HF Terrain Analysis

Why Terrain Analysis?
It doesn't take much operating experience for a Ham to realize that the terrain around their station has a significant impact on the strength of the signal radiated from their antennas. A Ham with a location in a valley isn't going to "get out" as well as the Ham located on a ridge line above surrounding terrain. But beyond these "obvious" cases, most Hams need help in understanding how the surrounding terrain impacts their station and how they may be able to mitigate these effects by choice and placement of antennas.

Fortunately, the ARRL Antenna Handbook companion CD (you know!, the one in the back of the book!) contains a software package called HFTA or HF Terrain Analysis developed by Dean Straw, N6BV when he was the handbook Editor at ARRL.

HFTA models horizontal antennas (dipoles, simple yagis, stacked yagis) at specified heights over terrain and plots the modeled antenna gain at different elevation angles. Here is an example of a 2 element yagi at 10 meters modeled over both flat terrain and a chosen location.

Flat versus 10m 2 ele Yagi

Modeling an antenna over real terrain gives you a visual picture of how terrain impacts performance. You can use HFTA to:

  • Determine optimum height for antennas on an existing tower
  • Compare different tower locations for performance
  • Compare different sites for performance

HF Terrain Analysis
HFTA is a powerful but easy to use tool but requires a set of data files which describe a series of 2-D "slices" of terrain around a chosen location. The slice is a set of step distances from the center (the chosen location) together with the height of terrain at each step. These distance/height pairs are sometimes referred to as "spot heights" as they give the height of terrain at a given spot.

Ideally you need a set of profile files for all azimuth directions around the chosen location - from True North to 359 degrees. In the ARRL Antenna Handbook, N6BV describes a process for generating these profile files using a freely downloadable GIS (Geographic Information System) package called MicroDem to process Digital Elevation Model (DEM) data downloaded from the Internet. An updated version of that process is described in a PDF document from ARRL HFTA Product Notes.

Although the process to generate your own terrain files is not complex, it is cumbersome especially if you only execute the process sporadically. The steps of finding the right web site, specifying the data, downloading it in the right format and then running MicroDem can be lengthy and error-prone.

To add further complications, some of the DEM data sources on the Internet are old and contain data with known positional errors that can generate mis-leading or flat-out wrong results.

Generating an accurate terrain profile
After going through this process manually multiple times, I decided it was past time to automate it. K6TU.NET now has the capability to generate a set of profile files for your location by simply filling in a form and submitting the request. Much like generating a Propagation Prediction, the site captures the information necessary to fulfill the request and then generates the results in the backgound. Once the results are available, an email is sent to you with a link to the results.

Generating a Terrain Profile Request is very simple and starts by selecting Terrain Profile Request from the New menu in the Navigation bar on the right side of the page when you are logged in as a registered user. You do NOT need to be a subscriber to K6TU.NET to access this service - it is freely available to anyone with an activated account.

When you click on New|Terrain Profile Request, you will see a simple form to capture the location for the terrain profile in decimal degrees of Latitude and Longitude that looks like this:

Terrain Profile Request form
Enter a title for the Terrain Profile Request together with the latitude and longitude of the location for which you want the terrain profile. Specify the latitude and longitude in decimal degrees (i.e. convert degrees/minutes/seconds to decimal - you can use this calculator for the conversion).

For optimum results, specify the location to 6 decimal places.

There are several ways of finding the accurate coordinates for a location such as a GPS in position averaging mode, Google Maps etc.

Click on the Save button to store the Terrain Profile Request and then you will see a button Generate Profile - click on this button to submit the request to K6TU.NET to generate the Terrain Profile for the specified location.

K6TU.NET will process your request as a background task and will send you an email when the terrain profile request is complete. The time it takes will depend on how many other requests are pending together with the time to process an individual request.

The process of generating a profile takes the following steps:

  • Determine the terrain elevation data sources available for the requested location
  • Check the stored data on K6TU.NET (about 1 TByte is held locally)
  • Download any required data not found locally (typically 1-3 minutes if necessary)
  • Generate the terrain profile, store for user access and email the user

If all the required data is found locally, the entire process takes about 30 seconds.

When the request is completed, the site creates a Terrain Profile Result which has a link to the ZIP file containing the terrain profile files (360 of them, one for each degree of azimuth) together with an explanation of the source of the terrain data for this profile.

K6TU.NET uses two different sources of elevation data.

  • For locations within the United States (including Alaska, Hawaii and territories), the elevation data source is the National Elevation Dataset with 1/3rd arc-second resolution (~10m). This dataset is almost 500 GB and is stored locally in its entirety on K6TU.NET.
  • For locations outside the United States and for locations between latitude 60 degrees South and 60 degrees North, the elevation data source is the Shuttle Radar Topography Mission in either 3 arc-second (~90m) or 1 arc-second (~30m) resolution depending on location/availability.

Combining HF Terrain Analysis and K6TU.NET
HFTA is a great graphical tool but only models the antenna over at most four azimuth values at a time. The program is intended to evaluate different antenna heights over terrain in a specific direction ( a single azimuth value). But what we really want is to know how the antenna is impacted by terrain in ALL directions.

I developed a wrapper program around HFTA called HFTASweep. HFTASweep runs HFTA under programatic control in order to sweep the antenna around a full 360 degrees and generate an antenna model file that can be used with K6TU.NET. Click on this link to download HFTASweep and instructions. You can also click on the HFTASweep link at the bottom of the site pages under the Documentation block.