Dielectric Wave Guide at mmwave

hi,
I have simulated a silicon ( er=11.9 )dielectric slab at 300 Geg with slab thickness=0.1mm on hfss...
I expected single mode transmission, however i found 90% of power radiated? and the waveguidance s21 is veryyy low like -60dB or so?
I used lumped port excitation ... please advise,
thanks,
shehata