Tuesday, November 18, 2008

The VOACAP mystic, an interesting comment...

I received the following emails from Dave AB7E (reproduced here with his authorization). Think he is saying interesting things...Hi, Pat. Wow ... what a rigorous and interesting analysis. Very impressive.
My first reaction, though, is that an antipodal path (ZL-F) is possibly not the most valid for comparing VOACAP predictions to real life propagation. VOACAP assumes a single direct path (either SP or LP), whereas the actual path for an antipodal target could be almost any direction. VOACAP might render its calculation based upon a path over Asia while the actual signal might not take that route at all.
As a real life example, my Optibeam OB16-3 has a pretty narrow beam width on 20m (I can easily hear the difference in 20 degrees of beam heading), yet when working VQ9RD recently (close to antipodal for me), it was almost impossible to find a "best" beam heading within about a 70 degree spread. VOACAP, of course, is based upon heaps of experimental observations taken decades ago, with extrapolations based upon theoretical models of the ionosphere. It doesn't even come close to being a valid predictor of real-time conditions, a fact that the authors were very quick to point out but which hams have conveniently ignored.
It is especially weak in predicting the effects of takeoff angles, which can have a large impact. So overall I'm not surprised that your analysis would find large discrepancies between predicted and actual SNRs, but I suspect that the results would have been closer for a non-polar path of roughly 1/3 the circumference of the earth instead of 1/2.
73, Dave AB7E

pat_f6irf wrote:
Yes David agree... I think Jari that OH6BG summarized it very well in the part 4. The purpose of this article was to draw the attention of contesters and DX-men, on the fact that VOACAP is not suitable at all for low-band DX'ing... What the designers accept, but most hams ignore (including me b4 looking at it !); see http://www.voacap.com/lowband.html
Even the 10degrees vertical angle forecast may be wrong...

Hi again, Pat.
Yes, I think VOACAP is quite poor for low band use, especially since so much of it is gray line dependent. I'm convinced that much of gray line propagation is chordal hop, which is supported by the comments from K3LR and others (as well as my own observations) that high angle antennas often work better for such openings. When you draw a picture of the earth showing an F2 gradient at sunrise/sunset it is easy to see why that would be true. I don't think that VOACAP even considers chordal hop propagation mechanisms at all. I certainly hope you keep up the excellent work, and I think the diversity experiments should be very interesting as well. 73, Dave AB7E

Thanks Dave...
Your email reminded me that already ZL at long path (audio recording) was something for my "Cizirf-special" antenna, which was not designed to be a DX antenna.
Promised, I'll come back with the same type of analysis for shorter distance paths

By the way, I have updated the blog "best of" , an easy way to find articles...

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