Guilin Sun
Ansys Employee

In general, for periodic simulations, you should use monitors wide enough to pick up larger-angle diffraction. That said, the monitor spans should be larger than the simulation spans, which are the periods in the two diractions. The same is true for the plane wave source. Fortunately, the software has the precausious measure to automatically expand the plane wave spans larger than the simulation spans.

As for the distance, source and monitors can be around half wavelength away, or closer, to avoid large simulation volume. Due to numerical errors, far-away monitors may have a little smaller transmission, but the difference should be very small.

Please increase the monitor spans.