Feeding antenna in CST

Hi, I have an antenna model (planar type - the antenna is placed on a dielectric substrate) in CST and I want to analyze its operation in free space without the effect of ground so my model doesn't contain ground. My problem is that if I use coaxial cable to excite this antenna, is it OK without ground. I've constructed the coaxial cable in CST with the inner feed, dielectric, outer conductor (the outer conductor doesn't connect to ground).
Please help me. Thanks in advance.