Fabry-Perot microwave resonator modes

Hi, I'm trying to build a model of a cylindrical microwave cavity resonator with spherical mirrors. I'm new to COMSOL so I think there is something basic I am missing but when I try to compute the solution I'm given an electric field of zero everywhere despite having an active port.

I've built the model in 3D because eventually I want to half fill the resonator with dielectric. Could someone please try and help me fix my model.

Thanks,
Will