The two types of open boundary conditions are Approx Open and the Perfectly Matched Layer (PML) absorbing boundary condition. Approximate open is a zero-conductivity impedance boundary condition with the impedance of free space (376.73 Ω). As a result, it adds very little to the computational burden of a simulation, and it is sufficiently accurate for many applications. It is a good choice for non-radiating systems that are not bounded by conductors. It may also be used as a alternative to PML, when some level of accuracy may be sacrificed for solver speed. You should ensure that the boundaries of such systems are far enough away from the structures of interest that the fringing fields are attenuated to near-zero at the boundary. The approximate open boundary condition is useful because it does not fully reflect the low fields at the boundary, as PEC and PMC do, although some reflections still occur.
PML defines an auxiliary mesh that adjoins the main FEM mesh at the PML boundary. This auxiliary mesh is filled with a graded lossy medium that is impedance-matched to the adjacent main mesh, so that incident fields penetrate the PML mesh with low reflection, and those fields are attenuated in the PML material. Field attenuation within the PML depends on the incident angle of the fields that intersect the PML boundary. Normally-incident fields experience optimal attenuation, while glancing fields experience less attenuation. Materials that intersect the PML go to infinity. PML may be applied to either planar or spherical surfaces, but not both within the same geometry. When using symmetry planes with spherical PML, the geometric center of the PML sphere must lie on the plane (if only one), or at the intersection line/point if there are multiple planes. Using PML boundary conditions on a spherical boundary may result in a faster simulation than using PML on a rectangular boundary, because the auxiliary mesh typically adds fewer elements to the system than in the rectangular case. PML is more accurate than approximate open, but because PML directly increases the size of the mesh, PML adds substantially to the computational burden of the simulation. For this reason, PML is preferred when the degree of accuracy is more important than the simulation run time. See PML for details on PML parameters.
Open boundary conditions are required in antenna simulations. For more details, see Antenna Configuration.