HF Terrain Analysis (HFTA) is an excellent tool for modelling an antenna over real ground. Written by Dean Straw N6BV, HFTA is included on the CD that accompanies the ARRL Antenna Handbook. I don't remember which editiion of the handbook ARRL begain including the CD but its been there for a long time.
HFTA models the performance of an antenna over real ground by examining the diffraction and reflection that occurs in the near field around the antenna based on actual terrain. The near field in this case is the terrain surrounding the antenna out to 4.4 Km.
HFTA comes with extensive documentation describing its use but hasn't been updated for several years to reflect where and how to get terrain data.
SInce I don't use HFTA every day, I find myself having to relearn how to do this everytime - hence this blog post to serve to remind me (and others) what to do.
Getting the terrain data
HFTA needs elevation data that describes the terrain in spot heights above mean sea level around your antenna. You can get elevaton data from two sources depending on where on Earth (literally!) you need the data.
National Elevation Dataset - The NED database holds elevation data at various resolutions for the conterminous United States, Alaska, Hawaii, and territorial islands.
Shuttle Radar Topopgraphy - The SRTM database holds elevation data for over 80% of the Earth's land surface between 60° north and 56° south latitude. Elevation data is available at various resoultions with 1 arc-resolution (30m) for the United States and 3 arc-seconds for the rest of the world. At the time of this post, data at 1 arc-second resolution is being released for territories outside the US.
HFTA can use data at different resolutions including 10m (1/3 arc-second), 30m (1 arc-second) and 90m (3 arc-seconds). I use the highest resolution data that is available for the location I'm modelling.
Getting data from NED
Clicking on the link above will open the National Map Viewer. To access the elevation data, you need to click on the tab marked Overlays, then click on the + icon to expand the Elevation Availability and then select the NED 1/3 data source. Here's a screen shot:
To get the elevation data around the location of the antenna site, you want to click on the tool bar to bring up the "Bounding Box from Coordinates" tool.
This pulls up a dialog box that enables you to enter the boundaries of the geographic area for which you want the terrain data.
To define the rectangle you want, take the location of the antenna and define a box that is 0.1 degrees around the location of the antenna. For example:
Antenna at 37.4N, 122.2W, select boundaries that are south limit 37.3N, 122.2W, north limit 37.5N, 122.2W, east limit 37.4N, 122.1W, west limit 37.4N, 122.3W. Don't forget the note at the bottom of the form "Use the minus sign (-) to indicate Southern and Western Hemispheres"!
When you enter the coordinates and click on draw area, you will be given a choice of different overlays to order for download.
Click on Next and you will get a list of the available datasets to download. Scroll down through the list looking for data with resolution of 1/3 arc-second in IMG format. The form will look like this:
Fill in your email address, re-enter it for verification and then click on Place Order. The National Viewer will show you a form with the order number of your order and let you know that it will email you when the dataset is available for download. The email generally takes about 5 minutes to appear.
Click on the link to download the .ZIP file containing your data set.
Look in the .ZIP file and you wil see a good size file with a .IMG extension. This has to be converted to a GeoTiff file before you can run MicroDEM to generate the FAN data required for HFTA.
This intermediate step is required because the option to get the data in GeoTiff format is no longer offered. Fortuantely the conversion is simple using the GDAL tools which you can download from the GDAL.org website - click on the download link and then select the version of tools for your operating system. Download and install the tools on your system - following the instructions to make the tools available from a command (shell) window.
Open a command shell, locate the directory where you extracted image file and run the following command:
gdal_translate -oF GTiff filename.img filename.tif
Where filename is the name of the file you want to convert.
Now you can run MicroDEM per the instructions from the HFTA documentation and extract the FAN data HFTA needs to do its magic.
Getting Data from SRTM
Open the EarthViewer from the link above and then select the Search Criteria tab. To select the area for elevation data, you need to add the coordinates of the bounding box:
To add the corners of the bounding box, you need to click on the coordinates tab in the middle of the form and then click on Add Coordinate. Use this to add the four corners of your bounding box by adding each corner as a new coordinte.
Now click on the Results button to get the data set:
USe the tool bard to download the dataset you have found and then select the GeoTiff option from the list presented:
You do have to have an account on the system but this is free and can be done by clicking on the Register button at the top of the screen.
The resulting GeoTiff you download is usable by MicroDEM directly without any file conversion being required.
That's it! Refer to the HFTA documentation for using MicroDEM to generate the FANs and then use them in HFTA.