Oscar Kilo
I clearly remember from my GPS user manual that the suction cup antenna should be mounted with the curved surface facing the sky. Yours may be different but there ought to be instructions for this. The antenna certainly should be mounted with "a" flat surface facing the sky.
The coax used for VHF, UHF and GPS is all 50 ohm cable. This is common as muck; however for the connection to the GPS itself you may have to use something much smaller than a BNC plug and a rather thin cable might be needed; however you can still get 50 ohms. Go for a low loss type.
I used the suction cup antenna in a PA28 for every one of my 60 hours in them
I stuck it to the very top of the windscreen, where the material is almost horizontal. Worked great.