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Standing EM Wave on conductive bar

Discussion in 'Electronic Design' started by Jon Slaughter, Mar 7, 2008.

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  1. Is is possible to set up a relatively precise standing EM wave on a
    conductive bar(see below for how precise)?

    I am thinking that it could be used to measure the length of such things
    since L = c/f. Of course this assumes knowing c in the material and the
    frequency to get an accurate result but I'm just curious as to if it is even
    possible to do such a thing?

    Sounds like it would be very similar to antenna's but here I'm trying to get
    an accurate result. Maybe simply sweeping the frequency range and finding
    the resonant peak treating the bar as an antenna would work?

    Just looking for some ideas on it as I've been pondering lately how to
    measure things accurately and this was one idea I was thinking about.

    Another similar idea which probably won't work would be to setup a high
    frequency EM wave on the conductive bar and measure the EM field along the
    bar counting the peaks. (essentially the intensity plots out a ruler on the
    bar) Unfortunately for decent resolution one needs to use too high of a
    frequency. (to get even mm resolution one around a frequency of 100 GHz)

    Maybe one could send a pulse like radar/sonar or something and measure the
    time delay?

    Thanks,
    Jon
     
  2. Guest

    http://en.wikipedia.org/wiki/Time-domain_reflectometer

    If you want to do really precise length measurement, you have to go to
    really high frequency electromagentic radiation, and use optical
    interferometry.

    Hewlett-Packard (now Agilent) and Zygo sell spectacularly precise bits
    of gear (though it helps if your optical path is evacuated - speed of
    light in air depends on atmospheric pressure and can change enough to
    make a perceptible difference to a really precise measurement).

    http://www.zygo.com/?/products/zmi/

    http://cp.literature.agilent.com/litweb/pdf/5091-8435E.pdf
     
  3. http://en.wikipedia.org/wiki/Time-domain_reflectometer

    If you want to do really precise length measurement, you have to go to
    really high frequency electromagentic radiation, and use optical
    interferometry.

    Hewlett-Packard (now Agilent) and Zygo sell spectacularly precise bits
    of gear (though it helps if your optical path is evacuated - speed of
    light in air depends on atmospheric pressure and can change enough to
    make a perceptible difference to a really precise measurement).

    http://www.zygo.com/?/products/zmi/

    http://cp.literature.agilent.com/litweb/pdf/5091-8435E.pdf

    ----

    What about my specific ideas? I know there are other methods out there but I
    am not so much interested in measuring something as I am about how to
    measure it. I know the idea of linear optical encoders, magnetoresistive
    devices, magnetorestrictive devices, and a few others but I have not seen
    any that are based on the ideas I have given and I curious as to how
    practical they are. (obviously they hold in theory since its just basic
    physics but I'm more curious as to how precise in practice for relatively
    simple implementation)
     
  4. Wimpie

    Wimpie Guest

    Hello Jon,

    When the thickness of the bar << wave length, yes, you can get a
    standing wave pattern with sharp knots. However, from this you
    cannot directly determine the length with very high accuracy, even if
    you sweep en measure the shift of a knot.

    The reason is that there is a so-called end-effect (fringe). Your bar
    will appear somewhat longer then the physical length. The larger
    thickness/WL, the stronger the effect. In addition when objects are in
    the vicinity, the propagation speed along the bar will change more or
    less. For this you should add (empirical) corrections

    From a standpoint of accuracy you want a high frequency, but this will
    increase Thickness/WL. Because of this, standing wave pattern will be
    less sharp. At high frequency, departure from a straight line will
    give more radiation loss. This also will result in less sharp standing
    wave pattern.

    Best regards,

    Wim
    PA3DJS
    www.tetech.nl
     
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