HFSS15: Floquet Ports Theory

The Floquet port in HFSS is used exclusively with planar-periodic structures. Chief examples are planar phased arrays and frequency selective surfaces when these may be idealized as infinitely large. The analysis of the infinite structure is then accomplished by analyzing a unit cell. Linked boundaries most often form the side walls of a unit cell, but in addition, at least one "open'' boundary condition representing the boundary to infinite space is needed. The Floquet port is a specialized boundary condition to handle this case.

The Floquet port is closely related to a Wave port in that a set of modes, here "Floquet modes", is used to represent the fields on the port boundary. Fundamentally, Floquet modes are plane waves with propagation direction set by the frequency and geometry of the periodic structure. Just like Wave modes, Floquet modes have propagation constants and experience cut-off at a sufficiently low frequency.

When a Floquet port is present, HFSS performs a modal decomposition that gives additional information on the performance of the radiating structure. As in the case of a Wave port, this information is cast in the form of an S-matrix interrelating the Floquet modes. In fact, if Floquet ports and Wave ports are simultaneously present, the S-matrix will interrelate all Wave modes and all Floquet modes in the model. Floquet ports can be combined with lumped ports, but not with terminal ports.