So far, all wave ports have been applied to trace ends on the geometry exterior. There are situations where it would be simpler to apply the wave port to a trace that terminates in the geometry interior. When a wave port is applied to the end of a trace in the geometry interior, the solver cannot extract the port plane without more information about how large the port plane should be, and what shape it should have. Describe the size and shape of the port plane by using custom wave port extents on the end of the trace. Then, the solver will be able to extract the port plane, and the simulation can proceed.
The following image shows a trace that terminates away from the geometry boundary.
In this image, we have defined the custom extents with a rectangle, and the port attribute has been applied both the rectangle and the trace.
This situation is intrinsically nonphysical. You should exercise care when using it. The port solution extracts the mode that the trace would support if the trace extended to infinity, as would happen for a wave port on the end of a trace that terminated on the boundary exterior. The resulting fields are sourced back along the trace. At the same time, no fields are sourced in the other direction, as if the custom extent region were backed by a PEC plate. Similarly, any fields approaching from behind the custom extent rectangle will reflect off as if it were made of PEC.
The solver chooses the direction to source the fields based on the metal structures that touch the custom extent region. If multiple conductors touch the custom extent region, the solver will treat the side with the most conductors as the front. In this case, the bottom edge of the custom extent region contains PEC BCs, because it is touching the PEC boundary of the outer geometry. The other three edges of the custom extent region contain PMC.
All restrictions and caveats associated with using custom extents on wave ports still apply when used on internal surfaces. In addition, because the custom extent region behaves like a PEC plate, you should ensure that using such a construct will not disrupt your geometry.