Transmitter Models

Complex Retransmitter Model

The Complex Retransmitter allows you to select among a variety of analytical and realistic antenna models, and to define the characteristics of the selected antenna type. The following parameters can be set for this model:

Model Specs

Sat. Flux Density. The Saturation Flux Density is the amplifier's saturation point by the input flux density in dBW/m2. This represents the per carrier flux density for systems supporting multiple carriers per transmitter.

Sat. Output Power. The Saturation Output Power is the RF Power output of the transmitter as measured at the input to the antenna when the amplifier is at its saturated state.

Operational Mode. One of the following modes of operation:

  • Use constant output power.

    • This mode models the retransmitter with automatic gain control (AGC) circuitry. The power amplifier output is maintained at the optimum operating power level. This power output level is held constant independently of the input flux density. The AGC dynamic range is assumed to be enough to cover the amplifier's operating power levels.

  • Use receiver antenna gain delta adjusted flux density.

    • This model is suitable for modeling bent-pipe transponders with time-dynamic communication links. The positions of the user terminal transmitters and receivers may be rapidly changing. This necessitates adjustment of the incoming flux density by the receiver's delta gain (maximum gain - current gain) and accounts for the loss of signal power due to the link direction of the satellite's receiver.

  • Use unadjusted receive flux density.

    • This mode is generally used by fixed-geometry communication links (such as GEO satellites and ground stations). The satellite retransmitter's saturated flux density is usually read from the flux density and/or g/T contours. The saturated flux density inherently contains the appropriate receiver gain within the value used in the link analysis and does not change over time.

    Antenna

    You can select to embed an antenna model from the Component Browser or you can link to an antenna object. Antenna objects are listed in the Object Browser.

    To embed an antenna model, select Embed as the Reference Type. On the Model Specs subtab, click the Antenna Models ellipsis button to select an antenna model. You can define polarization and orientation parameters for an embedded antenna using the Polarization and Orientation subtabs. For parameter definitions, see Antenna Orientation Methods and Polarization.

    To link to an antenna object, select Linked as the reference type and select the antenna from the drop-down list. You can define polarization parameters for a linked antenna. For parameter definitions, see Polarization. You cannot modify the antenna's model specification and orientation parameters while in the receiver's basic properties. To modify these parameters, go to the antenna's basic properties.

    If a transmitter or receiver is using a linked antenna, the geometric and vector constraints are not available for that transmitter or receiver because STK pulls the geometric and vector constraints from the antenna.

    For information on antenna types and parameters, and how to link to an antenna on a sensor, see STK Antenna Models.

    • A linked antenna is the focus of the communications link, which means that all geometry and vector computations are carried out on the linked antenna instead of on the receiver to which that antenna is linked.
    • The reference type is only available for transmitters, receivers, and radar objects that are not a child of a Sensor object. If one of these objects is a child to a Sensor, the only option is to use the embedded antenna model. For more information, see Linking to an Antenna that Resides on a Sensor.

The Antenna's frequency is normally taken from the antenna's parent transmitter or receiver, but retransmitters do not have a frequency of their own and use the default frequency value for the computation of gain, beamwidth and the like.

Polarization for Complex UAN Format and Complex ANSYS *.ffd Format antenna models is pulled from the external file that you added to the Antenna's Model Specs tab. Select Use on the Antenna's Polarization tab to enable the polarization specified in the file. You can ignore the remaining options on the polarization tab as they do not apply to these antenna models.

Transfer Functions

For information on defining a frequency, power backoff, or carrier to intermod (C/Im) ratio coefficient, see Transfer Functions.

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.