How to use FEKO with FEST3D?
Altair EmployeeAnswers
-
FEST3D is a software tool for the synthesis and analysis of passive waveguide components. The performance of a structure designed in FEST3D can be verified by calculating the full-wave Method of Moments (MoM) solution with FEKO. The mode data from FEST3D can also be used for further analysis of more complex, or radiating, structures in FEKO. A two-way interface allows impressed modes and generalised scattering matrices (GSMs) to be exchanged between FEKO and FEST3D.
FEKO to FEST3D
The DA card is used in FEKO to write out a *.chr file that is then loaded into a 1-Port / N-Port User Defined element in FEST3D.
In FEKO
The DA card can be used in EDITFEKO to export the generalised scattering matrix to a FEST3D *.chr file. The SP card must be included in the *.pre file for the S-parameters to be calculated.
The calculated modes must be set up in order with AW cards in EDITFEKO, or can alternatively be defined using a *.fim file. The order of modes should be according to an increasing order of ports, with the modal expansion sorted according to the increasing value of cut-off frequency, starting with the fundamental mode. If degenerate modes are present, the TE modes should be listed before the TM modes and even modes (with reference to the x coordinate) should be listed before odd modes.
Modes are ordered by port and according to cut-off frequency at each physical port, e.g. for a three port structure with two modes at each port the order will be:
- AW MyWaveguideStructure.Port1 mode1
- AW MyWaveguideStructure.Port1 mode2
- AW MyWaveguideStructure.Port2 mode1
- AW MyWaveguideStructure.Port2 mode2
- AW MyWaveguideStructure.Port3 mode1
- AW MyWaveguideStructure.Port3 mode2
The same number of linear frequency points used for the computation of the matrix in FEKO must be used by the circuit containing the N-Port User Defined discontinuity in FEST3D.
In FEST3D
The *.chr file created with FEKO can be loaded into FEST3D using a 1-Port User Defined or N-port User Defined element, depending on the circuit to be analysed.
FEST3D to FEKO
A *.fim file is created with FEST3D and loaded into FEKO with the AW card.
In FEST3D
A one port circuit is needed to create a fim file that contains the impressed modes at the port. If the FEST3D circuit contains more than one port, all other ports should be set to be of SubType “Termination” (“Adapted”) instead of “Input/Output Port”. For a file with one open port, and any other ports adapted, the GSM can be computed (by pressing the “S” button in FEST3D). The GSM is written out to a *.chr file, e.g. MyAdaptedCircuit.chr. A new circuit should be created where a single waveguide is connected to a 1 Port User Defined element which is set up to load the GSM (MyAdaptedCircuit.chr). The impressed modes can now be calculated by pressing the “I” button in FEST3D. This will write out a *.fim file to disk, e.g. MyWaveguideCircuit.fim.
Partial FEST3D toolbar showing the 'S' and 'I' buttons that are used to calculate a GSM (write out a *.chr file) and calculate impressed modes (write out a *.fim file).
In FEKO
The solution frequency must be defined in FEKO to coincide with the number of frequency points used in the FEST3D calculation.
FEKO needs to associate ports of a *.fim file with port face labels in the FEKO model. The CB card can be used in EDITFEKO to change the labels to numbered port names, corresponding to the FEST3D numbering, for example:
CB: MyWaveguideStructure.Face1 : Port1 CB: MyWaveguideStructure.Face2 : Port2
The AW card is used in EDITFEKO to excite a waveguide structure with impressed modes calculated in FEST3D by loading the impressed modes from a FEST3D *.fim file.
Partial AW card dialog in EDITFEKO. The “Reference point for FEST3D model (named point)” input parameter on the AW card dialog is the origin of the reference coordinate system. This is chosen to be the bottom right corner of a rectangular port, and the centre of a circular port. In both cases the z-axis should be defined to point into the structure. This reference point is used to indicate the translation of the imported model in the FEKO coordinate system. It can be defined with the DP card.
Rectangular waveguide port reference coordinate system (reference point O). Circular waveguide port reference coordinate system (reference point O). Notes
Geometry import/export
Scaling could be required on importing STEP and IGES geometry files into FEKO. Exported 3D geometry files from FEST3D have been found to state dimensions are in ‘mm’ regardless of unit chosen for export (found in version 6.7.3 of FEST3D). The geometry could be incorrectly scaled upon import into FEKO. The user can scale the model in FEKO if necessary, e.g. if exported 3D geometry in FEST3D in ‘inches’, import in CADFEKO will pick up unit is ‘mm’ and incorrectly scale by 0.0393701 if the model unit in FEKO is ‘inches’. Select transform -> scale and scale the model by 25.4 (or 1/0.0393701) to correct. If the geometry was exported in mm no scaling would be required.
Symmetry
The use of symmetry is not officially supported for the interface. If symmetry is used, it is the user’s responsibility to ensure that symmetry conditions are set up to coincide between FEST3D and FEKO.
Possible symmetry conditions in FEKO are geometrical, electrical and magnetic symmetry and symmetry defined in the XY, XZ or YZ planes. Symmetry definitions supported by FEST3D are All-Inductive (H plane, constant height), All-Capacitive (E plane, constant width), All-Cylindrical (All-Centered Circular waveguides), X symmetric (symmetric under horizontal reflection), Y symmetric (symmetric under vertical reflection) and TEM (All-Centered).
Symmetry going from FEST3D to FEKO
FEKO can handle files for which symmetry was defined in FEST3D and will use the impressed modes that existed with the symmetry in FEST3D as excitation. Files containing modes that cannot exist due to the symmetry conditions in FEKO will cause an error. No checks are performed to ensure that similar symmetry conditions are set up in FEKO as were used in FEST3D.
Symmetry going from FEKO to FEST3D
FEKO will always write out <symmetries nsymmetries=”0”/> to a *.chr file. If symmetry planes are set up in FEKO, modes that cannot exist due to the symmetry conditions will have zero amplitude in the FEKO formulation. Modes that can exist given the symmetry conditions will be written out to the generalised S matrix in the *.chr file. However, FEST3D will be unable to use this kind of file as a component in a circuit where any global symmetry has been defined. In order for the file to be used successfully with symmetry in FEST3D, it can be manually edited by the user. The user should confirm that the correct modes are taken into account and symmetry is correctly defined.
The correct syntax can be obtained from the *.chr file that is written out for the circuit when selecting “Compute Generalized Z matrix” in FEST3D, for example:
<symmetries nsymmetries='1'> <symmetry/> </symmetries>
0