I Reflection (intrinsic and characteristic impedance effects) (1 Viewer)

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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?)
 

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tech99

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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.
 
145
1
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.
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.
 

tech99

Gold Member
1,549
492
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?
Yes. The ratio of diameter and spacing needs to be the same (assuming the same dielectric).
 
145
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Yes. The ratio of diameter and spacing needs to be the same (assuming the same dielectric).
Thanks now that answers my question
 

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