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Electromagnetic standing waves measurement

  1. Sep 5, 2009 #1
    Do you get electromagnetic standing waves? If so, do the electric and magnetic fields not cancel each other because of their opposite direction in the standing wave? Can such a thing be measured?
     
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  3. Sep 5, 2009 #2

    Hootenanny

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    Sure. Standing EM waves have a number of important applications such as lasers, fibre optical cables and other wave guides.

    http://en.wikipedia.org/wiki/Optical_cavity
    http://en.wikipedia.org/wiki/Waveguide
     
  4. Sep 5, 2009 #3
    Thanks for that. I presume that you mean INSIDE the laser body?
     
  5. Sep 5, 2009 #4

    Hootenanny

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    Of course.
     
  6. Sep 5, 2009 #5
    Sorry to be so dense, but these standing waves are surely not propagating and if not can they be measured or even detected?
     
  7. Sep 5, 2009 #6

    f95toli

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    I am not sure what you mean by "detected". It is fairly easy to e.g. map out the field distribution inside a cavity using some sort of field probe. There are of course also a number of other ways of doing it.
    This is pretty much just standard enginein ring since e.g. most passive microwave filters are based on connecting a number of resonators which -by definition- have standing waves in them when they are resonating; i.e. it is in no way "exotic" physics.

    Another example would be just about any simple antenna; the length of an antenna can e.g. be made to be exactly half a wavelength.
     
  8. Sep 5, 2009 #7
    Thanks for the info. I was under the impression that the fluctuating fields of the electromagnetic wave were necessary to "detect" the signal. Is it not those fluctuations that generate the electrical singnal that we read on our instruments? Do standing waves exhibit these fluctuations passing the "detection" point of the antenna for instance?
     
  9. Sep 5, 2009 #8
    Electromagnetic standing waves are extremely important in charged particle accelerators. Usually the RF or microwave cavities are resonant in the TM (transverse magnetic) mode so there is a longitudinal standing wave. There is a small hole at each end of the cylindrical cavity for the particles to enter and exit. The longitudinal standing wave has a voltage node at each end and is 1/2 wavelength long. Typical frequencies range from a few tens of MHz (Fermilab uses 53 MHz) up to microwave S and X band (SLAC uses 2856 MHz).
     
  10. Sep 5, 2009 #9
    Thanks guys. I now know that EM standing waves do exist and are used extensively. I think I now grasp the reversal of the of the field upon reflection at the node as well and that there is still a fluctuating field to measure. Thanks.
     
  11. Sep 6, 2009 #10

    Born2bwire

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    Yeah, when we talk about standing waves, we are talking about a standing wave in terms of the wave's spatial variations, not the temporal fluctuations. The wave is still going to fluctuate in time but the amplitude of the wave over space will be a standing wave (just like a standing wave on a string).
     
  12. Sep 6, 2009 #11
    Would it be possible that a LONG standing wave could exist in space if a continuous electromagnetic wave was reflected perfectly back on itself?
     
  13. Sep 7, 2009 #12

    Born2bwire

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    Yeah, I do not see any reason why the distance would matter. The only thing that would be a problem is the gradual loss of the wave due to the space loss factor. If you are talking about a LONG distance here, then it would probably be a safe assumption that the amplitude of the wave will decrease over the distance travelled in question. You will still have a standing wave since the frequency of the wave will not have changed and the perfect reflection will still force a node at the point of reflection. But the shape of the standing wave will be different than expected. At first glance, the amplitude of the peaks would expect to decrease as you moved away from the point of reflection since the reflected wave will weaken with distance, but I think that the peaks will actually increase since the incident wave will weaken as it moves towards the point of reflection and I think that these peaks will dominate more. Eventually though, the loss of the reflected wave in respect to the incident wave will mean that eventually the standing wave will disappear. The only time we do not consider the space loss factor over very large distances is if we have a true plane wave, which requires an infinite source.
     
  14. Sep 7, 2009 #13
    Thanks for your clear explanation. If this was a continuous emission and a continuous perfect reflection (theoretically) could it be said that the standing wave is standing still even though the constituent waves are travelling at 300 000 km/sec? If so, may I perhaps return to my initial question: do we use detectors that would detect such a "non propagating" standing wave?
     
  15. Sep 7, 2009 #14

    Born2bwire

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    Yeah, it can be detected. Like I stated before, the wave is still fluctuating in the temporal space and is a standing wave in the spatial space (is that even right to say...). As a wave propagates, the wave will fluctuate over time and space. In a travelling wave, the amplitude of the field will travel in time as the wave propagates. However, the field is still fluctuating in time as well, independent of the propagation of the wave. A standing wave is stationary in that the amplitude of the waves does not move in time, but the fields are still fluctuating. It is like generating a standing wave on a length of rope. The rope will still be oscillating up and down but the nodes and antinodes are fixed in space. As f95toli pointed out, we can measure these standing waves by simply using a probe. We stick the probe into the field and map the fields and we would discover the nodes and antinodes of the wave.
     
  16. Sep 7, 2009 #15
    Thank you for taking my questions seriously. I appreciate your responses. Regards.
     
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