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S11(f) - How to explain the minima

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elektr0

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Hi guys,
very basic questions: How do we explain the minima/maxima of S11 versus frequency, e.g. of a transmission line.
Thank you very much.
-e
 

... to be a little bit more precise:
Using S11(f) expressions of a transmission line with characteristic impedance ZW and termination impedances ZT, we can see 20*log10( abs(S11(f))) having maxima and minima.
For a simple transmission line, the input reflection coefficient r_input is the reflection coefficient of the output impedance rT and the complex exp function
r_input=rT * exp(-2*gamma*length)
plotting this (20*log10(abs(rinput))) over freuency does not include maxima and minima.

Thanks you for any comment....
 

The experiment prerequisites aren't completely clear. Perhaps you should clarify in which situation you observe |S11| maxima and minima.

If the transmission line impedance is equal to the S11 reference impedance, there's no ripple, S11 is just rotating around the origin and |S11| stays constant. But with different transmission line impedance, you get maxima and minima. You can visualize the problem using Smith chart.
 
But with different transmission line impedance, you get maxima and minima.

Exactly. And when the line impedance is n* lambda/2, you get Zin=Zload. That's one case for ripple in S11: 50 Ohm load, 50 Ohm reference impedance, but the cable impedance is somewhat different.

smith5025.PNG
 
Thank you very much. Sometimes the basic stuff gives me a headache ;)
 

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