Impedance Matching Coaxial Cables

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Discussion Overview

The discussion revolves around the problem of impedance matching between two coaxial cables with different dimensions but the same dielectric constant. Participants explore the implications of removing the dielectric on wave propagation velocity and characteristic impedance, as well as the mathematical relationships involved in determining the necessary dimensions for matching.

Discussion Character

  • Homework-related
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • One participant presents a homework problem involving the calculation of the outer conductor radius needed for impedance matching between two coaxial cables.
  • Another participant suggests that removing the dielectric would decrease the velocity of wave propagation, implying that the characteristic impedance would increase as a result.
  • A question is raised about the implications of matching two transmission lines with different characteristic impedances, seeking clarification on the concept of impedance matching.
  • A mathematical approach is provided to solve for the radius of the outer conductor, with a specific value proposed (12 mm), though uncertainty about its correctness is expressed.
  • Clarification is sought regarding the conceptual understanding of impedance matching rather than the correctness of the mathematical solution.

Areas of Agreement / Disagreement

Participants express differing views on the effects of removing the dielectric on wave propagation velocity and characteristic impedance, indicating that the discussion remains unresolved on these conceptual points. There is also uncertainty regarding the mathematical solution for the radius of the outer conductor.

Contextual Notes

Participants have not fully resolved the implications of the dielectric's removal on wave propagation and impedance, and there are unresolved mathematical steps in the calculations presented.

Who May Find This Useful

This discussion may be useful for students and practitioners interested in transmission line theory, impedance matching, and the effects of dielectric materials in coaxial cable applications.

DODGEVIPER13
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Homework Statement


We need to connect two coax cables with the same dielectric (εr = 9) but
with different dimensions as shown. How big must be the radius of the
outer conductor of the second line in order to match two lines?

How would the velocity of the wave propagation change, if we remove
the dielectric? What’s about the characteristic impedance?


Homework Equations


Zc=sqrt(μ/ε)((ln(b/a))/(2∏))
μ=μrμ0
ε=εrε0

The Attempt at a Solution


I so far have uploaded my answer to the radius part am I right? For the last conceptual parts I am not sure and need a bit of help.
 

Attachments

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For the conceptual part I am thinking that the velocity would decrease because Vp=C/(sqrt(με) so if the dielectric is removed the denominator will decrease and will lead to a faster propaagation. Furthermore the characteristic impedance is sqrt(μ/ε) so it will increase too.
 
First, what is implied when two transmission lines of different characteristic impedance are "matched"? In regards to the conceptual question, it looks like you have the right idea.
 
Zl=sqrt(μ/ε)(((ln(b/a))/(2∏))=sqrt(μ/ε)((ln(b2/a2))/(2∏)) so I can cancel 2 pi and sqrt(μ/ε) and both natural logs giving b1/a1=b2/a2 solving for b2 I get 12 mm is it still wrong I am lost?
 
I wasn't implying that it was wrong I was just seeing if you understood what was meant conceptually by impedance matching.
 

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