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Waveguides and 900 MHz

Discussion in 'Electronic Basics' started by Eric R Snow, Sep 19, 2004.

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  1. Eric R Snow

    Eric R Snow Guest

    It was mentioned that a 6 inch diameter steel pipe might act as a
    waveguide to a transmitter located 60 feet into the pipe. How would
    this be calculated? It doesn't really matter since I'm gonna be doing
    this. I'm just curious.
    Eric
     
  2. Hi,
    A circular guide of only 6in. in diameter (15.2cm) isn't a good
    choice for propagation at the wavelength corresponding to 900MHz
    (33.3cm), which is above the guide cut-off wavelength. This, in the
    dominant TE11 mode, is roughly equal to -

    sqrt(3) x guide diameter = 26.4cm

    and so you must expect some attenuation; the graphs also show that
    this increases very rapidly above cut-off. You will have to factor in
    material losses as well because you are expecting to use steel pipe.
    The only formula I could find assumes a copper guide which wouldn't be
    too accurate in this case.


    Cheers - Joe
     
  3. Eric R Snow

    Eric R Snow Guest

    Thanks Joe, I understand it a little better now. I know a little about
    light guides and how changing the refraction of the guide helps the
    light travel the length of the guide. Fiber optics and GRIN lenses
    work this way. There are layers in the guide so different diameters
    refract light differently. But a metal tube is akin to a single type
    of glass. How does the material the tube is made of affect the radio
    waves. Light must pass through a material to be refracted differently
    and so it is bent at different angles as it travels the length of the
    guide. But if it reflects the only thing that material changes is how
    well it reflects and how much light is absorbed. The angle is the same
    for all materials as long as it is reflected. I thought that wave
    guides also reflected the radio waves. Does the material absorbing the
    radio waves make a big difference the way it does with light? And
    since copper is a better conductor than steel wouldn't it absorb more
    radio energy? Just shows how little I know about this.
    Thanks again,
    Eric
     
  4. John Larkin

    John Larkin Guest


    If I have all this right,

    6" dia = 3" radius = 0.076 m

    The longest wavelength mode is TE11, 3.41 * r = 0.259 m

    Your wavelength is 0.33 m. So it's a waveguide beyond cutoff, and
    losses will be extreme.

    I think.

    John
     
  5. andy

    andy Guest

    If I remember right, it's the other way round - because it's a better
    conductor, it's better at carrying the induced currents inside
    the inner surface of the guide that mirror the EM field back into the
    space inside the guide, without dissipating energy through resistive loss.
     
  6. Hi,

    There is a continuous reaction between the E/M field and the
    guide walls such that currents flow in them which then re-radiate
    the wave. As a result any resistive losses there will absorb
    energy from them increasing guide attenuation. Copper, being a
    better conductor than steel, will thus afford lower attenuation.
    A further consideration is the surface roughness of the guide as
    any of that will make matters worse and I would expect commercial
    tubing to be only fair in this regard.

    Coupling into the TE11 mode is usually by a probe fitted into
    the wall of the pipe with a similar one parallel to it at the
    receiving end. For the impedances and tuning thereof, you will
    have to look in the books.


    Cheers - Joe
     
  7. Roy McCammon

    Roy McCammon Guest

    You can take conductivity to be an imaginary
    component added to the permitivity. Then,
    assuming monochromatic signal, you can put that
    complex permitivity into Snell's law and compute
    reflected and transmitted waves. You'll probably
    need to look at an E&M text book to understand
    the interpretation of the results, but the upshot
    is that waves travel very slowly in a good conductor
    so that in some sense, good conductors have very large
    indexes of refraction.
     
  8. Roy McCammon

    Roy McCammon Guest

    Rectangular wave guides are easier to compute
    and visualize than circular ones. Replace
    your 6 inch pipe with a 6 inch square wave guide.
    You won't get exactly the same numerical answers,
    but you get close and the qualitative behavior
    is similar.
     
  9. John Larkin

    John Larkin Guest


    The waves bounce off the walls of a waveguide, so stay inside the
    pipe. Think of copper as a shiny mirror, and steel as a dirty one.

    Radar absorbers are usually carbon or ferrite-loaded plastics, very
    bad conductors.

    John
     
  10. John Larkin

    John Larkin Guest

    Around here, they coat roofs with it.

    John
     
  11. what effect would the tube being grounded have?

    Charles
     
  12. john jardine

    john jardine Guest

    An old book says that "Pitch" is good at bending microwaves. The stuff
    seemed to figure in lots of electrical experiments late 1800's yet is not to
    be seen nowadays.
    regards
    john
     
  13. John Larkin

    John Larkin Guest

    None. The waves are inside, and don't care about the outside. A mirror
    doesn't need to be grounded.

    John
     
  14. Actually I was thinking the grounding could reduce the bounced energy.

    Charles
     
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