# Far Field Goal

Optimise the far fields that are solved as part of the Feko model.

On the Request tab, in the Optimisation group, click the  Add goal function icon. From the drop down list, select  Far field.

The Focus is identified based on the label of a Far fields request in CADFEKO or of an FF card in EDITFEKO.

The following focus types can be optimised:

E-field
The E-field focus type considers the radiated fields associated with a specific far field solution request directly. The fields are considered according to the settings of the far field request. For example if only the scattered fields from a single object are requested, then only these will be taken into account in the goal evaluation.
Directivity, Gain and Realised gain
With this focus type, only the directivity, gain or realised gain of the model is considered. This option can only be based on a far field request where the Calculate fields as specified option is chosen and is independent of whether Directivity or Gain is selected in the far field request.
This focus type is only valid for far field solutions that have been computed with a plane wave source. The RCS focus type delivers non-complex values (or an array of non-complex values) representing the derived RCS (see section) according to the options set in the far field calculation request. If no valid RCS information is found in the computation output, an error will be generated during the goal evaluation.
Note: Fields that are requested in invalid directions (for example fields requested below an infinite ground plane) are ignored during the Goal evaluation. If no valid far field results with the correct request label are found in the solver output, an error will be generated during the Goal evaluation phase of the first optimisation iteration.

## Polarisation

The Polarisation option allows the specification of the component of the far field to be considered in the goal.
Total
For the E-field focus type, the Total option provides a magnitude combination of the $\theta$ - and $\varphi$ -components of the far field. The total field is calculated as:
(1) ${\text{E}}_{\text{total}}=\sqrt{{|{E}_{\theta }|}^{2}+{|{E}_{\varphi }|}^{2}}$
This value is representative of the power in the far field.
For the Directivity, Gain and Realised gain focus types, the polarisation-independent quantities are considered. This is the only Polarisation option for RCS.
Horizontal (Phi)/Vertical (Theta)
These options allow specific selection of the $\theta$ - and $\varphi$ -directed components of the far field. For the Directivity, Gain and Realised gain focus types, only the component of the field with the selected polarisation is used in the calculation of the required quantity, delivering a non-complex value (or array of non-complex values).
LHC/RHC
These options allow specific selection of the left-hand-circular and right-hand-circular components of the far field (see section). For the Directivity, Gain and Realised gain focus types, only the component of the field with the selected polarisation is used in the calculation of the required quantity, delivering a non-complex value (or array of non-complex values).
S/Z
These options allow specific selection of the S- or Z-polarised components of the far field (see section ). For the Directivity, Gain and Realised gain focus types, only the component of the field with the selected polarisation is used in the calculation of the required quantity, delivering a non-complex value (or array of non-complex values).
Axial ratio
This option is only available for the E-field focus type. This provides the ratio between the magnitudes of the $\theta$ - and $\varphi$ -directed field components (see section).
For the purposes of optimisation, an additional sign is added to the Axial ratio value considered by the optimiser. The sign indicates the handedness of the radiated field, with a negative sign implying left-handedness, and a positive sign implying right-handedness. This makes provision for the inclusion of the required handedness directly in the Axial ratio optimisation.
Ludwig III (Co and Cross)
These options allow specific selection of the Ludwig III (Co) and Ludwig III (Cross) polarised components of the far field (see section).