The near-to-far-field calculation integrates the total field at the open boundary surface to produce far field results. Because the calculation uses the total field, it automatically accounts for the presence of any ground planes and symmetry planes. The exterior of the domain should be a combination of open boundary conditions, symmetry planes, and ground planes. As always, with open boundaries, any structure that touches the open boundary will electromagnetically appear to extend outward to infinity. In general this is desired behavior, but in antenna simulations it's best to avoid modeling semi-infinite structures aside from ground planes and symmetry planes. Semi-infinite waveguide feeds, for instance, are not likely to be a desired part of the radiation pattern.
Both Approximate Open and PML perform best for normally incident fields. For best results, the exterior boundaries of the system should be at least a quarter wavelength from the radiator, but not more than a few wavelengths to avoid excessive element counts. The resulting system may be large compared to the size of the radiator. Since this can add substantially to the computational burden, it is good practice to reduce the domain size whenever possible with symmetry planes. You can also potentially speed up the simulation with spherical PML.
The near-to-far-field calculation is sensitive to small errors in the fields at the outer boundaries of the model volume, so the generation of patterns requires attention to any potential sources of error at those boundaries. You may wish to use approximate open for fast results during the design phase, and then use PML for validation calculations.
The following list summarizes the recommendations for setting up a geometry to run an antenna simulation.
The domain exterior should be a combination of open boundary conditions, symmetry planes, and ground planes. It is best to avoid contact between any other structure and the open boundaries, in general.
The open boundaries should be at least a quarter-wavelength away from the radiating structure, but not more than a few wavelengths away.
Avoid assigning ports to the geometry exterior, except when the port is embedded in a ground plane. See the following section for details.