Steve London, N2IC, said:
> 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".
<snip>
N6BV: I remember marveling at the number of diffraction points on Steve's
terrain profile.
>
> 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.
N6BV: I agree with Steve that such results have to be looked at with a
jaundiced eye. My own recommendation when using HFTA is to vary the antenna
height by, say, 1-foot increments. If you get big changes, then you are
probably seeing an "aliasing artifact" in the ray tracing. From the HFTA.pdf
operator's manual:
"ACCURACY AND TESTING THE RESULTS
What would I estimate as the "accuracy" of HFTA elevation predictions? I
would say that I would trust the results within plus/minus 3 dB. In other
words, take HFTA results with a grain of salt. Don't obsess with changing
the height of your antenna by fractions of a foot to see what happens!!
Having said that, now I must state that it is a good idea to compare
elevation patterns in intervals of perhaps 1 foot to assess whether HFTA is
generating reasonably smooth results. Often, the ?? steps used in the
program don't align exactly and artificial "spikes" (or "holes") can be
created. This is inherent in any ray-tracing program and can only be
eliminated by using extremely small angular step increments-and doing so
would slow down execution even more."
N6BV: In Steve's case he, in effect, varied the terrain a small amount
rather than the antenna height. Same idea -- flush out the aliasing
artifacts.
Since my retirement, I've been much too busy to re-visit HFTA...! I do
intend, someday soon, to look back at it, and Steve's terrain will be one of
my benchmarks since it is so challenging.
73, Dean, N6BV
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