Fighting substrate/surface modes in planar antenna

I'm designing a planar antenna in the mm-wave range. I have to use thick substrate (membrane- or horn-type antenna is out of the question). Since the final target design is an antenna array for focal imaging, no lens must be used. I have to design an antenna on Si or Quartz substrate with back-side metalization.

I have been simulating (in HFSS) microstrip dipoles, slots, double slots and double dipoles and all have radiation pattern a lot worse than in the air. Multi-beam pattern, bad directivity, high sidelobes, etc. Impedance is not of high importance, but radiation pattern is.

The reason for poor radiation pattern is TM0 surface/substrate mode in the dielectric. At least i can avoid TE1 mode by using appropriately thin substrate.

Please share your experience, knowledge, suggestions about how to fight against TM0 mode.

1. Is there a way to surpress TM0 mode by PBG pattern on the bottom metalization?

2. Is there a symmetry trick to use to cancel the TM0 mode?

...

Thanks to anyone for constructive suggestions.

br,
rfmw