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Characteristic Impedance

This page explains basics of characteristic impedance and provides equations of characteristic impedance of various transmission lines which include parallel line,coaxial line,shielded parallel line,stripline and so on.

Wen the reflected wave in a transmission line is zero, the ratio of V(z)/I(z) in the direction of propagation(z) is called characteristic impedance. It is denoted by symbol Z0.

Characteristic impedance(Z0) = SQRT(Z/Y) = SQRT(R+jWL/G+jWC)

Characteristic impedance of a lossy transmission line:

Zo = SQRT( R+jwL / G+jwC)

Characteristic impedance of a lossless transmission line:

Zo = SQRT(L/C)

Characteristic Impedance of Transmission Lines

Characteristic Impedance for Parallel Line
Zo = (276/sqrt(ε))* log(2*S/d)
Where S is the spacing between two conductors and d is the diameter of the conductor. ε is the dielectric constant between the conductors.

Characteristic Impedance for coaxial Line
Zo = (138/sqrt(ε))* log(D/d)
Where, D is the diameter of the outer conductor and d is the diameter of the inner conductor.

Characteristic Impedance for shielded parallel Line
Zo = (276/sqrt(ε))* log (2A *(1-B2/1+B2))
A=s/d and B = s/D

Characteristic Impedance for stripline
Zo = (377/sqrt(ε) )*(T/W)
T is the thickness of the dielectric material(PCB), W is the width of the line to be etched.


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Types and basics of transmission line
Types and basics of coaxial line
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