How do you set the impedance in THz antenna simulation

Hi all:

I will start the project of design of the antennas at THz range. Our THz signal will be generated by FET device, and the antenna will mount on the FET. In this case, how do I set the impedance in the simulation software when I set the excitation? I don't know the impedance of the device is 50Ohm or not.

Thanks a lot.

could you be more specific

Which simulation software(s) can you use?

why not do a quick measurement on the impedance of the FET with VNAs. Then plug this value in your model.

To SunnySan: Thank you! I would like to try CST or XFDTD. I plan to make the antenna on the wafer, integrated with the FET. In my experience, HFSS is not good at very small size feature. Any suggestion?

To dazhen: Thank you! There is no vna can measure THz. device impedance depands on frequency. Any suggestion?