CST-MWS unidirectional farfield

Hi,
i am simulating planar antennas which should result in unidirectional farfield but so far all i got are bidirectional. i assigned open boundary for all except an open added boundary for Zmax.
can somebody please help me to fix this problem.
thanks a lot.
abti

What is the design principle of your antenna? If you have designed a microstrip patch antenna, then the groundplane is responsible for the uni-directional rad.pattern.
Try assigning a perfect E-plane to your groundplane... If you are simulating a finite groundplane with Open boundary (which is most probably your case now...) then some radiation will be inevitable backwards, due to spillover to the finite groundplane (to a certain degree)...
hope this helps...
mogwai.