How Do I Model a Turnstile, Yagi-Uda, or Other Custom Antenna in MATLAB and Export Them for Use in STK?

You can use the MATLAB Antenna Toolbox to model custom antennas in MATLAB. Then, with some scripting support, you can export the antenna radiation pattern as an External Antenna Pattern file, which you can import into STK. The attached PDF document describes how to do all this.
 
The document provides steps for modeling the following three antenna types: a basic turnstile antenna, a single Yagi-Uda antenna, and a cross-polarized Yagi-Uda antenna.
 
Document overview:

1. Modeling Antennas in MATLAB Antenna Toolbox
   1. The turnstile antenna
   2. The Yagi-Uda antenna
   3. The cross-polarized Yagi-Uda antenna
   4. Additional information and other antenna types
2. Formatting and exporting antenna radiation patterns for use in STK
   1. External Antenna Pattern file format
   2. Generating the correct antenna radiation pattern data
   3. Setting up the external antenna pattern file
   4. Writing the antenna radiation pattern data
3. Using an external antenna pattern file in STK
   1. Importing into an Antenna object
   2. Importing into a Transmitter or Receiver object
   3. Displaying the external antenna radiation pattern

Please direct any questions regarding this document to Support@agi.com.