The Transmission Line (tl) interface (), found under the Radio Frequency branch () when adding a physics interface, is used to study propagation of waves along one-dimensional transmission lines. The physics interface solves the time-harmonic transmission line equation for the electric potential.

When this physics interface is added, these default nodes are also added to the Model Builder — Transmission Line Equation, Open Circuit, and Initial Values. Then, from the Physics toolbar, add other nodes that implement, for example, boundary conditions. You can also right-click Transmission Line to select physics features from the context menu.

The physics-controlled mesh is controlled from the Settings window for the Mesh node (if the Sequence type is Physics-controlled mesh). In the table in the Physics-Controlled Mesh section, find the physics interface in the Contributor column and select or clear the checkbox in the Use column on the same row for enabling (the default) or disabling contributions from the physics interface to the physics-controlled mesh.

When the Use checkbox for the physics interface is selected, this invokes a parameter for the maximum mesh element size in the transmission line. The physics-controlled mesh automatically scales the maximum mesh element size as the wavelength changes in different transmission lines. If the model consists of multiple transmission lines, identical number of domain mesh elements are generated in each of them.

When the Use checkbox is selected for the physics interface, in the section for the physics interface below the table, choose one of the four options for the Maximum mesh element size control parameter — From study (the default), User defined, Frequency, or Wavelength. When From study is selected, 1/60 of the transmission line wavelength from the highest frequency defined in the study step is used for the maximum mesh element size. For the option User defined, enter a suitable Maximum element size in free space. For example, 1/5 of the vacuum wavelength or smaller, and it scales the value to the transmission line wavelength. When Frequency is selected, enter the highest frequency intended to be used during the simulation. The maximum mesh element size is 1/60 of the transmission line wavelength for the entered frequency. For the Wavelength option, enter the smallest vacuum wavelength intended to be used during the simulation. The maximum mesh element size is 1/60 of the transmission line wavelength.

The Label is the default physics interface name.

The Name is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.

The default Name (for the first physics interface in the model) is tl.

Select the Activate port sweep checkbox to switch on the port sweep. When selected, this invokes a parametric sweep over the lumped ports in addition to the automatically generated frequency sweep. The generated lumped parameters are in the form of an S-parameter matrix. For Activate port sweep enter a Sweep parameter name (the default is PortName) to assign a specific name to the variable that controls the port number solved for during the sweep.

For this physics interface, the S-parameters are subject to Touchstone file export. Click Browse to locate the file, or enter a filename and path. Select an Output format — Magnitude angle, Magnitude (dB) angle, or Real imaginary.

The dependent variable (field variable) is the Electric potential V (SI unit: V). The name can be changed but the names of fields and dependent variables must be unique within a model.

Select the shape order for the Electric potential dependent variable — Linear, Quadratic (the default), or Cubic. For more information about the Discretization section, see Settings for the Discretization Sections in the COMSOL Multiphysics Reference Manual.