enjunear thank you for the response. i plan to do everything you say but first can i ask you a question? i have the circuit in the schematic and is performing well. what procedure do you follow to build the layout? i generate/update the layout, i connect the components in the desired fashion and afterwards i upgrade the substrate from the schematic and i am trying to set up the ground plane and the holes. Can you explain the procedure requruired to do the last 2 (ground plane and holes)?
Momentum is somewhat removed from the schematic to/from layout functions (generate/update sch and/or layout). Once the metal is in the layout editor, you can think of the Momentum solver as basically importing the layout information, crunching the numbers, and spitting out a set of S parameter data.
You can define a ground plane, but the simplest way is to take advantage of the infinite ground plane that Momentum assumes by default. When you go into the Momentum layer editor, you need to define the substrate dielectric parameters. Once that's done, go to the second tab and link the layers in the stackup view with the layers from the layout editor (microstrip copper metal on cond layer, vias on via layer, etc). Select the top layer (-------) in the stackup editor, tell it to be a strip layer, then specify it to use the polygons from the cond layer.
Next, select the substrate from the stackup window, tell it to be a "via" layer, then specify it to use the polygons on the via layer; this tell Momentum to run vias through that dielectric layer, since you can have multiple dielectric layers, and vias going from layer 1 to 2, 2 to 3, 1 to 4, etc.
The bottom layer will be the infinite ground plane, so be sure you link your vias to it. One way to check that you got it correct is to layout a 50 ohm microstrip and drop a via right in the middle of it. When you run Momentum, it should look like a shorted line and some frequencies, and open at others (you basically make a shorted stub for a test). If the via isn't connected to the ground layer, then the microstrip will look like a pretty decent 50 ohm line, with a minor disturbance from an open-circuited via hanging in parallel with the 50 ohm thru line.