Far-field in air versus vacuum

I think the far-field is significantly different in vacuum versus air, is this true? More specifically, when we talk about far-field, we mean the electromagnetic wave as a whole etc. but in the near field we can have the magnetic component in one way or ways and the electric in another. For example one direction could be almost entirely magnetic with almost no electric field to it. Only when this interacts with electrons will it start to convert from one to the other, eventually having equal parts in electric and magnetic, so then we call it far-field. But what if there are never any electrons to encounter, I think that would extend the far-field, perhaps to infinity. Does this sound correct?

• No. The electrons are in the wire, not in the air. Once it radiates, they are photons, not electrons. The only difference air makes is that it might attenuate the signal by absorbing some frequencies slightly. Ground makes a bigger difference.
– user10489
Oct 10 at 23:44
• I think you are wrong. If the antenna itself was in space and maybe it was an array. Oct 11 at 2:55
• @Jack0220 your comment makes no sense. Both in air as in vacuum, there's no "free" electrons (or any charge carriers) with which your wave interacts. Oct 11 at 10:01
• The hypothesis of the question ignores basic discoveries by Faraday and Maxwell about E & M & EM fields.
– hotpaw2
Oct 11 at 15:30
• Whoa, let's not insult your question. You're just trying to understand and learn, and that's commendable. Oct 12 at 15:21

I think the far-field is significantly different in vacuum versus air, is this true?

No, exactly the same equations (Maxwell!) apply; only the $$\varepsilon_r$$ and $$\mu_r$$ are different – but to a very minor degree. Hard to measure even with lab equipment.

More specifically, when we talk about far-field, we mean the electromagnetic wave as a whole etc.

Hm, not really, no. "Far-field" describes the location. When I say "far-field wavefront", I describe the wavefront in all places that I consider far field.

but in the near field we can have the magnetic component in one way or ways and the electric in another.

That's always the case: E- and H-field are orthogonal in isotropic media.

For example one direction could be almost entirely magnetic with almost no electric field to it.

As said, practically always the case, unless your medium is "special".

Only when this interacts with electrons will it start to convert from one to the other, eventually having equal parts in electric and magnetic, so then we call it far-field.

No, none of this is correct.

But what if there are never any electrons to encounter, I think that would extend the far-field, perhaps to infinity. Does this sound correct?

No.

• Comments are not for extended discussion; this conversation has been moved to chat. Oct 12 at 15:24

Only when this interacts with electrons will it start to convert from one to the other

No! The EM field interacts with itself; it doesn't need any help from electrons. Maxwell's equations give the differential equations that govern this, and show that the wave propagates at the speed of light.

• I don't think so. I think the magnetic field is expanding from the source into infinity, same for the electric field. I see no reason why the 2 necessarily have to exist together. How would a spinning magnet in space create an electric field (of the same magnitude no less), it just comes out of nowhere or what? Tell me how and I might believe you. Oct 12 at 12:07
• @Jack0220 -- the term "electromagnetic field" refers to a combination of electric and magnetic fields. Each field reinforces the other, and its the combination of the two that makes up the wave. Oct 12 at 12:55
• @PeteNU9W each reinforces the other, I would agree with you in the presence of electrons but not out in space. What I think is, we always see them together so we assume they come from each other but did anyone ever pause to think that maybe both fields are just always created together but that it doesn't have to be that way. A magnetic field wiggling an electron also creates an electric and vice versa, but without electron I don't see how. Oct 12 at 13:01
• @Jack0220 -- no, each reinforces the other directly, without the need for electrons. You keep rejecting references to the mathematics that describe it, but without the math your intuition will lead you astray. That's the problem that 19th century physicists ran into, which led them to invent the "aether", which was the substance that light waves supposedly vibrated in. Experiments proved that there was no aether. To repeat the previous mantra: Maxwell's equations. Oct 12 at 13:04
• @Jack0220 -- there are standalone electric fields, and there are standalone magnetic fields, just as you've described. And, yes, those are two separate things. If you vibrate that balloon its electric charge combined with the motion produces a changing magnetic field, and that, in turn, produces a changing electric field. The two reinforce each other as they move in tandem away from the balloon, and that's an electromagnetic wave. Oct 12 at 17:05