How to locate and Calculate 50 ohm feed point of a multiband monopole structure? ?

I want to locate and calculate the feed point for a 50 ohm coaxial cable to excite a multiband monopole structure. My antenna consist of a partial ground plane at the lowest layer. Followed by a Rogers 5880 substrate with er=2.2 as the second layer and the monopole structure as the final top layer. The structure is shown below.

Can someone kindly advise on how to calculate the exact impedance match feed point using a coaxial connector from the bottom ground plane to the top structure.

Awaiting replies,

Regards

,, something like this ...