# 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, you must specify a vector to ensure the correct surface orientation. The U-Vector should be set to point into the direction of the U-Vector (or X vector in global coordinates) of the original characterised surface. This characterisation is performed either through solving with 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 original characterised surface.

The orientation of the U-Vector is only important when the characterised surface is anisotropic (properties dependent on the plane wave angle of incidence). Isotropic surfaces do not depend on the orientation of the U-Vector. Consequently the only requirement for the U-Vector is that there should be a valid projection of the vector onto the face. In essence this means that the U-Vector is not allowed to point into 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 Modify Face 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 (RL-GO) if it is not already set.
2. Click 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.