circular iris and rectangular cavity

Hello All,
i'm trying to simulate a rectangular cavity excited through a circular iris.
are there any guidelines about the size of the iris ? my simulation isn't working. i dont see the cavity modes in my S11 plot. i attached here my hfss file . the rectangular waveguide is 0.9' X 0.4'
Thank alot,
guy .

I've changed the cavity's walls to copper and the iris to rectangular and now how can see the modes on my S11 plot.