HFSS15: Wave Ports Theory

By default, the interface between all 3D objects and the background is a perfect E boundary through which no energy may enter or exit. Wave ports are typically placed on this interface to provide a window that couples the model device to the external world.

HFSS assumes that each wave port you define is connected to a semi-infinitely long waveguide that has the same cross-section and material properties as the port. When solving for the S-parameters, HFSS assumes that the structure is excited by the natural field patterns (modes) associated with these cross-sections. The 2D field solutions generated for each wave port serve as boundary conditions at those ports for the 3D problem. In addition to serving as a boundary condition, a wave port also supplies port impedances and propagation constants that are useful in describing waveguides or transmission lines.

A wave port is restricted to reside at an external boundary of a 3D problem. In some instances, it makes sense to get around this restriction by defining a wave port in the interior of a 3D domain by capping the wave port surface with a PEC object. In doing this, in effect the wave port is viewed as residing at the external boundary of a 3D problem. However, in general lumped ports should be used when defining ports in the interior of a 3D domain.

Often there is a need to have a particular field direction of a mode pattern for consistent results. There are two different methods in HFSS to accomplish this task. The most basic method is where, in the presence of an integration line, HFSS resolves 180 degree ambiguity by enforcing the electric field mode pattern to have a positive line integral along the integration line for that mode (Mode Polarity). The second more advanced option is where HFSS aligns electric field mode patterns, when appropriate, in specific directions (Mode Alignment). This second option consists of a manual but general approach and an automated approach for a few port types. For more in-depth discussions refer to the following topics:

• Mode Polarity

• Mode Alignment