HFTASweep is a program "wrapper" for the ARRL HF Terrain Analysis program (HFTA) which you can find on the CD-ROM which accompanies the ARRL Antenna Handbook. The program was released around 2004/5 so almost any edition of the Antenna Handbook in the last 10 years should have the current version of HFTA.
HFTASweep runs HFTA 90 times programmatically to model your HORIZONTAL polarized antenna over the actual terrain around your location. The program captures the results for each azimuth direction (4 different azimuth angles at a time) and at the end, builds a VOACAP type 13 antenna model as a file called antenna.13.
The antenna.13 file is a table containing the gain of the modeled antenna over actual terrain for all radiation angles from 0 to 34 degrees (the maximum radiation angle considered by HFTA). The remaining elevation angles simply contain the same data for angles 35 through 90 as the data shows for 34 degrees. This is a reasonable simplifcation for horizontal antennas that are at least a quarter wavelength or more above ground level.
You can upload type 13 antenna files to K6TU.NET and use them in the different prediction options by creating an Advanced Station Configuration from the New option under the Navigation menu. Full details on the Advanced Station Configuration are available on the Advanced Options in the Documentation block at the bottom of the site pages.
IMPORTANT NOTE: The link below contains the latest version of HFTASweep which has been tested on Windows 7, 8 and 10 and found to work on all. The source code for the HFTASweep application and the utiltiy to include HFTA in your PATH environment variable are included. This version of HFTASweep is a complete re-write using the AutoIt! scripting tools. You can find information of AutoIt at their Website via this link.