There are several valid ways to put a port on the end of a coaxial cable. Wave ports are recommended as they are more accurate than lumped ports, since they are intrinsically free of parasitics (see “Wave Ports on Metal Traces”). If you are using a wave port and the coaxial cable terminates on the geometry exterior, the recommended configuration is to place the port attribute on the center conductor face, as follows.
The wave port plane only extends to the outer conductor, if additional geometry is present on the port plane as follows.
If the coax terminates in the geometry interior, apply the port attribute to both the center trace and the insulator.
Again, the port mesh will extend only as far as the shield.
We do not recommend applying the port only to the insulator, as shown in the following figure. In this scenario, the structure is treated as a waveguide. While this setup is valid, the resulting port impedance will be different from the circuit impedance because positive and negative terminals have not been defined. So the generalized S-matrix will be valid, but the renormalized S-matrix may refer to an impedance that is different from what was intended.
Even though the port attribute has been applied to the dielectric, it appears to cover the center conductor as well because the center conductor overlaps the dielectric in the geometry.