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How does a dipole antenna work

  1. Jul 29, 2012 #1
    How does it interact with the magnetic and electric fields of the radio waves while receiving them?
     
  2. jcsd
  3. Jul 29, 2012 #2

    mfb

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    Radio waves are electromagnetic waves. Imagine a plane wave, polarized in some way. If you put a conductor parallel to the direction of the electric field, you get an electric field which changes direction with the frequency of the radio waves (or 2 times this frequency, if you just count direction switches). This electric field leads to an alternating current in the antenna, and therefore the potential at the ends of the antenna change - this can be detected by electronics.

    Most radio waves are not plane waves and not polarized nicely, but usually you get some component of radiation which has the correct orientation for the antenna to pick up.
     
  4. Jul 29, 2012 #3
    This may sound stupid but then what happens to the magnetic component of the waves? Does it also interact with the antenna somehow to create electrical current.
     
  5. Jul 29, 2012 #4

    sophiecentaur

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    A changing magnetic field will induce an emf in a wire (it needn't be a loop).
     
  6. Sep 14, 2012 #5
    There are many ways of interpreting that line, please specify which interpretation is the correct one.
     
  7. Sep 14, 2012 #6

    mfb

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    This thread is more than 1 month old.
    I cannot see multiple ways to interpret that line, can you show me some?
     
  8. Sep 14, 2012 #7
    Well the direction of the electric field is at a right angle to the direction the EM wave would be going. So parallel to that would be under or over the direction of the electric field? I'm pretty sure there's more ways of interpreting that but is that the correct one? I'm not very good at reading in general, but I want to make sure, thanks.
     
    Last edited: Sep 14, 2012
  9. Sep 14, 2012 #8

    davenn

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    what light ??

    we are talking about radio wave E-M field


    Dave
     
  10. Sep 14, 2012 #9
    When I wrote light I meant an EM wave, some animals may be able to see radio waves so I included them as light. I changed my previous post, it's more clear now that way.
     
  11. Sep 14, 2012 #10

    davenn

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    try the ARRL Radio Handbook
    it gives a very good background to radio transmission and reception including antenna theory


    cheers
    Dave
     
  12. Sep 15, 2012 #11

    mfb

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    What do you mean by "over" or "under" the electric field? In planar waves (or waves similar to them), the electric field is the same everywhere in the plane perpendicular to the propagation direction of the wave.
     
  13. Sep 16, 2012 #12
    So where exactly is parallel? Nevermind about the over and under thing.
     
  14. Sep 16, 2012 #13
    Thanks!
     
  15. Sep 16, 2012 #14

    mfb

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    Parallel

    As examples:
    If the polarization of light is vertical, the electric field is vertical, and your antenna has to be vertical to pick up the signal (at least with some vertical component).
    With a circular polarization, the electric field is vertical->horizontal->vertical->..., and your antenna direction does not matter unless it is orthogonal to the direction of propagation of the wave (or has some orthogonal component).
     
  16. Sep 16, 2012 #15
    So if the antenna is vertical and the electric field is vertical and the photon moves by the antenna but is a bit too far left to hit it directly, could the magnetic field induce current in the antenna?
     
  17. Sep 16, 2012 #16

    davenn

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    Even with the antenna and the electric field being at 90 degrees to each other there will still be some signal induced into the antenna .... just not very efficiently

    In the field, in pratical situations, we see a 25 to 30 dB difference in signal strength when the polarisation is out by 90 deg

    Dave
     
  18. Sep 16, 2012 #17
    If the polarization of the signal is vertical and the photon meets the vertical antenna straight on, will that generate the most current? I hope this has to do with dipole antennas.
     
    Last edited: Sep 17, 2012
  19. Sep 17, 2012 #18
    A dipole is when the two conductor feed line is attached at the center between two separate long elements. This configuration is usually horizontal. but may be vertical, sometimes an inverted "V" shape... etc.
    A true "vertical" antennae is distinguished by being configured vertically but with a single element, the feed line attaching one lead to the vertical element and the other to ground (usually a ground plane of radiating rods on the ground or similar, lots of designs).

    The noteworthy behavior of a dipole is the relationship between current and voltage at different parts of the antennae... it alternates from having high voltage and low current at the distal ends of the elements while having low voltage and high current at the center, to having low voltage and high current at the ends and high voltage and low current at the center.

    That is the basic mechanism in principle for both sending and receiving... how that relates to the emission and reception of EM is more complicated - ARRL books have great info and practical "theory", but may not get down to the level of a suitable "physics" answer.
     
  20. Sep 17, 2012 #19

    mfb

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    I think you have something like that in mind. Forget it. Light does not work that way. That is not a 3-dimensional picture! Imagine those fields everywhere in the (z,y)-plane, and it gets better. While it is possible to emit directed electromagnetic waves, you cannot really "miss" an antenna in the way you imagine it.
     
  21. Sep 17, 2012 #20

    sophiecentaur

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    Photons are not like little bullets and they don't "meet and antenna" in the conventional sense. It is really not helpful to look at it this way. How could you even start to consider how this single photon (dimensions / extent totally unspecified) will interact with a piece of metal, which consists of a distribution of charges all over it? Stick to waves if you want to understand most non-QM phenomena. There is nothing wrong with ( you are not making any compromises) using the wave approach.
     
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