1
$\begingroup$

In most explanations of how antennas work they say that because of kinks in the electric field of an antenna meets at one point and detaches... I looked up reasons for why this happens but all I got was an answer saying that electromagnetic waves travel in the direction of the Poynting vector.

So I have 2 questions:

  1. Why does the electric field detach from the antenna and propagate?

  2. Can that be explained by the Poynting vector?

$\endgroup$
1
  • $\begingroup$ In most explanation of how antennas work they say... Please provide a credible source. I have never seen an antenna described in this way. I don’t even know what it means for an electric field to “detach”. $\endgroup$
    – G. Smith
    Nov 22 '20 at 0:25
2
$\begingroup$

The build up of Maxwell's equations was long, and the experimental confirmation that light is the electromagnetic waves predicted by them took a while. There are "laws", relations which were defined from experimental observations, which are as an axiomatic underlayer so that the equations appear consistent and give the electromagnetic wave as part of its solutions, which eventually fitted light observations at the time. It also explains mathematically the classical electromagnetic wave from an antenna.

In classical electrodynamics , a changing current generates electromagnetic waves. A moving charge with a constant velocity is a current of charge.An accelerated charged particle is a changing current and thus has to give off electromagnetic waves according to the equations. That is how antennas work, by changing currents. At the time the current carriers were a hypothesis.

We now know that they are electrons and positive ions, depending on the set up. At the quantum mechanical level, a change in velocity for an electron means an interaction with the field , either giving up energy or radiating energy away and there are QM precise calculations on the probability of an accelerated electron to radiate a photon. There is continuity in the theories of physics and it can be shown mathematically that a confluence of photons builds up the classical electromagnetic wave of Maxwell equations.

$\endgroup$
10
  • $\begingroup$ What I don't understand is how a change in electric field makes it travel out at the speed of light rather than just oscillating back and forth around the antenna...can you explain that? $\endgroup$ Nov 23 '20 at 8:43
  • 1
    $\begingroup$ It is an observed experimental effect that electromagnetic waves leave the antenna. If you get down to electrons, they radiate a photon, photons are particles, andt they go their way. The mathematics is such that the zillions of photons radiated by the zilions of electrons (going up and down in an antenna, add up to make the classical wave of changing electric and magnetic fields macroscopically. . You need quantum field theory in order to really understand the last link( which I only handwavingly understand) $\endgroup$
    – anna v
    Nov 23 '20 at 9:16
  • 1
    $\begingroup$ How an electron radiates a photon is by interacting with the electric fiekd of the atoms of the antenna,scattering off, losing part of its energy into a real photon see this en.wikipedia.org/wiki/Bremsstrahlung . for a feynman diagram it will be similar to the bremsstrahlung one.i.stack.imgur.com/cPjvK.png , where the exchange will be with the ambient field $\endgroup$
    – anna v
    Nov 23 '20 at 9:55
  • $\begingroup$ thanx that helped a lot but that raised another question...radiation was emitted when the particle deccelrated...so when a particle accelerates shouldn't it absorb radiation? $\endgroup$ Nov 23 '20 at 10:21
  • 1
    $\begingroup$ From left to right, the electron is accelerated when the virtual photon hits it, the first vertex, and decelerated when the real photon leaves at the second upper vertex. (take the red electron line as representing the field of atoms in the case of an antenna) $\endgroup$
    – anna v
    Nov 23 '20 at 12:23
1
$\begingroup$

The "electric field detachs" from the antenna because it is not simple electrostatic field, but instead it is transverse wave generated by accelerated electrons in the antenna. Accelerated charged particles create such propagating waves of electric field.

Poynting vector is a measure of EM energy flow, by itself it does not explain how electric field behaves. Maxwell's equations (and boundary conditions) explain that.

$\endgroup$
11
  • $\begingroup$ Well can you please explain more about how accelerated electrons make such waves? $\endgroup$ Nov 22 '20 at 5:55
  • $\begingroup$ See animations here, when you change velocity of electron, you can see waves being created. compadre.org/osp/EJSS/4126/154.htm $\endgroup$ Nov 22 '20 at 12:55
  • $\begingroup$ the animation helped but the kink waves sort of get bigger... right? Why is that? $\endgroup$ Nov 23 '20 at 8:08
  • 1
    $\begingroup$ The wave does not get bigger in intensity, only size. It is because the wave is expanding radially from its source. Like waves from a stone thrown into pond, they get out and they get bigger. $\endgroup$ Nov 23 '20 at 15:25
  • 1
    $\begingroup$ The kinks are not "wave amplitude". What you see in the animation is lines of force (electric field). The patterns in these lines (kinks) do get bigger, but to notice this in real world, you have to measure the pattern in many points throughout a big region of space. The big pattern is invisible at any single point. Wave amplitude at any single point is instead proportional to spatial density of the lines of force. This density gets lower with distance $d$, it varies as function $1/d$. $\endgroup$ Nov 24 '20 at 11:37

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.