[TowerTalk] HFTA Accuracy / Usefulness

[Steve London]

Without passing judgment on the usefulness of HFTA, let me tell you my story...

I live on a hilltop, with a net dropoff of 400 feet in the first quarter mile, 
in every direction except west. The dropoff is not smooth and regular - lots of 
bumps in that quarter mile. When I showed Dean, N6BV, the terrain profile I used 
for HFTA, he commented "you have a very complex terrain profile".

When I first moved here, 5 years ago, I spent hours doing analysis with HFTA.

Then, something interesting happened. I was looking at the output of HFTA on 20 
meters using the same antenna and antenna height, with two nearly identical 
terrain profiles. And, I do mean nearly identical - no single data point on one 
terrain profile differed by more than 6 inches in elevation from the other 
terrain profile. Most of the differences were 1 to 3 inches. Certainly within 
the range of uncertainty of any map, or most surveying techniques. Now, you 
would expect that with nearly identical terrain profiles, you would get nearly 
identical results with HFTA. Here's the shocker - the HFTA results were vastly 
different - up to 8 dB difference at some vertical takeoff angles.

Now, I am an engineer. When you make trivial changes to the model input, and get 
non-trivial changes in the model output, you have to be very careful, even 
suspicious, of your interpretation of the model output.

I no longer use HFTA.

73,
Steve, N2IC