simulation time interval is called truncation error. The amplitude of the ripple increases with the truncation error.
Please note that this ripple does not move the location of minima or maxima in the S-parameter curves. Therefore, if you are only interested in the location of a peak, a larger truncation error is tolerable.
The level of the truncation error can be controlled with the Accuracy setting in the transient solver control dialog box. The default value of 30 dB will usually give sufficiently accurate results. However, to obtain highly accurate results for antenna structures, it is sometimes necessary to increase the accuracy to 40 dB or 50 dB.
Because increasing the accuracy requirement for the simulation limits the truncation error and increases the simulation time, the accuracy requirement should be specified with care. As a general rule, the following table can be used:
Desired Accuracy Level Accuracy Setting (Solver control dialog box) Moderate High Very high
The following rule may be useful, as well: If you find a large ripple in the S-parameters, increase the solvers accuracy setting.
2. Effect of the Mesh Resolution on the S-parameters Accuracy
Inaccuracies arising from the finite mesh resolution are usually more difficult to estimate. The only way to ensure the accuracy of the solution is to increase the mesh resolution and recalculate the S-parameters. When the results no longer significantly change when the mesh density is increased, then convergence has been achieved.
In the example above, you have used the default mesh that has been automatically generated by an expert system. The accuracy of the results is most easily tested with the full automatic mesh adaptation that can be switched on by checking the Adaptive mesh refinement option in the solver control dialog box (Solve
Transient Solver,
):
-30dB -40dB -50dB
Please note that the previously selected template has changed the default settings to the energy-based adaptive strategy that is more convenient for planar structures. Thus, you only have to activate the Adaptive mesh refinement tool in the Transient Solver Parameters dialog and start the solver again by pressing the Start button.
In this example, two adaptation passes are necessary to obtain a suitable result. This means that the maximum deviation of the S-parameters between the second and the third run is less than 2%.
The convergence process of the input reflection S1,1 during the mesh adaptation can be visualized by selecting 1D Results tree:
Adaptive Meshing
|S| dB
S1,1 from the navigation
You have the option of reducing the accuracy limit for the mesh adaptation or starting the adaptation with a finer starting mesh resolution to obtain even more accurate results. However, these options will certainly increase the simulation time and might be more recommendable after the basic design state of the antenna device is finished. Another possibility for obtaining an impression of the reliability of a solution is to perform a second simulation with a completely different solver and mesh type, as will be shown in the following chapter.
Please note: Refer to the Getting Started manual for more information on using Template Based Postprocessing for automated extraction and visualization of arbitrary results from various simulation runs.
Frequency Domain Solver
CST MICROWAVE STUDIOoffers a variety of frequency domain solvers that are specialized for different types of problems. They differ not only by their algorithms, but also by the type of grid on which they are based. The general purpose frequency domain solvers are available for hexahedral grids as well as tetrahedral grids.
The availability of a frequency domain solver within the same environment offers a very convenient method of cross-checking results produced by the time domain solver with minimal additional effort.
? Making a Copy of Transient Solver Results
Before performing a simulation with a frequency domain solver, you may want to keep the results of the transient solver to allow for an easy comparison of the two simulations. To obtain the copy
?
of the current results: Select, for example, the |S| dB folder in 1D Results, then press Ctrl+c and Ctrl+v. The copy of the result curve will be created in the selected folder. The name of the copy will be S1,1_1. You may rename it to S1,1_TD with the Rename command from the context menu. Use Add new tree folder from the context menu to create an extra folder. Please note that at the current time it is not possible to make a copy of 2D or 3D results.
? Optimize Structure for Tetrahedral Mesh
In the following section, the general purpose frequency domain solver is applied to the tetrahedral mesh. This solver is less efficient if there are PEC sheets with very small, but non zero thicknesses, as represented by the antenna patch in our example. Because this thickness has a rather small influence on the results compared to a zero thickness, we rebuild the patch as a PEC sheet, as shown in the following section.
First, select the patch in the navigation tree and then select the patchs bottom face using the face pick tool (Objects
Pick
Pick Face,
). Therefore rotate the structure as shown in the
picture below and double click on the patch.
A PEC sheet is easily created from the selected face by applying Objects Shape from Picked Faces,
):
Face Healing Tools
Enter a suitable name for the new shape (patch0) and confirm the creation by pressing the OK button. Finally, delete the old patch (component1 with zero height (component1
There may be old results present from the previous transient solver run that will be overwritten when changing the model. In this case, the following warning message appears:
patch0) remains.
patch) so that only the newly created patch
Press OK to acknowledge deletion of the previous results.
In order to allow a tetrahedral-based calculation, we change the Mesh type from hexahedral to tetrahedral in the Mesh Properties dialog box (Mesh
Global Mesh Properties,
). This
selection can also be performed directly in the Frequency Domain Solver Parameters dialog box when choosing the desired solver type, as demonstrated in the following chapter. Furthermore, regarding the circular geometry of the patch antenna and its coaxial feed, it is advisable to refine the default mesh settings. This ensures a homogenous size of the tetrahedra before starting the mesh adaptation. Therefore, please increase the Steps per wavelength to a value of 10 in the Mesh Properties dialog box: