How to know the correct impedance of antenna to design a filter?

I have a scattering param file of an antenna with mode impedance of the guide 670 ohm at access, should i simulate the file in ADS with 50 ohm termination or 670 ohm termination ( S param file has R=50 )?

Generally, the reference impedance given in the S-parameter file is 50 Ohm, which is the port impedance used when test the DUT. Don't care the impedance in the S-parameter file and add ports with any impedance you needed. Just regard the ADS component using the S-parameter file as the antenna in your simulation.

hmsheng thanx for reply,
I have to design a filter for this antenna, when i simulate the antenna in ADS with 50 ohm termination(Term in ADS) it gives different impedance as when i did with 670 ohm termination.
how should I know the correct impedance of the antenna to design the filter?

If you read the port impedance from the Smith chart you will get different result because that is normallized with the port impedance. When it is multiplied by the port impedance, eg, 50 Ohm or 670 Ohm, you should get the same results. That's the real impedance of your antenna.

If you do S-parameter simulation in ADS, you can use Zin conponent to get the input impedence directlly.