spiral antennas in cst

Hello,
I have a problem with CST MWS that how should i feed for Single Arm Spiral antenna
without the ground plane.

Does any one knows how boundary conditions are used in there?

Thanks,

hi..
u've to feed it by bottom. if u use ground plane then u can keep all boundary conditions open. else try making zmin electrical. see if it works.

hi
Another way is that u creat a pec object and set it infinite to simulate the ground