HFSS15: Lumped Ports Theory

Lumped ports resemble wave ports, but can be located internally and have a complex user-defined reference impedance. Lumped ports are restricted to single mode ports and the S-parameters are always based on the user defined reference impedance. This mainly because no transmission line is being modeled in the interior domain which suggests an interpretation of the lumped port as a measurement probe being connected to the surface of the lumped port with the reference impedance specified by the user.

When a lumped port is used internal to a 3D problem, it makes sense to place the lumped port at a location where the field distribution would approximately be the same as the dominant mode of the port definition in the absence of the lumped port. A typical example of this is when a rectangular port is drawn between a conductor trace and a ground plane with the face oriented perpendicular to both conductors. The port mode resulting from this case is a parallel plate mode which will often closely resemble the field distribution at that location in the absence of the port.

A perhaps unsuspected result of using a complex reference impedance is that the S-parameters can be greater than one even for a passive device.