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# How to get S-Parameter when use incident plane?

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#### JuanjoSF

##### Newbie level 5
s parameter, pencil

Hi all,

I have a simple doubt. When I apply a incicent wave in one of the faces of my structure in order to measure, for instance, the scattering behavior of a FSS, I only can measure fields. HFSS-->Create Report doesn't allow me to get the S-parameter. How can I obtain it? Should I define some wave ports? How?

Thank u very much

You can't plot the S parameters for the simple reason that they don't exist for an incident plane wave.

In electromagnetic problems, you need to be able to define a voltage wave to get S parameters. In HFSS you do this by setting the integration line of your port. So, yes, you need to define a wave port. (A lumped port would be fine as well.)

Thank u! I'd never used Incident Wave before. So, the wave ports have to be created at the top and the bottom of the structure, right? If you could have a look at the picture attached and tell me whether it's correct, I would solve my headache! Thank u!

you can try the following: (i am assuming you are illuminating the geometry with a normal incident plane wave)

- create a line along the excitation
- on this line calculate the electric field or magnetic field using the field calculator and data tables.
- then, on this data apply some numerical method for expanding it in terms of exponentials (matrix-pencil method or generalized pencil-of-function method)
- since hfss gives the scattered field by default, you will have one scattered plane wave and one transmitted plane wave.
- by using the coefficients of these waves, and by shifting them properly to the physical boundaries of your problem, you can get the scattering parameters defined by those boundaries.
- this method is a little bit complicated and cumbersome, but it is best since it allows any mode-conversions to decay sufficiently near the physical boundaries. alternatively, you can still use waveports, but be sure to place them sufficiently away from the problem geometry, and use deembeding...

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