Difficulty in using iterative method for Rf problems which has multiple ports condition one boundary

Hi everybody,

have anybody encountered the problem of keep on having ill conditioned preconditioner when you apply a lot of port condition on a single boundary (For grating structure simulations where many diffraction order expected)? Although it works fine with direct methods, the iterative method with multigrid keeps on not converging. I tried using iterative method on the example comsol model of plasmonic wire grating but i encountered the same problem. Anybody can give an advice how to tackle such problems with multiple ports on one boundary with iterative methods?

Best regards,


Aimi