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port Z0 is the characteristic impedance of the structure.
There are a few strange things I notice in your attached image - is b (= 39.5 mm) the width of your SIW? Using a dielectric constant constant with FR-4 = 4.4, the cutoff frequency should be 1.8 GHz, so I guess the mode should be supported at 2 GHz. I'm not sure why the characteristic impedance would have such a large imaginary component (~10 degrees?). Can you post some more information, such as the setup and scattering parameters, or perhaps attach the file?
That's something I did not know. Thanks for that information. Is there anywhere in the HFSS documentation I could find this?
At 2GHz which is above the cutoff frequency, the port Z0 given by HFSS is 3.96. Multiplying with a/b (53.74/0.6) gives me 350 ohms around. For an equivalent rectangular waveguide filled with dielectric of er=3.6 is, I remember I should ve got around 270 ohms.
So when I calculate the Z parameters from solution data, they are normalized to the Z0 which is the port Z0 displayed HFSS times (a/b). Am I correct?
For the antenna to be matched, the real part should be around 50 ohms and the imaginary part near zero.