[TowerTalk] HFTA Disc....

[N3AE]

Since a discussion has been started on HFTA, I'll throw in a few observations. 

The terrain around my QTH is complex, with many steep gullies and rapidly changing elevations. We're not talking cliffs or mountains, but rolling terrain intersected by many 30 to 50 ft deep gullies. 

I once downloaded terrain data from the USGS web site. I download both DEM and 3rd arc-second NED data and generated terrain profiles for both using MICRODEM. When plotting these two elevation profiles, they essentially lay on top of each other. But if you look closely, there are differences of up to 4 feet at various distances from the tower. 

When I generate the HFTA takeoff profiles, however, there are large differences between these two "essentially identical" elevation profiles. Some 14 MHz examples: 7.5 db difference at a 6 deg takeoff angle for a 30 ft antenna height (3 element Yagi) and 8 db difference at a 2.5 deg takeoff angle for a 40 ft antenna height. 

Sort of a butterfly effect ... small changes in elevation profiles cause large changes in computed takeoff angle. 

Interestingly, however, the HFTA Figure of Merit for these two elevation profiles isn't very different. 

I've had discussions with Dean on these observation and the preliminary conclusion is that my terrain may cause a lot of spacial aliasing in the HFTA algorithms. As we know, one must sample a sinusoid at a sample rate at least twice the sinusoid's frequency (Nyquist rate). Similarly, when you sample spacial variations (elevations along a radial, for example) one needs to sample more frequently in range if the elevation changes a lot and quickly with range. In my case, the 30 meter "range sample" may be too large to capture the "high frequency" terrain variations between sample points. Or there could be artifacts introduced by the terrain smoothing algorithms within the HFTA code. 

USGS now has 10 meter range resolution data available for many locations, but unfortunately HFTA can only accept 150 range points (including the tower coordinate) along a given azimuth radial. So 10 meter data doesn't "reach out" far enough in range to make a comparison of results with 30m and 10m range sets. 

I think HFTA is a wonderful program, but like any modelling code, it's important to understand the limitations of that code and situations were problems may arise. As Dean mentions in his HFTA instructions, trust the results to +/- 3db. And if something looks funny or unrealistic. make a small change in antenna height (a foot or two). If this small change in antenna height causes large differences in takeoff angle plots, there's probably some spacial aliasing and /or unrealistic diffraction going on along that azimuth radial. Change the antenna height until you find one where a +/- 1 ft change in height doesn't significantly change results. 

I do think it's time for someone to pick up the cloak and try to improve on HFTA. Being able to use 10m resolution data would be one improvement, and perhaps some adjustments of the internal terrain profile smoothing filters. 

As a practicing engineer, I prefer to know what's going on "under the hood" of a model before I fully trust the results, or alternatively have its results compared with a widely accepted model code's output. I don't know if there is a "professional grade" electromagnetic ray tracing code out there one could use for verification, but I'd love to see how HFTA stacks up against one so we can avoid any pitfalls. Any takers? 

The Navy's Advanced Refractive Effects Prediction System (AREPS) package may be such an animal. See http://www.public.navy.mil/spawar/Pacific/AP/Documents/sofAREPS_36.pdf and 
http://www.public.navy.mil/spawar/Pacific/AP/Pages/SoftwarePrograms.aspx I'm not sure since my professional area of expertise is not electromagnetic modelling codes. 

If anyone wants to see my HFTA results and elevation profiles behind this discussion, send me an email off-reflector. 

N3AE