Talk:Near and far field

Misleading

The text claims that the 1/r³ term is the electrostatic field term, and the 1/r² term is the induction term.

1. These should be called "magnetic" and "electric" terms; not electrostatic. Near and far field have no meaning in electrostatics
2. The terms switch meanings when you talk about a current loop. The assignments referred to in the text are only about dipoles

This article is confusing, otherwise, and there's no reason it couldn't be made accessible to laymen. — Omegatron 02:22, 25 November 2006 (UTC)

I have a BS in Physics and I have a copy of the Jackson text somewhere... I'll put this on my todo list. I'm not sure that this can be accessible to a true "layman", but it could be somewhat simpler. The main point of the article should be that the "near field" (in classical theory at least) is the part of an EM field (or also sound waves, etc) that does not propagate (though does theoretically extend "infinitely") and concentrates its energy nearby, whereas the "far" field allows the energy to propagate arbitrarily far, falling off as an inverse square of the distance (unless perfectly collimated, which I suppose is theoretically possible). - JustinWick 23:11, 7 December 2006 (UTC)

I believe the far field falls of with distance rather than distance squared because the radiant lobes are roughly conic sections rather than spherical so shell volume is dependent on R rather than R squared. Bill field pulse (talk) 20:44, 28 January 2024 (UTC)

The Feynman explanation is here. It is, as well as I know, not the way that other books do it, but then again, he got the Nobel prize for QED. There are three terms. The first is the field at the retarded time. The second is nature's attempt to correct for the motion, and the third is what we use as the far field term. The terms are based on the charge distribution, derivative of charge distribution, and second derivative of charge distribution. The system is that you keep taking derivatives until you get a wave equation. Gah4 (talk) 02:14, 30 January 2024 (UTC)

I picture a thing called instantaneous charge distribution that forms a spherical shell around a charge. If the charge moves the next shell is thinner in front equal at the sides and thicker at the rear. The thicker the instantaneous shell the less intense the field is. Shells always establish first at the origin (at the charge) then outward from that center as fast as possible (the speed of light). Another charge of the same sign always seeks lowest charge distribution directly (electric attraction) and curves towards lowest charge distribution as it moves forward (magnetic attraction). The direction of lowest charge distribution is where a second charge is pulled electrically. The minimum derivative with respect to displacement (or time) corresponds with magnetism. But the far field is due to photons being produced perpendicular to the highest velocity electrons at the magnetic maxima where acceleration is zero. The magnetic maxima occurs where the second derivative with time is zero so I guess the photons of the far field are produced where the electrons stop accelerating and begin to deaccelerate.

However, I imagine that the shape and dimensions of an element determines if we have a current carrying wire (mostly near field) or a powerful transmitter (strong far field). Bill field pulse (talk) 20:57, 2 February 2024 (UTC)

Somewhat confusing. Sometimes he writes v and sometimes v/c. So, I think that in some instances v = v_actual/c. Especially see Table 26-2 and 26-3. On the second table he says "We have also put back the c’s". So, sometimes when you see v you are supposed to think v/c. Constant314 (talk) 13:36, 3 February 2024 (UTC)

Power density of the far field goes as 1/r². The amplitude of E goes as 1/r. Constant314 (talk) 13:47, 3 February 2024 (UTC)

The article explicitly states: The amplitudes of the far-field components fall off as $1/r$, the radiative near-field amplitudes fall off as $1/r^{2}$, and the reactive near-field amplitudes fall off as $1/r^{3}$. Constant314 (talk) 13:50, 3 February 2024 (UTC)

Also I would dispute your claim that near/far fields have no meaning in electrostatics. Consider an electrostatic dipole, or quadrapole, etc. The net "flux" through a sphere centered on a monopole is finite and constant, no matter how large or small the sphere is. Any higher order moment is necessarily neutral, rendering its net flux zero - this is, IMHO, a fundamental difference (see plasma physics for an example of this), as it allows electrostatic monopoles to feel interactions from arbitrarily far away, as the amoun tof available monopoles at any given distance would typically go up as the square of the distance, balancing out the weakening of the E field. Higher order moments only interact at short range (as they fall off too quickly to be offset by the greater abundance of sources as distance goes up). But if you can find something in Jackson that states right out that near/far field are purely electrodynamic concepts, I won't argue :) Gosh, it's been a long time since I thought about any of this - hope I make sense! - JustinWick 00:17, 8 December 2006 (UTC)

The far field term is based on the second time derivative of the charge distribution. That is zero for static fields. But yes, there are various limits of the fields, but not ones that have a time derivative in them. Gah4 (talk) 02:14, 30 January 2024 (UTC)

Hope you will but could you please read a bit of the text in Planck vs Stirling talk (just finished). Maybe too long but I guess it's for people who both like far/near fields and don't mind donuts (at least statistical donuts). To sum up the idea is that if you have:

a straight wire of 5 wavelengths grounded on both ends then you should have n=10 antinodes and with a high standing wave ratio this may be treated as an isolated system for "bench" (statistics). This could be one of the versions of the earliest statistical system Planck originally proposed for the black body radiation. Maybe the wire should be round the table though (read the talk there). If you suddenly disconnect one end from ground, but just to add one antinode, so that you have 11 antinodes, you will spoil the standing wave and some field should be radiated (like a droplet or a tornado in the examples there ) . Now the problem is that Planck wrote something about Stirling that nobody seems to have been able to understand so far. It is suggested that some higher Stirling terms should be somehow visible especially in the near field. Have you ever heard of anyone ever tried something like that? How to combine that with the near/far field? Would you mind leaving your comments here or there.--C. Trifle 09:57, 15 December 2006 (UTC)

I'm not sure that this can be accessible to a true "layman"

Anything can be accessible to anyone. Where "accessible" just means "they can figure out the context, and if they can't understand the details, they at least understand the basic idea, and know what prerequisites they'll need to research to fully understand it". Explaining practical applications helps make it accessible, too.

The main point of the article should be that the "near field" (in classical theory at least) is the part of an EM field (or also sound waves, etc) that does not propagate (though does theoretically extend "infinitely") and concentrates its energy nearby, whereas the "far" field allows the energy to propagate arbitrarily far, falling off as an inverse square of the distance

That's not very clear to me. Both "parts" of the EM field exist everywhere; from the source to infinity. It's just that one is stronger in one region and the other dominates in the other region, right? I wonder if a log-log plot would make this clearer... If it would make the differently powered terms into straight lines that intersect at the boundary between near and far, like the (unrelated except visually) frequency response plots I am more familiar with.

Consider an electrostatic dipole, or quadrapole, etc.

Hmmm... I'm not sure I understand that example. An electrostatic dipole would feel interactions from arbitrarily far away, too. — Omegatron 16:53, 15 December 2006 (UTC)

I wouldn't like to distract you but I am not quite sure if you see my point. It's about the near/far field and the series named after Stirling. There's the quantum theory which was introduced to science using the so-called "first order Stirling approximation" which was then used for at least 25 years by different writers.

The idea should be testable (and perhaps was tested ‹The template Talkfact is being considered for merging.›  I don't know) in antenna radiation. For example, if you look from the layman's point of view, there should be some phenomena that only depend on the number of quanta for big numbers n as well (not only for single photons). Quanta should also exist in UHF/VHF range if they do exist at all. So if you set a transmitter frequency to ν and the power level to P, say 110 MHz and 1W, you will have n=P/hν quanta per second, which is a certain very big number. For the chosen set you should have a certain radiation lobe pattern for the near field and usually a different pattern for the far field. Now if you divide the frequency by half and reduce the power level by half, and use an antenna twice as big, being the exact copy of the one you used before, you will have exactly the same number of quanta n and nothing should change in the far/near field lobe pattern. But if you change the power level for the same frequency, for example 1 W to 0.1 mW, there should be some effect related purely to the number. If there is no effect of any kind on the radiation field pattern depending on number n, one should conclude that the Stirling series is not related to radiation at all. Maybe the effect is only visible for very big differences in number n though, such as ten orders of magnitude or more, I don't know. Have you ever come across anything like that or should it rather be treated as a research topic to be proposed to some research institution? --C. Trifle 11:40, 18 December 2006 (UTC)

I looked at some graphs, one as old as 70 years (Terman), eg. Length of Wire in Wave Lengths , and some more recent. They give various radiation patterns but they were all practical measurements, accuracy is not stated. Here the problem is rather an exercise of different kind, a bit of philosophical (is h really a constant for radio frequency range and below, and are there separate quanta in that range. One can't say using those graphs.) This is last time from me about Stirling and antenna on this page (unless someone finds something really interesting).--C. Trifle 18:13, 18 December 2006 (UTC)

Sorry if I didn't really go into detail on which modes are "propagating" and which modes are "non propagating." Propagating modes are those in which the total energy of the field does not decrease with distance. Because we live in three regular spatial dimensions, a 1/(r^2) field will not decrease in total strength as the sphere of measurement gets larger (i.e. flux is conserved). So, waves resulting from the dipole effect allow their energy to extend arbitrarily far *without* the energy being reduced. The density is reduces, but the region it inhabits increases to counter this precisely. With other higher order moments, this is not true - the total energy of the field at a given distance goes down drastically with distance. That's why it's considered "bound" even though it does technically extend "infinitely" - JustinWick 18:46, 11 January 2007 (UTC)

The non propagating near field is the field directly associated with the moving electrons whereas the propagating field is the field associated with photons. The biggest difference is that for alternating current electrons magnetic and electric field intensity peaks and minimums are at opposite times while for photons both peaks and both minimums are simultaneous. Bill field pulse (talk) 15:32, 24 February 2024 (UTC)

Both regions of the field are associated with both moving charges and photons. In the near field, the peak of the electric component occurs at the same time the magnetic component transits through zero and vice versa. If there is energy transport in the near field, then you will find that the electric and magnetic components are not strictly in quadrature phase. In this case, the near field can be decomposed into two components: Q (quadrature) and I (in-phase). The in-phase component accounts for all the energy transport. Constant314 (talk) 16:15, 24 February 2024 (UTC)

There are two separate effects both exist in all regions except that near the non photon effect dominates and far the photon effect dominates. Bill field pulse (talk) 20:31, 24 February 2024 (UTC)

Re the discussion of whether the terms near-field and far-field are purely dynamic, the issue needs more detailed explanation. The key distinction between near field and far field is that in the near field, the wavelengths are long compared to the structures of interest (say antennas, or other objects interacting with the field). Observe that Maxwell's equations have no lengthscale in them, so the definition of near- and far-field has to be relative to a particular physical structure of interest. So, the near field is defined as the region in which the electromagnetic wavelength is long compared to the lengthscale of the objects in the field. The near field regime can also be descibed as the "electro-quasi-static" or "magneto-quasi-static" regime. What this means is that even though we are in fact talking about dynamic fields & AC circuits, the behavior you get is the same as in the static case, in the following sense. In the near field regime, you can predict the values of current and voltage throughout the system using "lumped component" values of capacitance and inductance, which are determined by geometry and have the same values whether in the DC or quasi-static case. For example, suppose the circuit includes two plates, both much smaller than the wavelength. In the near field case, we know that the AC current through the pair of plates is given by I = 2 pi f C V. (We would use this same value of C to compute stored charge from voltage in the electrostatic case using the expression Q=CV.) If we were to cut the frequency (f) by a factor of 2, the current would be reduced by the same amount. On the other hand, if we are not in the near field case, antenna-type effects must be considered. It would be possible to cut the frequency by 2 and have the current out of the "transmit" plate go up, if the wavelength happened to come into the proper relationship with the plate size (so that it became a more efficient radiator). You can find a great discussion of near-field, far-field, and the quasi-static / AC Circuits regime in a book by Fano, Chu, and Adler. 192.52.57.33 18:58, 7 February 2007 (UTC)JoshSmith

Adhering to the J.D. Jackson text is a good idea. Two articles that exemplify what this page could communicate are still availible in Google's cash. http://216.239.51.104/search?q=cache:RB7gPXONO9oJ:journals.iranscience.net:800/www.conformity.com/www.conformity.com/0102reflections.html+%22near+and+far+fields%22&hl=en&ct=clnk&cd=16&gl=us

http://216.239.51.104/search?q=cache:BCTF_Wny7BwJ:www.sm.luth.se/~urban/master/Theory/3.html+near+far+fields+urban&hl=en&ct=clnk&cd=3&gl=us

It might be possible to get rights to the material through Urban Lundgren or Isadore Strauss or http://www.conformity.com which formerly hosted the Strauss article. Sue... suzysewnshow@yahoo.com.au 65.41.254.175 14:52, 20 June 2007 (UTC)

I have to agree with Omegatron that the terms Near-field and Far-field have no meaning in electrostatics, and as far as I am concerned, when dealing with magnetic fields, they are useless too. The unit of measurement for near and far fields is the wave length; since there are no wavelengths in static fields, how can you apply this unit of measurement. Even when we are dealing with quasi-static fields, who is to say that energy transfer occurs at the speed of light. Has anyone ever measured it? If someone reading this is aware of it, please let me know. The experts usually grow silent when asked this question. Steinhauer 22:21, 22 September 2007 (UTC)

I think you are asking a self defeating question. Quasi-static means that we consider any movement to take place at much less than the speed of light. This means that any remote changes in field strength can be considered to happen infinitely quickly compared with the movements that we are considering (although the disturbance actually propagates at the speed of light). Martin Hogbin (talk) 11:08, 25 August 2008 (UTC)

I think that the first picture of Near and Far fields is false, as it represents fields for antenna array not for single antenna. At least it should be mentioned better. 91.154.1.4 (talk) 08:32, 7 February 2010 (UTC)

Any active discusion still?

It seems that this article could do with some improvement, is anyone active here at present? Martin Hogbin (talk) 11:01, 25 August 2008 (UTC)

Disambiguation Page?

For anyone looking for the mathematical concept of Near Field it is easy to overlook the small link at the top of the article; it seems to me that a disambiguation page would be more helpful. It is likely that the electromagnetic meaning is more common, but it is not clearly a primary meaning. For the word "rice" the primary meaning is the foodstuff, not simply in the sense that it is most common, but in the sense that it is clearly the meaning normally understood in the absense of reason to the contrary. By contrast, there is no normally understood meaning of "Near field": to a physicist working in electromagnetic field theory one meaning will appear normal, to a mathematician working in algebra the other will. I therefore propose to replace the redirect with a disambiguation page if no one argues a countercase. JamesBWatson (talk) 16:47, 8 November 2008 (UTC)

Nobody has disagreed with the above suggestion, so I have made the change. JamesBWatson (talk) 21:24, 25 November 2008 (UTC)

I was looking for acoustic definitions of near-field and far-field, which have similarities and differences with electromagnetics. Even if there is no entry for acoustic near/far-field, it would be nice if it were acknowledged to exist, like the mathematical near-field link, with an entry to eventually come. 192.91.171.34 (talk) 18:07, 30 July 2014 (UTC)John Morgenstern (john.morgenstern@lmco.com)

Meaning

I could easily have this wrong but... isn't the far-field the propogating / radiating bit, and the near field the bit that exponentially decays? William M. Connolley (talk) 11:48, 13 July 2009 (UTC)

Well, it decays faster than the radiating field (1/r^2 in power), but it's not really exponential. A better description is something like 1/r^3 or 1/r^4 in power, depending. The coupling from metal detector to detected object, or from one coil to another coil through vacuum, is of this sort. SBHarris 04:42, 17 November 2010 (UTC)

The exponential decay is the evanescent field when there is otherwise total internal reflection. Otherwise, near-field and far-field are simplifying limits. Oh, there is another (I believe) exponential decay, when the wave goes through a hole much smaller than the wavelength. There is near field microscopy, where you move a small hole along over the sample, and measure the signal coming through. You can resolve close to the hole diameter, instead of the more obvious wavelength. Gah4 (talk) 00:50, 25 February 2024 (UTC)

Absolutely TERRIBLE article

Could someone PLEASE re-write this entire article. It requires extensive simplification, verification and citation. It also suffers severely from 'look at my big brain' verbiage instead of being genuinely informative and descriptive. —Preceding unsigned comment added by 70.26.38.207 (talk) 01:54, 17 November 2010 (UTC)

Some work could be done to expand the lede to make it accessable to your average high school grad. However, electric, magnetic, and electromagnetic fields and the way they change in time and space, are not inherently "simple" concepts that can just be explained easily to anybody, because they aren't LIKE anything in everyday experience. So what prompted you to come to this article, what did you know already, what did you want to learn (but didn't), and what is your background? You can't just complain-- you need to supply specifics. You're the unhappy "audience" so tell us about your education, a bit. SBHarris 04:37, 17 November 2010 (UTC)

Agreed. This article is repetitious and unclear, and it turns immediately to jargon (or Maxwell's equations) to describe what happens in the real world. Just say what's different about near-field: it's short-range, it's denser, and its ratios of electric to magnetic fields are in flux. What else? I have no idea. The author couldn't explain without leaning on jargon, or by saying that the concepts aren't simple enough. That's just bad writing. — Preceding unsigned comment added by 144.191.148.3 (talk) 22:41, 3 October 2013 (UTC)

Contains numerous factual errors.

At the start of the article it says "The "far-field", which extends from about two wavelengths distance from the antenna to infinity,". Then there's a diagram showing this too. It it totally wrong. According to that theory, for the 305 m diameter dish at the Arecibo Observatory, operating at 2380 MHz, the far field would start 252 mm from the dish. According to the usual theory of 2 D^2/lambda, it would be 1.5 million metres!

It seems some people just love writing about things they don't have a clue about. — Preceding unsigned comment added by 193.93.38.6 (talk) 10:14, 10 November 2011 (UTC)

Clearly the thing needs to be qualified for conditions like dish antenna like Arecibo (or any radar dish) whose diameter D is much larger than lambda. In that case, you must use units of R = 2D^2/lambda and the near and far field roughly begin where this R is about lambda. However, for an antenna which is smaller than the wavelength (as often happens with whip antennas) then the approximation given at the beginning of the article, that near and far field start at roughly R = lambda, is correct. These are two different situations and you are correct that the article doesn't make that clear. I'll see what I can do in the short term to insert qualifiers that the simple "lamba approximation" only works with "short" antennas WRT wavelength. SBHarris 19:31, 10 November 2011 (UTC)

Thank you. I see you (or someone else) has edited this to make this a bit clearer, but there are no references given. I know about the 2 D^2/lamda case, but are unsure if that is a sufficient or even necessary condition at all times. It seems hard to find any hard evidence on this - I've looked at a number of antennas books, and can't seem to find a definitave answer. Drkirkby (talk) 15:08, 1 December 2011 (UTC)

Here is a simple version of the equations for fields from a "short" current segment--a bit of antenna, or actually the whole antenna if it is small compared with wavelength: [1]. It starts from Liénard-Wiechert potential of a moving charge and takes this charge as a differential element of current in an antenna excited by a harmonic voltage, and looks at the EM field terms, as a function of distance from the antenna current element. Much the same route is taken in chapter 9 section 1 of Jackson (in the references). In such cases, the reactive near-field zone is roughly λ/2π, the transition zone extends to about λ, and the far-field is past 2λ.

When the antenna starts to get longer with regard to wavelength, the various fields from different parts of the antenna start to interfere with each other at slightly off-angles, and the gain (directionality) of the antenna goes way up (this is true of long whip antennas, not just dishes). When this effect starts to get important for a linear center-fed dipole antenna, is calculated by Jackson in doing the full multipole expansion of the fields for such an antenna of length D in section 9 of chapter 9. He graphs the power outputs using various approximations and shows that they all diverge from reality, at about kD = π, where k is the wavenumber 2π/λ. Or in other words, this starts to be a bad approximation when the antenna D is larger than λ/2 (to answer your question in the next section of "how big is big"). It's really bad by D = λ. He graphs this out to kD = 25, meaning D = 25/k = (25/2π)λ ~ 4λ and you can see that the radiated power is more than 1.8 times as large as you'd expect from a simple small dipole already. The contribution of the higher multipole moments is an effect of the fact that long antennas act like multiple in-synch short antennas (antenna arrays), since they have a standing wave with multiple nodes, on a single antenna. Jackson doesn't cover antenna theory past the simple dipole, so the 2D²/λ criterion for far-fields from long/large antennas isn't derived in Jackson. You may be able to find it other places-- I have seen many sites on the net but haven't had time to examine many of them.

Finally, I fully agree with you that this article is in need of an expert. However, as it stands I'm convinced it isn't really badly wrong. The breakpoint for antenna length D really is about λ/2, in terms of when you have to start looking factoring in the physical length of the antenna itself. On the other hand, the static near-field effect of severe feedback to the antenna inside λ/2π would be true even of a "point source" of radiation, and is derived from that assumption. The extra effects of physically large antennas are constructive interferrence effects and they act almost like both a beam-focuser and telescope to extend the near field limit by a ratio D/λ focusing effects toward a point, and then D/λ again in telescopically looking at the feedback effect of fields at that distance. So the combined effect of a big antenna to extend near-field effects is about (D/λ)² as compared with small antennas. This is sort of intuitively believable, don't you think? BTW, this source [2] gives the equation for the reactive-near zone of a large antenna as πD²/8λ but the true far-field still as beginning at 2D²/λ. SBHarris 04:53, 2 December 2011 (UTC)

Short antenna

The article now says: A "short" antenna is defined in this context as one that is shorter than the wavelength of the radiation it emits" I don't think anyone would consider a half-wave dipole a short antenna, but that's 0.5 lambda. I've seen some references to "short" antennas being less than lambda/10. Drkirkby (talk) 15:22, 1 December 2011 (UTC)

See above. D = Lambda/2 is right at the boundary where things start to go wrong. Right there you still get near field effects where you don't need to worry about D/lamba ratios, but with any antenna larger than half a wavelength, you do. The antennas of lamba/10 are "short" only in the sense of being very capacitative, meaning that to match impedience they need some type of inductive load. SBHarris 05:03, 2 December 2011 (UTC)

I've added the "expert" tag

I feel this article needs editing by an expert. It's a complex topic, but it does not appear to me that anyone editing this is really very knowledgeable on the topic. I don't claim I am. I have an MSc in Microwaves and Optoelectronics and design antennas for a living, but don't feel I know enough about this topic to make it a decent article. (I've also got a Ph.D., but not in this area). It really needs someone who is an expert on the topic. Drkirkby (talk) 15:36, 1 December 2011 (UTC)

Also: Parts seem to have been taken almost verbatim from the second reference (OSHA article). Any electrical engineers around to help out? — Preceding unsigned comment added by Duckyphysics (talk • contribs) 01:14, 1 March 2012 (UTC)

Equations are not explanations

Is anyone else troubled by "The basic reason an EM field changes in character with distance from its source, is that Maxwell's equations prescribe different behaviors for each of the two source-terms of electric fields and also the two source-terms for magnetic fields." Maxwell's equations aren't the *reason* for anything, they *describe* EM fields. This could be rephrased along the lines "At greater distances from the source, the propagating EM wave becomes a larger component than the component due to the source currents and voltages", or something along those likes. --Wtshymanski (talk) 20:13, 17 December 2012 (UTC)

Well, I'm glad you brought the subject up. Certainly there is something wrong with the standard explanation that time-derivatives of fields CAUSE spacial derivatives of the other fields, and vice versa-- that's an association (as you can verify by simply changing your reference frame to see a different relationship). On the other hand, we really do think that movement of charges causes at least some changes in fields at a distance as a result, do we not? It's a lot clearer in the Liénard-Wiechert potential formulation of Maxwell's equations, where the constant velocity term of charge movement doesn't have a time-retardation component, so you can't tell cause from effect THERE. That's simply an *association*. IOW, if a charge moves along at constant velocity, I can't tell if the charge causes' the field that goes back and connects to it, or if they just "go together." To tell what is cause and effect I have to accelerate the charge, and that's the second term of the Liénard-Wiechert potential (LWP), and it turns out that my acceleration causes a type of change in the field which is what the second potential term describes, because my acceleration happens FIRST, and the change in the second term happens in a time-retarded way, the farther out I go. I can't pretend that the one doesn't cause the other, as there is that time-delay. That's the whole idea of radio communication, which depends on this second term.

So, we end up with the near field being primarily a result of the first term of the LWP, and isn't a cause and effect thing, but an association of magnetic and electric dipole fields in the antenna that to some extent (especially close to the antenna) are just "changes in statics" that may not require charge-acceleration (electric dipoles may change and currents flow for a time without charges accelerating much). But when charges DO accelerate, now dipoles not only change, but their rate of change changes, and the current changes also, and so this is a cause for an effect which travels outward only at a limited velocity of c. And that's the classic far-field. SBHarris 03:05, 20 December 2012 (UTC)

I'm not concerned about *which* equations one uses to describe EM fields, I'm more concerned about saying that the equations *cause* the EM fields. Surely that's backwards? Math models approximate a description of the world, not the other way around...--Wtshymanski (talk) 14:19, 20 December 2012 (UTC)

Saying equation effect A is caused by equation 's term B is just a shorthand for saying we think the equation models a cause-and effect relationship in reality. The force causes the mass to accelerate which is to say the F causes the m*a and remember the symbols stand for things. The idea that equation is correct model for world is understood and goes without saying. A major problem is causation direction is lost when we write the = sign. There's often more in reality then than survived in the math. Maxwell's equations famously don't prohibit time-advanced solutions where effect precedes cause. We have to toss those out of the model (or not!) for other reasons of our own. The same is true when we loosely talk about what causes an EM field. It's not the equation, it's the charge and charge motion described by the equation. But not the other way around for charge accelerations! We presume they cause the propagating changes in the EM field, not the other way around. SBHarris 15:49, 20 December 2012 (UTC)

Well, sometimes we get the equations wrong - even Ohm got his law wrong the first time around. I'd like to think a general purpose encyclopedia should not be explaining everything in terms of mathematics if there's any reasonably compact and correct way of expressing the ideas, even approximately, in English, first. If the relevant parts of the above discussion could be abstracted, it might make a good introduction for the maths part. "Here's the phenomenon, and here's what the math model says when we want to analyze the phenomenon". --Wtshymanski (talk) 14:35, 21 December 2012 (UTC)

• What we can say is that some properties have instantaneous values that are proportional to the time integral of other values. In a sense, you can think of velocity as being the cause of position, acceleration as the cause of velocity, etc.. Similarly, the cause of an accumulation of magnetic flux would have units of voltage (=webers per second) and the cause of an accumulation of charge would have units of current (= charge per second). A more familiar example, accumulation of energy, can be seen as being caused by power (=joules per second).
• If distance increases with time, then derivatives with respect to distances can be seen as causes of their corresponding distance integrals. Likewise for derivatives with respect to area or volume.
• Conversely, if distance decreases with time, then derivatives with respect to proximity (inverse distance) can be seen as the cause of their corresponding integrals.siNkarma86—Expert Sectioneer of Wikipedia
  ^{86 = 19+9+14 + karma = 19+9+14 + talk} 14:52, 10 May 2013 (UTC)

Inconsistent notation

In the section "Electromagnetically long antenas" the notation d_f (d subscript f) appears to be used to represent two separate things. First it is used to represent the Fraunhofer distance, and then to represent a "far-field region distance". One of these should be changed to a different symbol to avoid confusion.-- Tim314 (talk) 20:17, 9 May 2013 (UTC)

Contradiction

The introduction says "Near-field strength decreases with distance, whereas far-field strength decreases with the inverse square of the distance", but elsewhere in the article it seems to be the opposite, which makes more sense. — Preceding unsigned comment added by 88.96.79.118 (talk) 21:58, 12 January 2015 (UTC)

Last sentence of first paragraph has "decreases with distance" transposed with "decreases with the square of the distance"

Noting that you are talking about field strength, the sentence should read:

"Near-field strength decreases with the square of the distance,whereas far-field strength decreases with distance."

For a discussion of this, see page 21:

"Foundations of Antenna Theory and Techniques" by Vincent F. Fusco, 2005 Pearson Education Limited, ISBN 978-81-317-1125-5.

And thanks for writing the Wikipedia entry! — Preceding unsigned comment added by 68.47.85.67 (talk) 23:52, 12 July 2015 (UTC)

Causes of the two fields

I think we should say that the near field is the field directly associated with the movement of electrons while the far field is only indirectly associated to the electrons, and is carried via the photons being emitted. Some may say that this is obvious and unnecessary but I think it adds clarity. Bill field pulse (talk) 15:47, 24 February 2024 (UTC)

Needs reliable sources. Direct and indirect are simply points of view and in this case an arbitrary distinction. Constant314 (talk) 16:00, 24 February 2024 (UTC)

Do you agree that the near field can not possibly be photons because photons have the magnetic and electric fields simultaneous. Whereas the the near field has them at opposite points. Bill field pulse (talk) 20:27, 24 February 2024 (UTC)

No I do not agree. Constant314 (talk) 20:59, 24 February 2024 (UTC)

The Feynman explanation is here. The first two go as 1/r^2 in field, and so 1/r^4 in intensity (energy). The third term is the far field term, 1/r, and so 1/r^2 in intensity. The idea of far field is that we can simplify, take the large r limit, and ignore the other terms. And for near field, hopefully, ignore the far field term. But okay, the near field has position of the electron and first derivative, the far field has second derivative. There is a rule that says that you keep taking derivatives until you get to the wave equation. For gravity waves, you need one more derivative. Gah4 (talk) 01:08, 25 February 2024 (UTC)

I agree or in physical terms: If we consider that photons must cross over the near field to get to the far field it is easy to see that they are present but less intense than the near field. Since near field exactly matches electron motion magnetic field maximum as electrons move fastest and electric field maximum as electrons stop and change directions. The near field must be produced by all electrons causing imbalance hence it is stronger than the photons however as it falls off more quickly at a certain distance photons while only produced by certain electrons dropping energy levels eventually dominates. Now try explaining so Constant314 understands. He thinks photons could produce the near field which is quite impossible considering the nature of photons. Bill field pulse (talk) 20:31, 25 February 2024 (UTC)

It mostly doesn't depend on photons, just Maxwell's equations. Near field and far field are the small, and large, r limits, respectively. In between you are stuck, and have to use all the terms. Gah4 (talk) 22:11, 26 February 2024 (UTC)

It doesn't matter what I understand. What matters is that you have to paraphrase a reliable source to add material to the article. Constant314 (talk) 16:54, 27 February 2024 (UTC)