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.

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 example, if the first and second primitive unit cell vectors are a1 and a2 on the source port, the first and second primitive unit cell vectors on the listener port will be −a2 and −a1, respectively, as the cross product between the first and second primitive unit cell vectors must point in the direction of the port normal. On the listener port, the normal points in the opposite direction to the normal on the source port. With the sign changes and the primitive unit cell vector index swaps, between the source and the listener ports, also the grating vectors change sign and swap indices, comparing the source and listener ports. Thus, the mode numbers will also be different on the listener port compared to the mode numbers on the source port.

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| = −y and a2/|a2| = −x.