plz help in ADS

hi frnds iam designing a combline bandpass filter using ADS ..what is the characteristic impedance of the resonator?whether it is 50 ohm or greater than 50 ohm(why?)........i have to short circuit the resonators in layout window using via hole grounding .........plz explain how to make a via hole grounding in the layout window of ADS...how to change the momentum(substrate ..create/modify) for via hole grounding
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Near 50 ohm should be OK.
Read momentum--->substrate--->help manual for via definition