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I Reflection (intrinsic and characteristic impedance effects)

  1. Dec 27, 2018 #1
    Do we have reflection when the intrinsic impedance η=E/H between two media are matched but not necessarily the characteristic impedance (assuming a transmission line)?

    Basically, I have a case here shown below
    upload_2018-12-27_10-5-58.png
    I have two parts with different geometries (this may not be a transmission-line, if you want you can assume an equivalent structure for a parallel line to get a transmission-line). The material parameters μ, ε are the same between the two media but their geometry is different. Do we still have a reflection at the boundary in this case? (does the boundary condition result in a reflection?)
     
  2. jcsd
  3. Dec 27, 2018 #2

    tech99

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    Gold Member

    Allow me to talk in terms of transmission lines, and let me assume they have resistive characteristic impedances Z1 and Z2.
    LIne 2 is shown as transmitting energy to the right, so we assume it is terminated at the right hand edge with a matching resistor so there are no reflections. Alternatively it might be very long, so reflected energy is absorbed.
    Line 2 it therefore presents a resistive input impedance equal to its characteristic impedance, Z2. It looks like a resistor.
    Line 1 has different characteristic impedance to Z2, so it will always be mismatched and will have standing waves. There is always reflection at the boundary.
    It is possible to obtain a resistive source impedance into line 1 for lengths where line 1 is resonant (quarter or half wave in length), but line 1 itself will always have standing waves, showing it is mismatched.
    It is possible to insert a matching device at the junction, so that line 1 sees a ratio of E and H equal to its characteristic impedance, but that is not, I think, your question.
     
  4. Dec 31, 2018 #3
    Hi,

    Thanks for the reply. Do you mean to say that E/H ratios of two lines aren't matched if the dimensions are different? I was always thinking that the source of reflection was the mismatch of E/H ratio and the boundary condition. But, I read recently in a book that you can have same E/H ratios but different characteristic impedance, which may depend on dimension of the line, and this can result in reflections. Hence the confusion.
     
  5. Jan 1, 2019 #4

    tech99

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    Yes. The ratio of diameter and spacing needs to be the same (assuming the same dielectric).
     
  6. Jan 13, 2019 #5
    Thanks now that answers my question
     
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