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# Transmission Line Input Impedance by monitoring the voltage over the line

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#### Arend2

##### Newbie level 3
Dear,
Is possible calculate the imput impedance of a line (unknow Zo) just using the voltage values over the line (graph V versus position) and knowing the load impedance Zl (in my case pure resistive) ?
THANKS !!!!

Have you heard about the Smith Chart? Using Voltage Standing Wave Ratio, this is just meant for your case.

To measure the ratio of Z0 to Zi, the standing wave ratio on the line has sufficient information. But a slotted line can be also used to determine complex impedances by using the phase information. This has been in fact a state-of-the-art method of RF impedance measurements before vector network analyzers became available. https://en.wikipedia.org/wiki/Slotted_line

Smith chart ? Just some experiencies. Could you please check if the below steps are right ?
1- Calculate the reflection factor RF=(Vmax-Vmin)/(Vmax-Vmin);
2- Draw a circle (centralized) in the chart with r=RF (considering the chart size r=1);
3-As my load Zl is known and is real the Zo=Zl/(the circle intersection point with the real axis);
3- "walk" over this circle, from the real axis (as my load is pure real), clockwise (generator direction) the line length in number of wave length till the final point, getting Zin normalized;
4- Zin=Zin normalized*Zo.

THANK YOU !!!!!
4-

ops RF=RF=(Vmax-Vmin)/(Vmax+Vmin)

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