Multibeam Transmitter Model
The Multibeam Transmitter model and the GPS Satellite Transmitter model enable you to set up multiple antenna beams, each with its own specs and its own polarization and orientation properties.
Beams
Use the Beams tab to define parameters for one or more selected beams. The Beams tab, shown below, contains a Beam summary table, a Beam Selection Strategy field, and two additional rows of tabs: one for beam parameters, and one for antenna parameters. Use the summary table to perform an Add, Duplicate, Remove, and Orient operations on one or more beams. You can also use the summary table to modify the values for a selected beam. Use the Beams Specs tab and the Antenna tabs to modify values for one or more selected beams.
Beam selection strategy
Select the strategy to use when selecting a beam for a communications link:
- Aggregate Active Beams combines gains to model a single beam with a gain aggregating the gains of all individual beams. The beam powers are also combined with the respective gains to compute the aggregate EIRP for the antenna.
- Max Gain maximizes the gain in the communications link.
- Min Boresight Angle minimizes the angle between the beam's boresight and the line of sight to the other communications asset in the link.
- Script Plugin is a user-supplied script that defines the beam selection criteria. For more information, see Antenna Multibeam Selection Strategy Plugin Points. To develop a custom script from a sample template, select the appropriate sample plugin script, in MATLAB or VBScript, from <STK install folder>\CodeSamples\Extend\PluginScripts. The plugin script is not automatically reloaded after you make changes to it. To reload the script, click Reload.
These beam selection options have an impact on, among other things, link budget reports.
STK supports the 3D display of antenna gain patterns, including those of multibeam antennas. For a multibeam antenna using a Beam Selection Criterion other than Aggregate Active Beams, gain patterns (volume graphics) and 2D and 3D contours correctly depict the orientations of the beams and any relative offsets that have been introduced.
When you select the Aggregate Active Beams criterion, the gain/EIRP values for all active beams are aggregated and the aggregate pattern is depicted in 2D contours and 3D volume. Individual beam patterns are not visible. The multibeam antenna aggregate pattern is computed with respect to the body coordinate origin of the parent transmitter and is displayed in the direction of the antenna system's boresight.
Position offsets are ignored when you set the Beam Selection Strategy to Aggregate Active Beams.
Beam summary table
To perform an add, duplicate, remove, or orient operation on a selected beam or a selected group of beams, use one of the following edit options:
| Option | Description |
|---|---|
| Add | Use this to add a new beam with default parameters. A beam ID is automatically assigned. |
| Dupilicate | Use this to duplicate the selected beam or set of beams. A Beam ID is automatically assigned to each copy. |
| Remove | Use this to remove the selected beam or set of beams. |
| Orient | Displays the Antenna Beam Orientation window that enables you to modify the direction of the antenna boresight (Azimuth Angle and Elevation Angle) using a fixed value or incremental values. |
You can also modify the following parameters for a single beam in the Beam Summary table:
| Option | Description |
|---|---|
| Active | Select this check box to make the beam active or clear it to make it inactive. To change the beam status for one or more beams, select the Beam Specs tab. |
| Frequency/Power | To change the frequency or power value, double-click the value to make the field editable. To change this value for one or more beams, go to the Beam Specs tab. |
| Antenna Type | Go to the Antenna tab to change the type of antenna corresponding to the beam. The Beam table will be automatically updated with your change. |
| Polarization Type | Go to the Antenna tab and then the Orientation tab to change the polarization type of the beam. The Beam table will be automatically updated with your change. |
| AzimuthAngle/ElevationAngle | To change these values, click . You can also change these values by selecting the Antenna tab and then the Orientation tab. |
| X/Y/ZPositionOffset | Go to the Antenna tab and then the Orientation tab to change the beam's location in terms of its offset in Cartesian coordinates from the center of the coordinate system of the parent object. The Beam table will be automatically updated with your changes.
Position offsets are ignored when you set the Beam Selection Strategy to Aggregate Active Beams. |
AzimuthAngle/ElevationAngle. To change these values, click . You can also change these values by selecting the Antenna tab and then the Orientation tab.
X/Y/ZPositionOffset. Go to the Antenna tab and then the Orientation tab to change the beam's location in terms of its offset in Cartesian coordinates from the center of the coordinate system of the parent object. The Beam table will be automatically updated with your changes.
Beam Specs. Use to modify beam parameters for one beam or a group of beams. If you select two or more beams, any parameter for which the beams have different values will be designated 'Not Equal'.
Each beam has the following parameters.
- Active indicates whether the beam is active or inactive.
- Beam ID is a unique ID that can consist of any combination of characters.
- Frequency is the frequency of the beam.
- Power is the power of the beam.
- Antenna enables you to modify antenna parameters for one beam or a group of beams. If you select two or more beams, any antenna parameter for which the beams have different values will be designated 'Not Equal'.
For information on antenna types and parameters, see STK Antenna Models. Select the Polarization and Orientation tabs to define the antenna's polarization and orientation parameters for one beam or a group of beams. For parameter definitions, see Antenna Orientation Methods and Polarization.
For more information on modeling, designing, and using multibeam antennas in an STK scenario, see Multibeam Antennas.
Model Specs
Data Rate. The transmitter's data rate is a compound dimension with data bits and time as simple dimensions. For example, a data rate entered as 2.5 kb/msec will be converted to 2.5 megabits per second data rate. Valid data bit units are: bits (b), kilobits (kb), megabits (mb), gigabits (gb), and terabits (tb). Valid time units are seconds (sec), milliseconds (msec), microseconds (usec), nanoseconds (nsec), and picoseconds (psec). Changing the transmitter’s data rate could affect the values of the Modulator.
Modulator
STK Communications allows you to select from multiple modulators, including user-defined modulators. Each modulator has a defined modulation. The modulation determines two characteristics:
- The fraction of transmitter power contained within the receiver’s bandwidth (computed in the Bandwidth Overlap Factor).
- The translation between the signal-to-noise ratio (Eb/No) and the resulting bit error rate (BER). The BER curves used in STK represent theoretical performance curves. When modeling real demodulators, you may want to use an external modulation type with a slightly degraded BER curve. Typical systems run within 1-2 dB of the theoretical values at a given bit error rate. Perfect bit synchronization is assumed when demodulating the data to get a BER.
STK Modulators can be separated into five general categories: Common Analytical Modulators, Jammer/Interference Modulators, Coded Modulators, External File Modulators, and Script Plugin Modulators.
For more information on modulators, see one of the following categories:
- Common Analytical Modulators
- Jammer/Interference Modulators
- Coded Modulators
- External File Modulators
- Script Plugin Modulator
Filter
To specify a filter model, select Use and browse to a filter model. For more information, see Filter Models.
Additional gains and losses
During communications analyses, it is often necessary to model gains and losses that affect performance but are not defined using built-in analytical models. STK enables you to model these by specifying miscellaneous gains and losses to add to the equation.
Post Transmit Gains/Losses. To define a Gain or Loss, click Add. Enter a brief description of the Gain or Loss in the Identifier field and its value in the Gain field. Remember to make it negative if you are entering a Loss. Once you enter the values, you can click Remove to delete an entry from the table. To change an existing entry, simply edit the fields in the grid. The value in the Pre-Receive field will reflect the net value of all gains and losses recorded in the table.