Modes, Reflections, and Propagation

It is possible for a current solution generated by an excitation signal at one specific mode to contain reflections of higher-order modes which arise due to discontinuities in the high frequency structure. If these higher-order modes are reflected back to the excitation port or transmitted onto another port, the S-parameters associated with these modes should be calculated.

If the higher-order mode decays before reaching any port — either because of attenuation due to losses or because it is a non-propagating evanescent mode — there is no need to obtain the S-parameters for that mode. Therefore, one way to avoid the need for computing the S-parameters for a higher-order mode is to include a length of transmission line in the geometric model that is long enough for the higher order mode to decay.

For best results, include a small length of the transmission line that is connected to each port in the model of the structure. For example, if one port of a structure is connected to a microstrip line, the part of the geometric model leading up to the port must have the cross-section of the microstrip line.