HFSS15: Modes

For a waveguide or transmission line with a given cross-section, there is a series of basic field patterns, or modes, that satisfy Maxwell’s equations at a specific frequency. Any linear combination of these modes can exist in the waveguide. A user is responsible to specify the number of modes to be included in a wave port but not for lumped ports since they are restricted to contain only one mode. In general, in order to obtain accurate S-parameters the number of modes defined for a given port should be the same or greater than the number of propagating modes at the highest frequency of interest. Note that the port solver automatically determines what modes should be included based on the specified number of modes. The modes included in a port are based on a sorting criteria to extract the dominant set of modes out of the infinite set of possible modes. If there is no prior knowledge to the number of propagating modes, the user can do a ports only solution while specifying a reasonably large number of modes and view the 'Gamma' values in the Matrix Data panel to determine what modes are propagating.

If a port is defined on a surface where there is a geometrical discontinuity in the normal direction of the port, it may not be enough to include only the propagating modes for accurate S-parameters. If possible, it is recommended to avoid this case by modeling a short section of the port transmission line such that all the higher order modes have decayed and do not contribute to the field on the port surface. The length of the constant cross-section segment that has to be included in the model depends on the value of the mode’s attenuation constant, a.