![]() If it does change in that case, I have no idea. You should then be able to optimize your inset line and patch geometry to get a Zin=Zo with greater comfort that your results are physical. If it does not, then the issue you are seeing is due to the way you've been deembedding your port. You might try deembedding your port to flush with with beginning of the inset line (the end of your uniform feed line) and see if your Zin varies with length of feedline in that case. That ceases to be a valid operation when you try to deembed into a discontinuity. My understanding is that the software uses the calculated modal propagation constant for that port to change the phase (and attenuation related magnitude) of the calculated s-parameters. In other words, you cannot deembed into your antenna or inset feed and get accurate results. I work with HFSS mainly, but I'm going to assume CST is the same: my understanding is that deembedding (in either direction) assume that the cross section has not changed. Hence, a (1/8)-wavelength inset would decrease the input impedance by 50. As an example, if RL/4, then cos (piR/L) cos (pi/4), so that cos (pi/4)2 1/2. Hence, by feeding the patch antenna as shown, the input impedance can be decreased. So what you're doing (or at least part of it) is parametrically varying that length of uniform feed line (not the inset part) and then de-embedding (I'm pretty sure what CST means when it says "moving the reference plane") your port into the antenna? If that's the case, the results you're seeing are probably not physical. In the above equation, Zin (0) is the input impedance if the patch was fed at the end. ![]()
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