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Question about decoding information in electromagetic wave

  1. Jul 1, 2015 #1
    When electromagenetic wave is produced by dipole antenna, all the space exist EM waves like torus with different amplitude. Even though waves could reach destination, then how to decode information within it?
  2. jcsd
  3. Jul 1, 2015 #2


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    Staff: Mentor

    How much do you know so far about modulation and demodulation? Maybe start with this wikipedia article, and post any specific questions you have after reading that article:


  4. Jul 1, 2015 #3
    I have just read the article, thanks a lot, but I'm still confused. My question is after completing modulation of the signal, when ejecting it out via dipole antenna, it will create many kinds of signal at different points in space(This is my opinion, because at different points in space there exist different electrical and magnetic magnitudes, thus at different directions in space there exist different EM waves [if it is wrong,please correct it]), then when all these signal from the same source reach same destination, how to demodulate the real meaning from others?
  5. Jul 2, 2015 #4


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    The signal of a specified frequency (say your 101.7MHz FM station) and a specified modulation, in this case FM, (EM) is radiated by the transmitting antenna and picked up by any antennas within range. the antenna connects to a receiver and if the receiver can tune to the 101.7 MHz frequency, it will be amplified and sent to the demodulator circuit, again, in this case an FM demodulator, where audio will be recovered

    do some google searching on FM and AM modulation and demodulation and ask any specific questions of anything you don't understand
    Supply links to those places so we can also read what you are

  6. Jul 2, 2015 #5
    Thanks a lot, maybe I need to make my question clear. According to this[https://en.wikipedia.org/wiki/Dipole] [Broken], electric field of all the points in bisecting plane are dependent on the distance to axis. When the dipole is oscillating constantly, it seems electric field of all the points in the plane are changing with same frequency but amplitude related to distance to axis. Could all these points produce their EM waves? If so, many waves with different magnitude will reach destination if we use AM method, then how to modulate and demodulate signal? There must be wrong somewhere, but I don't know. Correct it please.
    Last edited by a moderator: May 7, 2017
  7. Jul 2, 2015 #6


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    Just so we're clear, the torus shape you see is a representation of the amplitude of the emitted wavefront or the gain of the antenna. The wavefront itself is NOT shaped like a torus.

    For the most part, only a very, very small part of the expanding wavefront will reach a receiver. You can have reflections and will have interference when the phase of these reflected parts aren't in phase with the main signal, but as long as the main signal is much larger in amplitude than the reflected parts you just demodulate it as normal. If the out of phase parts begin to approach the amplitude of the main signal then you start having problems. The amount of noise these reflected signals (or other sources of interference) also heavily depends on the type of modulation used. Amplitude modulation is notorious for being highly susceptible to noise, while FM is far less susceptible.

    For the exact details on modulation and demodulation you can see the following links or search google.

    Last edited by a moderator: May 7, 2017
  8. Jul 2, 2015 #7


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    The dipole doesn't oscillate .... there is RF AC (RF alternating current) oscillating in the dipole. The frequency of that RF current is emitted from the dipole as a changing electromagnetic field. The EM field strength drops off in intensity calculated by the
    inverse square law.

    all what points ?

    there is only one source of the RF energy --- the transmitting antenna

    as I said in my previous post....

    the modulator section in the transmitter circuit does the modulating
    the demodulator circuit section in the receiver does the demodulating

    the antennas know nothing about the types of modulation being used

  9. Jul 2, 2015 #8

    Thanks a lot to both of you, I got it. Your answers are fantastic(smile)
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