Antenna Design using CST microwavestudio

Hi guys, this is my first time doing antenna design. I need some help regarding the dimension of the antenna.
I am converting a 20GHz circular polarized helical antenna into square helix.
I am using CST MWS for the design.
1)How far should my coil be from the ground plane
2)Are there any formula to calculate the dimension of the ground plane thickness
3)Are there any formula to calculate the dimension of the dielectric. (I am using air)

1.) This distance makes a large difference in helical antenna performance. googling found me answers. look up stub-matched helicals
2.) As long as its larger than skin depth at the lowest frequency youre ok
3.) dimensions for the coax feeder? again google is ur best answer, and yes this makes a difference.