HFSS15: Guidelines for Assigning PML Boundaries

Keep the following guidelines in mind when assigning PML boundaries:

• When automatically creating PMLs, HFSS creates a new relative coordinate system for each PML object. This results in the z direction of the PML object coinciding with the normal direction of the base object’s face.

• HFSS treats PMLs uniformly with regard to thickness. If the PMLs in your design vary in thickness, create a separate PML group for each thickness.

You should manually create a PML in the following situations:

• The base object is curved.

HFSS calculates the PML material properties using the normal vector at the center of the base object’s face. If the face is curved, the normal vector changes with position. The PML materials will only be good approximations if the normal vector at each point on the face is close to the normal vector at the face center.

It is a good idea to segment the curved surface of the base object for greater accuracy. Create separate PMLs for each segment. Note that each segment’s thickness is treated uniformly. The view angle of the segments should be no wider than 45 degrees. The smaller the angle of each segment, the greater the accuracy of the corresponding PML.

• The material of the corresponding base object touching the PML is not homogenous. An example is a metal-shielded microstrip line with a substrate. One PML could be drawn to terminate the microstrip and another could correspond to the substrate.

Create as many PML objects as there are subsections of material properties in the base object.