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How does current flow in an antenna?

  1. Nov 9, 2009 #1
    Let's say we have a dipole antenna that we are using to radiate a signal. The dipole antenna (think rabbit ears) has two ends that appear to me to be open circuits.

    Maybe my problem is that I have too much circuit background and not enough EM, but how does current flow through these rabbit ears? There is no return path, no loop, so where does it "go"? I know that it's radiating, but I feel that I'm missing something here...


    Thanks
     
  2. jcsd
  3. Nov 9, 2009 #2
    It goes back and forth in the antenna. It does not have time to go beyond the ends of the antenna because it changes direction too fast. The length of the dipole antenna is proportional to the wavelength of the radio frequency. Remember, the electrons moving in the antenna are moving at the speed of light. The higher the frequency the shorter the dipole length. You can calculate the length with 468 / frequency (in Mhz).
     
    Last edited: Nov 9, 2009
  4. Nov 10, 2009 #3

    vk6kro

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    In this diagram, you can see the current and voltage patterns in a resonant dipole antenna.

    dipole voltage and current.PNG

    This is RF (high frequency AC) energy. The pattern is caused when one cycle of the incoming signal reaches the end of the antenna and is reflected and gets back to the center feedpoint just as a new cycle is entering the dipole. Just like standing waves in musical instruments.

    These are real voltages and currents which can be measured, but they seem pretty bizzare if you haven't seen them before.

    The alternating current flow in the middle of the dipole generates a magnetic field and the alternating voltage at the ends of the dipole generate an electrostatic field. These radiate out from the dipole as a radio signal.
     
  5. Nov 10, 2009 #4
    All of the above are good answers. I would only add the following 2 words - displacement current. As correctly stated above, the current moves back and forth in each conductor. The path is indeed open, but displacement current does not need a closed path.

    The only issue I have with the previous post is that the current and the magnetic field are mutual. There is no evidence that the current "generates" the H field. Likewise, voltage & E field are mutual. V does not generate E. Otherwise, I agree with the above.

    Claude
     
  6. Nov 10, 2009 #5

    vk6kro

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    I picture the magnetic component being generated like this:

    200px-Electromagnetism.svg.png
     
  7. Nov 10, 2009 #6
    Take a glass half full of water, then move it back and forth.
    Look at how the water moves, it's more or less how voltage and current move in a antenna.
     
  8. Nov 10, 2009 #7

    mheslep

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    You mean the electromagnetic field travels at the speed of light, not the electrons, which travel at the http://en.wikipedia.org/wiki/Drift_velocity" [Broken] in a conductor.
     
    Last edited by a moderator: May 4, 2017
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