ADS Momentum simulation of the patch antenna with partial ground

Is it possible to have a partial ground plane in a simple microstrip patch antenna simulation in ads momentum? I would appreciate if you could explain how to define the layers to achieve this.
Thank you and regards

Why don't you use a ground reference port?

Yes you can do this ... make a substrate without the ground...
to do this in layout window go to Momentum>Substrates>Create/Modify select the //////GND////// and make it and open boundry
now add the substrate layer below the required one as air(permittivity=1)
Now mark a metal layer as strip between Air and required substrate layer
Now the metal layer is your partial ground .. now on the partial ground place a pin .. any where and mark the pin marked on the patch antenna as single with Gnd reference to the one marked on the partial ground.... the designing part is over now simulate to enjoy the results .

@amitsinghjiit
Thank you, but I am not sure that I understood your explanation.

If port P1 is the port attached to the feed of the antenna (upper metalization layer), and port P2 is the port attached to the partial ground (lower metalization layer), does it mean that P1 port type should be internal and P2 should be Ground Reference (associate with port number 1)?
It is not possible to make P1 type single mode and P2 ground reference associated with P1.

Yes it is not possible to make P1 type single mode and P2 ground reference associated with P1, make it internal(P1) with ground reference P2. You are doing it correctly.