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Antenna and magnet?

  1. Apr 30, 2004 #1
    I am aware of the apperent presence of electrons on the [flat, top] base of an antenna such as one on a radio. I took the time to obtain a magnet with moderate strength. I placed the magnet about a centimeter away from the [flat] base of the antenna and found it vibrated. As I decreased the difference in distance between the magnet and the base of the antenna, I found that the antenna would increase in its rate of vibration. I am no physicist, but a curious explorer, therefore I know not what these observations are the products of. I have therefore reported it here in the hope that it will be explained. Thank you. :biggrin:
     
  2. jcsd
  3. Apr 30, 2004 #2
    An antenna is an antenna because in provides a conductive path for electrons to wiggle back and forth.
    This effect emits radio waves, depending on the frequency of the electrons moving back and forth.
    This effect also provides for a magnetic field surrounding the antanna, due to the alternating current through it.
    An external magnetic field will attempt to "couple" with the magnetic field generated by the antenna.
    Since the magnetic field of the antenna is changing(frequency) and yet the external magnet is not, the "vibration" is passed to the external magnet though not without some retort.
    I believe your observations are consistant with this.
     
  4. Apr 30, 2004 #3
    So, the external magnetic field is what is really vibrating? What kind of retort?
     
  5. May 1, 2004 #4
    Not initially. The alternating current of the antenna creates a weak but very real "vibrating" magnetic field around the antenna. A magnet placed close to the antenna has it's own magnetic field which interacts with the magnetic field of the antenna. The vibration(changes) of the antennas magnetic field would be passed to the magnet through it's coupled field, which then causes the magnet to vibrate.
    The "retort" I am refering to is that the magnetic coupling is not perfect, and so only a fraction of the vibration is transfered.

    Please note that I am not and expert on this, but it should be close to correct.
    I would encourage experts to jump in and assist this issue.
     
  6. May 1, 2004 #5

    Integral

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    You have not provided enough information to even make at guess at what you are observing. I sincerely doubt that the vibrations have anything to do with the fact that this is device is an antenna. Is this a powered device? A wild guess would say that you are feeling interaction with a poorly mounted device carnying 60Hz wall current. You best bet would be to open the antenna and look for what is vibrating.
     
  7. May 1, 2004 #6
    Thanks to Integral for making me re-read your question.

    Damn, I read your question wrong(I thought the magnet was vibrating instead of the antenna). But, the principle should remain the same: The magnet influences the physical nature of the antenna due to the alternating current going through the antenna.
    Consider this: Extend a 1-foot thin copper wire downwards from a support. Apply an electrical frequency to that wire, such that radio waves are emmited. Hold a magnet close to that wire and the wire will certainly vibrate.
     
  8. May 1, 2004 #7

    Integral

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    I cannot believe that RF will have the power to cause such interaction, also RF frequency's will be to high to feel. The only possible cause, that can also be felt, is, as I said, 60hz. Any RF present in a RECEIVING antenna will be in the milli or micro Watt range. This is way to low power to cause any detectable physical vibrations.


    Edit: Some punctuation and a small wording change (is -> in)
     
    Last edited: May 1, 2004
  9. May 1, 2004 #8
    Integral:
    Okay, you can do this experiment at home, if you like. Just get a magnet, put it on top of a (preferrably cheap) radio antenna. Then it'll vibrate.
     
  10. May 1, 2004 #9

    Integral

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    So you have tried this with every radio antenna made?

    It didn't on the one I just tried.
     
  11. May 2, 2004 #10
    Hmm. Good point(s) Integral. Surely I must stand corrected as you are no doubt much more of an expert than I. So, to get to the bottom of this I will ask him some questions in a following post.
     
  12. May 2, 2004 #11
    certainly not all the radio antennas ever made. But I did with the one in my room, and it did vibrate.
     
  13. May 2, 2004 #12

    Imparcticle, please explain your set-up. For example:

    1- Are you placing the magnet close to the top of a fully extended antenna, or close to the bottom?
    2- How long is the extended antenna?
    3- Is the antenna physically connected to the radio?
    4- Is the extension from the radio straight up and down, or can it also swivel?
    5- Is the radio turned on during this effect?
    6- Does it work if the radio is turned off?
    7- Is the radio an AM/FM reciever only type? And is it battery operated?
    8- What type of environment does this occur? In other words, does the effect happen only inside your home, or also outside your home, away from any electrical devices?
    9- To isolate potential vibrations of your hand holding the magnet, could you take the time to obtain a cardboard box, turn it on its side, put the radio with extended antenna inside, and put the magnet on the other side of the cardboard. Or any other method you can think of to hold the magnet without using your hands. Does it still vibrate?

    Any of these question you can answer at this time would be helpful.
     
  14. May 2, 2004 #13

    chroot

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    Does the antenna vibrate on its own, or do you have to get it started?

    (I'm with Integral in every way.)

    - Warren
     
  15. May 3, 2004 #14
    I'm with Integral too.
    There is absolutely now reason why placing a magnet near a receiving antenna should cause it to vibrate. The antenna is after all just a peice of metal.

    Even if you were talking about a 50 Watt transmitter, there still would be no vibrations, due to the extremely high frequency of the EM waves (as integral states). If you can feel it vibrate, the EM waves have a wavelength of several thousand kilometers...

    I also have trouble believing it is the power source. After all, something giving enough EM noise to shake a magnet would probably not be certified for sale anyway; there are after all standards. (also the engineer would have to make a hell of coil for no apparent reason to accomplish this task).

    I'm not trying to discredit your observation, but after all, it is a radio. radios have speakers. speakers have big magnets ... some sort of interaction there? I don't know.
     
  16. Feb 10, 2008 #15
    I have an ANTENNA related question......Could someone help me to understand what's happening here - when I take an ordinary auto antenna and gently wave it against a dark background I can observe a cobalt blue glow along the full length of the rod - extending out some two inches. This light resembles what some would say is an aura but thats not what I would call it, because that term is usually associated with living things. If the rod is pointed I can actually 'draw' with in the surrounding space in the sense that it leaves a trace of blue light behind by virtue of the motion of the rod. As I pass the rod through the blue light from left to right I observe a regular set of dark lines all along the full length of the rod. These lines are always at a 45 degree angle on the rod. Its as though the cobalt blue light is the cause of the lines as the rod slowly passes through. I also notice a faint white 'sparking' phenomenon that comes from the tip of the rod. Sometime the sparks are as far away from the tip of the rod as two inches. Most are much closer than that though.
    Incidentally - the rod does not have to be made of metal! A bamboo rod actually worked just as well or better, but the sparking effect was not evident with the bamboo.
    Additionally - the only thing that came to mind and that seemed at all reasonable was the photoelectric effect, but with the bamboo rod? I think not! Certainly not the Casimir effect because of the energies involved.
     
    Last edited: Feb 10, 2008
  17. Feb 10, 2008 #16
    Do you live under a power line? A really big one?
     
  18. Feb 10, 2008 #17

    chroot

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    This has nothing to do with the physics of electricity or anything else -- it's some kind of a visual illusion. Try using different lighting.

    - Warren
     
  19. Feb 11, 2008 #18
    Or perhaps a vision problem?
     
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