# Applying a Characterised Surface to a Face

Apply a characterised surface to a face bordering free space or dielectric regions.

When a characterised surface definition is applied to an RL-GO face, a vector is required to ensure the correct surface orientation. The U vector that is provided should point in the direction of the U vector (or X vector in global coordinates) when the surface was characterised (either through simulation using periodic boundary conditions, an infinite ground plane or measurements). The projection of the U vector onto the face correspond to the U vector (or principal direction) of the characterised surface.

The orientation of the U vector vector is only important when the characterised surface is anisotropic (has different properties, depending on the plane wave angle of incidence). Isotropic surfaces do not depend on the orientation and then the only requirement for the U vector is that there should be a valid projection of the vector onto the face (not allowed to point in the direction of the face normal).
Note: The face normal vector is the vector that is perpendicular to the face. For flat faces, the normal is the same everywhere on the face, but for curved faces, the normal changes as a function of the position on the face.

Curved surfaces such as radomes have to be split into smaller faces so that a valid U vector can be defined for each surface. As an illustration, consider a sphere. There is no single vector that has a valid projection onto the surface of a sphere, since at two points, the vector points in the direction of the face normal.

1. Select a face to apply a characterised surface.
2. From the right-click context menu, select Properties.
3. On the Face properties dialog, click the Properties tab.
4. In the Medium drop-down list, select the characterised surface to apply to the face.

The U-Vector is defined as the reference direction projected onto the face.

1. Under U vector, specify the start point and end point for the U-Vector. The vector is not required to be exactly in the plane of the face, since it is projected onto the face, but it should be approximately parallel to the face.

Ensure that the RL-GO solution method is selected on the Solution tab. The characterised surface feature is only supported in conjunction with the RL-GO solution method.

1. Click on the Solution tab and set the face solution method to ray launching geometrical optics if it is not already set.
2. Click the OK to apply the characterised surface and to close the dialog.
The U-Vector can be displayed in POSTFEKO to verify that all faces have the correct settings and U-Vector orientations applied.