define a port

Don't worry, this question is asked repeatedly by people new to HFSS.

Here is the definition:

Assuming the structure is a simple two-ports microstrip line structure. So it has 3 layers, the top being the stripline, the middle being the substrate and the bottom being the ground plane. The port should be large enough to accommomdate field pattern of the TEM microstrip mode. Ansoft recommends that the port be 5 to 10 times as wide as the width of the trace and 5 times as high as the thickness of the substrate.

If the stripline is 5 mm wide, so let's make the port 30 mm wide. The dielectric is 1.6 mm thick, so let's make the port 8 mm high. Furthermore, one edge of the port (i.e. the bottom edge) should coincide with the ground plane. Of course, center the stripline along the width the port.

Hope this help!

Important information about waveports in HFSS!
Characteristics
-Wave ports solve actual field distributions for one or more propagating or non-propagating Modes
+For ?nodal? port excitation references, optional Terminals may also be defined, permitting a single Wave Port to extend across multiple coupled signal conductors. Terminal excitations are built from superposition of Mode excitations.
-Port boundaries take on the boundary characteristics of the faces which share its edges
+Edges touching perfect_e or finite conductivity faces, such as ground planes, take on that definition
+Edges touching perfect_h faces become perfect_h edges for the port computation
+Edges touching symmetry faces take on the definition of the appropriate perfect_e or perfect_h symmetry type
+Edges touching radiation faces, however, default to perfect_e conductive boundary conditions!
++The environment variable ZERO_ORDER_ABC_ON_PORT = 1 can set them to 377 ohms instead
-Wave ports solve for characteristic impedance and propagation constants at the port cross-section
-Impedance and Calibration line assignments are optional for further mode and output reference control
Limitations
-Wave ports must have only one surface normal exposed to the 3D field volume
+Assign to exterior faces (2D objects or faces of solids) of the modeled geometry, or cover one face with a perfect conductor cap object if internal
-Due to the port bounding edges, which may not match boundaries on field behavior in the full 3D volume around the transmission line past the port plane, proper port sizing and location is crucial
-Ports cannot touch master or slave boundaries.
-Due to sizing requirements, Wave ports may not fit between closely spaced yet still isolated transmission lines (e.g. moderately spaced parallel traces)
-Due to the cap object, internalized Wave ports may present undesirable perturbations for antenna or EMI field analysis within the modeled volume
-Wave ports must exist at a sufficient distance from discontinuities in the transmission line structure so that their 2D field solution is appropriate in the context of the 3D, steady-state fields
+The port extension is the distance between a Wave port and any alteration of the transmission line

Thank you very much.
You are so kind.
I think I need more knowledge about EM field.
Now I have to simulate a pallrall plane capacitance of two layers by HFSS to see the S-parameter.On each plane,I want to set a port.If I directly set the wave port on the surface of the two plane,Simulation process ended for material assign fault.When I set an air cube on the top layer of the capacitance,Simulation can proceed well but with a faulty result.I'm really confused how to deal it.

two parallel planes as capacitor, just create a lumped source at the location you want to excite this capacitor. I can not figure out what is the meaning of "directly set the wave port on the surface of the two planes".
Anyway, this structure is quite challange in HFSS, because of high Q factor, memory and time consuming.
About material assign fault, please check whether u properly setup all materials, metal Plane (pec or lossy metal), substrate (between planes), outer boundary (radiation boundary or PML). I suspect your port setting overlap with the outer radiation bounday, so you got such error prompted out.
Best Regards,

thank you very much for your reply.
Formerly. I select the surface of the upper plane of the capacitance with nothing on which at that time. then I use command: assign excitation> wave port.I do the same to the lower plane.I don't think I define material badly.The material of the two plane is copper,and the board between the layers is FR4 substrate with a relative permittive of 4.3. Generally speaking,if I assign a wave port on a surface of metal without cover other nonmetal surface,HFSS will always end with those words of faulty material assignment.
I will attempt to set the lumped source on the face and I will reply you of the result.Thank you again!!

I think you misunderstood the definition of wave port, you can get all the information about the port settting from the HFSS manual and related materials which you can download from this forum.
Some hints: put a small piece of metal connecting two planes at the location where you excite this capacitor (BUT the width of this rectangular metal should be very small, the smaller the better, otherwise it interference the field distribution of two planes), then assign this metal into lumped source, then u can proceed ur simulation.

Best Regards.

I think you misunderstood the definition of wave port, you can get all the information about the port settting from the HFSS manual and related materials which you can download from this forum.
Some hints: put a small piece of metal connecting two planes at the location where you excite this capacitor (BUT the width of this rectangular metal should be very small, the smaller the better, otherwise it interference the field distribution of two planes), then assign this metal into lumped source, then u can proceed ur simulation.

Best Regards.