Most are probably familiar with HFTA. A software package written by Dean Straw in 2003 and included with the ARRL Antenna book. This nifty software package uses terrain files based on GPS coordinates of your tower or antenna support location. It generates a model of how your antennas, and more importantly, antenna heights might perform to various locations around the globe.
It used to be a long and tedious process to generate the files needed to use HFTA. Fortunately, K6TU has done an amazing job automating the process that is easy and simple. His website is: k6tu.net
Once you have the necessary files, the fun begins. Here is a current snapshot of what HFTA predicts on 20m to Europe with my 6/6 stack at 118 feet and 50 feet:
Arrow “A” is the current height of my 6/6 stack. Notice the deep null starting at about 7 degrees and ending around 11 degrees. Almost 20% of signals fall in that range.
Arrow “B” is the proposed height of my 6/6 stack. I haven’t completely eliminated the null, but it’s a significant improvement.
The terrain profile above shows a steep drop off to Europe. My tower sits 180 feet above salt water on a bluff, which this graph perfectly depicts. As I ran different scenarios, it showed high antennas in this direction aren’t necessarily the best option.
It took some time playing with different heights and spacing to find the best outcome at 110 feet and 30 feet in an effort to remove that deep null.
My terrain to Japan is a different story. The graph here shows moving the lower antenna from 50 feet to 30 feet, isn’t impacting my signal much as coverage still looks pretty good. Here is the terrain profile for Japan:
I enjoy a nice downhill slope for about 1/2 mile until it climbs again to a hill about one mile away. The hill isn’t great for very low angle work, but fortunately, it’s far enough away that I still have good coverage.
Here’s a graph for U.S. signals:
Not much difference here, except the proposed heights of 30 feet and 110 feet have a better higher angle takeoff after about 16 degrees. Here’s how the terrain looks:
Here’s what caught my eye when comparing 30 feet with my current bottom antenna at 50 feet:
Even at 50 feet, gain makes a downward slope after about 16 degrees, where as the 30 foot antenna stays consistently high throughout the entire range.
If you compare gain at the lowest angles of 0 to 6 degrees, the stack is better. However, HFTA says there aren’t many signals on 20m that fall in that lowest range, where as signals above 15 degrees represent a whopping 58% of all U.S. signals! Huge!
Practical, on air experience tells me it’s not always as simple as just leaving my antennas in one configuration. For example, when the band opens or closes, lower angles rule, but as the day progresses, higher angles come into play. I believe the trick is to have both, and that’s why I’ve embraced HFTA to help figure out what works best for my location.
I haven’t made any proposed changes yet, but plan to once the weather improves and tower work can move forward. If you have your own experiences with HFTA, let me know how it turned out. Until then, stay tuned!