Gary,
When I have more than 149 range and elevation pairs in a .pro file, my V1.04
HFTA reports that it is truncating to 149 points but in fact blows up with an
error message when I try to run the takeoff angle computation. HFTA is unhappy
with more than 149 range-elevation pairs along a given azimuth radial.
So if you are successfully using equally spaced 10 meter resolution range data,
then your max range from the tower must be 10*149 = 1490 meters. But when using
equally spaced 30 meter range data, then the max range is 30*149 = 4470 meters.
Perhaps if your terrain beyond 1490 meters doesn't vary too much, it doesn't
make a difference that you're only looking out to 1490 meters. Am I missing
something?
The HFTA instructions shows that you can deal with a distant mountain beyond
the typical 4400 max range by deleting a few of the last few range-elevation
pairs and replacing with the distant mountain's range and elevations.
As an experiment this afternoon, I took a 30 meter resolution DEM data file and
truncated it beyond 1470 meters so I could compare the results with 149 pairs
of 10 meter resolution data that ended at 1490 meters from the tower. Once
again, the two elevation profile plots were essentially on top of each other,
differing by less than 2 ft at any range. But the takeoff angle plots still
differed quite a bit. Not sure what's going on inside HFTA that causes this
"butterfly effect" sensitivity to differences in terrain elevations, at least
in the elevation profiles for my QTH.
Shawn
N3AE
----- Original Message -----
From: "StellarCAT" <rxdesign@ssvecnet.com>
To: "N3AE" <n3ae@comcast.net>, "TowerTalk" <towertalk@contesting.com>
Sent: Saturday, July 16, 2016 6:03:10 PM
Subject: Re: [TowerTalk] HFTA Disc....
I use the 10M range data with HFTA exclusively! The 30 is far too long of a
sample point. I'd prefer even smaller if available. I set up microdem at 5°
increments and never have a problem getting the full data set. Works great.
Gary
K9RX
-----Original Message-----
From: N3AE
Sent: Saturday, July 16, 2016 4:59 PM
To: TowerTalk
Subject: [TowerTalk] HFTA Disc....
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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