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[SOLVED] CST impedance and scattering matrix explanation

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Feb 3, 2012
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Hi everyone,

I would like to ask you for explanation. I investigated simple (ideal, lossless) microstrip transmission line (MTL) in CST Design Studio (analytical model) with characteristic line impedance approximately 54 Ohm (poor real-valued) and I matched all port reference impedances to this value. Therefore one can expect that input impedance of MTL will be independent on length itself and only slightly dependent on operational frequencies (if any). This presumption is correct when I done a transient simulation (attached pictures) where input impedance had the same value like load and characteristic line impedance. However if I investigated Z matrix parameter (attached pictures), the real part of input impedance is close to zero and imaginary part exist too and I really don't understand why, how it's possible :'( ???. I took this values (real and imaginary part) and calculated input reflection coefficient (S11 parameter) by hand. The achieved results didn't agree with results from S-parameter task simulation. Therefore I am absolutely confused, please can someone tell me where I do a mistake? I also simulated investigated structure in CTS Microwave Studio and I got the same result :( .

Thank you very much for any advice.

Project files and pictures can be find at my cloud:

You used a quite new version of CST, so unable to open. Recall the definition of z11. it's the input voltage/input current with the output open. Then take a equivalent circuit for your transmission line (easiest: series inductance and parallel capacitance) and voila: real(Z1,1) should almost zero and im(Z1,1) should be some frequency dependent value.
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