Port

The boundary condition is used to excite and absorb elastic waves that enter or leave waveguide structures. A given port condition supports one specific propagating mode. To provide the full description, combine several port conditions on the same boundary, for example, one for longitudinal, one for torsional, and one for transverse modes. Make sure that all relevant propagating modes in the studied frequency range are included (all modes that have a cutoff frequency in the frequency range). By doing this, the combined port conditions provide a superior nonreflecting condition for waveguides to, for example, a perfectly matched layer (PML) configuration or the Low-Reflecting Boundary. The same port boundary condition feature should not be applied to several waveguide inlets and outlets. The port condition supports S-parameter (scattering parameter) calculation, but it can also be used as a source to just excite a system.

The node is only available with some COMSOL products (see https://www.comsol.com/products/specifications/).


	The Port boundary condition exists only for 3D models.


	See the tutorial Mechanical Multiport System: Elastic Wave Propagation in a Small Aluminum Plate. Application Library path: Structural_Mechanics_Module/Elastic_Waves/mechanical_multiport_system

On a given boundary, a combination of ports will define the displacement field u (sum of incident and outgoing waves) as

where the summation “i” is over all ports on the given boundary “bnd”, Sij is the scattering parameter, Ain is the amplitude of the incident field (at port “j”), ϕ is a possible phase, and i is the mode shape of the ith port. The mode shape i is normalized to carry a unit power of 1 W. This definition means that the scattering parameter Sij defines the amplitude of mode i when a system is exited at port j (with mode j). This corresponds to a multimode expansion of the solution on the given port. The scattering parameters are automatically calculated when a model is set up with just one port exciting the system. To get the full scattering matrix, The Port Sweep Functionality can be used.


	Only one port should be excited at a time if the purpose is to compute S-parameters. The S-parameters are defined as solid.Smatrix11, solid.Smatrix21, and so on, and can be used in postprocessing. In other cases, having more than one port exciting the system might be wanted, but the S-parameter variables cannot be correctly computed. When several ports are excited, the S-parameter output is turned off.

Port Properties

Enter a unique . Only nonnegative integer numbers can be used as as it is used to define the elements of the S-parameter matrix. The numeric port names are also required for the port sweep functionality. The port name is automatically incremented by one every time a port condition is added.

Select a : or . Depending on the selection, different options appear in the section (see below).

Port Mode Settings

Depending on the option selected in the (see above):

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In the case of a port, a boundary mode problem is solved on the port face to compute the desired propagating mode. This option requires the use of a step in the study. It should be placed before the step. In the study, add one step for each port and make sure to reference the proper in the study step. When using the port, it is not possible to perform a frequency sweep in the study step. Only one frequency can be used and it should correspond to the entered in the step(s). One option is to add a and define a parameter for the frequency used in both the steps. In this case, care should be taken when setting up the search criteria in the mode analysis.

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For , enter user defined expressions for the un and the kn. The mode shape will automatically be scaled to unit power before it is used in the port condition. Use the user defined option to enter a known analytical expression.


	When the Numeric port option is used and the boundary mode analysis is run, the boundary conditions from the Solid Mechanics model are automatically inherited in the boundary problem. For this automatic procedure, there is only support for the Free, Fixed, Roller, Symmetry, Antisymmetry, and Spring Foundation conditions.

Incident Mode Settings

Activate if the given port is excited by an incident wave of the given mode shape. For the first condition added in a model, the is set to . For subsequent conditions added the excitation is set to per default. If more than one port in a model is excited, the S-parameter calculation is not performed.

When the is set to , then select how to define the incident wave. Select the : or .

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For enter the amplitude Ain of the incident wave. This is in general defined as the maximum amplitude for a given mode shape.

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For enter the power Pin of the incident wave. This is in general defined as the RMS power of the incident wave.

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For both options enter the phase ϕ of the incident wave. This phase contribution is multiplied with the amplitude defined by the above options. The input can be a complex number.

Note that when the option is selected at the physics level, the options in the section are deactivated. This is because this option automatically sends in a mode of unit power, sweeping through one port at the time.


	All ports with incident wave excitation turned off have an arrow symbol, added in the Graphics window, that points outward. When incident wave excitation is turned on, the arrow symbol points inward.


	If a waveguide is cut by one or several symmetry planes, care should be taken when setting up the port modes and when postprocessing. When postprocessing, remember that absolute values like, for example, the outgoing power at port 1, solid.port1.P_out, needs to be multiplied with an appropriate factor. Multiply with 2 if one symmetry plane is used, and so on.

Constraint Settings

To display this section, click the button (

) and select .

The Port Sweep Functionality

The port sweep functionality is used to reconstruct the full scattering matrix Sij by automatically sweeping the port excitation through all the ports included in the model. When the port sweep is activated, the options in the in the port conditions are deactivated and COMSOL controls which port that is excited with an incident mode.

The port sweep functionality is activated at the main physics interface level by selecting in the Port Sweep Settings section. Enter the , the default is PortName. Create a parameter with the same name under . This is the name of the parameter to be used in a parametric sweep, here it should represent the integer values (defined when adding the port conditions). Add a parametric sweep study step and run the sweep over the PortName parameter with an integer number of values representing all the ports in the model. Once the model is solved the full scattering matrix can be evaluated using the defined global variables solid.Smatrix11, solid.Smatrix21, solid.Smatrix12, ans so on.


	Use the Global Matrix Evaluation under Derived Values to evaluate the full scattering matrix solid.Smatrix.

Location in User Interface

Context Menus

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Physics tab with selected: