You have disproven the statement:
"Diffraction is the same, regardless of asymmetries in the terrain".
But I didn't say that. I only said it was RECIPROCAL.
Meaning the same on transmit as receive. If it wasn't, HFTA would
ask you whether you wanted to do a transmit analysis or a receive
analysis.
Rick N6Rk
David Gilbert wrote:
> Richard (Rick) Karlquist wrote:
>
> "Diffraction is reciprocal, regardless of asymmetries in the terrain."
>
>
>
> HFTA would seem to suggest otherwise.
>
> I generated four arbitrary terrain profile files and fed them into HFTA
> last evening. Each of the four terrain profiles had a peak 660 feet
> high (I said it was arbitrary) 5,000 feet distant from the antenna. The
> peak for the first profile was broad and smooth on both the near and far
> side. The peak for the second profile was sharp and steep on both near
> and far side. The peak for the third profile was sharp on the near side
> and smoothly broad on the far side, and the peak for the fourth profile
> was smoothly broad on the near side and steep on the far side. I
> assumed a yagi antenna on 14 MHz (8 elements to get more gain
> visibility) at 70 feet above ground.
>
> HFTA shows markedly different takeoff angle profiles for the four
> terrains. In general, HFTA says a peak with a steep near side slope and
> a sharp peak will diffract the signal lower than a more rounded near
> side slope and a broad peak. A peak that is symmetrically sharp appears
> to bend the signal the most, although not much more than if only the
> near side is steep. A peak that is broad on the near side and steep on
> the far side is almost identical to a peak that is broad on both sides.
> For the heights, distances, and terrain shapes I arbitrarily chose,
> either of the terrain profiles with a steep near side slope gave at
> least ten db stronger signals than either of the terrain profiles with a
> broad near side slope at all takeoff angles of six degrees or less. At
> higher angles, the plots tended to be similar with lots of crossing back
> and forth among them.
>
> I played around a bit with different antenna heights and the decibel
> difference between the plots varied somewhat, but the general
> relationships held. I haven't tried to change the distance or height of
> the peak to see what combinations might have the most effect ... I
> simply picked some numbers for a first pass comparison.
>
> Unless I messed up (probable), the two relevant plots from HFTA should
> (might) be available by clicking on the links below:
>
> http://www.mediamax.com/ab7e/Hosted/Diffraction1.jpg
> http://www.mediamax.com/ab7e/Hosted/Diffraction2.jpg
>
> The links for the four terrain files are:
>
> http://www.mediamax.com/ab7e/Hosted/rnd_near-shp_far.PRO
> http://www.mediamax.com/ab7e/Hosted/round_symmetric.PRO
> http://www.mediamax.com/ab7e/Hosted/sharp_symmetric.PRO
> http://www.mediamax.com/ab7e/Hosted/shp_near-rnd_far.PRO
>
> If those files aren't accessible, someone please let me know and I'll
> try to fix the links. I can also send them as attachments directly to
> anyone who asks.
>
> I'd appreciate any comments on this quick and maybe questionable
> analysis, but it seems to suggest that signals approaching a
> non-symmetrically shaped terrain feature from different directions could
> skew differently, and therefore that a signal going one way along a
> specific path might have a different strength than a signal going the
> other direction along that exact same path. It has occurred to me that
> a signal approaching a terrain feature from slightly above the horizon
> might behave differently than a signal approaching that same feature
> from below, but since HFTA won't handle negative takeoff angles I can't
> really check that out, and at six degrees or less I wouldn't think the
> difference would be large anyway.
>
> Simple diffraction aside, real life terrain contains more than one
> feature that would make full reciprocity even less likely, in my
> opinion. According to the algorithms built into HFTA, the net energy
> leaving at any particular angle is the result of multiple combinations
> of reflections and refractions, refractions of reflections, reflections
> of refractions, and so on. Intuitively, it seems impossible to me that
> a ray arriving from a distance at that same angle can somehow split
> itself into those same components in reverse. As a minimum, some of the
> reflecting surfaces available to the outgoing ray might be totally
> shadowed to the return signal by secondary terrain features.
>
> But again, if I'm wrong here I'd appreciate someone correcting me with
> enough explanation that I can understand it.
>
> 73,
> Dave AB7E
>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|
