SP Card

This card defines an S-parameter (S-matrix) request for active sources.

On the Request tab, in the Configurations group, click the  S-parameter (SP) icon.

Figure 1. The SP - Calculate S-parameters for active sources dialog .


Request name
The name of the request.
Always add port impedance to existing loads
When S-parameters are computed, each port is automatically loaded by Feko with the S-parameter reference impedance of the port. If this option is checked, and the user has manually defined a load at a port, then the S-parameter load will be added to the existing load at the port. If this option is not checked, then Feko will automatically add the S-parameter reference loads at the various ports, but possible user defined loads of the same load type (see discussion below) will be overwritten (not added) at these ports.
Restore loads after calculation
As discussed above, Feko automatically adds loads to ports when computing S-parameters. With this option the behaviour can be controlled after the SP card processing is finished. When this option is enabled, the loads that were automatically added will be removed, and the load situation (for instance for a subsequent far field request) is the same as if the SP card was not used. Otherwise, all the loads as set during the SP card processing will remain in place afterwards.
System impedance
The reference impedance in Ohm. This is used for all sources for which no impedance value is specified when defining the source. If this field is empty, it defaults to 50. Note that for waveguide sources (AW card) S-parameters are always related to the corresponding waveguide impedance.

All the ports must be defined before using the SP card. They are identified simply by defining excitation cards. Currently only A1, A2, A3, AE, AF, AN, AB and AW sources are supported. A1, A2 and A3 sources must be selected by label (not with position), and unique labels must be used (no other segments or triangles may have a label which is used for a port).

If the amplitude of any port is set to zero, it will be used as a receive port (or sink) but not as a source. For example, if only S21 and S11 are required for a two port network, one may set the amplitude of the source defining port 2 exactly to zero. Then S12 and S22 are not calculated. In some cases this could save considerable computation time.

The S-parameter load impedance for each of the port sources can be specified at the source itself. If no such impedance was specified, the system impedance ( Ω ) value specified with the SP card will be used (if this value is not specified it defaults to 50 Ω ). This S-parameter load impedance will be added automatically to each port. The only exception is the waveguide port (AW card) and the modal port (AB card) where S-parameters are related directly to the corresponding waveguide impedance.

It must be noted that except for waveguide ports the SP card adds load impedances to all the ports. For A1, A2 and A3 sources it uses LZ type loads, for AN sources it uses LN type loads and for AE sources it uses LE type loads. If any similar loads were applied to the source position before the SP card these loads will either be added or overwritten. The addition or overwriting is set with the Always add port impedance to existing loads check box.

When execution continues after processing the SP card these loads will either be removed or kept, as controlled by the check box, Restore loads after calculation. This makes a difference when, for example, after the SP card the far field is computed with the FF card. If the loads are removed then the result for the far field pattern is the same as if there was no SP card (far field computed with ports unloaded by the S-parameter reference impedance). The disadvantage of restoring the loads is that the loads change after the SP card processing. For the MoM this means that the MoM matrix changes, and in order to compute the far field pattern, a full extra matrix computation and LU decomposition must be done. If the loads are kept, then further results are readily available (by re-using the LU decomposed matrix).

The original amplitudes and phases of the excitations will always be restored. It should, however, be noted that unlike near- or far field computations or other results, the amplitudes and phases of the excitations at the various ports do not influence the S-parameter results (except for the special case of setting the amplitude of a port to zero which indicates to Feko that this is a passive port only). This behaviour is consistent with the definition of S-parameters (results are normalised by the incident port signal). It should in particular also be noted that setting a phase of 180° for the excitation of a port does not change the direction of this port. One rather physically has to define the port with opposite orientation. When viewing a model in POSTFEKO then the arrows always indicate the positive source direction and the arrows will also not change direction when setting a 180° phase on the excitation.

Note that a request to compute S-parameters is not required for 1-port networks as the S11 (reflection coefficient for a 1-port network) data will be available since it is always calculated for voltage and current sources. For current and voltage sources, an additional S-parameter block will not be computed if the model consists of a 1-port network and the user requested an SP card (with unchanged reference impedance). For waveguide and modal ports, S-parameters are calculated by default in the same way that port impedances are calculated for voltage and current sources. When a redundant S-parameter request has been made, a warning will be displayed to indicate to the user that the SP card will be ignored. For n-port networks (with n > 1) while processing an S-parameter request, source, power and impedance data is not written to the .out and .bof files since this would be misleading as the effect of port loads would be included in the results.