To this point, only the structure itself has been modeled. Now it is necessary to define some solver-specific elements. For an S-parameter calculation, you need to define input and output ports. Additionally, the simulation needs to know how the calculation domain should be terminated at its bounds.
Define Ports
Each port that is defined will simulate an infinitely long waveguide (here a coaxial cable) that is connected to the structure at the port’s plane. Waveguide ports are the most accurate way to calculate the S-parameters of filters and should therefore be used in this case.
Since a waveguide port is based on the two dimensional mode patterns in the waveguide’s cross-section, the port must be defined large enough to entirely cover these mode fields. In the case of a coaxial cable, the port has to completely cover the coaxial cable’s substrate.
Before you continue with the port definition, please clear the selection by either double- clicking on the view’s background or selecting Components in the navigation tree.
The port’s extent can be defined either numerically or, as is more convenient here, by picking the face to be covered by the port. Therefore, activate the pick face tool (Objects
Ö Pick Ö Pick Face, f ) and double-click the substrate’s port face at the first port as shown in the pictures below:
Please open the waveguide dialog box (Solve Ö Waveguide Ports, ) to define the first port 1:
Pick first port’s substrate face
Whenever a face is picked before the port dialog is opened, the port’s location and size will automatically be defined by the picked face’s extent. Thus the port’s Position (transversal as well as normal) is initially set to Use picks. You can accept this setting. The next step is to choose how many modes should be considered by the port. For coaxial devices, we usually have only a single propagating mode. Therefore, you should simply keep the default of one mode.
Finally, check the settings in the dialog box and press the OK button to create the port:
Now you can repeat the same steps for the definition of the second port:
1. Pick the corresponding substrate’s port face (Objects Ö Pick Ö Pick Face, ). 2. Open the waveguide dialog box (Solve Ö Waveguide Ports, ).
3. Press OK to store the port’s settings.
Define Boundary Conditions and Symmetries
Before you start the solver, you should always check the boundary and symmetry conditions. This is most easily accomplished by entering the boundary definition mode by pressing the tool bar item or selecting Solve Ö Boundary Conditions. The boundary conditions will then become visualized in the main view as follows:
Here, all boundary conditions are set to “electric” which means that the structure is embedded in a perfect electrically conducting housing. These defaults (set by the template) are appropriate for this example.
Due to the structure’s symmetry to the XY plane and the fact that the magnetic field in the coaxial cable is perpendicular to this plane, a symmetry condition can be used. This symmetry reduces the time required for the simulation by a factor of two. You should also refer to the example in the Getting Started manual for more information on symmetry conditions.
Please enter the symmetry plane definition mode by activating the Symmetry Planes tab in the dialog box. The screen should then look as follows:
By setting the symmetry plane XY to magnetic, you force the solver to calculate only the modes that have no tangential magnetic field component on these planes (thereby forcing the electric field to be tangential to these planes).
Please note that you also could double-click on the symmetry plane’s handle and choose the proper symmetry condition from the context menu.
Finally, press the OK button to complete this step.
In general, you should always make use of symmetry conditions whenever possible to reduce calculation times by a factor of two to eight.
Define the Frequency Range
The frequency range for the simulation should be chosen with care. Different considerations must be made when using a transient solver or a frequency domain solver (see next chapter for details).
For this example, we will choose a frequency range from 0 to 8 GHz. Open the frequency range dialog box (Solve Ö Frequency, ) and enter the range of 0 to 8 (GHz) before pressing the OK button (the frequency unit has previously been set to GHz and is displayed in the status bar):
Define Field Monitors
CST MICROWAVE STUDIO® uses the concept of “monitors” to specify which field data to store. In addition to choosing the type, you can also choose whether the field is recorded at a fixed frequency or at a sequence of time samples (the latter case applies only to the transient solver). You may define as many monitors as necessary to obtain the fields at various frequencies. For the transient solver, all monitors are calculated from a single simulation run by the Fourier Transform. Consequently, the computational overhead for a defined monitor is relatively small.
Please note that an excessive number of field monitors may significantly increase the memory space required for the simulation.
Assume that you are interested in the current distribution on the coaxial cable’s conductors at several frequencies (2, 4, 6 and 8 GHz). To add field monitors, select
Solve Ö Field Monitors from the main menu or press the corresponding icon in the toolbar .
In this dialog box, you should first select the Type H-Field / Surface current before specifying the frequency for the monitor in the Frequency field. Afterwards, press the
Apply button to store the monitor’s data. Please define monitors for the following
frequencies: 2, 4, 6, 8 (with GHz being the currently active frequency unit). Please make sure that you press the Apply button for each monitor (the monitor definition is then added in the Monitors folder in the navigation tree).
After the monitor definition is completed, you can close this dialog box by pressing the
S-Parameter Calculation
A key feature of CST MICROWAVE STUDIO® is the Method on Demand approach that allows a simulator or mesh type that is best suited to a particular problem. Another benefit is the ability to compare the results obtained by completely independent approaches. We demonstrate this strength in the following two sections by calculating the S-parameters with the transient solver as well as the frequency domain solver. The transient simulation uses a hexahedral mesh while the frequency domain calculation is performed with a tetrahedral mesh. Both sections are self-contained parts and it is sufficient to work through only one of them, depending on what solver you are interested in. The chapter ends with a comparison of the two methods.
Please note that not all solvers may be available to you due to license restrictions. Please contact your sales office for more information.