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

When the physics interface is added, three 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, Transient 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 three options for the Maximum mesh element size control parameter — User defined (the default), Frequency, or Wavelength. When User defined is selected, 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 tlt.

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.