# S11(f) - How to explain the minima

1. ## S11(f) - How to explain the minima

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

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2. ## Re: S11(f) - How to explain the minima

... 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....

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3. ## Re: S11(f) - How to explain the minima

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.

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4. ## Re: S11(f) - How to explain the minima

Originally Posted by FvM
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.

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5. ## Re: S11(f) - How to explain the minima

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

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