Bent Electric Field Explains Antenna Radiation

We all use antennas for radios, cell phones, and WiFi. Understanding how they work, though, can take a lifetime of study. If you are rusty on the basic physics of why an antenna radiates, have a look at the very nice animations from [Learn Engineering] below.

The video starts with a little history. Then it talks about charges and the field around them. If the charge moves at a constant speed, it also has a constant electric field around it. However, if the charge accelerates or decelerates, the field has to change. But the field doesn’t change everywhere simultaneously.

Where the field changes, there is a kink in the electric field. That kink explains the radiation. From that idea, the video builds to dipole antennas and more. Watching this video won’t get you ready to design the next broadband log periodic antenna, but it will help you get more of a gut feel for how antennas work.

This is one of those topics that is tough to approach even with sophisticated math. We’ve looked at other videos on the topic. We are pretty sure this is one of those topics that is much harder to learn without animations.

15 thoughts on “Bent Electric Field Explains Antenna Radiation

  1. Well, the graphics are pretty.
    I’m not sure you can explain a really complicated subject like this in a short video.
    Why would the propagation velocity be variable along the elements of a dipole? How is a sinewave with a variable rate of change? (think circular representation instead of oscilloscope).
    Free space impedance @377 ohms is good but the purpose of a satellite dish feedhorn to match that is a big leap.
    I’m not sure I’d recommend referencing this video if you have an EE test coming up.
    Every time you see radiated electrostatic you also have electromagnetic field. Every time you see a charge on a dipole you also have a current making magnetic field which is what all this stuff about reactance and impedance is all about.
    Very nice presentation and definitely kudos for trying, but…..

    1. I agree there is a lot to the subject and it is not often explain with enough mathematical or physical rigor. Strictly speaking there is no such thing as “radiated electrostatic”. EM radiation is anything but static, and in fact “electrostatics” itself is a bit of a misnomer since charges can and do move. In my opinion we should talk about static fields when we mean just that and in what context, never for any other purpose. For example a static electric field in a frame of reference is a varying electromagnetic (electric AND magnetic) field in a moving reference frame. The same for a static magnetic field. Yes, this may sound pedantic but I believe a lot of confusion arises misuse of terminology.

      1. I must correct myself again if I talk about rigour:

        a static electric field in a frame of reference is an electromagnetic (electric AND magnetic) field in a moving reference frame in relation to the former frame. Whether in the latter frame the electric and magnetic fields vary in space, time or both, depends on the type or relative motion between the frames and the configuration of the field in the former frame. Mathematically one can think of all types of situations (e.g. static, constant uniform fields in all space), realised or not in practice.

    2. +1 while graphical interpretations can be helpful for basic understandings to really conceptualize nothing beats reading experiments regarding electron behavior, such as electron slit. Ect

    3. Pretty screwed up explanation. It jumps from analogy to analogy and switches representations of waves mid-stream. And of course, anyone new to this will want to know what those positive charges are that are moving in the wires. Plus, any accelerating charge produces radiation. You don’t need + and -. They did a lot of work, but no.

  2. Hermann Haus had a paper in 1986 with a beautiful and straight forward derivation of the radiation condition from a moving charge.
    To quote the abstract: “The interpretation of the present derivation is that a charge at constant velocity v̄(‖v̄‖<c) does not radiate, not because it is unaccelerated, but because it has no Fourier components synchronous with waves traveling at the speed of light." Very nice paper but the math is a little tough, but you can follow it high level without getting into the extreme details.

    1. Also if a constant velocity particle did radiate it would lose energy ( velocity ) and would tend toward zero speed. But this would go against Einsteins non locality because it would dictate a universal speed for space.

    1. Photons are fundamentally a quantum mechanical concept; but not actually the same as the wave-function of QM. A photon is a particle that you get when you make an ‘observation’ in QM jargon. You can observe the behavior of the complex numbers used in QM through circular polarization.

      I suspect particles are actually data points without a real existence of their own. You seem to acquire them by intersecting fields of various kinds.

      1. I think this explanation is wrong.

        The movie shows, and the argument depends on, electrons making an excursion from one end of the half-wave dipole to the other end in the period of the wave.

        Please see Kenneth Macleish’s article (QST November 1992) who states the total peak-to-peak travel of a free electron as “no more than a hundred-millionth of an inch” during any half cycle. The fact of the relatively short motion of the electron as compared to the length of any practical dipole for radio frequencies proves that the high-speed passing of electrons (real things) and positive-charge carriers (imaginary things) at the midpoint of the dipole is close to total nonsense.

        There isn’t a single wave per cycle being radiated by the antenna, but cajillions (yes, I made that word up) of them. It is the individual acceleration and deceleration of each of the individual electrons that generates waves. The individual quanta of radiation only organize themselves into an organized field of expanding wave-fronts as the collective emissions traverse the near-field and enter the far-field, after which the co-mutual interference pattern appears organized.

        I really do think the presentation has lovely graphics, and it is refreshing to find an explanation of antenna emissions that understands that the fluctuating electric and magnetic fields around the antenna do NOT explain how the E/M wave is radiated from the antenna!

        1. Exactly.

          “What gets us into trouble isn’t what we don’t know, it’s what we know for sure that just ain’t so.”
          Attributed to Mark Twain, but probably by someone else which kind of proves the premise.

          Not a fan of the explanation of radio waves popping off the conductor like bubbles with the ends bending around corners. Sounds like a dream I had after eating a weird pizza too late at night.

          Funny thing is you can actually see the creation of the magnetic components of the radiated signal with low frequency AC and powdered iron filings.

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