HFSS15: Mode Alignment

In some cases the requested set of modes to be used with a wave port contains degenerate modes. For that case, the modes that are degenerate need special care in obtaining a unique orientation of the mode patterns which is important, for example, when a specific orientation for a given mode is intended to excite the structure.

For example, in the case of a square waveguide the first two dominant modes form a degenerate mode set. Therefore, in order to guarantee that the typically desired orientation of the modes (TE₀₁ and TE₁₀) are used for an HFSS simulation, the user needs to enable one of the two supported mode alignment options. The first option is general but manual and requires detailed user knowledge of the mode patterns while the other is restricted to a few basic port types where HFSS automatically aligns the modes.

The general mode alignment option is enabled by defining appropriate integration lines for the modes that should be aligned and selecting Align modes using integration lines in the Modes tab of the Wave Port setup window. Note that not all modes requested for a wave port need to have an integration line defined - only the ones for which mode alignment is desired. It is important that you draw integration lines that make sense for the modes to be aligned along and that all the modes that form a set of degenerate modes have a line defined. For example, if the first two modes of a square waveguide should be TE₁₀ and TE₀₁, you should draw two lines through the center of the waveguide where one line starts at the bottom and ends at the top while the other line starts on the left side and ends on the right side.

You enable the automated mode alignment option by selecting Align modes analytically using coordinate system in the Modes tab of the Wave Port setup window. By selecting this option, the user will be asked to draw a local coordinate system which is used to automatically align the modes in pre-defined directions. This option is only available for a select few port types discussed in Analytic Port Types.

In order to get intended outcome of either alignment option, it is necessary to include all modes in a set of degenerate modes. For example, assume that modes 3, 4, 5, and 6 are degenerate. You should either include all of them by specifying at least 6 modes or none of them by specifying at most 2 modes.