Please now open the waveguide dialog box (Solve
Waveguide Ports,
) to define the port:
Here, you have to choose how many modes should be considered by the port. For a simple coax port with only one inner conductor, usually only the fundamental TEM mode is of interest. Thus, you should simply keep the default setting of one mode.
Please confirm your port settings with the OK button to finally create the port. After rotating the model again to the top face, your model should now look as follows (please use again Ctrl+w or
to toggle the wireframe visualization mode on and off):
? 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).
In this example, the S-parameters are to be calculated for a frequency range between 2 and 3 GHz. Open the frequency range dialog box (Solve ? Frequency, the selected template and is displayed in the status bar):
) and enter the range from 2
to 3 (GHz) before pressing the OK button (the frequency unit has previously been set to GHz by
? Boundary Conditions
Because the calculation domain is only a limited volume it is necessary to define boundary conditions that incorporate the influences of the outer space. Please open the Boundary Conditions dialog box by selecting Solve
Boundary Conditions (
):
When the dialog box opens, all currently selected boundary conditions are simultaneously displayed in the main view:
When you selected the Antenna (on planar substrate) template at the beginning of this tutorial the boundary conditions were already properly set for this structure. At the ground plane, an electric boundary condition has been set that behaves like an infinite solid PEC brick. All other boundary planes are set to open or open (add space); they realize free space behind their boundary planes. Free space means that the electromagnetic fields are absorbed at these boundaries with virtually no reflections.
Please note: As a general rule, the open boundary conditions work best if they are at least 1/8 wavelength apart from the field source. Open (add space) already incorporates this rule and automatically adds the correct amount of background space to the structure.
Because the open (add space) boundary condition only adds background material to the structure, it should not be used if there is material that crosses the boundary plane and should practically extend to infinity (such as the substrate and ground solids in this example). In these cases, the open boundary condition must be invoked.
Please close this dialog box without any changes.
? Define Farfield Monitor
Besides the S-parameters, the main result of interest for antenna devices is the farfield distribution at a given frequency. The solvers in CST MICROWAVE STUDIOoffer the possibility of defining several field monitors to specify at which frequencies the field data shall be stored.
Please open the monitor definition dialog box by selecting Solve
Monitors (
):
?
In this dialog box, you should first select the Type Farfield/RCS before specifying the frequency for this monitor in the Frequency field. Afterwards, press the Apply button to store the monitors data. Please define a monitor at the frequency of 2.4 (with GHz being the currently active frequency unit). However, you may define additional monitors at other frequencies, each time
pressing the Apply button to confirm the setting and add the monitor in the Monitors folder in the navigation tree. After the monitor definition is complete, please close this dialog box by pressing the OK button.
S-Parameter and Farfield Calculation
A key feature of CST MICROWAVE STUDIOis the Method on Demand approach that allows specification of a simulator or mesh type that is best suited to a particular problem. Another benefit is the ability to compare the results from completely independent approaches. We demonstrate this strength in the following two paragraphs by calculating the S-parameters and the farfield of the constructed antenna device with both the transient and frequency domain solvers. The transient simulation uses a hexahedral mesh while the frequency domain calculation is performed with a tetrahedral mesh in this case. However, because both methods are self-contained, it is sufficient to work through only one of them. 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.
?
Transient Solver
? Frequency Range Considerations for the Transient Solver
We recommend using reasonably large bandwidths of 20% to 100% for the transient simulation. In this example, the S-parameters are to be calculated for a frequency range between 2 and 3 GHz. With a center frequency of 2.5 GHz, the bandwidth (3 GHz 2 GHz = 1 GHz) is 40% of the center frequency, which is inside the recommended interval. Thus, you can simply choose the frequency range as desired between 2 and 3 GHz.
Please note: In a case where you just cover a bandwidth of less than 20%, you can increase the frequency range without losing accuracy. This extension of the frequency range could speed up your simulation by more than a factor of three!
In contrast to frequency domain solvers, the lower frequency can be set to zero without any problems! The calculation time can often be reduced by half if the lower frequency is set to zero rather than e.g. 0.01 GHz.
? Transient Solver Settings
The solvers parameters are specified in the Transient Solver Parameters dialog box that can be opened by selecting Solve corresponding icon (
Transient Solver from the main menu or pressing the
) in the toolbar: