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Wavelength -> antenna, waveguides, mirrors ets

  1. Sep 7, 2009 #1
    Hi
    I have this question for sometime, and until i found a recent post here
    on this forum (wavelength and aperture) i thought i was the only person
    who just plain didn't get it.

    My questions arises firstly when i consider antennae and as roughly quote from
    the many texts i have read "to receive the maximum power radiated the antenna
    should have a physical size in multiples of the wavelength of the signal to be received"
    that is for example 1/4lambda etc

    I then take a look at waveguides and the general idea here besides all the math and
    different type of modes. The waveguide will have dimensions in width that relate to the
    wavelength of the signal typically microwave.

    It is clear also that as the frequency goes up the required physical size of the antennae or satellite dish become smaller (the obvious reason why we don't transmit signals using sound waves)?

    The above 2 examples are confusing conceptually because i can't get away from the idea that amplitude is more likely to have a bearing on the size of the antenna or waveguide.
    I can visually see the "wavelength" reaching out to the parallel walls of a waveguide and somehow affecting or being affect by the waveguide.

    I do have a question which relates the same problem i have with mirrors, wont go into it right now.


    Appreciate any suggestions on this.
     
  2. jcsd
  3. Sep 7, 2009 #2

    Born2bwire

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    The fields of an electromagnetic wave are not a spatial disturbance. If I have a field being produced only at a point in space, then the fields will be confined to that point, the amplitude does not stretch out in space. The amplitude of the electromagnetic wave relates the amplitude of the electric and magnetic fields. It's like painting a wall, I can change the color of the paint on the wall, make it brighter or darker, but none of these properties affect the spatial location of the paint.
     
  4. Sep 8, 2009 #3
    Thanks for your response
    I like your analogy. Should i really be trying to break away from the images of
    a cosine wave for E & B entering some waveguide? Or even as a traveling wave?
    Iam started to figure that this are an inadequate representation of the EM wave?
    I have to admit i am still unsure of how the physical dimension of wavelength relates to
    the physical dimensions of an antenna (sorry)!!!

    Thanks
     
  5. Sep 8, 2009 #4

    Born2bwire

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    The sinusoidal nature of the wave is still relevant. The amplitudes of the field vary in space. With a travelling wave, these amplitudes travel in space as well. The waveguide imposes boundary conditions that any propagating wave must satisfy to travel through as a supported mode. If the waveguide is made of perfect electrical conductors then the tangential electric fields and the normal magnetic fields must be zero on the surfaces of the waveguide. This is where the wavelength comes into play because it will specify the distance between the zero crossings of the fields. If the wave has a wavelength much larger than the physical dimensions of the waveguide, then it is highly improbable that the wave could be constrained in the waveguide in such a way that it can match the boundary conditions without severe distortion. This results in the wave travelling through while a large amount of its energy gets dumped, attenuating it quickly. An electromagnetic wave that has a wavelength smaller than the physical dimensions can orient itself to bounce around in the wave guide. It can travel in an off-axis direction that will allow it to satisfy the boundary conditions while still having a net movement in the guided directions.

    Most waves will bounce around in a waveguide because there are few wavelengths (and not every waveguide can do this) where the wave will satisfy all of the boundary conditions perfectly and be able to pass straight on through. Usually the wave has to travel at an angle to the propagating direction of the waveguide to make the wave match the boundary conditions. All of this is usually solved implicitly when we solve for the modes of a waveguide.
     
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