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[TowerTalk] HFTA Disc....

To: TowerTalk <towertalk@contesting.com>
Subject: [TowerTalk] HFTA Disc....
From: N3AE <n3ae@comcast.net>
Date: Sat, 16 Jul 2016 20:59:21 +0000 (UTC)
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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 
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