Hi Paul,
Below is from pages 12 and 13 of Dean's document, perhaps
this might help to explain what you have observed there?
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 ¼steps used in the program
don’t align exactly and artificial spikes (or holes) can be created. This is
inherent in any raytracing program and can only be eliminated by using
extremely small angular step increments —and doing so would slow down execution
even more.
After I do an evaluation for a particular antenna height, I will often specify
an overlay of three heights separated by one foot each. For example, if you are
interested in a single antenna at a height of 80 feet on 14.0 MHz for the
K5MA330.PRO terrain, you might first compare three heights of 79, 80 and 81
feet, bracketing that height. The three curves overlaid on each other look
relatively smooth, except there is a 1.4dB “bump” for the 79foot height.
Now, run three heights of 80, 79 and 78 feet. Now, the curves for 78 and 79
feet look smooth, but the 80foot curve has a noticeable dip. This means that
spurious artifacts of the raytracing process are occurring at 80 feet in the
program —but these would not occur in the real world. The solution: don’t use
the 80 foot point in the computer analysis, but you would mount your real
antenna at that 80foot height if you like the response at 79 or 81 feet.
Hope this help!
73, Billy AA4NU
> On 06/22/2022 1:09 PM Paul Christensen <w9ac@arrl.net> wrote:
>
>
> >"It's an interesting question."
>
> And a good one for Dean Straw to answer. When we conduct an HFTA analysis of
> a single antenna that results in extremely high ground reflection gain
> between 0 and 5 degrees elevation, minor changes in height, even as little as
> 12 inches, result in a significant reflection gain changes at the horizon.
> So, I have to believe that a much more complex stacking model is required
> when using HFTA to get an accurate result.
>
> Paul, W9AC
>
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