The Periodic Port Reference Point subnode is available only in 3D. When the Type of Port is set to Periodic under Port Properties, this subnode is available from the context menu (right-click the Port parent node) or from the Physics toolbar, Attributes menu.

The Periodic Port Reference Point is used to uniquely identify two primitive unit cell vectors, a1 and a2, and two reciprocal lattice vectors, G1 and G2. These reciprocal vectors are defined in terms of the unit cell vectors, a1 and a2, tangent to the edges shared between the port and the adjacent periodic boundary conditions. G1 and G2 are defined by the relation

where n is the outward unit normal vector to the port boundary. If there are multiple points defined in the selection list, only the last point is used.

The primitive unit cell vectors, a1 and a2 are defined from two edges sharing the Periodic Port Reference Point on a port boundary. The two vectors can have unequal lengths and are not necessarily orthogonal. They start from the Periodic Port Reference Point.

For listener (passive, observation, and not excited) ports, if the outward normal vector on the listener port boundary is opposite to that of the source port, the listener port reference point needs to be mirrored from the source port reference point based on the center coordinate of the model domain. For example, if the source port reference point is at {-1,-1,1} in a cubic domain around the origin, the mirrored listener port reference point is {1,1,-1}. In this case, if the Azimuth angle of incidence at the source port boundary is α2, the Azimuth angle of incidence at the listener port boundary π/2 − α2 and the signs of the diffraction order on the source and listener ports are opposite. See also Periodic for the angle definition.

If the lattice vectors are collinear with two Cartesian axes, then the lattice vectors can be defined without the Periodic Port Reference Point. For the port where n points along a positive Cartesian direction, a1 and a2 are also assigned to point along positive Cartesian directions. Conversely, for the port where n points along a negative Cartesian direction, a1 and a2 are assigned to point along negative Cartesian directions. The condition a1 × a2 || n is true on both ports. For example, if n = z, then a1/|a1| = x and a2/|a2| = y and if n = −z, then a1/|a1| = −x and a2/|a2| = −y.