I have a great deal of respect for N6BV's HFTA (the improved successor
to YT) and the rather sophisticated ray analysis that went into it, but
I think it may be prudent to be a bit careful how rigorously we accept
the takeoff angle overlays. The takeoff angle data was derived by
making thousands of runs with VOACAP for each month over a simulated 11
year sunspot cycle for various paths. While that is probably the best
available source for that kind of data, even the primary author of the
VOACAP program points out that of all the dozens of parameters predicted
by VOACAP, takeoff angle has the least supporting data and has the
highest uncertainty for accuracy. It seems entirely possible that there
may be anomalies in the results for certain paths and/or angles.
That being said, I don't quite get the comment that the 1 and 2 degree
elevation angles represent a similar percentage of openings as the rest
of the higher angles. I've reproduced below a portion of the actual
text data file used by HFTA for the W0-CO-US paths and I don't see
anything at 1 degree and not much at 2 degrees. Maybe the data files
were significantly updated between YT and HFTA.
I also think it is possible that there may indeed be a difference
between North Dakota and Colorado on the results. The takeoff angle
data for DX paths was generated strictly point-to-point, but obviously
some sort of multi-point average must have been used to generate data
for US-to-US paths. If I were generating that data, I'd pick a few key
population centers around the U.S. for the destinations and calculate
some sort of weighted average for the summary based upon population.
If that was done here, the northeast corner of the U.S. would certainly
rank quite high, and it seems possible to me that the path from Colorado
to New York might prefer some very low angles some percentage of the
time whereas energy radiated at those same low angles from North Dakota
might skip over New York almost all of the time. I think N6BV
occasionally lurks here, so maybe he could clarify that for us.
I also agree with K9YC's comment about diffraction. I've played around
with various terrain profiles in HFTA and interestingly enough a hill
with a sharp ridgeline of the right height at the right distance from
the antenna can actually help bend a signal closer to the horizon than
if the hill were not there at all. I would definitely fire up HFTA with
the actual terrain profiles of interest to make the comparison before
making a decision.
Take care es 73,
Dave AB7E
Gary Slagel wrote:
> My understanding is that the YT W0-CO-US elevation angle statistics reflect
> the percentage of time for an opening from CO to the rest of the US for each
> elevation angle. It shows that the 1 and 2 degree elevation angles are open
> to the rest of the US about as much as the rest of the higher elevation
> angles. I assume that means that there would be a certain percentage of
> openings to the rest of the US that I would not be able to take advantage of
> if I have no very low angle radiation. On the other hand the W0-ND-US and
> W0-IA-US statistics show absolutely no openings from these locations to the
> rest of the US at the 1 thru 3 degree elevation angles. I assume that means
> that from these locations, for US/North America contacts and contests, there
> would be pretty much no disadvantage to not having any antenna radiation at
> the 1 to 3 degree elevation angle. So, if my Black Hills location mimics the
> ND and IA statistics there is no harm being a mile from that big elevation.
> But
> if it mimics the CO statistics I am losing a certain percentage of openings
> to the east. And... it surprised me there is so much difference in the
> statistics between CO and ND/IA being that they're in the same general
> geographic area.
>
>
Denver, Co to USA
Elev 80m 40m 30m 20m 17m 15m 12m 10m
1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
2 0.0 0.6 1.0 1.9 2.8 2.2 1.0 0.0
3 2.4 2.9 3.8 6.4 11.3 9.7 7.1 4.1
4 4.1 7.2 3.1 3.4 6.3 8.7 10.2 8.8
5 1.6 2.7 4.6 1.8 4.6 5.6 7.1 6.8
6 0.8 0.4 2.3 1.5 3.9 5.1 7.1 9.5
7 0.4 0.3 1.4 2.6 3.3 4.8 10.0 12.2
8 0.2 0.0 2.2 4.4 3.3 5.2 9.8 14.9
9 0.4 0.0 0.4 4.1 4.5 2.6 4.4 9.1
10 0.0 0.0 0.2 1.6 2.7 2.2 3.8 4.7
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