statement that wasn't an approximation. Anyway, I hope we can all agree on the final wording I introduced, which is sufficient inasmuch as this is an article about radio antennas and the philosophers can debate whether it is proper to say that a loop detects curl E.Interferometrist (talk) 00:21, 20 October 2021 (UTC)

The article currently says that the small loop is insensitive to the electric field of an incoming radio wave - which is incorrect. See my two comments above. Hddharvey (talk) 00:25, 20 October 2021 (UTC)

I agree with Hddharvey. It is incorrect to say that the loop antenna is insensitive to the E-field. However, the following are correct statements: “the loop antenna may be analyzed as if it were insensitive to the E-field” and “The E-field may be ignored when calculating the response of a loop antenna.” Constant314 (talk) 00:46, 20 October 2021 (UTC)

True. I'd prefer the second variation since it is simpler and closer to what actually happens. Or even "the response of a small loop antenna is determined by the magnetic flux through it". Hddharvey (talk) 00:58, 20 October 2021 (UTC)

At the risk of prolonging this discussion, note that the peak signal of the electrically small loop responding to a passing plane wave occurs where the H-field is zero, because where the derivative of the H-field peaks and that is where the curl of the E-field is obvious.Constant314 (talk) 02:35, 20 October 2021 (UTC)

True. This is why (as you can see) I edited the article to say the antenna response "is determined by" magnetic flux. I liked this because it was nice and concise while also being vague enough to leave room for dependency on the derivative (and not just the flux at a single instant). Feel free to make the wording more precise if you think my chosen wording was misleading. Hddharvey (talk) 02:49, 20 October 2021 (UTC)

Perhaps this illustration will help. It is easy to see that you can ignore the H-field and calculate the voltage from the line integral of the E-field. I hope this makes it obvious that a loop antenna can respond to the E-field.

A propagating plane wave inducing voltage on a loop antenna.

Constant314 (talk) 03:01, 20 October 2021 (UTC)

References related to "receiving predominantly the magnetic component of the electromagnetic wave"

@Constant314: Hello Constant314, Hello Everyone. I think that the following reference is exactly about the subject matter of this discussion . 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)

Yes, thanks. I didn't look at that reference, but it's exactly what I've been saying (and what the equations say). You talk about a loop of a given size with the frequency increasing, which qualitatively is identical to speaking of a loop that grows larger with the frequency constant (the relevant quantity being the loop's size relative to wavelength). The discussion you jumped into, if it wasn't clear, had to do with the limit as a loop grows tiny compared to the wavelength, correctly described according to your reference as you quoted it.Interferometrist (talk) 22:07, 31 October 2021 (UTC)

@FreddyOfMaule: Thanks for the reference. In section IV the authors state "the line integral is zero if $i(s)$ is uniform in the thin wire and if the length of the arbitrary path over the gap is zero. It may therefore be ignored if the frequency is sufficiently low to allow us to consider that $i(s)=I_{0}$ everywhere in the thin wire, and if the length of the arbitrary path over the gap is sufficiently short." Furthermore, formula (75) implies that the response of the antenna is simply given by the flux of H if the current distribution is uniform. I think these parts of the reference support statements already written in the article. I have added that as a citation and expanded upon the existing statements a bit. Hddharvey (talk) 00:10, 1 November 2021 (UTC)

@Interferometrist:@Hddharvey: The authors are glad to see that, in addition to the reviewers of IEEE Transactions on Antennas and Propagation, they have some satisfied readers. Thanks :-) FreddyOfMaule (F5OYE) FreddyOfMaule (talk) 08:31, 1 November 2021 (UTC)

Are you one of the authors? Your name "Freddy" matches. Hddharvey (talk) 09:54, 1 November 2021 (UTC)

Yes. In fact, a radio amateur call sign, like F5OYE (mentioned in the bio) is more unique than a given name, at a given time. Thus, if you type F5OYE on your prefered search engine, you easily find out to whom it is currently attributed. FreddyOfMaule (F5OYE) FreddyOfMaule (talk) 11:34, 1 November 2021 (UTC)

@Constant314:@Interferometrist:@Hddharvey: Hello Everyone. We have published a new article which may be relevant to this discussion. This is an improved version of and it can be dowmloaded at no cost. Moreover, Section V of this new article was written to specifically address the topic of this discussion. FreddyOfMaule (talk) 17:01, 6 January 2023 (UTC)

Is there anything new or unexpected in your publication? Is it an exercise showing mathematical rigor? Constant314 (talk) 10:05, 7 January 2023 (UTC)

@Constant314: Yes, there is a lot of new material in the new article. The original 2020 article was less than 9 two-column pages long, the new article is more than 19 two-column pages long. Some of the added material is very technical, like sections VIII and IX. Some of the added or improved material is less computational and more relevant to this "receiving predominantly the magnetic component of the electromagnetic wave" discussion (and was introduced to address questions raised in this discussion), for instance Section II.B, Section IV and Section VII.C. For the Wikipedia readership, the fact the new article is open access may also be an advantage. Being one of the authors, though, I am not the best qualified to assess the new article.

Both articles accurately explain what a planar wire loop antenna actually receives, at any frequency. They are consequently fully relevant to the present "receiving predominantly the magnetic component of the electromagnetic wave" discussion. However, the new paper is a greatly improved and expanded version of the old one. In my opinion, the new paper should replace the old one in the "References" section of the "Loop antenna" article of Wikipedia. FreddyOfMaule (talk) 07:28, 10 January 2023 (UTC)

Can you summarize the results in a few sentences? Constant314 (talk) 19:04, 12 January 2023 (UTC)

@Constant314:@Evemens: Both articles use "electromagnetic field" to designate an electric field and a magnetic field which satisfy Maxwell's equations, and define a decomposition of an arbitrary incident time-harmonic electromagnetic field ${\mathcal {F}}_{i}=(\mathbf {E} _{i},\mathbf {H} _{i})$ into four elementary time-harmonic electromagnetic fields ${\mathcal {F}}_{A}=(\mathbf {E} _{A},\mathbf {H} _{A})$, ${\mathcal {F}}_{B}=(\mathbf {E} _{B},\mathbf {H} _{B})$, ${\mathcal {F}}_{C}=(\mathbf {E} _{C},\mathbf {H} _{C})$ and ${\mathcal {F}}_{D}=(\mathbf {E} _{D},\mathbf {H} _{D})$. The important point is that each of the elementary time-harmonic electromagnetic fields could exist independently of the others. ${\mathcal {F}}_{A}$ is the mirror-symmetric part of the transverse electric component of ${\mathcal {F}}_{i}$, ${\mathcal {F}}_{B}$ is the mirror-symmetric part of the transverse magnetic component of ${\mathcal {F}}_{i}$, ${\mathcal {F}}_{C}$ is the mirror-antisymmetric part of the transverse electric component of ${\mathcal {F}}_{i}$, and ${\mathcal {F}}_{D}$ is the mirror-antisymmetric part of the transverse magnetic component of ${\mathcal {F}}_{i}$. The decomposition is the basis of formula (70) of the new paper that gives the open-circuit voltage of an arbitrary planar wire loop antenna used for reception as a function of only $\mathbf {H} _{A}$, $\mathbf {E} _{A}$ and $\mathbf {E} _{B}$. This formula is applicable to any planar wire antenna (circular, square, polyhedral, etc), any incident field configuration, and valid at any frequency at which the thin wire approximation applies. It is possible, especially at low frequencies, to consider that ${\mathcal {F}}_{A}$ causes the intended response of the antenna, while ${\mathcal {F}}_{B}$ may cause an unwanted response. In formula (70) of the new paper, the effect of ${\mathcal {F}}_{A}$ is subdivided into a surface integral of $\mathbf {H} _{A}$, which may be viewed as the intended response of the antenna, and a line integral of $\mathbf {E} _{A}$, which can be viewed as a correction term for the gap width and the nonuniformity of the high-frequency current distribution, since this line integral vanishes if the current is uniform over the integration path. ${\mathcal {F}}_{C}$ and ${\mathcal {F}}_{D}$ have no effect on the antenna. This analysis explains the characteristics and limitations of a planar wire loop antenna used as a measuring antenna or as a direction finder, and is an accurate answer to the "receiving predominantly the magnetic component of the electromagnetic wave" discussion. FreddyOfMaule (talk) 09:06, 13 January 2023 (UTC)

@FreddyOfMaule: @Hddharvey: @Interferometrist: I think we are getting off topic for this article. Let's continue the discussion on your talk page and invite everyone who is interested. Constant314 (talk) 20:58, 13 January 2023 (UTC)

@FreddyOfMaule: @Constant314: I agree that the "new paper" better explains why the formula (70) is exact and applicable to any incident field, in the presence of any nearby items; and that the open-circuit voltage corresponding to the surface integral of $\mathbf {H} _{A}$, which may be viewed as the intended response of the antenna, is only an approximation. Evemens (talk) 09:27, 24 January 2023 (UTC)

References