ErrorHFSS: A radiation boundary face has solid objects with SolveInside on both sides

I am doing filter design using microstrips. I have assigned airbox from T+H, but error appears when airbox is assigned as radiation boundary. (T-thickness of gnd, H-thickness of substrate). A radiation boundary face has solid objects with SolveInside on both sides of it. Only substrate has solve inside set in the design..What is the problem here?

Assign radiation boundary to the faces of the airbox not to the object.

I was able to assign Rad boundary to faces of air box..

For the microstrip filter design, without assigning boundary, S21 was around -0.5dB and after assigning rad boundary, S21 dipped to -20dB. Why so?

did you reassign a conductor (bottom) as radiation instead of conductor? double check ports. something is wrong in the -20dB case. look through the help pages about radiation boundaries for the conditions you should meet. there's something about minimum distance from conductors. Also with no boundaries assigned the outer box is perfect conductor.

Lathas, attach a file would be a good idea.