# CF Card

This card sets the type of integral equation for perfectly conducting metallic surfaces.

In the Solve/Run tab,in the Solution settings group, click the  Integral equation (CF) icon.

## Parameters:

Type of integral equation for metallic surfaces
Chose between the EFIE (electric field integral equation) and the CFIE (combined field integral equation). See the comment below for more details.
Apply to all labels
The selected type of integral equation is applied globally to all metallic surfaces, irrespective of their label.
Apply to single label only
Here the selection of the type of integral equation applies to a single label only, which is entered into the field From label.
Apply to label range
Here the selection of the type of integral equation applies to a range of labels, which is entered into the fields From label and to label.
Factor of CFIE
In the CFIE formulation electric and magnetic terms are combined with a factor. Leaving this input field empty will select the default value of 0.2. It is generally not recommended to change this value.
The EFIE is the default in Feko if no CF card is used or for labels where no setting is made at the CF card. It is the most general formulation and can be applied to both open and closed bodies. The CFIE can only be used in connection with closed objects. The advantage is that the conditioning of the system of linear equations is better. In particular in connection with MLFMM the convergence can be improved if the CFIE is used for closed parts of an object. Note that the CFIE can be used together with the EFIE on the same object.
Note: All triangle normals should point away from the zero field region.
For the CFIE in addition to the fundamental restriction that the surface must be closed, these further conditions apply:
• The normal vector must point outwards (from the closed field free region into the domain of interest where there are sources and fields to be computed).
• Using symmetry in order to reduce the memory or run-time is not supported (it will be switched off automatically).
• The CFIE formulation for the MoM cannot be used together with the MoM/PO, MoM/UTD, or MoM/FEM hybrid methods.
• The CFIE formulation can be used only with the free space Green’s function - using the spherical or planar multilayer Green’s functions is not supported.
• When using the CFIE, dielectric bodies (solved using SEP, FEM or MoM/MLFMM) may be present in the model, though all of the CFIE surfaces must be perfectly conducting (no coating or skin effect and so forth).
Note that multiple CF cards can be used in order to specify for instance that the CFIE shall be used at multiple distinct labels which do not form a range. The setting for Factor for CFIE is global (not per label), the value read from the last CF card will be used.