hfss infinite

Hi, everyone. I have a question about HFSS boundary setting for infinite extended objects. In my project, I am going to simulate the scattering field for a infinite long dielectric cylinder (see attachment).

So I set the four side walls as PML, but how do I set for the top and bottom walls? Since HFSS doesn't allow PML boundary contact with two objects, so the only option is radiation boundary? But it seems the simulation result is quite different from the exact analytical solution...

Any information and reference will be appreciated. Thanks.